Heat Engines
Internal Combustion
Engines
Engine Fuels
Engine fuels may be in gaseous, liquid or solid form. But, liquid fuel is
most dominantly used. They are derived mostly from petroleum.
However, sources are depleting. Therefore, some are using alternative
fuels such as biofuels.
Petroleum Fuels
•composed primarily of hydrogen (~14%) and carbon (~86%)
•derived from crude oil.
Essential Characteristics of Engine Fuels
1.Must have a reasonably high energy value
2.Must vaporize at least partially at comparatively low temperatures
3.Fuel vapors must ignite and burn readily when mixed in proper
proportions with oxygen
4.Must be of such nature that it can be handled and transported with
comparative ease and safety
Combustion
Chemical reaction of carbon and hydrogen in the fuel with oxygen in
the air to form water and other exhaust products
Liberates heat which raises the cylinder pressure and drives the piston
downward during the power stroke
Actual combustion is complex, There are 3 types of combustion:
Ideal/Theoretical/Stoichiometric Combustion
All of the hydrogen in the fuel is converted into water and all of the
carbon is converted into carbon dioxide
Rich Combustion
There is not enough oxygen to convert all of the carbon to CO2
CO appears in the exhaust
Lean Combustion
Excess oxygen is supplied for combustion
O2 appears in the exhaust
Useful Values:
Approximate Air Composition By Weight:
• 23.2% oxygen
• 76.8% nitrogen and other trace elements
By Volume/Mole:
• 21% oxygen
• 79% nitrogen and other trace elements
• i.e. in air, one mole of O2 is always accompanied by 3.76 moles of nitrogen
Air to Fuel Ratio (A/F)
• It is the ratio of the quantity of air supplied to the quantity of fuel used in
combustion
• Can be expressed in “by mass/weight” and “by volume” basis
(A/F)m =(mass of air)/(mass of fuel)
(A/F) = (volume of air)/ (volume of fuel) = (moles of air)/ (moles of fuel)
Chemical Reaction of Fuel
Combustion of a fuel is said to be complete when all the carbon and
hydrogen are burned to form carbon dioxide (CO2) and water (H2O).
Combustion is the process by which a fuel unites chemically with oxygen,
producing an oxide and often generating heat of considerable intensity,
and sometimes light.
Specific Gravity of Fuels, γ
the density of the fuel relative to the density of water at the same
temperature densities must be measured at a standard temperature
(15.6C or 60F) or must be corrected at that temperature. A hydrometer
for measuring the specific gravity of a fuel
API Gravity
a gravity scale devised by the American Petroleum Institute (API) convenient
for refinery use
°API = (141.5/γ) - 131.5
where γ is the specific gravity of fuel at 15.6°C (60°F)
API - American Petroleum Institute which is one of the organizations that set the
engine oil performance standards for each type of vehicle, both gasoline and diesel engines
Heating Value
amount of heat energy contained in any fuel
can be expressed as higher/gross heating value (HHV) or lower/net
heating value (LHV)
when computing for engine efficiencies, state whether LHV or
HHV was used.
Estimation of Heating Value
Based on API gravity and applicable to petroleum fuels
HHV = KF1 + KF2 (API-10)
LHV = 0.7190 HHV+ KF3
where:
HHV = higher heating value in kJ/kg (BTU/lb)
LHV = lower heating value in kJ/kg (BTU/lb)
API = API gravity in degrees
KF1= 42,860 (18,440)
KF2= 90 (40)
KF3= 10,000 (4,310)
(Karl Fischer) KF PRO Oil is the perfect instrument to measure ppm water in oils and fuels
without the worry of interference side reactions caused by additives .
Fuel Properties
1. Volatility
Refers to the ability of fuels to vaporize
Can be determined by Reid vapor pressure and distillation curves
Fuels that vaporize easily at lower temperatures are more
volatile than fuels that require higher temperatures to
vaporize
Gasoline refiners adjust gasoline volatility to suit season and
location
Gasoline sold for use in summer or at high elevations is made
less volatile because evaporation is easier under such
conditions and vapor lock must be avoided
Fuel Properties
Vapor lock - formation of fuel vapor bubbles in the fuel line which
prevents the fuel pump from working.
Diesel fuel must be volatile enough to ensure vaporization in the
combustion chamber
If too volatile, fuel droplets will evaporate too quickly to permit
adequate spray penetration
2. Flash Point
Lowest temperature to which a fuel must be heated to produce an
ignitable vapor-air mixture above the liquid fuel when exposed to an
open flame
Relates to safety precaution that must be taken when handling a fuel
Gasolines have flash points well below the freezing point of water
(-40°C) and can readily ignite in the presence of spark or flame
Commercial diesel fuels have flash points above 38°C-52°C
Fuel Properties
3. Cloud Point
Temperature at which crystals begin to appear as the liquid
is cooled
4. Pour Point
Highest temperature at which the fuel ceases to flow
The cloud point typically occurs 5°C - 8°C above
the pour point.
Combustion In I.C.E.
1. Combustion by explosion
occurs when the air and fuel are mixed, compressed and ignited
2. Boundary combustion
occurs when fuel is injected into heated air and is burned as the
surface of the stream or droplets contacts the air
3. Detonation – sometimes called engine knocking.
A blow in the piston like hammering, occurs when a combustible
mixture is heated by compression to the ignition temperature
Undesirable combustion that results to sudden rise in pressure, loss
of power, and overheating.
It is harmful to the engine and causes the engine running shaky.
To prevent this event, use a high octane fuel.
Combustion In I.C.E.
Causes of Detonation
A. Improper combustion chamber
High compression pressure
B. Improper fuel
C. Inadequate cooling
Self-Ignition Characteristics of Fuel
Self-Ignition Temperature, SIT
Temperature at which an air-fuel mixture will self-ignite
without the need of a spark plug or external igniter
Ignition Delay/Lag, ID
Delay time in air-fuel mixture ignition
Octane Rating
measure of knock resistance of gasoline
numerical scale generated by comparing the self-ignition
characteristics of the fuel to that of standard fuels in a specific test
engine at specific operating conditions
the higher the octane number of a fuel, the less likely it will self-
ignite.
based upon the percentage by volume of isooctane in an isooctane-
heptane mixture.
Isooctane( C8 H18)- has excellent antiknock qualities, rated at 100
Heptane (C7 H16)- knock excessively even under low-
compression conditions, rated at 0
(Fuel having a knock characteristic as 90-10 isooctane-heptane
mixture is called a 90 octane fuel)
Cetane Rating
Indicator of anti-knock qualities of diesel fuels
The higher the cetane number, the shorter is the ignition delay
(ID) and the quicker the fuel will self-ignite in the combustion
chamber environment
Fuels with high cetane numbers have low octane numbers and
the reverse is true
based upon the percentage by volume of n-cetane and HMN
mixture
n-cetane (hexadecane), C16H34, given a value of 100,
heptamethylnonane (HMN), C12H34, a value of 15
CN of fuel = (percent of n-cetane) + (0.15) (percent of HMN)
Cetane Rating
Normal cetane number range is about 40 to 60
If the cetane number is too low (long ID) more fuel than, desirable
will be injected into the cylinder before the first fuel particles ignite,
causing a very large, fast pressure rise at the start of combustion
results in low thermal efficiency and a rough-running engine.
If the cetane number is too high (short ID), combustion will start
too soon in the cycle
Inadequate mixing of fuel and air, hence incomplete combustion
Pressure will rise before TDC and more work will be required during
compression stroke.
results in low thermal efficiency and a rough-running engine
Cetane Boosters
Ignition-accelerating additives for diesel fuel include
organic peroxides, nitrates, nitrites, and various
sulphur compounds.
most commercially important are the alkyl nitrates
(isopropyl nitrate, primary amyl nitrates, primary
hexylnitrates, octylnitrate).
Typically, about 0.5 percent of these additives by volume
in a distillate fuel gives about a 10 cetane number
increase in a fuel's ignition quality.
I. Fuel System – responsible for supplying fuel in the combustion
chamber
A. Functions of the fuel system of compression ignition engines
To meter the correct amount of fuel as demanded by the load
To accurately time the beginning and ending of the fuel injection
To inject fuel against very high pressure in the cylinder
Major parts of diesel fuel-injection system:
✓ Fuel tank,
✓ Fuel transfer pump,
✓ Primary fuel filter,
✓ Two-stage secondary filter,
✓ Injection pump,
✓ Injection nozzles, and
✓ Governing mechanism
B. Functions of the fuel system of spark ignition engines
To assist in properly vaporizing the fuel
To mix the vaporized fuel in the correct proportions with air
To supply the engine with the proper quantity of mixture
Essential components are:
✓ Fuel-supply container or tank,
✓ A carburetor,
✓ Connecting lines, pump, filter, etc., and
✓ Intake manifold to conduct the mixture from the carburetor to
the cylinder
II. Electrical System (gasoline engines only)
The essential functions of any electric ignition system are:
✓ The generation of a large, hot spark in the cylinder, and
✓ The production of this spark at the right instant in the travel of the
piston
Two kinds of ignition systems:
✓ Battery-ignition system, and
Magneto-ignition system.
III. Valve system (Four stroke cycle engine only)
The operation of an ICE necessitates the admission, trapping, and
exhausting of the working medium. These functions are
accomplished by means of the valves.
IV. Cooling System
Classification of cooling methods and systems:
1. Air cooling
This is usually applied to small cylinder engines. Multiple cylinder
engines are cast individually to maximize cooling effect.
It is difficult to maintain proper cooling under all conditions and
almost impossible to fully control cylinder temperature.
2. Liquid cooling
3. Combination of liquid and air cooling
✓ Thermosiphon
✓ Forced-circulation system
V. Lubrication System
Functions:
1) Reduces friction and wear by separating rubbing
parts
2) Assists in cooling the engine by absorbing heat
from parts
3) Acts as the final seal between the piston and
cylinder walls
4) Assists in cleaning the engine
V. Lubrication System
Types of Lubricants:
1. Crankcase oil – thin free flowing, SAE 30 or 40, oil for engine internal
parts
2. Transmission oil – heavier and higher viscosity, oil for transmission
gears
3. Grease – slow, non-flowing lubricant, used in wheel bearings
Types of Lubrication systems:
1. Splash system
The lubrication of all the principal engine parts depends directly on
the splashing of the oil by a dipper on the bottom side of the
connecting rod cap that dips into the crankcase oil each time the
piston reaches the bottom dead center.
V. Lubrication System
Types of Lubrication systems:
2. Pressure-feed and splash system
Oil is forces directly to the main crankshaft, connecting rod, and camshaft
bearings. Drilled passages in the crankshaft carry the oil from the main
bearings to the connecting-rod bearings. The oil oozing out of these
bearings creates a spray that lubricates the cylinder walls, pistons and
piston pins. The valve mechanism is oiled by the pressure from the
crankcase.
3. Full-pressure system
Oil is forced not only in the crankshaft, connecting rod, and camshaft
bearings but also to the piston pin bearings through passages. Cylinders and
pistons receive oil from the piston pins and from the mist creates by the oil
issuing from various bearings. The valve mechanism is also oiled by
pressure.
VI. Governing System
A governor is a device that automatically regulate the speed of an
engine. In general, governors are of centrifugal spring-loaded type.
For electric ignition engines, the governing mechanism is connected to
the throttle butterfly of the carburetor in such a manner that it controls
and varies the throttle opening according to the amount of fuel mixture
needed to maintain the desired engine speed and power output.
The control of the diesel engine speed and load involves the control of
the charge fuel injected. The mechanism is connected and becomes a
part of the injection pump. No attempt is made to vary the air charge,
and it remains constant at all loads.
VI. Power Transmission System - responsible for transferring the
power generated from the combustion chamber (engine) to the drive shaft to the
rear wheels. The system consist the Clutch, Gear box, Differential and Differential,
(for some tractors it includes the rear axle and final drive). Combination of all these
components is responsible to the movement of a motorized vehicle.
VI. Power Transmission System - sometimes called as sequence of
gears and shafts, through which the engine power is transmitted to the wheels. The
system consist of various devices that cause backward and forward movements.
The complete path of engine power to the wheels is called powertrain.
.
VI. Power Transmission System - a speed reducing mechanism,
equipped with several gears. responsible for transferring the power generated from
the combustion chamber to the drive shaft.
.
VI. Power Transmission System - responsible for transferring the
power generated from the combustion chamber to the drive shaft.
Clutch
Transmit power from the flywheel
Absorbs impact load
Used for gradual application of load to engine
Permits the change in gears without stopping the engine
Gear box
Supply different speeds of travel
Supply necessary torque at different loads
Used for idling
Differential
Supply different speeds of ground wheel when turning
Functions as a second speed reduction point in the power train
Brake system
Used to reduce the speed or stop the vehicle.
The following conditions must exist if the engine is to
operate efficiently:
a. The cylinder should have good compression. To ensure good
compression:
1. Valves must sit properly on the valve seat;
2. Good fit must exist between piston rings and cylinder wall;
3. Use of recommended oil to enable it to seal minute gaps
between the piston rings and cylinder wall;
4. Cylinder head gasket must be tightly secured.
b. The valves must be correctly timed. Engine manufacturers fix
the opening and closing points of the valves with respect to the
position and direction of motion of the piston. It is fixed by:
1. Providing timing marks between the crankshaft and camshaft
timing gears;
2. Fixing the clearance between the valve and tappet.
c. The correct air-fuel mixture must be supplied to the cylinder. A
15:1 air-fuel ratio can be provided by an efficiently air cleaner
and properly adjusted carburetor. In general, there are two
carburetor adjustments, Carburetor adjustments should always
be made on hot engine and operating and generally, the low
speed is made first.
d. The occurrence of the spark must be correctly timed.
The combustible mixture must be ignited at the right moment
so that its maximum effect would be felt when the piston is at
TDC.
There is definite time, though short, between the ignition of the
charge with maximum expansion.
If the engine is at low speed, the spark may occur at TDC.
However, as engine speed increases, the spark must occur
before TDC so that maximum expansion will be felt at TDC.
The higher the speed, the earlier the spark should occur before
TDC. This is referred to as spark advance and designated in
degrees rotation of the crankshaft.
e. The moving parts must be properly lubricated. Lubricating oil
does not only reduce friction by separating rubbing parts but also
assists in cleaning the engine of carbon and dirt.
This is one reason why there is a need for periodic oil changes.
The temperature of the engine must be maintained at optimum
level.
The main bulk of the heat rejected by the engine is dissipated
through the cooling system.
For air cooled engines, it is necessary that the passages are free
from dirt for more efficient heat transfer.
For liquid-cooled engines, there should be sufficient amount of
water available in the cooling system to carry away or dissipate the
unwanted heat.
Servicing of Engines
Servicing is a term which includes both the essential daily
and periodic maintenance work.
Daily maintenance involves the simple activities such as
checking the levels of fuel and oil, condition of the
cooling system and transmission belt, and the tightness
of the bolts and nuts and screws.
Periodic maintenance on the other hand, involves the
scheduled cleaning, replacement or adjustment of parts
related to the valve, ignition, fuel, lubrication and cooling
systems.
