THEORY AND DESIGN

OF
AUTOMOTIVE ENGINES
I Introduction
1 General - Historical development of automobiles, Types of power plant, Principle of engine operation,
Classification of engines.
2. Two stroke & four stroke engines; Principles of engine operation (SI & CI), Scavenging - systems,
theoretical processes, parameters, relative merits & demerits; Port timing diagrams, port design.
Relative merits & demerits compared to petrol & diesel engines, scavenging pumps.
II Engine components –
Classification/types, function, materials, construction details, design and manufacturing processes of
the following engine components
3. Cylinders and liners - design, cylinder wear and corrosion, details of water jacket, dry and wet liners,
Cylinder head - design;
4. Piston, piston rings, piston pin - design - stress analysis, methods of manufacture, compensation of
thermal expansion in pistons, heat treatment, piston ring selection, limits of fit for pins 5. Connecting
rod - design, effects of whipping, bearing materials, lubrication
6. Crank shaft - design, firing order, balancing and torsional vibration analysis, vibration dampers,
bearings,. Lubrication
7. Flywheel - design; Camshaft - drives of cams, materials, Types (only descriptive) 8. Valve and valve
mechanism - design, types of valve operating mechanisms, valve springs, guides, push rods, rocker
arms, tappets, valve timing diagrams
9. Crank Case- Design of crank case, oil sumps and cooling features
10. Manifolds-construction and design of inlet and exhaust manifolds.

TEXT BOOKS:
I. High Speed Engines - P .M.Heldt, Oxford & IBH , 1965
2. Auto Design - R.B Gupta, Satya Prakashan, New Delhi 1999

REFERENCE BOOKS:
I.A course in I.c. Engine - Mathur & Sharma, Dhanput Rai & Sons, Delhi, 1994
2.Automobile Engineering VoU & II - Kirpal Singh, Standard publications, New Delhi,
1972 3. Modem Petrol Engine ~ A.W.Judge, B.I. Publications. 1983
4. I.c. Engine - Maleev &Litchy, McGrawHill
5. I.C.Engines - H.B.Keshwani, Standard Pub New Delhi., 1982
6. Fundamentals of I.C.Engines - J.B.Heywood
7. Machine design exercises - S.N.Trikha, Khanna publications, Delhi
8. Automotive mechanics - N.K.Giri, Khanna publications,Delhi
9. Automotive mechanics - William H. Crouse, Tata Mc,Graw Hill Publications Co. New Delhi
10. I.C.Engines and Air Pollution - B.P.Obel'rlntext harper & Roni Pub, New york )

THEORY AND DESIGN OF AUTOMOTIVE ENGINES

Theory and Design of Automotive Engines
CHAPTER - 1
HISTORY
Automobiles through the Years - Since they originated in the late 1800s, automobiles have
changed and developed in response to consumer wishes, economic conditions, and advancing
technology. The first gas-powered vehicles looked like horse buggies with engines mounted underneath
because this was the style to which people were accustomed. By 1910, however, features like the front
mounted engine were already established, giving the automobile a look that was all its own. As public
demand for cars increased, the vehicles became more stylized. The classic cars of the 1920s and 1930s
epitomize the sleek, individually designed luxury cars called the “classic cars.” During the 1940s and
1950s, automobiles generally became larger until the advent of the “compact” car, which immediately
became a popular alternative. The gasoline crisis is reflected in the fuel efficient cars made in the 1970s
and 1980s. Current designs continue to reflect economy awareness, although many different markets
exist.
 The history of the automobile actually began about 4,000 years ago when the first wheel was
used for transportation in India.
In the early 15th century the Portuguese arrived in China and the interaction of the two cultures
led to a variety of new technologies, including the creation of a wheel that turned under its own power.
By the 1600s small steam-powered engine models had been developed, but it was another century
before a full-sized engine-powered vehicle was created.
In 1769 French Army officer Captain Nicolas-Joseph Cugnot built what has been called the first
automobile. Cugnot’s three-wheeled, steam-powered vehicle carried four persons. Designed to move
artillery pieces, it had a top speed of a little more than 3.2 km/h (2 mph) and had to stop every 20
minutes to build up a fresh head of steam.
Cugnot Steam Tractor
-the first self-propelled road vehicle, thus,
the earliest automobile. Powered by steam,
the three-wheeled tractor- invented in 1769
by Nicolas-Joseph Cugnot. designed to
carry artillery, but similar vehicles soon
found many other uses in industry.

As early as 1801, successful

but very heavy steam automobiles
were introduced in England. Laws
barred them from public roads and
forced their owners to run them like
trains on private tracks.
In 1802 a steam-powered coach designed by British engineer Richard Trevithick journeyed more than
160 km (100 mi) from Cornwall to London. Steam power caught the attention of other vehicle builders.
In 1804 American inventor Oliver Evans built a steam-powered vehicle in Chicago, Illinois. French
engineer Onésiphore Pecqueur built one in 1828.
British inventor Walter Handcock built a series of steam carriages in the mid-1830s that were
used for the first omnibus service in London.
By the mid-1800s England had an extensive network of steam coach lines. Horse-drawn
stagecoach companies and the new railroad companies pressured the British Parliament to approve
heavy tolls on steam-powered road vehicles. The tolls quickly drove the steam coach operators out of
business.
During the early 20th century steam cars were popular in the United States. Most famous was
the Stanley Steamer, built by American twin brothers Freelan and Francis Stanley. A Stanley Steamer
established a world land speed record in 1906 of 205.44 km/h (121.573 mph). Manufacturers produced
about 125 models of steam-powered automobiles, including the Stanley, until 1932.

2
Theory and Design of Automotive Engines
Internal-Combustion Engine
Development of lighter steam cars during the 19th century coincided with major developments
in engines that ran on gasoline or other fuels. Because the newer engines burned fuel in cylinders inside
the engine, they were called internal-combustion engines.
In 1860 French inventor Jean-Joseph-Étienne Lenoir patented a one-cylinder engine that used
kerosene for fuel. Two years later, a vehicle powered by Lenoir’s engine reached a top speed of about
6.4 km/h (about 4 mph).
In 1864 Austrian inventor Siegfried Marcus built and drove a carriage propelled by a two
cylinder gasoline engine.
American George Brayton patented an internal-combustion engine that was displayed at the
1876 Centennial Exhibition in Philadelphia, Pennsylvania.
In 1876 German engineer Nikolaus August Otto built a four-stroke gas engine, the most direct ancestor
to today’s automobile engines. In a four-stroke engine the pistons move down to draw fuel vapor into
the cylinder during stroke one; in stroke two, the pistons move up to compress the vapor; in stroke three
the vapor explodes and the hot gases push the pistons down the cylinders; and in stroke four the pistons
move up to push exhaust gases out of the cylinders. Engines with two or more cylinders are designed so
combustion occurs in one cylinder after the other instead of in all at once. Two-stroke engines
accomplish the same steps, but less efficiently and with more exhaust emissions.
Automobile manufacturing began in earnest in Europe by the late 1880s.
German engineer Gottlieb Daimler and German inventor Wilhelm Maybach mounted a gasoline
powered engine onto a bicycle, creating a motorcycle, in 1885.
In 1887 they manufactured their first car, which included a steering tiller and a four-speed
gearbox. Another German engineer, Karl Benz, produced his first gasoline car in 1886.

Early Car
The first practical car, built by German engineer Karl Benz in 1885,
initiated the era of automobile manufacturing. Benz made
improvements to the internal combustion engine and invented the
differential drive and other automotive components. The company Benz
founded grew into one of the largest automobile manufacturers in
Germany.

In 1890 Daimler and Maybach started a successful car manufacturing company, The Daimler
Motor Company, which eventually merged with Benz’s manufacturing firm in 1926 to create Daimler
Benz. The joint company makes cars today under the Mercedes-Benz nameplate.
3
Theory and Design of Automotive Engines
In France, a company called Panhard-Levassor began making cars in 1894 using Daimler’s
patents. Instead of installing the engine under the seats, as other car designers had done, the company
introduced the design of a front-mounted engine under the hood. Panhard-Levassor also introduced, a
clutch and gears, and separate construction of the chassis, or underlying structure of the car, and the car
body. The company’s first model was a gasoline-powered buggy steered by a tiller.
French bicycle manufacturer Armand Peugeot saw the Panhard-Levassor car and designed an
automobile using a similar Daimler engine. In 1891 this first Peugeot automobile paced a 1,046-km
(650-mi) professional bicycle race between Paris and Brest.
Other French automobile manufacturers opened shop in the late 1800s, including Renault.
In Italy, Fiat (Fabbrica Italiana Automobili di Torino) began building cars in 1899.
 4
Theory and Design of Automotive Engines
American automobile builders were not far behind. Brothers Charles Edgar Duryea and James
Frank Duryea built several gas-powered vehicles between 1893 and 1895. The first Duryea, a one
cylinder, four-horsepower model, looked much like a Panhard-Levassor model.

Horseless Carriage
The original
horseless
carriage was
introduced in
1893 by
brothers
Charles and
Frank
Duryea. It
 was
America’s
first internal
combustion
motor car,
and it was
followed by Henry Ford’s first experimental car that same year.

In 1893 American industrialist Henry Ford built an internal-combustion engine from plans he
saw in a magazine. In 1896 he used an engine to power a vehicle mounted on bicycle wheels and
steered by a tiller.
Early Electric Cars
For a few decades in the 1800s, electric engines enjoyed great popularity because they were
quiet and ran at slow speeds that were less likely to scare horses and people. By 1899 an electric car
designed and driven by Belgian inventor Camille Jenatzy set a record of 105.8810 km/h (65.79 mph).
Early electric cars featured a large bank of storage batteries under the hood. Heavy cables connected the
batteries to a motor between the front and rear axles. Most electric cars had top speeds of 48 km/h (30
mph), but could go only 80 km (50 mi) before their batteries needed recharging. Electric automobiles
were manufactured in quantity in the United States until 1930.

Automobiles in the 20th century

For many years after the introduction of automobiles, three kinds of power sources were in
common use: steam engines, gasoline engines, and electric motors.
In 1900 more than 2,300 automobiles were registered in New York City; Boston, Massachusetts; and
Chicago, Illinois. Of these, 1,170 were steam cars, 800 were electric cars, and only 400 were gasoline
cars. Gasoline-powered engines eventually became the nearly universal choice for automobiles because
they allowed longer trips and faster speeds than engines powered by steam or electricity.
Improvements in the operating and riding qualities of gasoline automobiles developed quickly
after 1900. The 1902 Locomobile was the first American car with a four-cylinder, water-cooled, front
mounted gasoline engine, very similar in design to most cars today. Built-in baggage compartments
appeared in 1906, along with weather resistant tops and side curtains. An electric self-starter was
introduced in 1911 to replace the hand crank used to start the engine turning. Electric headlights were
introduced at about the same time.
Most automobiles at the turn of the 20th century appeared more or less like horseless carriages.
In 1906 gasoline-powered cars were produced that had a style all their own. In these new models, a
hood covered the front-mounted engine. Two kerosene or acetylene lamps mounted to the front served
as headlights. Cars had fenders that covered the wheels and step-up platforms called running boards,
which helped passengers, get in and out of the vehicle. The passenger compartment was behind the

5
Theory and Design of Automotive Engines
engine. Although drivers of horse-drawn vehicles usually sat on the right, automotive steering wheels
were on the left in the United States.
 6
Theory and Design of Automotive Engines
In 1903 Henry Ford incorporated the Ford Motor Company, which introduced its first
automobile, the Model A, in that same year. It closely resembled the 1903 Cadillac, which was hardly
surprising since Ford had designed cars the previous year for the Cadillac Motor Car Company. Ford’s
company rolled out new car models each year, and each model was named with a letter of the alphabet.
By 1907, when models R and S appeared, Ford’s share of the domestic automobile market had soared to
35 percent.
 Ford Model T

A Ford Model T rolls off the assembly line. Between 1908 and 1927,
Ford built 15 million Model Ts.

Ford’s famous Model T debuted in 1908 but was called a 1909 Ford. Ford built 17,771 Model
T’s and offered nine body styles. Popularly known as the Tin Lizzy, the Model T became one of the
biggest-selling automobiles of all time. Ford sold more than 15 million before stopping production of
the model in 1927. The company’s innovative assembly-line method of building the cars was widely
adopted in the automobile industry.

Silver Ghost

7
Theory and Design of Automotive Engines
One of the highest-rated early luxury automobiles, the 1909 Rolls
Royce Silver Ghost’s features included a quiet 6-cylinder engine,
leather interior, folding windscreens and hood, and an aluminum body.
Generally driven only by chauffeurs, the emphasis of the luxury car was
on comfort and style rather than speed.

By 1920 more than 8 million Americans owned cars. Major reasons for the surge in automobile
ownership were Ford’s Model T, the assembly-line method of building it, and the affordability of cars
for the ordinary wage earner.
 8
Theory and Design of Automotive Engines
Improvements in engine-powered cars during the 1920s contributed to their popularity:
synchromesh transmissions for easier gear shifting; four-wheel hydraulic brake systems; improved
carburetors; shatterproof glass; balloon tires; heaters; and mechanically operated windshield wipers.
Phaeton
Cars of the 1920s exhibited design refinements such as balloon tires,
pressed-steel wheels, and four-wheel brakes. Although assembly lines
(which originated with Henry Ford in 1908) continued to bring the price
of automobiles down, many cars in this time were one-of-a-kind vintage
models, made to individual specifications. The 1929 Graham Paige DC
Phaeton shown here featured an 8-cylinder engine and an aluminum
body.

From 1930 to 1937, automobile engines and bodies became large and luxurious. Many 12- and
16-cylinder cars were built. Independent front suspension, which made the big cars more comfortable,
appeared in 1933. Also introduced during the 1930s were stronger, more reliable braking systems, and
higher-compression engines, which developed more horsepower. Mercedes introduced the world’s first
diesel car in 1936.
Automobiles on both sides of the Atlantic were styled with gracious proportions, long hoods,
and pontoon-shaped fenders. Creative artistry merged with industrial design to produce appealing,
aerodynamic automobiles.

9
Theory and Design of Automotive Engines
 De Luxe Sedan
The roomy interior and rear-hinged back door of this 1937 Pontiac De
Luxe sedan represent a move toward a car more suited to the needs of
families. With these consumers in mind, cars were designed to be
convenient, reliable, and relatively inexpensive. Vehicles in the 1930s
were generally less boxy and more streamlined than their
predecessors.

Some of the first vehicles to fully incorporate the fender into the bodywork came along just after
World War II, but the majority of designs still had separate fenders with pontoon shapes holding
headlight assemblies. Three companies, Ford, Nash, and Hudson Motor Car Company, offered postwar
designs that merged fenders into the bodywork. The 1949 Ford was a landmark in this respect, and its
new styling was so well accepted the car continued in production virtually unchanged for three years,
selling more than 3 million. During the 1940s, sealed-beam headlights, tubeless tires, and the automatic
transmission were introduced.
Two schools of styling emerged in the 1950s, one on each side of the Atlantic. The Europeans
continued to produce small, light cars weighing less than 1,300 kg (2,800 lb). European sports cars of
that era featured hand-fashioned aluminum bodies over a steel chassis and framework.

10
Theory and Design of Automotive Engines
 Studebaker
T
his

1940 Studebaker Champion two-door sedan was designed by

Raymond Loewy and built by Studebaker craftsmen. Features
emerging in the 1940s include automatic transmission, sealed-beam
headlights, and tubeless tires.

In America, automobile designers borrowed features for their cars that were normally found on
aircraft and ships, including tailfins and portholes. Automobiles were produced that had more space,
more power, and smoother riding capability. Introduction of power steering and power brakes made
bigger cars easier to handle. The Buick Motor Car Company, Olds Motor Vehicle Company
(Oldsmobile), Cadillac Automobile Company, and Ford all built enormous cars, some weighing as
much as 2,495 kg (5,500 lb). The first import by German manufacturer Volkswagen AG, advertised as
the Beetle, arrived in the United States in 1949. Only two were sold that year, but American consumers
soon began buying the Beetle and other small imports by the thousands.

VW Beetle
The

Volkswagen Beetle dominated the market for several years, during which few
modifications were made on the original design. Volkswagen’s name means “car
for the people,” and the car served at least two important consumer needs. The
rear-mounted engine and small, rounded, buglike shape of the European car
represented an appealing combination of look and economy that remained
popular for more than four decades.

That prompted a downsizing of some American-made vehicles. The first American car called a compact
was the Nash Rambler. Introduced in 1950, it did not attract buyers on a large scale until 1958. 11
Theory and Design of Automotive Engines
More compacts, smaller in overall size than a standard car but with virtually the same interior body
dimensions, emerged from the factories of many major manufacturers. The first Japanese imports, 16
compact trucks, arrived in the United States in 1956.
In the 1950s new automotive features were introduced, including air conditioning and
electrically operated car windows and seat adjusters. Manufacturers changed from the 6-volt to the 12-
volt ignition system, which gave better engine performance and more reliable operation of the growing
number of electrical accessories.
By 1960 sales of foreign and domestic compacts accounted for about one-third of all passenger
cars sold in the United States. American cars were built smaller, but with increased engine size and
horsepower. Heating and ventilating systems became standard equipment on even the least expensive
models. Automatic transmissions, power brakes, and power steering became widespread. Styling
sometimes prevailed over practicality—some cars were built in which the engines had to be lifted to
allow simple service operations, like changing the spark plugs. Back seats were designed with no
legroom.

12
Theory and Design of Automotive Engines

Gullwing
Powerful
high-performance cars
such as this 1957
Mercedes-Benz
300SL were built on
compact and
stylized lines. Also called the
Gullwing because its doors open
upward into the shape of a gull’s
wings, the 300SL was capable of
230 kmh (144 mph), its on-road
performance matching its racing
capacity.

El Dorado

This 1957 Cadillac El Dorado convertible epitomizes the large cars of the
“American Dream” era. Tail fins are an example of a trend in car design.
Although the feature did little for the performance of the vehicle, consumers
loved the look, and demanded fins of increasing size until the 1960s.

Mustang
More than 100,000 Ford Mustangs sold during
first four months the model was on the market
in 1964, making it Ford’s best early sales
success since the introduction of the Model T. A
vehicle from the “muscle car” category, the
Mustang’s popular characteristics included a
small, fast design, excellent handling, a
powerful engine, and a distinctive look.

13
Theory and Design of Automotive Engines

In the 1970s American manufacturers continued to offer smaller, lighter models in addition to
the bigger sedans that led their product lines, but Japanese and European compacts continued to sell
well. Catalytic converters were introduced to help reduce exhaust emissions.
Digital speedometers and electronic prompts to service parts of the vehicle appeared in the
1980s. Japanese manufacturers opened plants in the United States. At the same time, sporty cars and
family minivans surged in popularity.
Advances in automobile technology in the 1980s included better engine control and the use of
innovative types of fuel. In 1981 Bayerische Motoren Werke AG (BMW) introduced an on-board
computer to monitor engine performance. A solar-powered vehicle, SunRaycer, traveled 3,000 km
(1,864 mi) in Australia in six days.

MR-2 Turbo

Modern cars like the Japanese 1992 MR-2 Turbo T-bar Toyota are generally
light, aerodynamically shaped, and compact. Japanese imports changed the
automobile industry significantly. The generally reliable, inexpensive cars
increased competition between manufacturers dramatically, to the benefit of
consumers.

New technologies

Gas-Electric Hybrids

The Toyota Prius,

a four-seat hybrid electric vehicle (HEV), was the first HEV to be
marketed when Toyota introduced it in Japan in 1997.

The Honda Insight,

a two-seat HEV, followed in 1999 when it was sold in both Japan and
the United States. The Prius had its U.S. debut in 2000.

14
Theory and Design of Automotive Engines

Gas-Electric Hybrids The Toyota Prius, a four-seat hybrid electric vehicle (HEV), was the first
HEV to be marketed when Toyota introduced it in Japan in 1997. The Honda Insight, a two-seat HEV,
followed in 1999 when it was sold in both Japan and the United States. The Prius had its U.S. debut in
2000.
Pollution-control laws adopted at the beginning of the 1990s in some of the United States and in
Europe called for automobiles that produced better gas mileage with lower emissions. In 1996 General
Motors became the first to begin selling an all-electric car, the EV1, to California buyers. The all
electric cars introduced so far have been limited by low range, long recharges, and weak consumer
interest.
Engines that run on hydrogen have been tested. Hydrogen combustion produces only a trace of
harmful emissions, no carbon dioxide, and a water-vapor by-product. However, technical problems
related to the gas’s density and flammability remains to be solved.
Diesel engines burn fuel more efficiently, and produce fewer pollutants, but they are noisy.
Popular in trucks and heavy vehicles, diesel engines are only a small portion of the automobile market.
A redesigned, quieter diesel engine introduced by Volkswagen in 1996 may pave the way for more
diesels, and less pollution, in passenger cars.
While some developers searched for additional alternatives, others investigated ways to combine
electricity with liquid fuels to produce low-emissions power systems. Two automobiles with such
hybrid engines, the Toyota Prius and the Honda Insight, became available in the late 1990s. Prius hit
automobile showrooms in Japan in 1997, selling 30,000 models in its first two years of production. The
Prius became available for sale in North America in 2000. The Honda Insight debuted in North America
in late 1999. Both vehicles, known as hybrid electric vehicles (HEVs), promised to double the fuel
efficiency of conventional gasoline-powered cars while significantly reducing toxic emissions.
Computer control of automobile systems increased dramatically during the 1990s. The central
processing unit (CPU) in modern engines manages overall engine performance. Microprocessors
regulating other systems share data with the CPU. Computers manage fuel and air mixture ratios,
ignition timing, and exhaust-emission levels. They adjust the antilock braking and traction control
systems. In many models, computers also control the air conditioning and heating, the sound system,
and the information displayed in the vehicle’s dashboard.
Expanded use of computer technology, development of stronger and lighter materials, and
research on pollution control will produce better, “smarter” automobiles.
In the 1980s the notion that a car would “talk” to its driver was science fiction; by the 1990s it
had become reality.
Onboard navigation was one of the new automotive technologies in the 1990s. By using the
satellite-aided global positioning system (GPS), a computer in the automobile can pinpoint the vehicle’s
location within a few meters. The onboard navigation system uses an electronic compass, digitized
maps, and a display screen showing where the vehicle is relative to the destination the driver wants to
reach. After being told the destination, the computer locates it and directs the driver to it, offering
alternative routes if needed.
Some cars now come equipped with GPS locator beacons, enabling a GPS system operator to
locate the vehicle, map its location, and if necessary, direct repair or emergency workers to the scene.
Cars equipped with computers and cellular telephones can link to the Internet to obtain constantly
updated traffic reports, weather information, route directions, and other data. Future built-in computer
systems may be used to automatically obtain business information over the Internet and manage
personal affairs while the vehicle’s owner is driving.
During the 1980s and 1990s, manufacturers trimmed 450 kg (1,000 lb) from the weight of the
typical car by making cars smaller. Less weight, coupled with more efficient engines, doubled the gas
mileage obtained by the average new car between 1974 and 1995. Further reductions in vehicle size are

15
Theory and Design of Automotive Engines
not practical, so the emphasis has shifted to using lighter materials, such as plastics, aluminum alloys,
and carbon composites, in the engine and the rest of the vehicle.
Looking ahead, engineers are devising ways to reduce driver errors and poor driving habits.
Systems already exist in some locales to prevent intoxicated drivers from starting their vehicles. The
technology may be expanded to new vehicles. Anti-collision systems with sensors and warning signals
are being developed. In some, the car’s brakes automatically slow the vehicle if it is following another
vehicle too closely. New infrared sensors or radar systems may warn drivers when another vehicle is in
their “blind spot.”
Catalytic converters work only when they are warm, so most of the pollution they emit occurs in
the first few minutes of operation. Engineers are working on ways to keep the converters warm for
longer periods between drives, or heat the converters more rapidly.
 16
Theory and Design of Automotive Engines
Types of power plant
An engine is a device which transforms one form of energy into another form. However, while
transforming energy from one form to another, the efficiency of conversion plays an important role.
Normally, most of the engines convert thermal energy into mechanical work and therefore they are
called 'heat engines'.
Heat engine is a device which transforms the chemical energy of a fuel into thermal energy and
utilizes this thermal energy to perform useful work. Thus, thermal energy is converted to mechanical
energy in a heat engine.
Heat engines can be broadly classified into two categories:
(i) Internal Combustion Engines (IC Engines) (ii) External Combustion Engines (EC Engines)
 Table 1.1 Classification of heat engines

Engines whether Internal Combustion or External Combustion are of two types, viz.,
(i) Rotary engines (ii) Reciprocating engines
Of the various types of heat engines, the most widely used ones are the reciprocating internal
combustion engine, the gas turbine and the steam turbine. The steam engine is rarely used nowadays.
The reciprocating internal combustion engine enjoys some advantages over the steam turbine due to the
absence of heat exchangers in the passage of the working fluid (boilers and condensers in steam turbine
plant). This results in a considerable mechanical simplicity and improved power plant efficiency of the
internal combustion engine.

Fig.1.1 Classification of heat engines

17
Theory and Design of Automotive Engines
Another advantage of the reciprocating internal combustion engine over the other two types is
that all its components work at an average temperature which is much below the maximum temperature
of the working fluid in the cycle. This is because the high temperature of the working fluid in the cycle
persists only for a very small fraction of the cycle time. Therefore, very high working fluid temperatures
can be employed resulting in higher thermal efficiency.
Further, in internal combustion engines, higher thermal efficiency can be obtained with
moderate maximum working pressure of the fluid in the cycle, and therefore, the weight of power ratio
is less than that of the steam turbine plant. Also, it has been possible to develop reciprocating internal
combustion engines of very small power output (power output of even a fraction of a kilowatt) with
reasonable thermal efficiency and cost.
The main disadvantage of this type of engine is the problem of vibration caused by the
reciprocating components. Also, it is not possible to use a variety of fuels in these engines. Only liquid
or gaseous fuels of given specification can be efficiently used. These fuels are relatively more
expensive.
Considering all the above factors the reciprocating internal combustion engines have been found
suitable for use in automobiles, motor-cycles and scooters, power boats, ships, slow speed aircraft,
locomotives and power units of relatively small output.
External Combustion and Internal Combustion Engines
External combustion engines are those in which combustion takes place outside the engine
whereas in internal combustion engines combustion takes place within the engine. For example, in a
steam engine or a steam turbine, the heat generated due to the combustion of fuel is employed to
generate high pressure steam which is used as the working fluid in a reciprocating engine or a turbine.
In case of gasoline or diesel engines, the products of combustion generated by the combustion of fuel
and air within the cylinder form the working fluid.

Principle of engine operation (4 stroke & 2 stroke operating cycles)

In reciprocating engines, the piston moves back and forth in a
cylinder and transmits power through a connecting rod and crank
mechanism to the drive shaft as shown in Fig1.2. The steady rotation of
the crank produces a cyclical piston motion. The piston comes to rest at
the top center (TC) crank position and bottom-center (BC) [These crank
positions are also referred to as top-dead-center (TDC) and bottom-dead
center (BDC)] crank position when the cylinder volume is a minimum or
maximum, respectively. The minimum cylinder volume is called the
clearance volume.
The volume swept out by the piston, the difference between the
maximum or total volume Vt and the clearance volume, is called the
displaced or swept volume Vd. The ratio of maximum volume to minimum
volume is the compression ratio rc. Typical values of rc are 8 to 12 for SI
engines and 12 to 24 for CI engines.

Fig 1.2
Basic geometry of the reciprocating
internal combustion engine.
Vc, Vd, and Vt, indicate clearance,
displaced, and total cylinder volumes.

18
Theory and Design of Automotive Engines

Fig.1.3 :-The f our-stroke operating cycle.

The majority of reciprocating engines operate on what is known as the four-stroke cycle. Each
cylinder requires four strokes of its piston-two revolutions of the crankshaft-to complete the sequence of
events which produces one power stroke. Both SI and CI engines use this cycle which comprises 1. An
intake stroke, which starts with the piston at TC and ends with the piston at BC, which draws fresh
mixture into the cylinder. To increase the mass inducted, the inlet valve opens shortly before the stroke
starts and closes after it ends.
2. A compression stroke, when both valves are closed and the mixture inside the cylinder is compressed
to a small fraction of its initial volume. Toward the end of the compression stroke, combustion is
initiated and the cylinder pressure rises more rapidly.
3. A power stroke, or expansion stroke, which starts with the piston at TC and ends at BC as the high
temperature, high-pressure, gases push the piston down and force the crank to rotate. About five times
as much work is done on the piston during the power stroke as the piston had to do during compression.
As the piston approaches BC the exhaust valve opens to initiate the exhaust process and drop the
cylinder pressure to close to the exhaust pressure.
4 An exhaust stroke, where the remaining burned gases exit the cylinder: first, because the cylinder
pressure may be substantially higher than the exhaust pressure; then as they are swept out by the piston
as it moves toward TC. As the piston approaches TC the inlet valve opens. Just after TC the exhaust
valve closes and the cycle starts again.
Though often called the Otto cycle after its inventor, Nicolaus Otto, who built the first engine
operating on these principles in 1876, the more descriptive four-stroke nomenclature is preferred. The
four-stroke cycle requires, for each engine cylinder, two crankshaft revolutions for each power stroke.
To obtain a higher power output from a given engine size, and a simpler valve design, the two
stroke cycle was developed. The two-stroke cycle is applicable to both SI and CI engines.

Figure 1.4 shows one of the simplest types of two-stroke engine designs. Ports in the cylinder
liner opened and closed by the piston motion, control the exhaust and inlet flows while the piston is
close to BC. The two strokes are:
A compression stroke, which starts by closing the inlet and exhaust ports, and then compresses
the cylinder contents and draws fresh charge into the crankcase. As the piston approaches TC,
combustion is initiated.

19
Theory and Design of Automotive Engines

Fig.1.4 The two-stroke operating cycle.

A crankcase-scavenged engine
 A power or expansion stroke, similar to that in the four-stroke cycle until the piston approaches
BC, when first the exhaust ports and then the intake ports are uncovered. Most of the burnt gases exit
the cylinder in an exhaust blow down process. When the inlet ports are uncovered, the fresh charge
which has been compressed in the crankcase flows into the cylinder.
The piston and the ports are generally shaped to deflect the incoming charge from flowing directly into
the exhaust ports and to achieve effective scavenging of the residual gases.

Each engine cycle with one power stroke is completed in one crankshaft revolution. However, it
is difficult to fill completely the displaced volume with fresh charge, and some of the fresh mixture
flows directly out of the cylinder during the scavenging process. The example shown is a cross
scavenged design; other approaches use loop-scavenging or uniflow systems

20
Theory and Design of Automotive Engines
Engine classifications

Fig.1.5
IC engine classification

There are many different types of internal combustion engines. They can be classified by:
1. Application.
Automobile, truck, locomotive, light aircraft, marine, portable power system, power generation

2 Basic engine designs

Reciprocating
engines (in
turn subdivided by
arrangement of
cylinders: e.g., in-line, V,
radial,
opposed-ref, fig1.6.),
rotary
engines (Wankel and
other
geometries)

Fig1.6.Engine Classification by Cylinder Arrangements

21
Theory and Design of Automotive Engines
3. Working cycle.
Four-stroke cycle: naturally aspirated (admitting atmospheric air), supercharged (admitting pre
compressed fresh mixture), and turbocharged (admitting fresh mixture compressed in a compressor
driven by an exhaust turbine),
Two-stroke cycle: crankcase scavenged, supercharged, and turbocharged,
Constant volume heat addition cycle engine or Otto cycle engine -SI engine or Gasoline
engine, Constant pressure heat addition cycle engine or Diesel cycle
engine-CI engine or Diesel engine.

4 Valve or port design and location.

Overhead (or I-head) valves,
under head
(or L-head) valves, rotary valves,
cross
scavenged porting (inlet and exhaust
ports on
opposite sides of cylinder at one
end), loop
scavenged porting (inlet and exhaust
ports on
same side of cylinder at one end),
through- or
uni-flow scavenged (inlet and
exhaust ports or
valves at different ends of cylinder)
 (C)

Fig1.7 (a)Cross, (b) Loop, (c) Uniflow Scavenging

classification of SI engine by port/ valve location

22
Theory and Design of Automotive Engines
5. Fuel
Gasoline (or petrol), fuel oil (or diesel fuel), natural gas, liquid petroleum gas, alcohols
(methanol, ethanol), hydrogen, dual fuel

6. Method of mixture preparation.

Carburetion, fuel injection into the intake ports or intake manifold, fuel injection into the engine
cylinder

7. Method of ignition
Spark ignition (in conventional engines where the mixture is uniform and in stratified-charge
engines where the mixture is non-uniform), compression ignition (in conventional diesels, as well as
ignition in gas engines by pilot injection of fuel oil)

8. Combustion chamber design.

Open chamber (many designs: e.g., disc, wedge, hemisphere, bowl-in-piston), divided chamber
(small and large auxiliary chambers; many designs: e.g., swirl chambers, pre-chambers)

9. Method of load control.

Throttling of fuel and air flow together so mixture composition is essentially unchanged, control
of fuel flow alone, a combination of these

10. Method of cooling.

Water cooled, air cooled, un-cooled (other than by natural convection and radiation)

. All these distinctions are important and they illustrate the breadth of engine designs available from a
fundamental point of view. The method of ignition has been selected as the primary classifying feature.
From the method of ignition-spark-ignition or compression-ignition-follow the important characteristics
of the fuel used, method of mixture preparation, combustion chamber design, method of load control,
details of the combustion process, engine emissions, and operating characteristics. Some of the other
classifications are used as subcategories within this basic classification. The engine operating
cycle--four-stroke or two-stroke--is next in importance.

23

Theory
and
Design
of

Automotive Engines
Table 1.2

24
Theory and Design of Automotive Engines
Table 1.3 Engine characteristics Emphasized by Type of Service
References:
1. Microsoft Encarta
2. Fundamentals of IC Engines By J B Heywood
3. Theory & Practice in IC Engines By C F Taylor
4. I C Engines By M L Mathur & RP Sharma
5. I C Engines By Ganesan

25
Theory and Design of Automotive Engines
CHAPTER 2

FOUR-STROKE CYCLE S-I ENGINE - PRINCIPLE OF OPERATION

 Fig: cross section of a SI Engine
In Four-stroke cycle engine, the cycle of operation is completed in four-strokes of the piston or
two revolutions of the crankshaft. Each stroke consists of 180°, of crankshaft rotation and hence a cycle
consists of 720°of crankshaft rotation. The series of operations of an ideal four-stroke. SI engine are as
follows (see Fig.2.1 & 2.2)
1. Suction stroke
Suction stroke 0-1 starts when the piston is at top dead centre and about to move downwards.
The inlet valve is open at this time and the exhaust valve is closed. Due to the suction created by the
motion of the piston towards bottom dead centre, the charge consisting of fresh air mixed with the fuel
is drawn into the cylinder. At the end of the suction stroke the inlet valve closes.
2. Compression stroke.
The fresh charge taken into the cylinder during suction stroke is compressed by the return stroke
of the piston 1-2. During this stroke both inlet and exhaust valves remain closed. The air which occupied
the whole cylinder volume is now compressed into clearance volume. Just before the end of the
compression stroke the mixture is ignited with the help of an electric spark between the electrodes of the
spark plug located in combustion chamber wall. Burning takes place when the piston is almost at top
dead centre. During the burning process the chemical energy of the fuel is converted into sensible
energy, producing a temperature rise of about 2000°C, and the pressure is also considerably increased.
3. Expansion or power stroke.
Due to high pressure the burnt gases force the piston towards bottom dead centre, stroke 3-4,
and both the inlet and exhaust valves remaining closed. Thus power is obtained during this stroke. Both
pressure and temperature decrease during expansion.
4. Exhaust stroke.
At the end of the expansion stroke the exhaust valve opens, the inlet valve remaining closed, and
the piston is moving from bottom dead centre to top dead centre sweeps out the burnt gases from the
cylinder, stroke 4-0. The exhaust valve closes at the end of the exhaust stroke and some 'residual' gases
remain in the cylinder.
Each cylinder of a four-stroke engine completes the above four operations in two engine
revolutions. One revolution of the crankshaft occurs during the suction and compression strokes, and
second revolution during the power and exhaust strokes. Thus for one complete cycle, there is only one
power stroke while the crankshaft turns by two revolutions. Most of the spark-ignition internal
combustion engines are of the four-stroke type. They are most popular for passenger cars and small
aircraft applications.

26
Theory and Design of Automotive Engines
Fig.2.1-The four-stroke spark-ignition (SI) engine cycle (Otto cycle or constant volume cycle)

Fig.2.2-Ideal and actual indicator diagrams for four-stroke SI engine

27
Theory and Design of Automotive Engines
Fig. 2.3 Four-stroke petrol engine valve timing diagram in relation to the pressure volume diagram

Actual Valve Timing Of Four-Stroke Petrol Engine.

Valve timing is the regulation of the points in the cycle at which the valves are set to open and
close. As described above in the ideal cycle inlet and exhaust valves open and close at dead centres, but
in actual cycles they open or close before or after dead centres as explained below. There are two
factors, one mechanical and other dynamic, for the actual valve timing to be different from the
theoretical valve timing.

(a) Mechanical factor.

The poppet valves of the reciprocating engines are opened and closed by cam mechanisms. The
clearance between cam, tappet and valve must be slowly taken up and valve slowly lifted, at first, if
noise and wear is to be avoided. For the same reasons the valve cannot be closed abruptly, else it will
'bounce' on its seat. (Also the cam contours should be so designed as to produce gradual and smooth
changes in directional acceleration). Thus the valve opening and closing periods are spread over a
considerable number of crankshaft degrees. As a result, the opening of the valve must commence ahead
of the time at which it is fully opened (i.e., before dead centres). The same reasoning applies for the
closing time and the valves must close after the dead centres. Fig.2.3 shows the actual valve timing
diagram of a four-stroke engine in relation to its pressure-volume diagram.
b) Dynamic factor;
Besides mechanical factor of opening and closing of valves, the actual valve timing is set taking
into consideration the dynamic effects of gas flow.
Intake valve timing.

28
Theory and Design of Automotive Engines
Intake valve timing has a bearing on the actual quantity of air sucked during the suction stroke
i.e. it affects the volumetric efficiency. Fig.2.4 shows the intake valve timing diagram for both low
speed & high speed SI engines.
 Fig:2.4 Valve timing for low and high speed four-stroke SI engine

It is seen that for both low speed and high speed engine the intake valve opens 100before the
arrival of the piston at TDC on the exhaust stroke. This is to insure that the valve will be fully open and
the fresh charge starting to flow into the cylinder as soon as possible after TDC. As the piston moves
out in the suction stroke, the fresh charge is drawn in through the intake port and valve. When the piston
reaches the BDC and starts to move in the compression stroke, the inertia of the entering fresh charge
tends to cause it to continue to move into the cylinder. To take advantage of this, the intake valve is
closed after BDC so that maximum air is taken in. This is called ram effect. However, if the intake valve
is to remain open for too long a time beyond BDC, the up-moving piston on the compression stroke
would tend to force some of the charge, already in the cylinder, back into the intake manifold. The time
the intake valve should remain open after BDC is decided by the speed of the engine.
At low engine speed, the charge speed is low and so the air inertia is low, and hence the intake
valve should close relatively early after BDC for a slow speed engine (say about 100 after BDC). In high
speed engines the charge speed is high and consequently the inertia is high and hence to induct
maximum quantity of charge due to ram effect the intake valve should close relatively late after BDC
(up to 600 after BDC).
For a variable speed engine the chosen intake valve setting is a compromise between the best
setting for low and high speeds.
There is a limit to the high speed for advantage of ram effect. At very high speeds the effect of
fluid friction may be more than offset the advantage of ram effect and the charge for cylinder per cycle
falls off.

Exhaust valve timing

The exhaust valve is set to open before BDC (say about 250before BDC in low speed engines
and 550before BDC in high speed engines). If the exhaust valve did not start to open until BDC, the
pressures in the cylinder would be considerably above atmospheric pressure during the first portion of
the exhaust stroke, increasing the work required to expel the exhaust gases. But opening the exhaust
valve earlier reduces the pressure near the end of the power stroke and thus causes some loss of useful

29
Theory and Design of Automotive Engines
work on this stroke. However, the overall effect of opening the valve prior to the time the
piston reaches BDC results in overall gain in output.
The closing time of exhaust valve effects the volumetric efficiency, By closing the exhaust
valve a few degrees after TDC (about 150in case of low speed engines and 200in case of high
speed engines) the inertia of the exhaust gases tends to scavenge the cylinder by carrying out
a greater mass of the gas left in the clearance volume. This results in increased volumetric
 efficiency.
Note that there may be a period when both the intake and exhaust valves are open at the same
time. This is called valve over-lap (say about 150in low speed engine and 300in high speed
engines). This overlap should not be excessive otherwise it will allow the burned gases to be
sucked into the intake manifold, or the fresh charge to escape through the exhaust valve.

Table2.1–Typical valve timings for four-stroke SI engines

Note.
Valve timing is different for different makes of engines.
b-before, a-after TDC-Top dead centre,BDC-Bottom dead centre. 30

Theory and Design of Automotive Engines

FOUR-STROKE CI ENGINES- PRINCIPLE OF OPERATION
The four-stroke CI engine is similar to four-stroke SI engine except that a high compression
ratio is used in the former, and during the suction stroke, air alone, instead of a fuel-air mixture, is
inducted. Due to high compression ratio, the temperature at the end of compression stroke is sufficient
to ignite the fuel which is injected into the combustion chamber.
In the CI engine a high pressure fuel pump and an injector is provided to inject fuel into
combustion chamber.
The carburettor and ignition system, necessary in the SI engine, are not required in the CI
engine.
 The ideal sequence of operation for the four-stroke CI engine is as follows:

Fig.2.5 Ideal
P-V Diagram Fig.2.6 Cycle of Operation
1.Suction stroke
Only air is inducted during the suction stroke. During this stroke intake valve is open and
exhaust valve is closed.
2.Compression stroke
Both valves remain closed during compression stroke.
3. Expansion or power stroke
Fuel is injected in the beginning of the expansion .stroke. The rate of injection is such that the
combustion maintains the pressure constant. After the injection of fuel is over (i.e. after fuel cut off) the
products of combustion expand. Both valves remain closed during expansion stroke. 4. Exhaust stroke.
The exhaust valve is open and the intake valve remains closed in the exhaust stroke. Due to
higher pressures the CI engine is heavier than SI engine but has a higher thermal efficiency because of
greater expansion. CI engines are mainly used for heavy transport vehicles, power generation, and
industrial and marine applications.

The typical valve timing diagram for a four-stroke CI engine is as follows

IVO about 300 before TDC

IVO up to 500 after BDC

EVO about 450 before BDC

EVO up to 300 after TDC

Injection about 150 before TDC

31
Theory and Design of Automotive Engines
TWO-STROKE CYCLE ENGINE-PRINCIPLE OF OPERATION
In two-stroke engines the cycle is completed in two strokes, i.e., one revolution of the crankshaft
as against two revolutions of four-stroke cycle. The difference between two-stroke and four-stroke
engines is in the method of filling the cylinder with the fresh charge and removing the burned gases
from the cylinder. In a four-stroke engine the operations are performed by the engine piston during the
suction and exhaust strokes, respectively. In a two stroke engine suction is accomplished by air
compressed in crankcase or by a blower. The induction of compressed air removes the products of
combustion, through exhaust ports. Therefore no piston strokes are required for suction and exhaust
operations. Only two piston strokes are required to complete the cycle, one for compressing the fresh
charge and the other for expansion or power stroke.

Types of two stroke engines

• Based on scavenging method
i) Crankcase & ii) Separately scavenged engine
• Based on scavenging process (air flow)
 i) Cross flow scavenging,
ii) Loop scavenging (MAN, Schnuerle, Curtis type)
iii) Uni-flow scavenging (opposed piston, poppet valve, sleeve valve)
• Based on overall port-timing
i) Symmetrical & ii) Unsymmetrical

Crankcase-scavenged two-stroke engine

Figure 2.7 shows the simplest type of two-stroke engine – the crankcase scavenged engine.
Fig.2.8 shows its ideal and actual indicator diagrams. Fig.2.9 shows the typical valve timing diagram of
a two-stroke engine. The air or charge is sucked through spring-loaded inlet valve when the pressure in
the crankcase reduces due to upward motion of the piston during compression stroke. After the
compression, ignition and expansion takes place in the usual way: During the expansion stroke the air in
the crankcase is compressed. Near the end of expansion stroke piston uncovers the exhaust port, and the
cylinder pressure drops to atmospheric as the combustion products leave the cylinder. Further motion of
the piston uncovers transfer ports, permitting the slightly compressed air or mixture in the crankcase to
enter the engine cylinder. The top of the piston sometimes has a projection to deflect the fresh air to
sweep up to the top of the cylinder before flowing to the exhaust ports. This serves the double purpose
of scavenging the upper part of the cylinder of combustion products and preventing the fresh charge
from .flowing directly to the exhaust ports. The same objective can be achieved without piston deflector
by proper shaping of the transfer port. During the upward motion of the piston from bottom dead centre,
the transfer ports and then the exhaust port close and compression of the charge begins and the cycle is
repeated.

Fig.2.7-Crankcase-scavenged
two-stroke engine

32

Theory and Design of Automotive Engines

Fig. 2.8 Ideal and actual indicator diagrams for a two-stroke SI engine
 Fig.2.9. Typical valve timing diagram of a
two-stroke engine

33
Theory and Design of Automotive Engines
Separately scavenged engine

In the loop-scavenged engine (Fig. 2.10) an external blower is used to supply the charge, under
some pressure, at the inlet manifold.
During the downward stroke of the
piston exhaust ports are uncovered
at about 65° before bottom dead
centre. At about 100later the inlet
ports open
and the scavenging process takes
place.
The inlet ports are shaped so
that most
of the air flows to the top of the
cylinder for
proper scavenging of the upper part of the
cylinder. Piston deflectors are not used as they
are heavy and tend to become overheated at
high output. The scavenging process is
moreefficient in properly designed loop
scavenged engine than in the usual crank-case
compression engine with deflector piston.
Fig.2.10. Loop-scavenged two-stroke engine (separately scavenged
engine)

Opposed piston or end to end scavenged engine (uniflow scavenged) two stroke engine.

In this type of engine the

exhaust ports or
exhaust valves are opened first. The
inlet ports give
swirl to incoming air which
prevents mixing of fresh
charge and combustion products
during the
scavenging process. Early on the
compression stroke
the exhaust ports close. In loop
scavenged engine the
port timing is symmetrical, so the
exhaust port must
close after the inlet port closes.
These timings prevent
this type of engine from filling its
cylinder at full inlet
pressure. In the end-to-end
scavenged engines counter
flow within the cylinder is
eliminated, and there is
less opportunity for mixing of fresh charge and burnt
gases. The scavenging should therefore be more
efficient.

Fig. 2.11. 'End to end' scavenged or uniflow two-stroke

engine

34
Theory and Design of Automotive Engines
Valvetiming for two-stroke engines
 Fig. 2.12(a), (b) and (c) show typical valve timing diagram for a crankcase-scavenged two
stroke engine and supercharged two-stroke engine and a four-stroke engine, respectively.

Fig 2.12
In case of two-stroke engine the exhaust port is opened near the end of the expansion stroke.
With piston-controlled exhaust and inlet port arrangement the lower part of the piston stroke is always
wasted so as far as the useful power output is concerned; about 15% to 40% of the expansion stroke is
ineffective. The actual percentage varies with different designs. This early opening of the exhaust ports
during the last part of the expansion stroke is necessary to permit blow down of the exhaust gases and,
also to reduce the cylinder pressure so that when the inlet port opens at the end of the blow down
process, fresh charge can enter the cylinder. The fresh charge, which comes from the crankcase for
scavenging pump, enters the cylinder at a pressure slightly higher than the atmospheric pressure. Some
of the fresh charge is lost due to short-circuiting. For petrol engine this means a loss of fuel and high
unburnt hydrocarbons in the exhaust.
By comparing the valve timing of two stroke and four-stroke engines, (Fig. 2.12), it is clear that the
time available for scavenging and charging of the cylinder of a two stroke engine is almost one-third
that available for the .four-stroke engine. For a crankcase-scavenged engine the inlet port closes before
the exhaust port whilst for a supercharged engine the inlet port closes after the exhaust port [Fig. 2.12
(b)]. Such timing allows more time for filling the cylinder.

35
Theory and Design of Automotive Engines
Scavenging process

At the end of the expansion stroke, the combustion chambers of a two-stroke engine is left full
of products of combustion. This is because, unlike four-stroke engines, there is no exhaust stroke
available to clear the cylinder of burnt gases. The process of clearing the cylinder of burned gases and
filling it with fresh mixture (or air}-the combined intake and exhaust process is called scavenging
process. This must be completed in a very short duration available between the end of the expansion
stroke and start of the charging process.

The efficiency of a two-stroke engine depends to a great degree on the effectiveness of the
scavenging process, since bad scavenging gives a low mean indicated pressure and hence, results in a
high weight and high cost per bhp for the engine. With insufficient scavenging the amount of oxygen
available is low so that the consequent incomplete combustion results in higher specific fuel
consumption. Not only that, the lubricating oil becomes more contaminated, so that its lubricating
qualities are reduced and results in increased wear of piston and cylinder liners. Poor scavenging also
leads to higher mean temperatures and greater heat stresses on the cylinder walls.

Thus it goes without saying that every improvement in the scavenging leads to improvement in
engine and its efficiency in several directions and hence, a detailed study of scavenging process and
different scavenging systems is worthwhile.

The scavenging process is the replacement of the products of combustion in the cylinder from the
previous power stroke with fresh-air charge to be burned in the next cycle. In the absence of an exhaust
stroke in every revolution of the crankshaft, this gas exchange process for a two-stroke engine must take
place in its entirety at the lower portion of the piston travel. Obviously, it cannot occur instantaneously
at bottom dead centre. Therefore, a portion of both the expansion stroke and the compression stroke is
utilized for cylinder blow-down and recharging.

The scavenging process can be divided into four distinct periods

Fig. 2.13 show the pressure recordings inside the cylinder for a Flat 782 S engine. When the
inlet port opens the gases expanding in the main cylinder tend to escape from it and to pre-discharge
into the scavenge air manifold. This process, called pre-blowdown, ends when the exhaust port opens.
As soon as the exhaust ports are open, the gases existing in the cylinder at the end of expansion stroke
discharge spontaneously into the exhaust manifold and the pressure of the main cylinder drops to a
value lower than that existing in the scavenge air manifold. This process, called blowdown, terminates
at the moment the gas pressure inside the cylinder attains a value slightly lower than the air-pressure
inside the scavenge manifold. During the third phase, called scavenging, which starts at the moment the
spontaneous exhaust gases from the cylinder terminates and ends at the moment the exhaust ports are
closed; the scavenge air sweeps out all residual gases remaining in the main cylinder at the end of the
spontaneous exhaust and replaces them as completely as possible with fresh charge. After scavenging is
complete the fresh charge continues to flow till the scavenge ports are open and the pressure in the
cylinder rises. This results in better filling of the cylinder. This last part of the scavenging process is
called additional-charging.

36
Theory and Design of Automotive Engines
Fig. 2.13 Fiat 782 S engine standard scavenging & typical valve timing diagram of a two-stroke
engine

37
Theory and Design of Automotive Engines

Fig.2.14shows, a typical
pressure-volume diagram
for a two-stroke engine. In this diagram
the total piston
stroke has been divided into power stroke and scavenging
stroke (This division is arbitrary). The area of the p-v
diagram for the power stroke depends very much on the
scavenging efficiency. With proper scavenging efficiency
the pressure rise due to combustion is lower and hence
this area is smaller and lower thermal efficiency is
obtained.
Fig. 2.14 Typical pressure-volume for a two-stroke engine.

Theoretical scavenging processes

Fig. 2.15 Three theoretical scavenging

processes.

Fig.2.15 illustrates three

theoretical
scavenging processes. They are
• Perfect scavenging,
• Perfect mixing and
• Complete shortcircuiting.

mass of delivered air (or mixture) per cycle

Rdel = , compares the actual
{ The delivery ratio
reference mass
scavenging air mass (or mixture mass) to that required in an ideal charging process. (If scavenging
is done with fuel-air mixture, as in spark-ignition engines, then mixture mass is used instead of air
mass.)
The reference mass is defined as displaced volume ⋅ ambient air (or mixture) density. Ambient
air (or mixture) density is determined at atmospheric conditions or at intake conditions. This definition
is useful for experimental purposes. For analytical work, it is often convenient to use the trapped
cylinder mass mtr as the reference mass. OR in other words the delivery ratio is a measure to the air
(mixture) supplied to the cylinder relative to the cylinder content.
If Rdel = 1, it means that the volume of the scavenging air supplied to the cylinder is equal to the
cylinder volume (or displacement volume whichever is taken as reference).
Delivery ratio usually varies between 1.2 to 1.5, except for closed crankcase-scavenged, where it
is less than unity.
mass of delivered air (or mixture) retained
=
ηsc ,
The scavenging efficiency
mass of trapped cylinder charge indicates to what extent the
residual gases in the cylinder have been replaced with fresh air. Ifηsc =1, it means that all gases
existing in the cylinder at the beginning of scavenging have been swept out completely.}

38
Theory and Design of Automotive Engines
(I)Perfect scavenging.
Ideally, the fresh fuel-air mixture should remain separated from the residual combustion products with
respect to both mass and heat transfer during the scavenging process. Fresh air pumped into the
cylinder by the blower through the inlet ports at the lower end of the cylinder pushes the products of
combustion ahead of itself and of the cylinder through the exhaust valve at the other end. There is no
mixing of air and products. As long as any products remain in the cylinder the flow through the exhaust
valves consists of products only. However, as soon as sufficient fresh .air has entered to fill the entire
cylinder volume (displacement plus clearance volume) the flow abruptly changes from one of products
to one of air. This ideal process would represent perfect scavenging with no short -circuiting loss.

(ii) Perfect mixing.

The second theoretical scavenging process is perfect mixing, in which the incoming fresh charge mixes
completely and instantaneously with the cylinder contents, and a portion of this mixture passes out of
the exhaust ports at a rate equal to that entering the cylinder. This homogeneous mixture consists
initially of products of combustion only and then gradually changes to pure air. This mixture flowing
through the exhaust ports is identical with that momentarily existing in the cylinder and changes with it.
For the case of perfect mixing the scavenging efficiency can be represented by the following equation:
− Rdel
sc e
η = 1− , where ηsc and Rdel are scavenging efficiency and delivery ratio respectively. This is plotted in
Fig. 2.15. The result of this theoretical process closely approximates the results of many actual
scavenging processes, and is thus often used as a basis of comparison.

(iii)Short-circuiting.
The third type of scavenging process is that of short-circuiting in which the fresh charge coming from
the scavenge manifold directly goes out of the exhaust ports without removing any residual gas. This is
a dead loss and its occurrence must be avoided.
The actual scavenging process is neither one of perfect scavenging nor perfect mixing. It probably
consists partially of perfect scavenging, mixing and short-circuiting.
Fig. 2.16shows the delivery ratio and trapping efficiency variation with crankangle for three different
scavenging modes., i.e.,perfect scavenging (displacement), perfect mixing and intermediate
scavenging.
Fig. 2.17shows the scavenging parameters for the intermediate scavenging. This represents the actual
scavenging process. It can be seen from this Fig. that a certain amount of combustion products is
initially pushed out of the cylinder without being diluted by fresh air. Gradually, mixing
and short circuiting causes the out flowing products to be diluted by more and more fresh air until
ultimately the situation is the same as for perfect mixing, i.e., the first phase of the scavenging process
is a perfect scavenging process which then gradually changes into a complete mixing process.

Fig,2.16 Delivery ratio and efficiency variation with ` Fig. 2.17 Scavenging parameters for crankcase for
three different scavenging modes. intermediate scavenging

39
Theory and Design of Automotive Engines

Scavenging parameters ..
The delivery ratio - The delivery ratio represents the ratio of the air volume, under the ambient
conditions of the scavenge manifold, introduced per cycle and a reference volume. This reference
volume has been variously chosen to be displacement volume, effective displacement volume, total
cylinder volume or total effective cylinder volume. Since it is only the quantity or charge in the
remaining total cylinder volume at exhaust port closure that enters into the combustion, the total
effective cylinder volume should be preferred. The delivery ratio is mass of fresh air delivered to the
cylinder divided by a reference mass,
mass of delivered air (or mixture) per cycle
Rdel = ,
i.e.,
reference mass
The delivery ratio compares the actual scavenging air mass (or mixture mass) to that required in
an ideal charging process. OR The delivery ratio is a measure to the air (mixture) supplied to the
cylinder relative to the cylinder content.
If Rdel = 1, it means that the volume of the scavenging air supplied to the cylinder is equal to the
cylinder volume (or displacement volume whichever is taken as reference).
Delivery ratio usually varies between 1.2 to 1.5, except for closed crankcase-scavenged, where it
is less than unity.
(If scavenging is done with fuel-air mixture, as in spark-ignition engines, then mixture mass is
used instead of air mass.) The reference mass is defined as displaced volume ⋅ ambient air (or mixture)
density.
Ambient air (or mixture) density is determined at atmospheric conditions or at intake conditions.
This definition is useful for experimental purposes. For analytical work, it is often convenient to use the
trapped cylinder mass mtr as the reference mass.
The trapping efficiency - The amount of fresh charge retained in the cylinder is not same as
that supplied to the cylinder because some fresh charge is always lost due to short-circuiting. Therefore,
an additional term, trapping efficiency, is used to indicate the ability of the cylinder to retain the fresh
charge. It is defined as the ratio of the amount of charge retained in the cylinder to the total charge
(or mixture) retained
ηtr =
mass of delivered air
delivered to the engine, i.e.,
mass of delivered air (mixture)
Trapping efficiency indicates what fraction of the air (or mixture) supplied to the cylinder is
retained in the cylinder. This is mainly controlled by the geometry of the ports and the overlap time.
The scavenging efficiency Scavenging efficiency is the ratio of the mass of scavenge air which remains
in the cylinder at the end of the scavenging to the mass of the cylinder itself at the moment when the
scavenge and exhaust ports of valves are fully closed. It is given by mass of delivered air (or mixture)
retained
=
ηsc ,
mass of trapped cylinder charge
indicates to what extent the residual gases in the cylinder have been replaced with fresh air. If ηsc =1, it
means that all gases existing in the cylinder at the beginning of scavenging have been swept out
completely.
mass of air in trapped cylinder charge purity =
, indicates
The purity of the charge:
mass of trapped cylinder charge the degree of dilution, with
burned gases, of the unburned mixture in the cylinder. mass of delivered air (or mixture)
retained
=
ηch , indicates
The charging efficiency
displaced volume x ambient density how effectively the
cylinder volume has been filled with fresh air (or mixture)
Relative cylinder charge.- The air or mixture retained, together with the residual gas,
remaining in the cylinder after flushing out the products of combustion constitutes the cylinder charge.
Relative cylinder charge is a measure of the success of filling cylinder irrespective of the composition
of charge. The relative cylinder charge may be either more or less than unity depending upon the
scavenging pressure and port heights.

40
Theory and Design of Automotive Engines
Excess air factor,λ - The value (Rdel-1) is called the excess air factor. If the delivery ratio is 1.4,
the excess air factor is 0.4.
Classification based on scavenging process
The simplest method of introducing the charge into the cylinder is to employ crankcase
compression as shown in Fig.2.7. This type of engine is classified as the crankcase scavenged engine. In
another type, a separate blower or a pump (Fig.2.8) may be used to introduce the charge through the
inlet port. They are classified as the separately scavenged engines.

Fig.2.16 Methods of Scavenging (a)Cross Scavenging (b) Loop Scavenging, M.A.N. Type (c)Loop Scavenging
Schüürle Type, (d) Loop Scavenging, Curtis Type
Another classification of two-stroke cycle engines is based on the air flow.
Based on a transversal air stream, the most common arrangement is cross scavenging, illustrated
in Fig.2.16 (a). Most small engines are cross-scavenged. The cross scavenging system employs inlet and
exhaust ports placed in opposite sides of the cylinder wall. The incoming air is directed upward, to
combustion chamber on one side of the cylinder and then down on the other side to force out the exhaust
gases through the oppositely located exhaust ports. This requires that the air should be guided by use of
either a suitably shaped deflector formed on piston top or by use of inclined ports. With this
arrangement the engine is structurally simpler than that with the uniflow scavenging, due to absence of
valves, distributors, and relative drive devices. The inlet and exhaust of gases is exclusively controlled
by the .opening and closure of ports by piston motion. The main disadvantage of this system is that the
scavenging air is not able to get rid of the layer of exhaust gas near the wall resulting in poor
scavenging. Some of the fresh charge also goes directly into the exhaust port. The result of these factors
is poor bmep of cross-scavenged engines.
Based on a transversal air stream, with loop or reverse scavenging, the fresh air first sweeps
across the piston top, moves up and then down and finally out through the exhaust. Loop or reverse
scavenging avoids the short -circuiting of the cross-scavenged engine and thus improves upon its
scavenging efficiency. The inlet and exhaust ports are placed on the same side of the cylinder wall.
In the M.A.N. type of loop scavenge, Fig.2.16(b), the exhaust and inlet ports are on the same side, the
exhaust above the inlet.
In the Schnuerle type, Fig.2.16(c), the ports are side by side. the inlet ports are placed on both
sides of the exhaust ports so that the incoming air enters in two streams uniting on the cylinder wall
opposite the exhaust ports, flows upwards, turns under the cylinder head, then flows downwards the
other side to the exhaust ports. Such a system of air deflection reduces the possibilities of short
circuiting to minimum. With this system flat-top pistons without deflectors are used. The speed of loop
or reversed scavenged engine is not restricted by mechanical limitations because valves are not used,
the charging process being controlled by the piston only. The speed can thus, exceed that of valve
controlled two-stroke engines. Owing to the absence of cams, valves and valve gear, engines are simple
and sturdy. They have a high resistance to thermal stresses and are, thus, well suited to higher
41
Theory and Design of Automotive Engines
supercharge. The major mechanical problem with a loop scavenged two-stroke engine is that of
obtaining an adequate oil supply to the cylinder wall consistent with reasonable lubricating oil
consumption and cylinder wear. This difficulty arises because when the piston is at top dead centre there
is only a very narrow sealing belt available to prevent leakage of oil from crankcase into the exhaust
ports. Since for loop scavenging greater cylinder distance is necessary to accommodate scavenge-air
passage between the cylinder, a strong connecting rod and crankshaft need for supercharged engine can
be used.
The Curtis type of scavenging, Fig.2.16(d), is similar to the Schnuerle type, except that
upwardly directed inlet ports are placed also opposite the exhaust ports.
The most perfect method of scavenging is the uniflow method, based on a unidirectional air
stream. The fresh air charge is admitted at one end of the cylinder and the exhaust escapes at the other
end flowing through according to parallel flow lines normally having a slight rotation to stabilize the
vertical motion. Air acts like an ideal piston and pushed on the residual gas in the cylinder after the
blowdown period and replaces it at least in principle, throughout the cylinder. The air flow is from end
to end, and little short-circuiting between the intake and exhaust openings is possible. Due to absence, at
least in theory, of any eddies or turbulence it is easier in a uniflow scavenging system to push the
products of combustion out of the cylinder without mixing with it and short circuiting. Thus, the
uniflow system has highest scavenging efficiency. Construction simplicity is, however, sacrificed
because this system requires either opposed pistons, poppet valves or sleeve valve all of which increases
the complication.
The three available arrangements for uniflow scavenging are shown in Fig.2.17 A poppet valve
is used in (a) to admit the inlet air or for the exhaust, as the Case may be. In (b) the inlet and exhaust
ports are both controlled by separate pistons that move in opposite directions. In (c) the inlet and
exhaust ports are controlled by the combined motion of piston and sleeve. In an alternative arrangement
one set of ports is controlled by the piston and the other set by a sleeve or slide valve. All uniflow
systems permit unsymmetrical scavenging and supercharging.

Fig.2.17 Uniflow Scavenging

(a) Poppet Valve
(b) Opposed Piston
(c) Sleeve Valve

42
Theory and Design of Automotive Engines

Reverse flow scavenging is shown in Fig.2.17 In

this type the inclined
ports are used and the scavenging air is forced on to the
opposite wall of the
cylinder where it is reversed to the outlet ports. One
obvious disadvantage of
 this type is the limitation on the port area. For long stroke engines operating at
low piston speeds, this arrangement has proved satisfactory.

Fig2.17 Reverse Flow Scavenging

An interesting comparison of the

merits of two cycle engine air scavenging
methods is illustrated in Fig.2.18. In fact,
specific output of the engine is largely
determined by the efficiency of the
scavenging system-and is directly related to
the brake mean effective pressure. As
shown in Fig.2.18 scavenging efficiency
varies with the delivery ratio and the type of
scavenging. In this respect cross scavenging
is least efficient and gives the lowest brake
mean effective pressure. The main reason
for this is that the scavenging air flows
through the cylinder but does not expel the
exhaust residual gases effectively. Loop
scavenging method is better than the cross
scavenging method. Even with a delivery
ratio of 1.0 in all cases the scavenging
efficiencies are about 53, 67 and 80 per cent
for cross scavenging, loop scavenging and
uniflow scavenging systems with
corresponding values of bmep as 3.5,4.5 and
5.8 bar.

Fig.2.18 Scavenging Efficiency

Comparison of different scavengingsystems

Fig.2.19 compares the scavenging efficiencies of three different types of scavenging system. The
cross-scavenging system employs inlet and exhaust ports placed in opposite sides of the cylinder wall.
In the loop scavenging system, inlet and exhaust ports are in the same side of the cylinder wall and in
uniflow scavenging system, the inlet and exhaust port are at opposite ends of the cylinder. It can be seen
that uniflow scavenging gives by far the best scavenging, that loop scavenging is good, and that in
.general, cross-scavenging is the worst.
The scavenging curve for the uniflow scavenging is very near to that of perfect scavenging that
for loop scavenging is near the perfect mixing. With good loop scavenging the scavenging curve is
generally above the perfect mixing curve and that of cross-scavenging engines it is, generally, below the
perfect mixing curve.
Table 2.2 compares the port areas available for different scavenging systems. Largest flow areas
are available with uniflow system. In such a case the whole circumference of cylinder wall is available
43
Theory and Design of Automotive Engines
and the inlet port area can be as high as 35 per cent of the piston area. Due to the use of exhaust valve
the exhaust flow area is small - about 18 per cent. In cross-scavenging the size of the inlet and exhaust
ports is limited to about 25 and 18 per cent of piston area respectively because the ports are located on
the opposite sides of cylinder wall. Schurnle type of loop scavenging requires that both the ports must
be located within about three-quarters of the cylinder circumference. This limits the size of inlet and
exhaust ports to about 18 and 14 per cent of piston area only. The data for a typical four-stroke engine
are also given for comparison. However, while comparing with the four-stroke engine it must be kept in
mind that though the flow area is small, the time available for flow is almost three times more than that
available for the two-stroke engine.

Fig. 2.19 scavenging efficiency, versus delivery ratio of different scavenging system.

Table 2.2 Typical values for areas for different scavenging systems

Loop or cross-scavenged engines with their inlet ports limited half of the cylinder circumference
fall in low speed category. Uniflow scavenged engines with adequate air inlet port are and limited
exhaust port areas fall in medium speed category, whilst the opposed piston engine takes on to high
speeds because of its high rate of exhaust port opening, freedom from valve gear speed limits, good
scavenging and perfect balancing. Un-supercharged uniflow engine has a considerable higher mean
effective pressure than the loop-scavenged engine. There is more freedom in design of combustion
chamber for loop scavenging. This results in low fuel consumption and the engine is simple to make and
easy to produce. Table 2.3 compares the typical bmep values obtainable with different types of
44
Theory and Design of Automotive Engines
scavenging systems. The output of both uniflow and loop scavenged engines is limited 'by the thermal
stresses imposed. But the loop scavenged engine due to its simple cylinder head can better withstand the
thermal stresses.
 45
Theory and Design of Automotive Engines

Table 2.3 Typicalvaluesof bmep for the C.I. two-stroke oil engines
Table 2.4compares the representative port timings for different types of two-stroke engines.

Table 2.4. Port timings for different two-stroke engines

Port design
The Design of the inlet and exhaust ports for two stroke engines depends on various parameters.
Some of the important basic parameters are;
a) Scavenging method
b) Shape, inclination & width of ports
c) Amount of air/charge delivered
d) Scavenging pressure
e) Mean inlet velocity –fn. Of pr. Ratio, temp. of scavenging & scavenging factor f) Duration(crank
angle) of port opening & average port height uncovered by piston Blowdown time area (for
exhaust)–[which is a fn. of temperature of exhaust Gas, expansion end volume(fn. of displacement
volume), exhaust Gas pr., scavenging pr., & indicated mean effective pressure]
g) Inlet duration, exhaust lead* & hence exhaust duration
h) Number of ports & height of ports
*
during exhaust Lead, only exhaust port is kept open, & during super charging only inlet port is kept
open.

46
Theory and Design of Automotive Engines

• THE DIFFERENT SCAVENGING METHODS ARE AS FOLLOWS

 BASED ONSCAVENGING PROCESS( AIR FLOW )

I. CROSS FLOW -for low power o/p engines eg. Two wheelers,
Simple, but more short circuiting, hence more charge loss, super charging
is not possible. It is found that port position is limited with in 50% of
circumference.
 II. LOOP FLOW -for medium o/p engines.
Air takes loop, less short circuiting, hence less charge loss
A. MAN type -intake & exh. ports positioned one below the other. -Good n

B. SCHNURLE type-intake & exh. ports positioned side by side. -Better a

C. CURTIS type -intake on one side & exhaust on the other side. -Best o

III. UNIFLOW (BEST) –for very High o/p engines

Ex. large power marine engines, locomotive engines etc
As intake port is on one side & exhaust port on the other side. & the flow
is uni-directional, ports can be wider. Residual gases are low. Ports can be
located all around the circumference. Opposed piston engines also use this
type. Ports with poppet valves & Sleeve valves have been used.

 BASED ON SCAVENGING METHOD

I. CRANKCASE SCAVENGED ENGINE (crank case compression)

-petroil lubrication is adopted. Hence lubricating oil is also burnt. So
pollution is more. Compression is bad, more petrol consumption, and
more residual gases. Generally used along with symmetrically scavenged
engine, but lower delivery ratio (generally 0.7), Simple and suitable for
small engines. Suitable for low o/p engines (5-20bhp)

II. SEPARATE BLOWER / PUMP SCAVENGED ENGINE

-higher scavenging pressure & delivery ratio is possible. Residual gases
are low. Used in bulky arrangements i.e. above 100 hp engines

 BASED ON OVERALL PORT TIMING

I. SYMMETRICAL PORT TIMING - EPO-IPO-IPC-EPC

-Opening and closing of the ports by the piston is symmetrical.
Advantage-arrangement of the mechanism is very simple.
Disadvantage- more short circuiting, hence more charge loss, super
charging is not possible. Suitable for low power o/p engines up to 5bhp
i.e. scooters / moped engines.

II. UN-SYMMETRICAL PORT TIMING - EPO-IPO-EPC-IPC -Opening and closing of the ports
by the piston is un-symmetrical.
Mechanism is complex.
Advantages- super charging is possible - by the following ways
Supercharging valve-rotary valves,
Poppet valves by suitably designing the cam mechanism,
Using sleeve /slide valve, but it is mechanically
complicated,
47
Theory and Design of Automotive Engines
& using opposed piston

• The common different Shapes of ports are as follows

Rectangular -BEST
With rounded corners, which gives maximum flow area & smooth edges reduce friction
&
 Rhomboidal & Oblong -good w.r.to ring entrance avoidance

Circular-only some applications (only for intake)

Inclination -is given for better mixing, scavenging, turbulence, swirl and combustion. Width
-for Uniflow scavenging -0.6πD (entire circumference available for porting -for lLoop
scavenging -0.2πD (both ports are on same side of the wall)
-for Crossflow scavenging -0.3πD (50% of circumference is available for porting) Ports
should be sufficiently wider for max. flow area, But should not create problem of piston ring entrance
into it.

• Amount of air/charge delivered

The delivery ratio is a measure of the air (mixture) supplied to the cylinder relative to the cylinder
content.
mass of delivered air (or mixture) per cycle
Rdel = ,
The delivery ratio
reference mass
If Rdel = 1, it means that the volume of the scavenging air supplied to the cylinder is equal to the cylinder
volume (or displacement volume whichever is taken as reference).
Delivery ratio usually varies between 1.2 to 1.5, except for closed crankcase-scavenged, where it is less
than unity.
Rdel = 0.7 to 0.8 – for crank case scavenging
Rdel = 1.4 –normal value
Rdel = 1.3 –for fuel economy Rdel = 1.5 –for =
ηsc ,
high o/p
For separately scavenged engines

mass of delivered air (or mixture) retained

The scavenging efficiency
mass of trapped cylinder charge Indicates to what extent the residual
gases in the cylinder have been replaced with fresh air. Ifηsc =1, i.e. all gases existing in the cylinder at
the beginning of scavenging have been swept out completely}

• Scavenging pressure
Proper scavenging pressures to be adopted for the respective scavenging method

• Mean inlet velocity

Mean inlet velocity to be calculated, which is a function of pressure ratio, temp. of scavenging &
scavenging factor.

• Duration(crank angle) of port opening & average port height uncovered by piston With
Duration (crank angle) of port opening, average port height & port timing can be calculated.
48
Theory and Design of Automotive Engines

• Number of ports & height of ports.

No. of ports are selected to ensure enough (max.) width, with sufficient bridge to sustain mechanical
and thermal load & to avoid piston ring failure i.e. entering in port area. After selecting no. of ports,
width of the ports may be calculated and adopted. The height of ports is a major factor in timing of
ports.
 The flow of gases through a two-stroke cycle engine is diagrammatically represented in fig. The
hatched areas represent fresh air or mixture and the cross hatched areas represent combustion gases. The
width of the channels represents the quantity of the gases expressed by volume at NTP condition.

Fig. Scavenging Diagram for Two-stroke Cycle SI Engine

49
Theory and Design of Automotive Engines
Scavenging pumps

Since the pumping action is not carried out by the piston of a two-stroke engine, a separate
pumping mechanism, called the scavenging pump, is required to supply scavenging air to the cylinder.
Different types of scavenging pumps used range from crankcase compression, piston type blowers to
roots blower. The design of a two-stroke engine is significantly affected by the type of scavenging pump
used; hence a careful selection of the scavenging pump is a pre-condition to good performance.
Crankcase Scavenging. The most obvious and cheapest in initial cost is the use of crankcase for
compressing the incoming air and then transferring it to the cylinder through a transfer port. Fig.2.20
shows such a system. This system is, however, very uneconomical and inefficient in operation. This is
because the amount of air which can be used for scavenging is less than the swept volume of the
cylinder due to low volumetric efficiency of the crankcase which contains a large dead space. Thus, the
delivery ratio of a crankcase scavenged engine is always less than unity.
Since the delivery ratio is less than unity it
is not possible to scavenge the cylinder
completely of the products of combustion
and some residual gases always remain in
the cylinder. This results in low mean
effective pressure for the crankcase
scavenged engine. Typical values are 3 to 4
bar. The output of the engine is strictly
limited because the amount of the charge
transferred through the transfer port is only
40-50% of the cylinder volume.

(a) two ports (b) three ports

Fig. 2.20 Two-stoke crankcase scavenged engines
A further disadvantage is that the oil vapors from the crankcase mixes with the scavenging air.
This results in high oil consumption. Because of these disadvantages the crankcase scavenging is not
preferred and for high output two-stroke engines a scavenging pump is a must. Piston, Roots, and
Centrifugal blowers
Piston type blowers as shown in Fig.2.21(a) are used only for low speed and single or two
cylinder engines. For all other type of engines either roots or centrifugal blowers are used. The roots
blower is preferred for small and medium output engines. While the centrifugal blower, is preferred for
large and high output engines. From Fig. 2.22 it is clear that the centrifugal blower has a relatively flat
characteristic curve compared to the steep characteristic curve of the 'roots blower. An increase in the
flow-resistance due to deposits, etc., thus, has a much greater effect on the scavenging air; output of a
centrifugal blower than on that of a roots blower. If deposits accumulate, an engine having a centrifugal
blower will start smoking earlier than that having a roots blower. Therefore, roots blower is preferred
due to its lower sensitivity to flow resistance changes for systems where space for exhaust ports is
limited.
The control of air delivery of centrifugal blowers can be done by throttling the air on the intake side.
This, however, would not reduce the scavenging power required by the centrifugal blower. In the roots
blower the air delivery is controlled by a throttle-actuated by-pass valve between blower inlet and
outlet. Such a control divides the air-flow into two parts and only half the flow passes through the
engine. This saves a substantial amount of scavenging power and hence results in lower specific fuel
consumption.

50
Theory and Design of Automotive Engines
Fig. 2.21 Scavenging-pump types.

Fig. 2.22 Pressure characteristics of centrifugal and roots blower.

51
Theory and Design of Automotive Engines

Comparison of two-stroke SI and CI engines

The two-stroke SI engine suffers from two big disadvantages-fuel loss and idling difficulty. The
two-stroke CI engine does not suffer from these disadvantages and hence CI engine is more suitable for
two-stroke operation.
If the fuel is supplied to the cylinders after the exhaust ports are closed, there will be no loss of
fuel and the indicated thermal efficiency of the two-stroke engine will be as good as that of four-stroke
engine. However, in an SI engine using carburettor, the scavenging is done with fuel-air mixture and
only the fuel mixed with the retained air is used for combustion. To avoid the fuel loss instead of
carburettor fuel injection just before the exhaust port closure may be used.
The two-stroke SI engine runs irregularly and may even stop at low speeds when mean effect
pressure is reduced to about 2bar. This is because large amount of residual gas (more than in four-stroke
engine) mixing with small amount of charge. At low speeds there may be back firing due to slow
burning rate. Fuel injection improves idling and also eliminates backfiring as there is no fuel present in
the inlet system.
In CI engines there is no loss of fuel as the charge is only air and there is no difficulty at idling
because the fresh charge (air) is not reduced.

Advantages and disadvantages of two-stroke engines

Two-stroke engines have certain advantages as well as disadvantages compared to four-stroke
engines. In the following sections the main advantages and disadvantages are discussed briefly.

Advantages of Two-stroke Engines

(i) As there is a working stroke for each revolution, the power developed will be nearly twice that of a
four-stroke engine of the same dimensions and operating at the same speed.
(ii) The work required to overcome the friction of the exhaust and suction strokes is saved. (iii) As there
is a working stroke in every revolution, a more uniform turning moment is obtained on the crankshaft
and therefore, a lighter flywheel is required.
(iv) Two-stroke engines are lighter than four-stroke engines for the same power output and
speed. (v) For the same output, two-stroke engines occupy lesser space.
(vi) The construction of a two-stroke cycle engine is simple because it has ports instead of valves. This
reduces the maintenance problems considerably.
(vii) In case of two-stroke engines because of scavenging, burnt gases do not remain in the clearance
space as in case of four-stroke engines.

Disadvantages of Two-Stroke Engines

(i) High speed two-stroke engines are less efficient owing to the reduced volumetric efficiency. (ii) With
engines working on Otto cycle, a part of the fresh mixture is lost as it escapes through the exhaust port
during scavenging. This increases the fuel consumption and reduces the thermal efficiency. (iii) Part of
the piston stroke is lost with the provision of the ports thus the effective compression is less in case of
two-stroke engines.
(iv) Two-stroke engines are liable to cause a heavier consumption of lubricating oil. (v) With heavy
loads, two-stroke engines get heated due to excessive heat produced. Also at light loads, the running of
engine is not very smooth because of the increased dilution of charge.

52
Theory and Design of Automotive Engines
SI and CI Engine application
We have seen that both SI and CI engines have certain advantages and disadvantages. The
selection of a type of engine for particular application needs consideration of various factors. The SI
engine offers the following advantages:
(1) Low initial cost.
(2) Low weight for a given power output.
(3) Smaller size for a given power output.
(4) Easy starting.
(5) Less noise.
(6) Less objectionable exhaust gas odor and less smoke.
 The SI engine finds wide application in automobiles because passenger comfort and in small
airplanes because of low weight. Two stroke petrol engines finds extensive use in motor cycles,
scooters, mopeds, pleasure motor boats, etc., because of simplicity and low cost. The SI engine is also
used for light mobile duty like lawn movers, mobile generating sets, water pumps, air compressors,
etc...
The CI engine offers the following advantages.
(1) Low specific fuel consumption at both full load and part load conditions.
(2) Utilizes less expensive fuels.
(3) Reduced fire hazard,
(4) Long operating life.
(5) Better suited for supercharging.
(6) Better suited for two-stroke cycle operating, as there is no loss of fuel in scavenging. Because of fuel
economy the CI engine finds wide usage in buses, trucks, locomotives, stationary generating plants,
heavy duty equipment such as bulldozers, tractors and earthmoving machinery. Because of the reduced
fire hazard the CI engine is also used for confined installations and marine use. The great advantage of
the CI engine is lower fuel consumption which counteracts the disadvantage of higher initial cost, if the
engine is used for long duties. (Table 2.6a gives complete comparison of the two types of engines.)

Comparison of two-stroke and four-stroke- engines (table 2.5)

The two-stroke engine was developed to obtain valve simplification and a greater output from
the same size of engine. Two-stroke engines have no valves but only ports (some two-stroke engines are
fitted with conventional exhaust valve). This simplicity of the two-stroke engine makes it cheaper to
produce.
Theoretically a two-stroke engine will develop twice the power of a comparable four-stroke engine
because of one power stroke every revolution (compared to one power stroke every two revolutions of
four-stroke engine). This makes the two-stroke engine cheaper and more compact than a comparable
four-stroke engine.
In actual practice power is not exactly doubled but is only about 30% extra because of (a)
reduced effective stroke, and (b) due to increased heating caused by increased power strokes. The
maximum speed is kept less than 4-stroke engine. The other advantages of the two-stroke engine are
more uniform torque on crankshaft and complete exhaust of products of combustion.
However, when applied to spark-ignition engine the two-stroke cycle has certain disadvantages
which have restricted its use to only small engines suitable for motor cycles, scooters, mopeds, lawn
mowers, out-board engines, etc. In spark-ignition engine (petrol engine) the charge consists of a mixture
of air and fuel. During scavenging, as both inlet and exhaust ports are open simultaneously for some
time, some part of the fresh charge containing fuel escapes with exhaust. This results in high fuel
consumption and hence lower thermal efficiency. The other drawback of two-stroke SI engine is the
lack of flexibility- the capacity to run with equal efficiency at any speed. If the throttle is closed below
the best point, the amount of fresh mixture entering the cylinder is not enough to clear out all the
exhaust, some of which remains to contaminate the fresh charge. This results in irregular running of the
engine.

53
Theory and Design of Automotive Engines
The two-stroke diesel engine does not suffer from these defects. There is no loss of fuel with exhaust
gases as the intake charge in diesel engine is air only. The two-stroke diesel engine is therefore used
quite widely. Many of the biggest diesel engines work on this cycle. They are generally bigger than
60cm bore and are used in marine propulsion.
A disadvantage common to all two-stroke engines, petrol as well as diesel, is greater cooling and
lubrication requirements due to one power stroke in each revolution of crankshaft. Consumption of
lubricating oil is also high in the two-stroke engine due to higher temperatures.

Table 2.5 Comparison of four-stroke and two-stroke cycle engines

 54
Theory and Design of Automotive Engines

Fundamental differences between SI and CI engines

Both SI and CI engines are internal combustion engines and have much in common. However,
there are also certain fundamental differences that cause their operation to vary considerably. These are
given in Table 2.6

Table 2.6 Comparison of SI and CI engines

 55
Theory and Design of Automotive Engines
table 2.6a detailed comparison of SI & CI engines
 56
Theory and Design of Automotive Engines
References
6. Theory & Practice of I C Engines By C F Taylor
7. Fundamentals of I C Engines By J B Heywood
8. I C Engines By M L Mathur & RP Sharma
9. I C Engines By Ganesan

57
Theory and Design of Automotive Engines
 Chapter-3
Cylinder Block
Forms the basic frame work of the engine it houses engine cylinders, where combustion take
place & serves as a bearing & guide for piston reciprocating in it. It carries lubricating oil to various
components through drilled passages.
At lower end the crank case is cast integral with the block. At the top, is attached the cylinder
head. Besides, other parts like timing gear, water pump, ignition distributor, flywheel, fuel pump etc.
are also attached
Around cylinders, there are passages for circulation of cooling water

Cylinder heads, Cylinders & liners

Most modern automotive engines have all of their cylinders and the greater part of their
crankcase poured in a single casting, so that cylinders and crankcase form a single unit. However,
cylinders and crankcase perform different functions.

Separate Vs. Integral Cylinder Heads.

Cylinder heads now almost always are made separate castings, which are secured to the cylinder
block with studs and nuts, with a gasket in between to ensure a gas-tight joint. The cylinder head can be
cast integral with the block, and at one period in engine development that was the predominant practice.
With integral cylinder heads there is, of course, no machining of joint surfaces and no need for a
gasket, but the cylinder casting is much more difficult to produce, and. besides, with the design which
was usually employed, cooling of the combustion-chamber walls was less effective-the wall
temperature of each combustion chamber being less uniform-than in an engine with a detachable head.

58
Theory and Design of Automotive Engines
In the case of L-head engines with integral cylinder heads, the valves were introduced through
openings in the head which were closed by threaded plugs generally referred to as "valve caps." These
plugs presented to the hot gases in the cylinder a considerable surface which was not water-cooled, and
which therefore formed "hot spots." It was customary to screw the spark plug into one of these "valve
caps." Since the insulator of the plug naturally is a poor
conductor of heat, and the additional threaded joint also
formed an obstruction to heat flow, this further
aggravated the situation with respect to "hot spots" and
made it
necessary to keep the compression quite low.
With the valve-in-head type of cylinder there are
two
alternate designs of integral heads. With one of these,
exemplified
in Fig, 1, the valves seat directly on the metal of the
head, but this
has the disadvantage that when they are to be reground,
the whole
block has to be removed from the car. With the other, use
is made
of so-called valve cages, that is, cylindrical sleeves
which are set
into bores in the cylinder head and retained therein between a
shoulder and a ring nut. The valve seat is fom1ed on the inner end
of the cage, and there is a port in the wall of the latter through
which the gases flow from or into a valve passage cast in the
cylinder head. The objection to valve cages is that they add another
"joint" to the path for heat flow from the valve head to the jacket
water, and therefore result in higher valve temperatures
(particularly of the exhaust valve), which promotes detonation and
makes the construction unsuitable for high speed, high-compression engines.
Fig.1. Cylinder with integral head
When the cylinder head is a detachable casting, the cylinder and jacket cores can be more
securely supported in the mold, and the cylinder castings are likely to be more nearly true to pattern,
with the result that after the cylinder is finished, its walls will be more nearly uniform in thickness.
With an engine having a removable head it is possible to thoroughly clean the combustion
chamber of carbon, by scraping, after the head has been removed. If it is desired to locate the valves in
the head, they may be seated directly on a water-cooled surface.
One reason for the continued, limited use of integral heads is that they avoid trouble due to
distortion of the upper or outer end of the cylinder bore due to the drawing up of the cylinder-head
retaining nuts. Such trouble is experienced occasionally, with detachable cylinder heads (blow-by past
piston rings, leakage past valves, and excessive oil consumption), but it can be guarded against by
performing the final finishing operation on the bore with a dummy cylinder head in place~ This
produces a bore which is true when the retaining nuts are tightened.

Gaskets
Copper-Asbestos Gaskets.
Separate cylinder heads were
rendered practical by
the introduction of the copper-asbestos
gasket. This consists
of an asbestos sheet cut or stamped to the
required form,
which is armored with thin sheet copper.
There is a copper
sheet on each side of the asbestos sheet, and the two copper
sheets lap along the outer edges of the asbestos sheet, so that
the latter is completely encased. Copper grommets are
inserted in the waterway openings and sometimes also in the combustion-chamber openings. In heavy
duty engines the combustion-chamber grommet of the gasket may be reinforced by a copper-wire loop
or a copper washer. In these copper-asbestos gaskets the copper provides the tenacity and the asbestos
the compressibility needed in a packing. A gasket for a four-cylinder L-head engine is shown in Fig.2.
 59
Theory and Design of Automotive Engines

Steel-Encased and Other Gaskets.

Cylinder-head gaskets are made also of asbestos sheet encased in steel instead of copper. Cold
rolled, deep-drawing steel is used, and is rust-proofed to prevent trouble from corrosion. Among the
rust-proofing processes applied to sheet steel for gaskets are tinning, electro-galvanizing, and terne
plating. Steel, being harder, does not have as good sealing properties as copper, and a sealing coat of
some heat-resistant, non-hardening material is generally applied to the gasket, either in the
manufacturing process or during installation. The edges of the steel sheet, of course, are not rust
proofed, and some steel-encased gaskets are fitted with copper grommets at the waterways. The
principal advantage of steel- over copper-encased gaskets is that the production cost of the former is
about 20 per cent less.
Another type of gasket comprises a central steel core with a layer of .coated and graphited
asbestos on each side thereof, the asbestos being bonded to the core by means of integral steel tangs
clinched into it. These gaskets, which are used chiefly in the engines of low-priced passenger cars,
generally are provided with steel grommets at the combustion-chamber and waterway openings, one
manufacturer is using a cylinder head gasket consisting of a sheet of SAE No. 1010 steel 0.015 in. thick,
which is corrugated around the openings therein, including those for the cylinder-head studs. The
corrugations have a spring action. and the sealing properties of the gasket are further improved by
applying a coating of a heat-resistant lacquer to both sides.

Cylinder-Head Studs.
To obtain a gas-tight permanent joint with a cylinder-head gasket it is necessary to make
provision for an adequate number of studs distributed as nearly uniformly as possible. With L-head
cylinders from 16 to 20 studs are used for a four-cylinder block, from 24 to 26 for a six-cylinder, and
from 30 to 32for an eight-cylinder. With , valve-in-head cylinders only two rows of studs are required,
instead of three, and the total number therefore is less, viz., 12 for a four-cylinder block, 16 for a six
cylinder, and 20 for an eight-cylinder. To prevent distortion of the casting by drawing up the nuts, there
must be plenty of metal in the bosses for the studs, and the studs must not be too near the valve seats. In
the design of the heads careful attention must be given to the avoidance of pockets which might form
steam traps. It is not necessary to use very large water ports. Moderate-sized ports judiciously
distributed, are better, as they make it easier to prevent leaks.

Cylinder Material.
In the past automobile-engine cylinders have been generally cast of close-grained gray iron
approximating the following composition.
Percent
Silicon 1.9 to 2.2
Sulphur not over 0.12
Phosphorus not over 0.15
Manganese 0.6 to 0.9
Combined carbon 0.35 to 0.55
Total carbon 3.2 to 3.4
The SAE has standardized five grades of cast iron, of which four are recommended for cylinder
blocks and cylinder heads as follows: No. 111 for small cylinder blocks; No. 120 for cylinder blocks
generally. No.121 for truck and tractor-, and No. 122 for diesel engine cylinder blocks. Pistons also are
cast of these irons.
It was determined from tests conducted, that to obtain the better physical properties the total
carbon & silicon contents must be reduced and the phosphorus content held to a lower limit. Among
other points usually covered in specifications for cylinder castings arc the following: Castings must be
smooth, well cleaned and free from shrinkage cavities, cracks and holes, large inclusions, chills, excess
free carbides and any other defects detrimental to machinability, appearance, or performance. They
must finish to the size specified. When tensile tests are provided for, the portion of the casting from
which the test piece is to be machined is usually specified. .

60
Theory and Design of Automotive Engines
The use of steel for cylinders has often been suggested, and for racing and aircraft engines,
cylinders are sometimes made from hollow steel forgings. Several American manufacturers use cylinder
castings of semi-steel, more properly called high-test cast iron. This material is made by adding a
certain percentage of scrap steel to the melt of cast iron, which results in a finer grain and in somewhat
better tensile properties.
To make it possible to successfully cast a multiple-cylinder block with thin walls, the iron must
pour well and have a "long life" (as the foundry men call it). These characteristics are strengthened, by
high phosphorus content, but, unfortunately, this element tends to make the iron soft and less resistant to
wear.

Nickel-Chromium irons.
Certain iron ore mined in Cuba contains small percentages of nickel and chromium, and the
metal made from this are, known as Mayari iron, is sometimes added to gray iron for cylinder castings:
Mayan iron therefore is a natural alloy. It is claimed that it is free from oxidation & has a lower
solidification point, and that the "longer life" of the iron improves the "feeding" of castings when they
are properly gated, in spite of low phosphorus content. Castings when sectioned -show sound metal
even where there are heavy bosses and thick sections. Cylinder castings made of a mixture containing
10 per cent of Mayari iron showed a tensile strength of 36,740 psi, according to makers of the iron; a
transverse strength of 4250 lb, and a Brinell hardness of 223-229. The same iron is also used for
cylinder heads and pistons. Results similar to those from Mayari iron are being obtained by the addition
of small quantities of nickel and chromium, and such alloy irons are now used not only for cylinder
blocks, but also for pistons, particularly for heavy duty, commercial-vehicle engines.
The chief advantage of alloyed irons is that they possess greater hardness and wear resistance,
and that without being harder to machine. The machinability of grey iron is dependent upon the absence
of excess iron carbide of chilled or hard spots. Nickel acts to eliminate both, and so to improve
machinability. In many cases the alloyed iron, although having a Brinell hardness from 30 to 40 points
greater, is actually easier to machine than ordinary gray iron.
When nickel is used alone as an alloying element, the content usually ranges between 1.25 and
2.5%, whereas if it is used in combination with chromium, the nickel content ranges between o.50 and
1.50 % and that of chromium between 0.25 and 0.50 % it is claimed that a combed content of nickel and
chromium of 1 per cent will give cast iron with a Brinell hardness of 207-217; of 2 per cent, 223-235,
and of 3 per cent, 241-255.
Chromium and nickel, however, are not the only alloying elements purposely added to cylinder
irons; others added to improve the fluidity of the molten iron, the resistance of the iron to wear, its
machinability, or both of the latter qualities, include, molybdenum, vanadium and titanium.

Copper and Molybdenum Additions.

Copper is of value in cylinder irons in that it tends to prevent chill in thin sections and to give a
finer grain structure in the heavier sections, thus acting the part of a stabilizer, It also increases the
fluidity of the iron and acts as a "graphitizer"; it hardens and tightens up the matrix so that “sponginess”
is reduced. The improvement due to copper is well shown in transverse tests, and these additions are
particularly effective in the presence of high manganese and of nickel or chromium.
Molybdenum increases the resistance to wear of cast iron, especially at higher temperatures.
This results from the refining action it has on the grain, and from the finer division of graphite which it
brings about. It increases the Brinell hardness-although in this respect it is not as effective as an equal
proportion of chromium and it accomplishes this without rendering the metal less machinable. It also
increases the tensile strength and the toughness of the metal. Where there is a tendency for the castings
to crack owing to faults in either the design or the foundry technique-molybdenum is often of benefit. It
is mostly used in combination with either chromium alone or with both nickel and chromium.

Heat Cracks in Cylinder Walls.

Cracks in L-head cylinder castings (especially in large ones) sometimes start at the sharp edge
formed by the cylinder bore and the valve-passage wall. This edge reaches a very high temperature,

61
 Theory and Design of Automotive Engines
because the hot gases pass over it during the exhaust period, and a crack naturally starts easily at a sharp
edge. Rounding off this edge has been found a good preventative against heat fatigue cracks. Cracks
may start also at either the inlet- or exhaust-valve seat. It was shown that such cracks usually are the
result of pre-ignition. The latter causes local overheating of the combustion-chamber wall, and the crack
forms when the overheated metal cools again. By installing a "hot" spark plug in one cylinder and then
running the engine under full load at from 3000 to 3500 rpm, cracks could be produced at will. The
"hot" plug causes pre-ignition, and usually one 10-minute run under these conditions resulted in the
formation of a crack, though sometimes several such runs were required.

Cylinder Wear.
The characteristic which is most important in judging cylinder irons is their resistance to wear
under engine- operating conditions. As the cylinder bore wears, the engine loses power, consumes
excessive quantities of oil, and gives off smoke in the exhaust. In fact, the rate of oil consumption is
usually taken as an index of the state of wear of the cylinder bore.
It was observed many years ago that the wear of cylinder bores is very non-uniform. It is greatest at the
top end of piston travel (under the topmost ring with the piston at the end of its up-stroke), and
decreases rather rapidly from there down. (Fig. 3.) It has been pointed out that cylinder wear is due to
three separate causes, viz.,
• Abrasion, which is due to foreign particles in the oil film;
• Erosion, which is due to
metal-to-metal
contact between the cylinder
wall on the
one hand and the piston and
rings on the
other; and
• Corrosion, which results from
chemical
action on the cylinder walls by the
products of combustion.
The order of importance of the three
causes varies with conditions of operation.
That corrosion may play an important part in the wear of cylinder bores, it was found that accelerated
cylinder wear occurs at low cylinder temperatures and is attributable to corrosion resulting from
deposition of acid-bearing moisture on the cylinder walls. The reasons for assuming corrosion to be
responsible were briefly as follows:
1. The pitted and discolored appearance of the cylinder walls and piston rings after low-temperature
operation.
2. The fact that increased wear begins just below the calculated dew point.
3. The detection of acids in the water of combustion.
4. A large reduction in the rate of wear obtained with hydrogen fuel.
5. A reduction in wear obtained when using corrosion-resisting materials.
The research work showed that corrosion is largely due to carbonic acid formed by the solution
of carbon dioxide, a product of combustion, in water condensed from the gases of combustion. When
hydrogen is used as fuel there is no carbon dioxide in the exhaust, so that no carbonic acid can form.

Effect of Cylinder Material on Rate of Bore Wear.

The result of the
Brinell test is generally
regarded as bearing some
relation to the rate of
cylinder wear. That hardness
is a factor in wear
resistance is indicated by the
fact that heat
treated liners of alloyed iron with a Brinell
hardness of slightly over 500, have been found
to require reconditioning of the bore (by re
grinding) only one third as often as the bores of
gray-iron cylinder blocks with a Brinell hardness of around 200. Cylinders with soft or "porous" spots
which are readily detected by the Brinell test, usually show a high rate of wear, but differences in
62
Theory and Design of Automotive Engines
hardness within the usual range specified for gray-iron cylinder castings, say. 180 to 230 Brinell, have
little effect on the resistance to wear.

Cylinder Stress and Wall Thickness.

With the usual compression ratio of between 7 and 8 (for passenger-car engines) a maximum
explosion pressure of about 700 psi may be figured with. Now consider a section of a cylinder of b in.
bore and 1 in. long, as represented in Fig. 4. The pressure developed in the cylinder by the explosion
tends to rupture the wall along lines parallel with the cylinder axis and at opposite ends of a diameter.
With a maximum combustion pressure of 700 psi the rupturing force on the section of the cylinder
considered is 700b lb. If the wall has a thickness t and the material has a tensile strength of 35,000 psi,
the resistance to rupture of the two sections 1 in. long and t in. thick is 70.000t lb and the. factor of
safety then is
f =70000t/700b =100t/b
For a factor of safety of 4 the ratio of wall thickness to bore then evidently must be 1/25 This
rule when applied to cylinders of small bore gives values for the cylinder-wall thickness which, while
large enough so far as withstanding the stresses of a normal explosion is concerned, would be too small
from the standpoint of shop production. If the water jacket is cast integral, as it usually is, the cylinder
can be machined only on the inside, and the minimum thickness of the wall then depends upon the
accuracy with which the cores are set. Some allowance must be made for inaccurate core work, and a
good value for the wall thickness is
t = (b/25) +0.10in
This formula can be safely applied to the whole range of sizes of automotive engines with cast
iron cylinders.
The cylinder head must be quite stiff in order to resist the stresses of detonation. The wall itself
is usually made slightly thicker than the cylinder wall. In the case of an overhead-valve engine, the Wall
is normally stiffened by the vertical walls of the valve pockets. A similar stiffening effect is usually
obtained in the heads of L-head cylinders from the walls of spark-plug wells, but if there are any
extended flat surfaces in these heads, they should be stiffened by ribbing.

Details of Water Jacket.

For a long time it was the general practice to extend the water jacket down the cylinder wall
only to the level of the top of the piston when at the bottom of the stroke. As the lower part of the
cylinder is not contacted directly by the hot gases, it does not reach an excessive temperature, and
therefore does not seem to require water-jacketing. However, in modern high-speed engines the
crankcase oil often reaches an excessive temperature, which reduces the load-carrying capacity of the
oil film in the bearings, and may cause the latter to fail in hard service. It has been found that by
extending the water jacket all the way down the cylinder, the temperature of the oil in the crankcase
under extreme conditions may be lowered by as much as 50 Fahrenheit degrees, as compared with an
engine with "half-length" jackets, and "full-length" jackets have come into general use.
Some designers taper the jacket down from the top to the lower end, so as to place a larger body
of water around the compression chamber, where most of the heat must be absorbed. In most engines,
however, the depth of the water jacket is uniform from top to bottom. This depth varies somewhat in
different designs, but usually is equal to about one-eighth the cylinder bore. Certain parts of the jacket
which directly affect the over-all dimensions of the block can be made smaller in depth, including the
space between adjacent cylinders and that between a cylinder and a valve pocket or a tappet housing.
Liberal water spaces have the advantage that the core sand can be more effectively removed from the
casting. In engines of special design, such as those with "wet" liners, the jacket depth can be made less.
The jacket wall generally is made as thin as the foundry process permits. It can be made thinner,
of course, in a small cylinder than in a big one, because in the former the area is smaller. Average
practice with regard to jacket-wall thickness is as follows:
Cylinder bore, inches 3 4 5 6
Thickness of jacket wall inches 5/32 3/16 7/32 1/4
 63
Theory and Design of Automotive Engines
Jacket walls must be made heavier when cylinder liners (especially the "wet" kind) are used and
the tensile stresses due to the force of explosion are sustained chiefly or wholly by these walls. On the
cylinder head the water jacket is usually made of somewhat greater depth than around the cylinders, so
as to provide adequate heat-storage capacity over the area where most of the waste heat enters the
cooling water. There should be water spaces between all adjacent valve pockets (instead of common
walls), and the water should come quite close to the valve seats, as it is only in this way that uniform
cooling of the valve seat can be assured, and distortion and consequent leakage prevented. Cylinder
heads must be so designed that no steam pockets can form in them; that is, it must be possible for the
water to flow from any part of the jacket to the outlet along a continuously rising path. Trouble from
overheating is most likely to arise at the exhaust-valve seats, and it is therefore desirable that the cooling
effect of the circulating water be most intense at the valve pockets. This can be assured by inserting a
distributing header in the water jacket, the header connecting with the water entrance to the jacket at the
front of the block and having an outlet adjacent to each exhaust-valve pocket. The header is usually
made of sheet metal and set into the mold. Two arrangements are illustrated in Fig. 5. With
valve-in-head cylinders the location of the water outlet presents some difficulty: because the valve
mechanism on top of the engine is usually provided with a cover. One solution of the problem consists
in forming a number of outlet bosses on the head over to one side, so they come outside the valve cover,
and using a water-return manifold.
While this tends to promote
uniformity of circulation, it makes for
dissymmetry of appearance, which is
the more objectionable because the
manifold is
located very prominently on top of the
engine.
The more common plan is to have an
outlet at
the front end of the head, just outside
the valve
cover, and usually oblong in form, with the long
diameter across the engine, so as to minimize
the overhang.
In cylinders provided with "full-length"
jackets, the central portion of the barrel lacks
the reinforcement which with "half-length"
jackets is provided by the flange that forms the bottom of the jacket. If the barrel also happens to be of
minimum thickness its central portion will have very little rigidity and will distort easily, particularly if
during machining operations the tool strikes a “hard spot:” This makes it almost impossible to obtain a
true cylindrical bore. Conditions can be improved in this respect by providing the barrels of such
engines with one or two circumferential ribs at intermediary points of their length.

While the flange around the cylinder at mid-length in engines with half-length water jackets has
the advantage of affording the rigidity of structure desirable during machining operations it is
detrimental under certain operating conditions. For instance, when an engine is being run under full load
immediately after a cold start, the piston heats up much more rapidly than the cylinder block and is apt
to get tight in the cylinder and scuff. It has been observed that in engines with half-length jackets such
scuffing occurs particularly at the level of the water-jacket bottom flange, which latter prevents the
cylinder from expanding.
 64
Theory and Design of Automotive Engines

Guarding Against Cylinder Distortion.

It has been pointed out already that a frequent source of trouble in operation is distortion of the cylinder
bore which results in blow-by overheating and excessive
cylinder
wear. Cylinder distortion may he due to either mechanical or
thermal
causes. Mechanical distortion is most likely to result from
tightening
of the cylinder-head nuts, if the anchorages for the cylinder
head studs
are not properly supported. It .has been suggested that these
anchorages be either located in a wall which extends straight
down to
the cylinder bottom flange so that the pull of the stud
produces pure
tensile stresses on the material of the block, or else be cast on the
jacket wall rather than on the cylinder wall, as illustrated in Fig. 6. To
further reduce cylinder-wall distortion, this wall is thickened near the
top, while the thickness of the deck around the cylinder wall is
reduced.
In valve-in-head engines the bases for the brackets carrying the
rocker arms must he well supported, so they will not yield unduly under load which would make the
engine noisy.

Removable Liners.
In most engines the pistons hear directly on walls forming part of the cylinder block, hut in
some-and particularly in engines with large
cylinders-removable liners are used. There are two types
of these liners:
A "dry" liner is one which is in contact with metal of the
block
over its whole length, or nearly its whole length, while a
"wet"
liner is one which is supported by the block over narrow
belts only,
and is surrounded by cooling water between these belts.
In the United States "wet" liners came into use
first,
especially in the engines of farm tractors and
commercial vehicles.
Aside from the fact that any liner when worn or damaged can be
replaced at relatively low cost, the construction offers the
advantage that because of their uniform wall thickness (being
machined inside and lout) and because they are very little affected
by the tension of cylinder-head studs, separate liners distort less in
service than the integral barrels of conventional cylinder blocks.
Fig. 7 "Wet" cylinder liner with packing rings.

65
Theory and Design of Automotive Engines
At first the liners were made of the same gray iron that was used for cylinder blocks, but in the
course of time materials of greater wear resistance were developed, and as most of these were more
expensive than ordinary gray iron, they lent themselves particularly to use in liners. One method of
installing a removable "wet" liner in a cylinder block is illustrated in Fig. 7. At the top the liner is
provided with an external flange which enters a counter bore in the cylinder. The top of the liner is flush
with the top of the block, and the joint is sealed by the cylinder-head gasket. In some cases and
especially in Diesel engines-the hole in the gasket is made slightly larger than the cylinder bore, and a
ring or loop of copper is inserted to reduce the pressure on the gasket.
At the bottom the liner is enlarged in diameter and has three grooves for packing rings cut in it. Instead
of in the liner, the grooves may be cut in the block. These packing rings are made of synthetic rubber,
which is more resistant to mineral oil and other petroleum products than natural rubber. The packing
rings may be made of circular section, of a diameter slightly larger than the width of the grooves, and
insertion of the liner then will deform them so that they substantially fill the grooves. To permit easy
insertion of the liner, either it or the bore of the block is chamfered, depending on which part contains
the packing rings.
Inaccuracies in the section diameter of these packing rings are said to have been the cause of
some trouble. If the diameter is too small there may be leakage, whereas if it is too large the pressure
exerted when the liner is forced into place may crack it. To overcome this difficulty, a cork-synthetic
rubber composition of greater elastic compressibility has been developed. Packing ring of this material
are molded with a square section, and when inserted project slightly above the surface of the part in
which the grooves are cut. Insertion of the liner compresses them flush with that surface. Single and two
packing rings also are used, and in the case of two rings, a third groove sometimes is cut between the
two containing the packing rings, to collect any oil or water that may seep past the rings and allow it to
drain off.
"Dry" liners, which in Great Britain were used
practically exclusively from the beginning, seem to have
gained the ascendancy over the "wet" type in this country
after World War II. A typical "dry-liner" installation (in a
GMC engine) is shown in Fig. 8. In this engine the
cylinder block and crankcase are separate castings, and the
liner extends some distance into the crankcase. It is held in
position by a flange. at the top. In some other engines with
dry liners and a separate crankcase the retaining flange on the
liner is near the bottom and is held between the cylinder
block and the crankcase. A British manufacturer of Diesel
truck engines (Albion) copper-plates the dry liners on the
outside. The copper is said to act as a lubricant, facilitating the insertion of the liner, and also to
improve the heat flow
from liner to cylinder wall. Fig. 8 "Dry" cylinder liner in position.

66
Theory and Design of Automotive Engines
Materials for Cylinder Liners
For the engines of public-service vehicles, which latter run up enormous mileages in the course
of a year, it has been found advisable to use alloy iron for the liners and to heat-treat them. General
Motors Truck & Coach Division, for instance, uses such hardened liners in all of its larger engines, the
material being a nickel-chromium iron of the following composition:
Percent
Total carbon 3.10-3.40
Combined carbon 0.75-0.90
Manganese 0.55-0.75
Phosphorus 0.20 max.
Sulphur 0.10 max.
Silicon 1.90-2.10
Nickel 1.80-2.20
Chromium 0.55-0.75
In the "as cast" condition the liners show a Brinell hardness of 212-241, a transverse strength of
2400 lb on A.S.T.M. arbitration bars (bars of 1.2 in. diameter and 18 in. between supports), a transverse
deflection of 0.20-0.30 in., and a minimum tensile strength of 37,000 psi on test bars machined from-the
casting. A hydrostatic test also is applied to the liners, which must withstand 1500 psi for a wall
thickness of l/8 in. and. bores of 4-5 in. To increase their wear resistance, the liners are hardened, by
being heated to. 1540- 1560 F for 30 to 40 minutes and quenched in still oil. After this they must show a
Brinell hardness of at least tensile 512 while the strength must range between 28,000 and 36,000 psi and
the transverse strength between 2700 and 2900 lb for the arbitration bar. With these liners the mileage
between cylinder overhauls is said to be practically trebled, as compared. With solid cylinders of gray
cast iron showing from 230 to 240 Brinell. A minor disadvantage is that it takes up to 5000 miles for the
piston rings to wear in fully, hence the oil consumption is rather high during the early part of the life of
the liner.

Nitrided Cylinder Liners.

A process for nitrogen hardening or “nitriding” cast iron was developed in Europe. The process
consists in exposing cast-iron objects to be case- hardened to a current of ammonia vapor at about 900 F
for a considerable length of time, and then quenching. At this high temperature the ammonia breaks up
into its constituents. Nitrogen and hydrogen, and the nitrogen penetrate into the surface of the casting &
combines chemically with the metallic elements, forming very hard nitrides.
A Special alloy iron containing aluminum must be used. The liners are exposed to the ammonia
vapors for 65 hours at 950 F and then have a hardened case of 0.015 in. depth, the hardness tapering off
from the outside, where it is somewhere between 800 and 1000 Brinell.
A slight "nitride fuzz" produced on the surface of the liners during the process is removed before
they are shipped to engine builders. Some distortion is caused, and the effects of this are eliminated by
honing after the liners are inserted into the block, for which purpose an allowance of 0.002 in. on the
diameter is made. Nitriding also produces a slight "growth," of the order of 0.001 in., and this, too, is
allowed for in advance. Liners are installed in blocks with a press fit, an interference of 0.0015 to
0.0025 in. being allowed, depending on the bore.

Chromium Plating.
Another method of reducing the rate of wear consists in chromium plating the bore. The process
differs radically from that of chromium plating for ornamental purposes. .It gives a "porous" coating
which holds oil, while the so called bright plating process gives a dense coating to which oil will not
adhere & which for this reason is readily is scored in service. From 200 to 500 times as much chromium
as in conventional decorative plating is deposited per unit of area. If slightly too much should be
deposited, so that the bore is undersize by from 0.0005 to 0.001 in., the excess can be removed by
honing. Wear tests made on a plain gray-iron cylinder of 241 Brinell hardness and a similar cylinder
plated indicated that chromium plating reduces the rate of cylinder wear approximately in the
proportion of 7:1 and that the wear on the top piston ring is coincidentally reduced about 4:1.

67
Theory and Design of Automotive Engines
Such methods as nitriding and chromium plating of cylinder bores are applicable particularly to
bus and railcar gasoline engines and to Diesel engines, which have a much longer service life than
passenger-car engines. Cylinder bores in plain cast iron must be reconditioned about every 50,000
miles, and with either a nitrided or chromium-plated bore, if reconditioning is required at all, it will be
required only after a much longer interval.
The primary function of a cylinder of an IC Engine is to maintain the working fluid & the secondary
function is to guide the trunk piston.

Advantages of Dry liner

1. simpler to replace
2. no danger of water leakage either in to crankcase or the combustion chamber 3. due to
absence of heavy flanges at the top of the liner, cylinder centres can be reduced 4. better
cooling of upper part of the liner
Disadvantages of Dry liner
1. Complicated casting
2. decreased heat flow through the composite wall

Advantages of Wet liner

1. the foundry problem is considerably eased, since the large internal cores of the cylinder block
can be properly supported
2. cylinder block is relieved of the stresses due to longitudinal expansion of the liner
Disadvantages of Wet liner
1. difficult replacement
2. Danger of water leakage in to the crankcase& the combustion space if the casting is defective

Qualities of a good liner

1. Strength to resist the gas pressure
2. Sufficiently hard to resist wear
3. Strength to resist the thermal stresses due to the heat flow through the liner wall
4. Anticorrosive
5. Capable of taking a good bearing surface
6. Should be symmetrical in shape to avoid unequal deflection due to gas load & unequal
deflection due to thermal load
7. no distortion of inner surface due to restraining fixings
 68
Theory and Design of Automotive Engines
Valve Seats
Water jackets should be carried close to and all around the
valve seats. It has been shown that the heat absorbed by the valve
heads through their contact with the burning gases passes off chiefly
through the seats, and if the water comes close to the seats the heads
will be cooled more effectively, while if it extends all around, the
heads will be cooled more nearly uniformly and will not warp.
Particularly effective cooling of the valve seats and valve
guides is
claimed for the arrangement shown in Fig. 9. Here a water
distributor is cast in the block, and has discharge openings in both
the top and the side adjacent to the valve guide. The outlets in the
top discharge into the space between the exhaust valve pocket and
the cylinder wall, and the water discharged there is induced to flow
completely around the valve pocket, by scallops directly above the
valve passage, through which it passes into the cylinder-head jacket.
The outlets at the side discharge against the bottom of the valve
pocket adjacent to the upper part of the valve guide.
In laying out the valve pocket, enough clearance must be allowed all
around the valve seat so the gases will pass fairly uniformly through all
sections of the valve port.
Fig. 9
Valve-Seat Inserts.
In high-speed, heavy duty engines the exhaust-valve seats, if directly on the cast iron of the
block, are likely to erode or wear way rather rapidly, causing the valve to sink deeper into the seat,
reducing the valve clearance, and necessitating valve-stem adjustment. To eliminate the necessity for
frequent adjustments, valve-seat inserts of heat-resistant material were introduced in 1931, first for
commercial-vehicle engines and shortly thereafter also for passenger-car engines. Such inserts had long
been used on engines having the part containing the valves made of aluminum, as the ordinary
aluminum alloys are far too soft to sustain the pounding of the valve heads. In that case the inserts are
made of aluminum bronze -(90 percent copper, 10 per cent aluminum), which has about the same
coefficient of heat expansion as the aluminum alloys used for cylinder heads. Aluminum-bronze inserts
are forced into counter bores in the head with a shrink fit.
One of the requirements of a valve-insert material therefore is that it must have substantially the
same coefficient of heat expansion as the material of the block or head; another is that it must be
sufficiently hard to withstand the pounding of the valve head at high temperatures over long periods.
The materials commonly used include nickel-chromium iron with moderate alloy contents, and the
high-percentage tungsten steel known as high-speed steel. Where the conditions are too severe for these
materials the seat of the insert can be provided with a facing or veneer of a nonferrous, heat-resistant
alloy. Alloys available for the purpose include "Eatonite" (chromium, tungsten, nickel and cobalt),
"Elkonite" (tungsten and copper), and "Stellite" (cobalt, chromium and tungsten). These alloys are
applied to the seat portion of the inserts by "puddling" with a welding torch.
In most applications the inserts are shrunk in place, and to get the necessary shrink fit without the use of
too great pressure (which might cause distortion), the inserts are cooled to about -100 F in dry ice or
-220 F in liquid air, while the blocks are heated in water to about 200 F. The interference is made about
0.0015 in. Per inch in the case of steel inserts, and 0.003 in. per inch in the case of cast iron.
As a rule, the inserts are chamfered at both top and bottom-at the bottom to facilitate entering
them in the counter bore, and at the top so that the block material can be rolled over the edge of the
insert to help retain it. In addition to rolling the block material over the top chamfer, the insert
sometimes is provided with a number of axial grooves into which metal of the block is forced by the
rolling process.

69
Theory and Design of Automotive Engines
Fig. 10 shows a Stellite-faced,
threaded insert.
The Stellite is puddled onto the steel
base with
an acetylene torch. The insert is
provided with
splines in its throat, to take a tool for
screwing it
in place, and a 0.014-in. washer of
extra-soft
iron is placed underneath it to assure a good
path for heat flow. After the insert is screwed
home, it is-locked in place by rolling the metal
of the block around it. The shape of the rolling
tool and the method of rolling are illustrated in
Fig. 11.
Valve inserts have been
standardized by the S.A.E. (Fig. 12). The
standard includes two series, one intended
for- passenger-car, the other for heavy
duty engines. It specifies the diameter and
depth of the bore in the cylinder or head,
and the thickness of the insert. This leaves
the diameter of the insert-which determines the interference-to be set by the manufacturer. Valve-seat
inserts shrunk in place sometimes come loose in service, and this is particularly likely to occur if the
interference is relatively large. This is due to the fact that in severe service such high temperatures may
be reached that the resulting stresses exceed the elastic limit of the metal and produce a permanent set.
Then, when the: engine cools down, the insert will be loose. It is therefore recommended that the
interference be made no greater than needed to firmly hold the- insert in place when the engine is cold.
An insert specially designed to prevent trouble from distortion and loosening. in severe service
is illustrated in Fig. 13. The main portion, which has-a section similar to that of the S.A.E. standard
insert, is given a clearance of 0.004 in. around its circumference, so that the metal around-it can distort
freely without subjecting the insert to undue stress. In addition there is an extension or skirt, which is
made an interference fit in the head or block. Owing to its greater distance from the valve seat, this
portion will not reach as high a temperature, and therefore is not likely to take a permanent set. The
considerable length of valve port or throat required seems to be a disadvantage of this type.

Length of Bore
 In most modem engines of both the L-head and I-head type the combustion chamber is formed
in the cylinder head and at the end of the up-stroke the top of the piston is flush with the finished top
surface of the cylinder block. One reason for not making the piston overrun the end of the bore is that
that would bring the top ring beyond the upper end of the water jacket at the end of the up-stroke, where
it would not be so effectively cooled, in the ring groove. The lower end of the piston generally is made
to overrun the end of the bore slightly.
The total length of the finished bore evidently is equal to the length of stroke plus the length of
the piston minus any overrun of the piston at both ends, the overrun being considered negative when the
piston does not come quite to the end of the bore. To facilitate getting the piston rings into the cylinder,
the bore is chamfered at the end from which the piston is entered

70
Theory and Design of Automotive Engines
Location of Spark-Plug Bosses
In L-head cylinders the sparkplug bosses usually are located over the passage between the valve
chamber and the cylinder barrel, as this location reduces the tendency to detonation to a minimum. The
spark points must never be directly in line with the cylinder wall, where they would become fouled by
oil thrown off by the piston.
The length of thread of standard spark plugs is considerably less than the average thickness of
the cylinder head, and to compensate for this difference, a conical depression is formed in either the
inner or outer wall of the head,
as shown
in Fig. 14. There appears to be
one
objection to each arrangement,
which is
probably the reason designers
have not yet
agreed on one or the other of
these
designs. If there is a depression
in the
outer wall, any water getting onto the top
of the engine will collect in it and tend to
cause rusting of the spark-plug shell and
its thread. A conical depression in the
inner wall adds to the cooling surface of
the combustion chamber. Besides, the
mixture at the spark points, near the bottom of the depression, may be less ignitable. In valve-in-head
engines the depth of the head is altogether too great to permit of having the spark-plug bosses extend
through it vertically, and in such engines they extend through the head from the side at an angle, a recess
being formed in the side of the head to obtain a square seat (Fig. 15). In larger cylinders, of course, the
vertical depth of the compression space is sufficient to allow of the plug being screwed horizontally into
the compression chamber wall.
Fouling of spark plugs by lubricating oil is most likely to occur in valve-in-head engines, where
the plugs are located directly over the pistons. One method of combating trouble from this source
consists in reducing the diameter of the spark-plug hole by about one-half at the inner end (see Fig. 15),
and making the thread of the plug slightly shorter than the depth of the threaded hole, so that a small
chamber is formed at the inner end. In engines which do not have this provision the same effect can be
obtained by the use of "adapters," which screw into the spark-plug hole and have an internal thread
which will take the plug.

Optimum Combustion-Chamber Form.

It was found that the combustion chambers illustrated in Figs. 16 and 17 work out satisfactorily, and that
with these the small compression volumes required can be obtained without difficulty. The various
combustion chambers represented by Fig. 16 differ from each other with respect to slope over the piston
and with respect to clearance ahead of and behind the valves, as shown in Fig. 17. A clearance of 1/16
to 3/32 in. is satisfactory above the valve, and 3/32 to 1/4 in. behind the valve, while as much clearance
as possible should be provided ahead of the valves. The width of the valve passage should be about 75
per cent of the bore, and the ratio between swept volume and passage area should be between 15 and 20.
The "masked" area of the piston may vary considerably; a satisfactory angle of slope over the piston
should be aimed at, and the "masked" area allowed to come what it will; in the design shown it is about
50 per cent of the piston-head area.
Although this study of the best combustion-chamber form was made on engines with aluminum heads,
the results obtained and the conclusions reached are equally applicable to high-compression engines
with cast-iron heads.

71
Theory and Design of Automotive Engines

Fig. 16 Forms of combustion

chamber used in
experimental aluminium cylinder
head.

Fig.17 Clearance ahead & behind valves.

Production of Engine Blocks
In the design of engine block or cylinder block it is well to consult with the foreman of the
pattern shop, because a casting of this kind is a difficult piece of mold, and the advice of an experienced
mechanic may obviate trouble later.
Cylinders must be molded with the head downward, for the reason that blowholes, porous spots,
etc are most likely to occur near the top of the casting, & the head of the cylinder, which is the working
end, must of necessity be of sound metal.
When the castings have cooled the core sand is removed, the seams etc., are chipped off, & the
castings are then put through a cleaning process. [Either by pickling & neutralizing or by blast cleaning
(blast cleaning by sand or small granules of chilled iron or steel) & then normalizing & cleaning]
Further the cast & cleaned blocks would undergo other operations in sequence like Milling, Drilling,
Cylinder boring, Precision boring, Finish of bore, Honing, Lapping, followed by measurement of
quality of surface finish, Water test Finishing of valve seats & guides & surface broaching. Transfer
 machines were adopted since world war-II to perform the operations automatically.

72
Theory and Design of Automotive Engines
DESIGN OF CYLINDER AND CYLINDER HEAD
Cylinder should be
- designed to withstand the high pr. & temp. conditions.
- be able to transfer the unused heat effectively so that metal temp. does not approach the
dangerous limit.
The Cylinder wall is subjected to gas pressure & the piston side thrust.
-Piston side thrust tends to bend the wall but the stress in the wall due to side thrust is very small
& can be neglected.
-The gas pressure Produces 2 types of stresses;
-longitudinal and circumferential - which act at right angle to each other & the net stress in each
direction is reduced. The longitudinal stress is usually small & can be neglected.
2
π p
D
force
=( )
4 stress=
area
=longitudinal
f max

l
22
π O−
DD
4
pD
max ⋅
=circumferential force=
f t
c

2
D=cylinder diameter, DO= cylinder outside diameter,
p =max. gas pr.
max
f c f
f - mf & - mfl,
c
Net
Net l =
f 1
1 = c
l
 where= m
poision’sratio=4

73
Theory and Design of Automotive Engines

CYLINDER WALL THICKNESS

The Wall Thickness is usually calculated by applying the formula for a thin
cylinder, p D
⋅

thus k
t f
= max c
2 +

Where t=wall thickness, mm,

pmax = max. gas pr.,N/mm2 (3.1 to 3.5N/mm2),
D=cy. bore, mm,
f =max. hoop stress and is equal to 35 to 105 N/mm2depending on the size and material, larger values
c

are used for smaller bores,

Cylinder bore, mm 75 100 150 200 25 300 350 400 450 500
0

k =reboring factor, mm 1.5 2.3 4.0 6.0 7.5 9.5 10.5 12.5 12.5 12.
5

The thickness of the cylinder wall usually varies from 4.5mm to more than 25mm, depending upon the
cylinder size.
According to an empirical relation,
For liners of oil engines,
 ≥ D
t 15 near the top portion & through 20% of the stroke.
For dry liners,
The total thickness ‘t’ is the thickness of the liner & that of the cylinder wall.
The thickness of the Dry liner is given as 't =0.03D to 0.035D
The thickness of the inner walls of the automobile engine cylinders is usually given empirically as t
=0.045D+1.6mm
The thickness of Jacket wall is given as = 1 to 3
3 4 t , larger ratio for smaller cylinder or
=0.032D+1.6mm
The water space between the outer cylinder wall & inner jacket wall is =10mm for a 75mm cylinder to
about 75mm for a 750mm cylinder
or =0.08D+6.5mm

CYLINDER DIAMETER AND LENGTH

pLAn
,
The o/p of a given cylinder can be written as - Power= W
60
Where L=stroke in m,
A=piston area, mm2,
n=no. of working strokes per minute
for 2 stroke engines and N
=N 2 for 4 stroke engines
p=imep-if power is indicated &
bmep if o/p is in brake power, N/mm2
* As a guide, the max. gas pr. can be taken as 9 to 10 times the bmep

74
Theory and Design of Automotive Engines
CYLINDER FLANGES AND STUDS
The cylinder is either cast integral with the upper half of the crankcase or attached to it with the help of
flanges, studs and nuts.
The cylinder flange is made thicker than the wall of the cylinder.
Flange thickness should not be less than 1.1 to 1.25t
Common value for flange thickness = 1.2 to 1.4t
Or =1.25 to 1.5 d where d =bolt diameter, nominal
The distance of the end of the flange from the center of the stud or bolt should not be < d +6mm, and
not > 1.5 d .
The use of studs decreases the bending stress at the flange root since the moment arm can be made very
small.
The material of the studs or bolts is usually nickel steel with a yield point of 630 to 945MPa. The
diameter of the bolt or stud is calculated by equation of the gas load to the area of all the studs at the
root of the threads multiplied by the allowable fibre stress.
∴, π
4πmax.
2
D ⋅ p = ct 2
D ⋅ p = ct
2
z⋅d⋅f
2
4 z⋅d⋅f
∴ max.
 Core

DiameterOutside

p
∴ dc = zf Diameter
t
D max
⋅ ,

f = allowable fibre stress, 35 to 70 N/mm2,

where t
dc = core diameter
f is taken since there is already high stress in the studs due to tightening of the nuts.
Low value of t
D
, D in mm
D

The number of studs 'z' may be taken as 
 
 +4

4 

+
100 
to 50
Or the pitch of the bolts may be taken as 19 d to 28.5 d , where d is in mm. In
generally varies from ( 3
practice d 4 to 1) times the thickness of the flange.
In no case d should be < 16mm

75
Theory and Design of Automotive Engines
CYLINDER HEAD
Usually a separate cylinder cover or head is provided with all but the smallest engines. A box type
section is employed of considerable depth to accommodate ports. The general design of the cover is
governed by the following factors along with the strength consideration.
 Air and gas passages
 Accommodation of valves and their gear
  Accommodation of the atomizer at the centre of the cover in the case of the diesel engines.
Cylinder head is the most difficult part to be designed and manufactured. The cylinder heads are usually
made of close grained cast iron or alloy cast iron containing nickel, chromium and molybdenum, for
small and medium sized engines, while for large engines, the material is low carbon steel.
The thickness of the cylinder wall ranges from about 6.5mm for small engines to proportionately
larger values for large engines. The thickness depends on the shape of the head. If the cylinder head is
approximately a flat circular plate, the thickness can be determined by the relation: Cp
t Dmax
= N/mm2
f
t

f =allowable stress, taken to be 35 to 56

Where C=const., in this case equal to 0.1, t
A low value of ' 't
f is taken because both pr. & temp. stresses are induced in the cylinder head and
the above equation is based upon only the cylinder pressure. The heat transfer through the head is about
5 to 13 times as much heat per unit area as the cylinder walls, depending on the design and amount of
cooling.

76
Theory and Design of Automotive Engines
• Example -1
Determine the thickness of a cast iron cylinder wall & the stressesfor a 300mm petrol engine,
2
with a maximum gas pressure of
3.5N/mm • Solution :
Given
D cylinder bore 300mm, max.gas pr. 3.5N/mm2
==p==
max

WallThicknessis usually calculated by applying formula for a thin cylinder,

WallTh, t ⋅ + ,
Thus ickness = max p D k
2

cf
2 N/mm = =
where p
,D cy.bore, mm,
max max.gas pr., ), 2
(3.1 to 3.5N/mm
2
fc max.hoop stress and is equal to 35 to105 N/mm

depending on the size and material,larger values areused for smaller bores,

Cylinder bore, mm 75 100 150 200 250 300 350 400 450 500
Reboring factor, mm 1.5 2.3 4.0 6.0 7.5 9.5 10.5 12.5 12.5 12.5

From above table k mm

= 21.5 ,
2
Assume f N mm
c
D
=
45 / , p ⋅ 3.5 300 ⋅

t 9.5 21.5
WallThickness , + = longitudinalstress
k mm , 2 45 ⋅
= 2
max
fc +=
Now apparent

2
2
==
ππ
[( / 4) ] D p pD

⋅
force max = max ⋅
f
l 22
− DD−
area [ ]/ 4 D D 2 2
()()
oo

Where, D cylinder diameter,

=
D cylinder outside diameter & max.gas pr. = =
p
o max

Now Do D 2t 300 2t 300 (2 21.5) 343mm = + = + = + ⋅

 =
2
3.5 300
Apparent longitudinalstress
⋅
2
f N mm
l
343 300 − =
= 22
11.45 /
()

Now apparent circumferentinalstress,

force pD⋅ 3.5 300 ⋅
max 2
f ==
2 = = N mm
2 21.5 ⋅24.4 /
c
area t
ll
1 1
f , where m poision's
Net = - c

ff m ratio = 4
=
24.4
Net =11.45-
2
f N mm = − =
l
11.45 6.1
4 5.35 /

11.45
& Net = 24.4 -
2
f N mm = − =
c
24.4 2.86 21.54 /
4

77
Theory and Design of Automotive Engines
Example 2
Α vertical 4 stroke CI Εngine has the following specifications: 2
Βrake power 4.5kW, Speed 1200rpm, imep 0.35Ν mm ,η 0.80 = = = =

/.
mech

Detrmine the dimensions of the cylinder.

Solutio :
n
Brake Power
Since η
mech
Indicat Power
=
ed
Brake Power 4.5

Indicated Power
∴=== 5.625 kW
 η o mech .8
22
P Ν mm L m A mm n rpm
Indicated
Power =
imep 60
m /
[1 1 ] Watt N ⋅ ⋅ ⋅ Watt
n
=
1200
s
n
= = = for single acting stroke Engine

2 4 600
2

0.35 600
5.625 10 ⋅ ⋅ ⋅

3
∴ ⋅ = watt 60
3

LA 5.625 10 60
2
⋅= = ⋅ 1.608 10
or L m A mm
⋅⋅ 0.35 600 ⋅
3

D ⋅ mm 23 =⋅
or L m
π
⋅ 2
4 1.608 10 L
Stroke
Now
ie
assuming
. . 1.35, 1.35 ratio as or L D
Bore D =
∴ ⋅ 1.35 D 23 =⋅
Dm 2
⋅
π
mm
1.35 mm π D 1.608
D ⋅ 4 2
10
or ⋅ mm 23 =⋅
1000 4 mm
or Bore Diameter D 1.608 10
= 115 ,
∴ = = ⋅ = Stroke Length L D mm
1.35 1.35 115 155
NowLength of Cylinder Stroke clearance onboth sides
=+
=+
Stroke to of the stroke
10 15%
∴=+⋅
Length of 178.5 mm
Cylinder = 155 (155 0.15)
78