9th & 10th Day of Class (MEE2004)

16/02/2024 & 19/02/2024

Lecture 5 & 6
(Module 1)

Thermal Engineering
Systems
by
Dr. Rohit Sharma
School of Mechanical Engineering
Outline of this class

Combustion stages in CI
Engine

Knocking and Detonation

COMBUSTION IN CI ENGINE
STAGES OF COMBUSTION IN CI ENGINE
• Ignition Delay period /Pre-flame combustion
i. Physical delay
ii. Chemical delay
• Uncontrolled combustion
• Controlled combustion
• After burning
 1.Ignition Delay period
Immediately upon injection into the
combustion chamber. There is a definite period
of inactivity between the time of injection and
the actual burning this period is known as the
ignition delay period.
The ignition delay period can be divided into
two parts, the physical delay and the chemical
delay.
When the fuel is injected into the
combustion chamber the fuel mix with
air. The time spend in this process is
known as physical delay.

Certain pre flame reactions starts and

after some time fuel burns automatically
and the time consuming during this
process is known as chemical delay.
2.Period of Rapid Combustion
During total delay period more fuel droplet
come from the injector into the combustion
chamber. This group of fuel droplets burn
together and this produces uncontrolled
combustion is known as knocking and this
occurs at the end of delay period or the
beginning of combustion.
The rate of heat-release is
maximum during this period. This is also known
as uncontrolled combustion phase, because it is
difficult to control the amount of burning/
injection during the process of burning.
3.Period of Controlled Combustion

The rapid combustion period is

followed by the third stage, the controlled
combustion. The temperature and pressure in
the second stage are so high that fuel droplets
injected burn almost as they enter and any
further pressure rise can be controlled by
injection rate. The period of controlled
combustion is assumed to end at maximum
cycle temperature.
4.Period of After-Burning
Combustion does not stop with
the completion of the injection process. The
unburnt and partially burnt fuel particles left in
the combustion chamber start burning as soon
as they come into contact with the oxygen. This
process continues for a certain duration called
the after-burning period. This burning may
continue in expansion stroke up to 70 to 80%
of crank travel from TDC.
Stages of Combustion in CI Engines

https://www.youtube.com/watch?v=Y8l6AEquy94
Parameters controlling abnormal combustion in
CI engines

•Abnormal combustion means knocking.

1.Compression ratio:
• Greater the compression the temperature in the
cylinder are higher and hence due to higher
molecular activity the ignition delay will be less and
the chances of abnormal combustion are less at
higher compression ratios.
2. Cylinder wall temperature:
If the cylinder wall temperatures are high
the delay period will be less and hence
knocking tendency will be less.

3. Delay period:
The delay period must be small. If the
delay period is large more fuel will be
accumulated in the cylinder and may burn
suddenly.

4. Inlet temperature:
For smaller delay period the inlet
5. Self ignition temperature:
Self ignition temperature must be low, so
that less time is required to reach that
temperature. Therefore only small amount of
fuel burning in a controlled manner.

6. Ignition advance angle:

If the ignition angle is advanced the fuel
injection starts at low pressure and low
temperature and due to this low temperature
the delay period will be large and hence
more fuel is accumulated in the cylinder and
may burn in an uncontrolled manner.
7. Engine speed:
The fuel injector is geared to
the engine and hence the injection remains
constant interms of crank angle. If the speed
is increases the duration of injection time
decreases and hence at higher speed more
fuel is accumulated in less time and hence in
CI engines the chances of abnormal
combustion are high at high speeds.
wt = Ѳ = constant
↑w X t↓ = constant
8. Using a better fuel:
Higher CN fuel has lower delay
period and reduces knocking tendency
.
9. Knock reducing fueltype
This injector :
of injector avoidthe
sudden increase in pressure inside the
combustion chamber because
of
accumulated fuel. This can be done by
arranging the injector so that only small
amount of fuel is injected first. This can be
achieved by using two or more injectors
arranging in out of phase.
COMPARISON OF KNOCKING IN SI AND CI
ENGINES
SI NO FACTORS SI ENGINE CI ENGINE
AFFECTING
KNOCKING
1 SIT HIGH LOW

2 DELAY PERIOD LONG SHORT

3 CR LOW HIGH

4 INLET TEMPERATURE LOW HIGH

5 INLET PRESSURE LOW HIGH

6 SPEED HIGH LOW

7 COMBUSTION CHAMBER LOW HIGH

WALL TEMP.
1) In spark ignition engines, auto ignition of end gas away
from the spark plug, most likely near the end of combustion
causes knocking.
But in compression engines the auto ignition of charge
causing knocking is at the start of combustion.
2. In order to avoid knocking in SI engine, it is
necessary to prevent auto ignition of the end
gas to take place at all. In CI engine, the
earliest auto –ignition is necessary to avoid
knocking
3. The knocking in SI engine takes place in
homogeneous mixture, therefore , the rate of
pressure rise and maximum pressure is
considerably high. In case of CI engine, the
mixture is not homogenous and hence the
rate of pressure is lower than in SI engine.
4. In CI engine only air is compressed, therefore
there is no question of Pre-ignition in CI
engines as in SI engines.
5. SI fuels should have long delay period to
avoid knocking. CI fuels should have short
delay period to avoid knocking.
 Engines
Difference in Comb. Process of SI and CI
 Abnormal Combustion in SI Engine

Pre-Ignition Knocking
Pre-ignition (self- Knocking is due to
ignition) occurs when
the fuel mixture in the auto-ignition of end
cylinder burns before portion of unburned
the spark-ignition event charge in combustion
at the spark plug chamber
PRE-IGNITION

Pre-ignition is the ignition of the

homogeneous mixture of charge as it comes
in contact with hot surfaces, in the absence of
spark.

Pre-ignition is initiated by some overheated

projecting part such as the sparking plug
electrodes, exhaust valve head, metal
corners in the combustion chamber, carbon
deposits or protruding cylinder head gasket
rim etc.

Auto ignition may overheat the spark plug

and exhaust valve and it remains so hot
FYI: PRE-IGNITION

Engine efficiency will decrease due to pre-

ignition
pre-ignition causes holes melted in pistons,
spark plugs melted away, and engine
failure happens pretty much immediately.
 PHENOMENON OF KNOCK IN SI ENGINES
✔ The Figure shows the cross-section of the combustion
chamber with flame advancing from the spark plug location
✔ In the normal combustion the flame travels across the
combustion chamber from A towards D. The advancing flame
front compresses the end charge BB'D farthest from the
spark plug thus raising its temperature.
✔ The temperature is also increased due to heat transfer
from the hot advancing flame-front.
 PHENOMENON OF KNOCK IN SI ENGINES

✔ If the temperature of the end charge had not reached

its self-ignition temperature, the charge would not
auto-ignite and the flame will advance further and
consume the charge BB‘D.
KNOCKING (https://youtu.be/EuHy_Vpx514)

✔ Now, if the final temperature is

than and equal
greater to the auto-ignition
temperature, the charge BB´D
auto-
ignites (knocking).
✔A second flame front develops and moves
in opposite direction, where the collision
occurs between the flames. This causes
severe pressure pulsation.
Link: https://skill-lync.com/blogs/what-is-engine-knocking-its-causes-easy-diagnose-fixes
ABNORMAL COMBUSTION
PHENOMENON OF KNOCK IN SI ENGINES

⦿ However, if the end charge BB'D reaches its auto-ignition temperature

and remains for some length of time equal to the time of preflame
reactions the charge will auto ignite, leading to knocking combustion.
⦿ In Figure, it is assumed that when flame has reached the position BB',
the charge ahead of it has reached critical auto ignition temperature.
During the preflame reaction period if the flame front could move
from BB' to only CC’ then the charge ahead of CC' would auto-ignite.
Combination of high temperature and strong pressure damages the
engine

ENGINE DAMAGE FROM SEVERE KNOCK

Piston Piston crown

Cylinder head gasket Aluminum cylinder head

Pre-Ignition and Knocking
COMPARISON OF ABNORMAL COMBUSTION

Pre-ignition Knocking
Pre-ignition is the ignition of the
homogeneous mixture of charge as Knocking is due to auto ignition of end
it comes in contact with hot portion of unburned charge in
surfaces, in the absence of spark. combustion chamber.

Pre-ignition is initiated by some The pressure and temperature of

overheated projecting part such as unburned charge increase due to
the sparking plug electrodes, compression by burned portion of
exhaust valve head, metal corners in charge. This unburned compressed
the combustion chamber, carbon charge may auto ignite under certain
deposits or protruding cylinder head temperature condition and release the
gasket rim etc. energy at a very rapid rate compared
to normal combustion.
Auto ignition may overheat the spark
plug and exhaust valve and it remains This rapid release of energy during
so hot that its temperature is sufficient auto ignition causes a vibrations and
to ignite the charge in next cycle pinging noise.
during the compression stroke before
spark occurs and this causes the
pre-ignition of the charge.
 IMPORTANCE OF FLAME SPEED AND
EFFECT OF ENGINE VARIABLES

Factors affecting/influencing the flame

speed:
1. Turbulence
2. Fuel-Air Ratio
3. Temperature and Pressure of
Intake
4. Compression Ratio (CR)
5. Engine Output
6. Engine Speed
7. Engine Size
Knocking

https://www.youtube.com/watch?v=EuHy_Vpx514
CETANE NUMBER:
The cetane number of a diesel fuel
is a measure of its ignition quality.

OCTANE NUMBER:
The octane number of a petrol fuel
is a measure of its ignition quality.
Thank You
 IC Engines, V Ganesan

Lubrication
System
12.9 LUBRICATION SYSTEM

The function of a lubrication system is to provide sufficient quantity of cool,

filtered oil to give positive and adequate lubrication to all the moving parts of
an engine. The various lubrication systems used for internal combustion
engines may be classified as

(i) mist lubrication system

(ii) wet sump lubrication system

(iii) dry sump lubrication system

12.9.1 Mist Lubrication System

This system is used where crankcase lubrication is not suitable. In two-stroke
engine, as the charge is compressed in the crankcase, it is not possible to have
the lubricating oil in the sump. Hence, mist lubrication is adopted in practice.
In such engines, the lubricating oil is mixed with the fuel, the usual ratio being
3% to 6%. The oil and the fuel mixture are inducted through the carburettor.
The fuel is vaporized and the oil in the form of mist goes via the crankcase into
the cylinder. The oil which strikes the crankcase walls lubricates the main and
connecting rod bearings, and the rest of the oil lubricates the piston, piston
rings and the cylinder.
 (a) (b)

(c)

Fig. 12.8 Lubrication of wristpin bearings

The advantage of this system is its simplicity and low cost as it does not
require an oil pump, filter, etc. However, there are certain disadvantages
which are enumerated below.

(i) It causes heavy exhaust smoke due to burning of lubricating oil partially
or fully and also forms deposits on piston crown and exhaust ports which
affect engine efficiency.
(ii) Since the oil comes in close contact with acidic vapours produced dur-
ing the combustion process gets contaminated and may result in the
corrosion of bearing surface.
(iii) This system calls for a thorough mixing for effective lubrication. This
requires either separate mixing prior to use or use of some additive to
give the oil good mixing characteristics.
(iv) During closed throttle operation as in the case of the vehicle moving
down the hill, the engine will suffer from insufficient lubrication as the
supply of fuel is less. This is an important limitation of this system.
 In some of the modern engines, the lubricating oil is directly injected into
the carburettor and the quantity of oil is regulated. Thus the problem of
oil deficiency is eliminated to a very great extent. In this system the main
bearings also receive oil from a separate pump. For this purpose, they will be
located outside the crankcase. With this system, formation of deposits and
corrosion of bearings are also eliminated.

12.9.2 Wet Sump Lubrication System

In the wet sump system, the bottom of the crankcase contains an oil pan or
sump from which the lubricating oil is pumped to various engine components
by a pump. After lubricating these parts, the oil flows back to the sump by
gravity. Again it is picked up by a pump and recirculated through the engine
lubricating system. There are three varieties in the wet sump lubrication
system. They are

(i) the splash system

(ii) the splash and pressure system

(iii) the pressure feed system

Splash System: This type of lubrication system is used in light duty engines.
A schematic diagram of this system is shown in Fig.12.9.

Oil pressure gauge

Camshaft

Connecting rod bearing

Main bearing

Lower oil pan Oil troughs

Oil pump
Oil strainer
Fig. 12.9 Splash lubrication system

The lubricating oil is charged into in the bottom of the engine crankcase
and maintained at a predetermine level. The oil is drawn by a pump and
delivered through a distributing pipe extending the length of the crankcase
into splash troughs located under the big end of all the connecting rods.
These troughs were provided with overflows and the oil in the troughs is
therefore kept at a constant level. A splasher or dipper is provided under each
connecting rod cap which dips into the oil in the trough at every revolution
of the crankshaft and the oil is splashed all over the interior of the crankcase,
into the pistons and onto the exposed portions of the cylinder walls. A hole
is drilled through the connecting rod cap through which oil will pass to the
bearing surface. Oil pockets are also provided to catch the splashing oil over
all the main bearings and also over the camshaft bearings. From the pockets
the oil will reach the bearings surface through a drilled hole. The oil dripping
from the cylinders is collected in the sump where it is cooled by the air flowing
around. The cooled oil is then recirculated.
The Sp lash and Pressure Lubrication System: This system is shown
in Fig.12.10, where the lubricating oil is supplied under pressure to main and
camshaft bearings. Oil is also supplied under pressure to pipes which direct
a stream of oil against the dippers on the big end of connecting rod bearing
cup and thus the crankpin bearings are lubricated by the splash or spray of
oil thrown up by the dipper.

Oil pressure gauge

Camshaft
Connecting rod bearing
Main bearing

Oil jet of dip

directed on rod

Oil pump
Oil strainer
Fig. 12.10 Splash and pressure lubrication system

Pressure Feed System: The pressure feed system is illustrated in Fig.12.11

in which oil is drawn in from the sump and forced to all the main bearings of
the crankshaft through distributing channels. A pressure relief valve will also
be fitted near the delivery point of the pump which opens when the pressure
in the system attains a predetermined value. An oil hole is drilled in the
crankshaft from the centre of each crankpin to the centre of an adjacent main
journal, through which oil can pass from the main bearings to the crankpin
bearing. From the crankpin it reaches piston pin bearing through a hole
drilled in the connecting rod. The cylinder walls, tappet rollers, piston and
piston rings are lubricated by oil spray from around the piston pins and the
main and connecting rod bearings. The basic components of the wet sump
lubrication systems are (i) pump (ii) strainer (iii) pressure regulator (iv) filter
(v) breather.

Tappet and cam receive oil

thrown from connecting rod
Oil pressure gauge

Camshaft

Connecting rod bearing End leakage

Main bearing

Header line

Oil pump
Oil strainer
Fig. 12.11 Pressure feed lubrication system

A typical wet sump and its components are shown in Fig.12.12. Oil is
drawn from the sump by a gear or rotor type of oil pump through an oil
strainer. The strainer is a fine mesh screen which prevents foreign particles
from entering the oil circulating systems. A pressure relief valve is provided
which automatically keeps the delivery pressure constant and can be set to
any value. When the oil pressure exceeds that for which the valve is set, the
valve opens and allows some of the oil to return to the sump thereby relieving
the oil pressure in the systems. Fig.12.13 shows a typical gear pump, pressure
relief valve and by-pass. Most of the oil from the pump goes directly to
the engine bearings and a portion of the oil passes through a cartridge filter
which removes the solid particles from the oil. This reduces the amount of
contamination from carbon dust and other impurities present in the oil. Since
all the oil coming from the pump does not pass directly through the filter,
this filtering system is called by-pass filtering system. All the oil will pass
through the filter over a period of operation. The advantage of this system is
that a clogged filter will not restrict the flow of oil to the engine.
 Filter

To engine bearings Pressure regulator

Breather
Oil pump

Oil strainer
Engine crankcase
Wet sump

Drain plug
Fig. 12.12 Basic components of wet sump lubrication system

Pump inlet To engine

Pressure
relief valve
By-pass

Fig. 12.13 Gear type lubricating pump

12.9.3 Dry Sump Lubrication System

A dry sump lubricating system is illustrated in Fig.12.14. In this, the supply
of oil is carried in an external tank. An oil pump draws oil from the supply
tank and circulates it under pressure to the various bearings of the engine.
Oil dripping from the cylinders and bearings into the sump is removed by a
scavenging pump which in turn the oil is passed through a filter, and is fed
back to the supply tank. Thus, oil is prevented from accumulating in the base
of the engine. The capacity of the scavenging pump is always greater than
the oil pump. In this system a filter with a bypass valve is placed in between
the scavenge pump and the supply tank. If the filter is clogged, the pressure
relief valve opens permitting oil to by-pass the filter and reaches the supply
tank. A separate oil cooler with either water or air as the cooling medium, is
usually provided in the dry sump system to remove heat from the oil.
 Pressure Vent
To bearings relief valve
Oil cooler

Supply tank
Oil pump

Engine crankcase

Filter by-pass
pressure relief valve
Dry pump

Filter
Strainer
Scavenging pump
 IC Engines, V Ganesan

Cooling
System
13.9 NEED FOR COOLING SYSTEM

From the discussion on heat rejection in the previous sections, it may be noted
that during the process of converting thermal energy to mechanical energy,
high temperatures are produced in the cylinders of the engine as a result of the
combustion process. A large portion of the heat from the gases of combustion
is transferred to the cylinder head and walls, piston and valves. Unless this
excess heat is carried away and these parts are adequately cooled, the engine
will be damaged. A cooling system must be provided not only to prevent
damage to the vital parts of the engine, but the temperature of these compo-
nents must be maintained within certain limits in order to obtain maximum
performance from the engine. Adequate cooling is then a fundamental re-
quirement associated with reciprocating internal combustion engines. Hence,
a cooling system is needed to keep the engine from not getting so hot as to
cause problems and yet to permit it to run hot enough to ensure maximum
efficiency of the engine. The duty of cooling system, in other words, is to keep
the engine from getting not too hot and at the same time not to keep it too
cool either!

13.10 CHARACTERISTICS OF AN EFFICIENT COOLING SYSTEM

The following are the two main characteristics desired of an efficient cooling
system:

(i) It should be capable of removing about 30% of heat generated in the

combustion chamber while maintaining the optimum temperature of the
engine under all operating conditions of the engine.
(ii) It should remove heat at a faster rate when engine is hot. However,
during starting of the engine the cooling should be minimum, so that
the working parts of the engine reach their operating temperatures in a
short time.

13.11 TYPES OF COOLING SYSTEMS

In order to cool the engine a cooling medium is required. This can be either
air or a liquid. Accordingly there are two types of systems in general use for
cooling the IC engines. They are

(i) liquid or indirect cooling system

(ii) air or direct cooling system

13.12 LIQUID COOLED SYSTEMS

In this system mainly water is used and made to circulate through the jackets
provided around the cylinder, cylinder-head, valve ports and seats where it
extracts most of the heat.
The diagrammatic sketch of water circulating passage, viz., water jacket is
shown in Fig.13.6. It consists of a long flat, thin-walled tube with an opening,
facing the water pump outlet and a number of small openings along its length
that direct the water against the exhaust valves. The tube fits in the water
jacket and can be removed from the front end of the block.
The heat is transferred from the cylinder walls and other parts by convec-
tion and conduction. The liquid becomes heated in its passage through the
 Spark plug

Cylinder head
Water jacket
Valve ports
Cylinder wall

Gasket

Piston
Water transfer port

Water jacket

Fig. 13.6 Cooling water passages

jackets and is in turn cooled by means of an air-cooled radiator system. The

heat from liquid in turn is transferred to air. Hence it is called the indirect
cooling system.
Water-cooling can be carried out by any one of the following five methods:

(i) Direct or non-return system

(ii) Thermosyphon system

(iii) Forced circulation cooling system

(iv) Evaporative cooling system

(v) Pressure cooling system

13.12.1 Direct or Non-return System

This system is useful for large installations where plenty of water is available.
The water from a storage tank is directly supplied through an inlet valve to
the engine cooling water jacket. The hot water is not cooled for reuse but
simply discharged.
13.12.2 Thermosyphon System
The basic principle of thermosyphon can be explained with respect to Fig.13.7.
Heat is supplied to the fluid in the tank A. Because of the relatively lower
density, the hot fluid travels up, its place being taken up by comparatively
cold fluid from the tank B through the pipe p2 .

Hot
p
1

B
A
Cold

p
2

Heat
Fig. 13.7 Principle of thermosyphon system

The hot fluid flows through the pipe p1 to the tank B where it gets cooled.
Thus the fluid circulates through the system in the form of convection currents.
For engine application, tank A represents the cylinder jackets while tank
B represents a radiator and water acts as the circulating fluid. In order to
ensure that coolest water is always made available to cylinder jackets, the
water jackets are located at a lower level than the radiator.
The main advantages of the system are its simplicity and automatic cir-
culation of cooling water. The main limitation of the system is its inability to
meet the requirement of large flow rate of water, particularly for high output
engines.

13.12.3 Forced Circulation Cooling System

This system is used in a large number of automobiles like cars, buses and
even heavy trucks. Here, flow of water from radiators to water jackets is by
convection assisted by a pump.
The main principle of this system is explained with the help of a block
diagram shown in Fig.13.8. The water or coolant is circulated through jackets
around the parts of the engine to be cooled, and is kept in motion by a
centrifugal pump which is driven by the engine. The water is passed through
the radiator where it is cooled by air drawn through the radiator by a fan
and by the air draft due to the forward motion of the vehicle. A thermostat
is used to control the water temperature required for cooling. This system
mainly consists of four components, viz., a radiator, fan, water pump and a
thermostat. The details of these components are shown in Fig.13.9.
Radiator: The purpose of a radiator is to provide a large amount of cooling
surface area so that the water passing downward through it in thin streams is
 Radiator
Thermostat Engine

Pump
Fig. 13.8 Principle of Forced Circulation cooling system using the thermostat

Header tank
Valves
Thermostat

Fan
Radiator

Drain Water pump Cylinder

Fig. 13.9 Cooling of an automobile

cooled efficiently. To accomplish this, there are many possible arrangements.

One such arrangement is shown in Fig.13.10.
The radiator consists essentially of an upper tank (header tank) and a
lower tank. The upper tank in some design may contain a removable filter
mesh to avoid dust particles going in into the radiator while filling water in
the radiator. Between the two tanks is the core or radiating element. The
upper tank is connected to the water outlets from the engine jacket by rubber
hose, and the lower tank is connected by another rubber hose to the jacket
inlet through the pump.
Radiator cores are classified as tubular or cellular. A tubular radiator,
consists of a large number of elliptical or circular brass tubes pressed into a
number of suitable punched brass fins. The tubes are finned to guard against
corrosion and are staggered as shown in Fig.13.10. The main disadvantage is
the great inconvenience to repair any of the damaged tubes. But initial cost of
the system is comparatively less. The other type of radiator core arrangement,
called honey comb or cellular radiator core is shown in Fig.13.11.
 Plan Air flow
Water flow

Tinned brass tube Brass fins

Elevation
Fig. 13.10 Radiator construction

Air space

Thin brass or copper tubes

Fig. 13.11 Honey comb radiator core

The water used for cooling should be soft. If hard water is used, it forms
sediments on water jackets and tubes, which acts as insulator and make the
cooling inefficient. If soft water is not available, 30 g of sodium bichromate
should be added for every 10 litres of water.
Fan: The fan mounted on the impeller spindle driven by a suitable belt pulley
arrangement as shown in Fig.13.9 draws air through the spaces between the
radiator tubes thus bringing down the temperature of the water appreciably.
Pump: The pump maintains the circulation of the water through the system.
The bottom of radiator is connected to the suction side of the pump. The
power is transmitted to the pump spindle from a pulley mounted on the end
of the camshaft or crankshaft. A positive supply of water is achieved in all
conditions by centrifugal pump placed in this system (Fig.13.9). This ensures
good velocity of water circulation. Consequently less quantity of water and a
smaller radiator would suit the purpose.
 A pump is mounted conveniently on the engine and driven by the crankshaft
with a fan belt. Adj ustable packing glands are provided on the driving shaft
to prevent water leakage. Lubrication of bearings is done by using high melt-
ing point grease. In certain cases special bushes are used which do not require
lubrication. In case of multi-cylinder engines a header is usually employed to
provide equal distribution of water to all the cylinders. The header is supple-
mented by tubes or ducts which give high rate of flow around critical sections
of the engine such as the exhaust valve seats. This system is employed on
most diesel and automotive spark-ignition engines. The rate of circulation is
usually 3 to 4 litres per minute per kilowatt.
In some engines the pump is installed between the outlet of the radiator
and the engine block and forces cool water from the radiator into the engine
j acket. On automobiles, however, this arrangement would result in such a
low location of the pump that the fan could not be well placed on the pump
shaft. A disadvantage of this installation would seem to be that in case of
loss of water, circulation stops as soon as the level drops to the bottom of the
cylinder head jacket while with the pump in the supply line continues as long
as there is any water left in the system.
Thermostat: Whenever the engine is started from cold, the coolant tem-
perature has to be brought to the desired level in order to minimize the
warmup time. This can be achieved by a thermostat fitted in the system
which initially prevents the circulation of water below a certain temperature
through the radiator so that the water gets heated up quickly. When the pre-
set temperature is reached the thermostat allows the water to flow through
the radiator. Usually a Bellow type thermostat is used, the details of which
are shown in Fig.13.12. In modern engines, a wax-element type thermostat is
normally employed.

Valve

Linkage

Bellows

Closed Open
Fig. 13.12 Bellows type thermostat

The unit consists of a closed bellows with volatile liquid under reduced
pressure. When the bellows is heated the liquid vaporizes and creates enough
pressure to expand the bellows. The movement of bellows operates a link-
age which opens the valve. When the unit is cooled, the gas condenses, the
pressure is reduced and the bellows collapses to close the valve.

13.12.4 Evaporative Cooling System

This system is predominantly used in stationary engines. In this, the engine

will be cooled because of the evaporation of the water in the cylinder jack-
ets into steam. Here, the advantage is taken from the high latent heat of
vapourization of water by allowing it to evaporate in the cylinder jackets. If
the steam is formed at a pressure above atmospheric the temperature will be
above the normal permissible temperature.
Figure 13.13 illustrates evaporative cooling with air-cooled condenser. In
this case water is circulated by the pump A and when delivered to the overhead
tank B part of it boils out. The tank has a partition C. The vapour rises above
the partition C and because of the condensing action of the radiator tubes D,
condensate flows into the lower tank E from which it is picked up and returned
to the tank B by the small pump F. The vertical pipe G is in communication
with the outside atmosphere to prevent the collapsing of the tanks B and E
when the pressure inside them due to condensation falls below atmospheric.

C A

F
Fig. 13.13 Evaporative cooling with air-cooled condenser

Figure 13.14 illustrates evaporative cooling with the water-cooled con-

denser. In this case condensation of the vapour formed in the overhead tank
B occurs in the heat exchanger C cooled by a secondary water circuit and the
water returns to B by gravity. The pump A circulates the cooling water to the
engine and the heated water from the engine is delivered to tank B thereby
the circulation is maintained.
 C

Fig. 13.14 Evaporative cooling with water-cooled condenser

13.12.5 Pressure Cooling System

As already mentioned, the rate of heat transfer depends upon the temper-
ature difference between the two mediums, the area of exposed surface and
conductivity of materials. In case of radiators, in order to reduce the size of
radiator, it is proposed to seal the cooling system from the atmosphere and
to allow a certain amount of pressure to build-up in the system, so that the
advantage may be taken of the fact that the temperature of the boiling point
of water increases as the pressure increases. Boiling point of water at various
pressures is shown in Table 13.1.
Table 13.1 Boiling Point of Water at Various Pressures

Pressure (bar) 1.0 2.0 5.0 10.0

Temperature (◦ C) 100 121 153 180

In pressure cooling system moderate pressures, say upto 2 bar, are com-
monly used. As shown in Fig.13.15, a cap is fitted with two valves, a safety
valve which is loaded by a compression spring and a vacuum valve. When the
coolant is cold both valves are shut but as the engine warms up the coolant
temperature rises until it reaches a certain preset value corresponding to the
desired pressure when the safety valve opens; but if the coolant temperature
falls during the engine operation the valve will close again until the tempera-
ture again rises to the equivalent pressure value. When the engine is switched
off and the coolant cools down vacuum begins to form in the cooling system
 Safety valve Vacuum valve

Pressure operated servo

to actuate shutters

Shutters

Water pump

Fig. 13.15 Pressure cooling

but when the internal pressure falls below atmospheric the vacuum valve is
opened by the higher outside pressure and the cooling system then attains
atmospheric pressure.
A safety device is incorporated in the filler cap so that if an attempt is
made to unscrew it while the system is under pressure, the first movement of
the cap at once relieves the pressure and thus prevents the emission of scalding
steam or the blowing off the cap due to higher internal pressure.

13.13 AIR–COOLED SYSTEM

In an air-cooled system a current of air is made to flow past the outside of the
cylinder barrel, outer surface area of which has been considerably increased
by providing cooling fins as shown in Fig.13.16. This method will increase the
rate of cooling.
Application : This method is mainly applicable to engines in motor cycles,
small cars, airplanes and combat tanks where motion of vehicle gives a good
velocity to cool the engine. In bigger units a circulating fan is also used. In
addition to these engines, air-cooling is also used in some small stationary
engines. The value of heat transfer coefficient between metal and air is appre-
ciably low. As a result of this the cylinder wall temperatures of the air-cooled
cylinders are considerably higher than those of water-cooled type. In order
to lower the cylinder wall temperature the area of the outside surface which
directly dissipates heat to the atmosphere must be sufficiently high.

13.13.1 Cooling Fins

Cooling fins are either cast integral with the cylinder and cylinder head or can
be fixed with the cylinder block separately. Various shapes of cooling fins are
shown in Fig.13.17. The heat dissipating capacity of fins depends upon their
 Cooling fins between walls

Cooling fins

Fig. 13.16 Cooling fins on an engine cylinder increase the surface area
of cooling
1.0
a
0.8
b d
0.6
T/T root

c
b
0.4
c
0.2 a

d 6 mm 0.0
40 mm
40 mm
Fig. 13.17 Types of cooling systems

cross-section and length. At the same time as heat is gradually dissipated

from the fin surface, the temperature of the fin decreases from its root to
its tip. Hence, the fin surface nearer to the tip dissipates heat at a lower
rate and is less efficient. On the other hand as the quantity of heat flowing
towards the tip gradually decreases, the thickness of the fin can be decreased.
The material of the fin is used most efficiently if the drop in temperature
from the root to the tip is constant per unit length. A comparison of fins of
different cross-sections is shown in Fig.13.17 with drop in temperature from
root to tip. The rectangular section has least temperature drop whereas the
maximum temperature drop is for the fin marked ‘a’.
Fins are usually given a taper of 3 to 5 degrees in order to give sufficient
draft to the pattern. The tip is made 0.5 to 1.25 mm thick and a clearance of
2.5 to 5 mm is allowed at the root. The fins are made 25 to 50 mm long. Too
close spacing of the fins is undesirable as mutual interference of the boundary-
layers of adjacent layers restricts the air flow and results in small quantity of
heat dissipated.
13.13.2 Baffles
The rate of heat transfer from the cylinder walls can be substantially increased
by using baffles which force the air through the space between the fins. Figure
13.18 shows various types of baffles commonly used on engines. The arrange-

(a) (b) (c) (d)

Fig. 13.18 Types of baffles in air-cooled engines

ment at ‘a’ has got the highest pressure drop. It is always desired to have
negligible kinetic energy loss between the entrance and the exit. Usually the
normal type of baffle, ‘b’, is used on petrol engines. The arrangement ‘c’ for
minimizing the kinetic energy loss is shown with a well rounded entrance to
reduce the entrance loss and an exit section that will transform the velocity
head into pressure head and thus decrease the pressure drop. Arrangement
‘d’ is adopted for diesel engines.

13.14 COMPARISON OF LIQUID AND AIR–COOLING SYSTEMS

In view of the wide spread use of these two alternative cooling systems for
petrol as well as diesel engines it is of interest to summarize the respective
advantages and limitations of these systems.

13.14.1 Advantages of Liquid-Cooling System

(i) Compact design of engines with appreciably smaller frontal area is pos-
sible.

(ii) The fuel consumption of high compression liquid-cooled engines are

rather lower than for air-cooled ones.

(iii) Because of the even cooling of cylinder barrel and head due to jacketing
makes it possible to reduce the cylinder head and valve seat tempera-
tures.

(iv) In case of water-cooled engines, installation is not necessarily at the

front of the mobile vehicles, aircraft etc. as the cooling system can be
 conveniently located wherever required. This is not possible in case of
air-cooled engines.

(v) The size of engine does not involve serious problems as far as the design
of cooling systems is concerned. In case of air-cooled engines particularly
in high horsepower range difficulty is encountered in the circulation of
requisite quantity of air for cooling purposes.

13.14.2 Limitations
(i) This is a dependent system in which water circulation in the jackets is
to be ensured by additional means.

(ii) Power absorbed by the pump for water circulation is considerable and
this affects the power output of the engine.

(iii) In the event of failure of the cooling system serious damage may be
caused to the engine.

(iv) Cost of the system is considerably high.

(v) System requires considerable maintenance of its various parts.

13.14.3 Advantages of Air-Cooling System

(i) The design of the engine becomes simpler as no water jackets are re-
quired. The cylinder can have identical dimensions and be individually
detachable and therefore cheaper to renew in case of accident etc.

(ii) Absence of cooling pipes, radiator, etc. makes the cooling system sim-
pler thereby has minimum maintenance problems.

(iii) No danger of coolant leakage etc.

(iv) The engine is not subject to freezing troubles etc., usually encountered
in case of water cooled engines.

(v) The weight of the air-cooled engine is less than that of water-cooled
engine, i.e., power to weight ratio is improved.

(vi) In this case, the engine is rather a self-contained unit as it requires no

external components like radiator, header, tank etc.

(vii) Installation of air-cooled engines is easier.

13.14.4 Limitations
(i) Can be applied only to small and medium sized engines

(ii) In places where ambient temperatures are lower

(iii) Cooling is not uniform

(iv) Higher working temperatures compared to water-cooling

 (v) Produce more aerodynamic noise

(vi) Specific fuel consumption is slightly higher

(vii) Lower maximum allowable compression ratios

(viii) The fan, if used absorbs as much as 5% of the power developed by the
engine
13th Day of Class (MEE2004)
28/02/2024

Lecture 8
(Module 1)

Thermal Engineering
Systems
by
Dr. Rohit Sharma
School of Mechanical Engineering
Outline of this class

Ignition system

Battery, Magneto, and Electronic

systems
Ignition system

Sistemas Automóveis
• The ignition system is one of the most important systems used in the

IC engines.

• The spark-ignition engine requires some device to ignite the

compressed air-fuel mixture.

• The ignition takes place inside the cylinder at the end of the

compression stroke, the ignition system serves this purpose.

Note: Compression ignition engine does not have such an ignition system. In
a compression ignition engine, only air is compressed in the cylinder. And at
the end of the compression stroke, the fuel is injected which catch fire due to
the high temperature and pressure of the compressed air.
Ignition system

Sistemas Automóveis
• It is a part of the electrical system which carries the electrical current

to a current plug. It gives the spark to ignite the air-fuel mixture at the

correct time.
Types of Ignition System

Sistemas Automóveis
• Following are the types of ignition system:

1. Battery ignition system or coil ignition system

2. Magneto ignition system

3. Electronic Ignition System

The battery ignition system is mostly used in passenger cars and light
trucks.
Induction

Sistemas Automóveis
• Electromagnetic Induction or Induction is a process in which
a conductor is put in a particular position and magnetic field keeps
varying or magnetic field is stationary and a conductor is moving.
This produces a Voltage or EMF (Electromotive Force) across the
electrical conductor. Michael Faraday discovered Law of Induction
in 1830.

• In the battery ignition system, the current in the primary winding is

supplied by the battery.
• In the magneto ignition system, the magneto produces and
supplies the current in the primary winding
How Ignition system works

Sistemas Automóveis
Reference: https://www.youtube.com/watch?v=W94iksaQwUo
Ignition System parts

Sistemas Automóveis
1. Battery

2. Switch ignition distributor

3. Ignition coil

4. Spark plugs and

5. Necessary wiring

Some system uses transistors to reduce the load on the

distributor contact points. Other systems use a combination of
transistors and magnetic pickup in the distributor.
Ignition System parts

Sistemas Automóveis
1. Battery

2. Switch ignition distributor

3. Ignition coil

4. Spark plugs and

5. Necessary wiring

Some system uses transistors to reduce the load on the

distributor contact points. Other systems use a combination of
transistors and magnetic pickup in the distributor.
An Ignition in The Vehicle

Sistemas Automóveis
• The ignition system supplied high voltage surges of current (as high as
30,000 volts).
• These surges produce the electric sparks at the spark plug gap.
• Spark ignite to set fire to the compressed air-fuel mixture in the
combustion chamber.
• The sparking must take place at the correct time at the end of the
compression stroke in every cycle of operation.
• At high speed or during part throttle operation, the spark is advanced.
So that it occurs somewhat earlier in the cycle, the mixture thus has
time to burn and deliver its power.
• The ignition system should function efficiently at the high and low
speeds of the engine.
Battery Ignition System

Sistemas Automóveis
Battery Ignition System

Sistemas Automóveis
• The figure shows the battery ignition system
for a 4 cylinder engine.
• A battery of 12 volts is generally employed.
There are two basic circuits in the system
primary and secondary circuits.
• The first circuit has the battery, primary
winding of the ignition coil, condenser, and
the contact breaker forms the primary circuit.
• Whereas the secondary winding of the
ignition coil, distributor, and the spark plugs
forms the secondary circuits.
How Battery Ignition system works

Sistemas Automóveis
Reference: https://www.youtube.com/watch?v=OMLSNwQiiKg
Battery Ignition System

Sistemas Automóveis
When the ignition switch is closed, current flows from the battery through the
primary winding of the ignition coil, provided contact breaker points arc closed.
They produce magnetic field around the winding. When the piston is at the end of
compression stroke, the contact breaker point opens. Thus the flow of current in
primary winding causes the magnetic field to collapse. As the field collapses, its
lines of force cut the wire turnings of the secondary winding. This increases the
voltage across the secondary winding terminals to a value of 20 to 24 thousand
volts. The high-voltage surge is delivered to the centre terminal of the distributor
cap where it is picked up by the rotor and directed to the proper spark plug. A
spark jumps the plug gap and ignites the compressed air-fuel mixture.

The battery ignition system

has massive use in cars, light
trucks, buses, etc.
Magneto Ignition System

Sistemas Automóveis
• The magneto ignition system has the same principle of working like that of the

battery ignition system In this no battery is required as the magneto acts as its

own generator.
Magneto Ignition System

Sistemas Automóveis
• It consists of either rotating magnets in fixed coils, or
rotating coils in fixed magnets.
• The current produced by the magneto is made to
flow to the induction coil which works in the same as
that of the battery ignition system.
• This high voltage current is then made to flow to the
distributor which connects the sparking plugs in
rotation depending upon the firing order of the
engine.
• This type of ignition system is used small
spark-ignition engines for example Scooters,
Motorcycles and small motorboat engines.
Magneto Ignition System

Sistemas Automóveis
• It consists of either rotating magnets in fixed coils, or
rotating coils in fixed magnets.
• The current produced by the magneto is made to
flow to the induction coil which works in the same as
that of the battery ignition system.
• This high voltage current is then made to flow to the
distributor which connects the sparking plugs in
rotation depending upon the firing order of the
engine.
• This type of ignition system is used small
spark-ignition engines for example Scooters,
Motorcycles and small motorboat engines.
Electronic Ignition System

Sistemas Automóveis
Electronic Ignition System

Sistemas Automóveis
• The conventional electro-mechanical ignition
system uses mechanical contact breakers.
Though it is very simple, it suffers from certain
limitations as follows:

❑ The contact breaker points handle the heavy current. This resulting in burnout

of contact points. Thus it requires periodical servicing and settings.

❑ The mechanically operated contact breaker has inertial effects. Hence at

higher speeds, the make or break of contact may not be timed.

❑ At higher speeds, the dwell time for building up the current in the coil to its

maximum value is low. Thus the spark strength may be reduced.

 Electronic Ignition System

Sistemas Automóveis
• To overcome the above drawbacks, in the modern automobiles, electronic

ignition systems are used.

• This electronic ignition system has its best performance at all varying

conditions and speed, unlike electro-mechanical systems.

• The electro ignition system consists of transistors, capacitors, diodes, and

resistors.

• These acts as heavy-duty switches in controlling the primary current for the

high voltage ignition coil.

 Electronic Ignition System

Sistemas Automóveis
• To overcome the above drawbacks, in the modern automobiles, electronic

ignition systems are used.

• This electronic ignition system has its best performance at all varying

conditions and speed, unlike electro-mechanical systems.

• The electro ignition system consists of transistors, capacitors, diodes, and

resistors.

• These acts as heavy-duty switches in controlling the primary current for the

high voltage ignition coil.

 Reference
1. Internal Combustion Engines. Third Edition, V Ganeshan.
2. Various online sources.
Thank You
15th Day of Class (MEE2004)
04/03/2024

Lecture 9
(Module 2)

Thermal Engineering
Systems
by
Dr. Rohit Sharma
School of Mechanical Engineering
Outline of this class

PERFORMANCE PARAMETERS
Engine Performance

Sistemas Automóveis
• The performance of an engine is an indication of the degree of success

with which the conversion of chemical energy contained in the fuel is

done into useful mechanical work

• An engine is selected for a particular application on the basis of its

power output and rated speed

• Other factors include capital cost and operational cost

• Therefore, certain measurements and calculations are required to

judge the performance of an engine

Performance parameters

Sistemas Automóveis
• Evaluation of engine performance is done based on the following

performance parameters

• These are

❑ Indicated power
❑ Brake power
❑ Frictional power
❑ Fuel consumption
❑ Air consumption
❑ Brake thermal efficiency
❑ Indicated thermal efficiency
❑ Mechanical Efficiency
❑ Volumetric efficiency
❑ Air–fuel ratio
Distribution of energy produced by fuel combustion

Sistemas Automóveis
Indicated Power (IP)

Sistemas Automóveis
• Rate of work done on the piston by burning of charge inside the cylinder.
Evaluated from an indicated diagram obtained from the engine Gross power
produced by the engine is calculated as:
Brake Power (BP)

Sistemas Automóveis
• Net power available at the engine shaft for external use
• Measured by the dynamometer (rope brake dynamometer), which can
be loaded to measure the brake power of the engine
• It is calculated as
 Sistemas Automóveis
Brake Power (BP)
Friction Power (FP)

Sistemas Automóveis
• It is the part of the indicated power which is used to overcome the frictional
effects within the engine
• The friction power also includes power required to operate the fuel pump,
lubrication pump, valves, etc
• Therefore, it is given as the difference between the indicated power and
brake power
Problem 1

Sistemas Automóveis
A rope-brake dynamometer was used to measure
the brake power of a single cylinder, four stroke
cycle petrol engine. It was found that the torque
due to brake load was 175 Nm and the engine
makes 500 rpm. Determine the brake power
developed by the engine.
Problem 2

Sistemas Automóveis
A four-cylinder, four-stroke petrol engine
develops indicated power of 14.7 kW at 1000
rpm. The mean effective pressure is 5.5 bar.
Calculate the bore and stroke of the engine, if
the stroke is 1.5 times the bore.
Thank You