Internal Combustion Engines

Prime mover:

A prime mover is a self moving device which converts the available natural source of energy
to mechanical energy of motion to drive the other machines. The various types of prime
movers which convert heat energy produced by the combustion of fuels into mechanical
energy are: Steam engines, Steam turbines, Gas turbines and Internal Combustion engines out
of which steam engine have become obsolete.

Different parts of an I.C. Engine:

The various important parts of an I.C. Engine are shown in Fig. 1.

a) Cylinder: The heart of the engine is the cylinder in which the fuel is burnt and fuel power
is developed. The inside 3iameter is called bore. To prevent the wearing of the cylinder
block, a sleeve will be fitted tightly in the cylinder. The piston reciprocates inside the
cylinder.
b) Piston: The piston is a close fitting hollow-cylindrical plunger moving to and-fro in the
cylinder. The power developed by the combustion of the fuel is transmitted by the piston
to the crank shaft through the connecting rod.
c) Piston Rings: The piston rings are the metallic rings inserted into the circumferential
grooves provided at the top end of the piston. These rings maintain a gas-tight joint
between the piston and the cylinder while the piston is reciprocating in the cylinder. They
also help in conducting the heat from the piston to the cylinder.
d) Connecting Rod: It is a link that connects the piston and the crankshaft by means of pin
joints. It converts the rectilinear motion of the piston into rotary motion of the crankshaft.
e) Crank and Crankshaft: The crank is a lever that is connected to the end of the connecting
rod by a pin joint with its other end connected rigidly to a shaft, called crankshaft. It
rotates about the axis of the crankshaft and causes the connecting rod to oscillate.
f) Valves: The valves are the devices which controls the flow of the intake and the exhaust
gases to and from the engine cylinder. They are also called poppet valves. These valves
are operated by means of cams driven by the crankshaft through a timing gear or chain.

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EXHAUST VALVE

OUTLET
INLET VALVE

CYLINDER

INLET
PISTON RINGS

PISTON
CAM

CONNECTING ROD

FLYWHEEL
CRANKCASE

CRANK

Fig. 1: Parts of I. C. Engine

I.C. Engine Terminologies:

The following are the important I.C. Engine terminologies (also shown in Fig. 2)
a) Bore: The inside diameter of the cylinder is called the bore diameter.
b) Stroke: the linear distance along the cylinder axis between the two limiting positions of
the piston is called stroke or stroke length.

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c) Top Dead Centre (T.D.C): The top most position of the piston towards cover end side of
the cylinder is called top dead centre. In case of horizontal engine, it is called as inner
dead centre (I.D.C).
d) Bottom Dead Centre (B.D.C): The lowest position of the piston towards the crank end
side of the cylinder is called bottom dead centre. In case of horizontal engine, it is called
outer dead centre (O.D.C).
e) Clearance Volume: The volume contained in the cylinder above the top of the piston,
when the piston is at the top dead centre is called clearance volume.
f) Compression Ratio: It is the ratio of total cylinder volume to the clearance volume.

CLEARANCE
VOLUME TDC
EXTREME TOP
POSITION OF PISTON

STROKE
LENGTH

BORE
BDC
EXTREME BOTTOM
POSITION OF PISTON

Fig. 2: I.C. Engine Terminologies

Heat Engine & I.C. Engine:

A heat engine is a prime mover which derives the heat energy from the combustion of fuels
any other source and converts this energy into mechanical work. In the heat engines, the
mechanical work produced is a linear work which in turn is converted into rotational work by
the elements such as cylinder, piston, connecting rod, crank, etc.
The heat engines are mainly classified into: (1) External combustion engines and (2) Internal
combustion engines. In the external combustion engines known as E.C. Engines, the

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combustion the fuel takes place outside the engine cylinder, ex: steam engine. In the internal
combustion engines known as I. C. Engines, the combustion of the fuel takes place inside the
engine cylinder, ex: petrol engines, diesel engines.
I.C. Engines: An internal combustion engine more popularly known as I.C. Engine, is a heat
engine which converts the heat energy released by the combustion of the fuel taking place
inside the engine cylinder into mechanical work. Its versatile advantages such as high
efficiency, light weight, compactness, easy starting, adaptability, suitability for mobile
applications, comparatively lower initial cost has made its use as an universal prime mover.

Classifications of I.C. Engines:

I.C. Engines are classified according to:

(i) Nature of Thermodynamic Cycle,

1. Otto cycle engine.
2. Diesel cycle engine.
3. Dual combustion cycle engine.
(ii) Type of the Fuel- used,
1. Petrol engine.
2. Diesel engine.
3. Gas engine.
4. Bi-fuel Engine.
5. Dual Fuel Engine
(iii) Number of Strokes,
1. Four stroke engine.
2. Two stroke engine.
(iv) Method of Ignition as..
1. Spark ignition engine, known as S.I. Engine.
2. Compression ignition engine, known as C.I. engine.
(v) Number of Cylinders,
1. Single cylinder engine.
2. Multi-cylinder engine.
(vi) Position of the Cylinder as,
1. Horizontal engine
2. Vertical engine.

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3. Vee engine.
4. Opposed cylinder engine.
5. Radial engine.
(vii) Method of Cooling,
1. Air cooled engine.
2. Water cooled engine.
(viii) Speed of the Engine,
1. Low speed engine.
2. Medium speed engine.
3. High speed engine.
(viii) Uses as…
1. Stationary engine.
2. Automobile engine.
3. Aero engine.
4. Locomotive engine.
5. Marine engine, etc.

Four-Stroke Petrol Engine:

The four-stroke cycle petrol engines operate on Otto (constant volume) cycle. Since ignition
in these engines is due to a spark, they are also called spark ignition engines. The four
different strokes are:
i) Suction stroke,
ii) Compression stroke
iii) Working or power or expansion stroke
iv) Exhaust stroke.
The construction and working of a four-stroke petrol engine is shown in Fig.4.
Figure 3 shows a theoretical Otto cycle.

Suction Stroke: During suction stroke, the piston is moved from the top dead centre to the
bottom dead centre by the crank shaft. The crank shaft is rev loved either by the momentum
of the flywheel or by the electric starting motor. The inlet valve remains open and the exhaust
valve is closed during this stroke. The proportionate air-petrol mixture is sucked into the
cylinder due to the downward movement of the piston. This operation is represented by the
line AB on the P-V diagram.
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C
Pressure

A B

Volume
Fig. 3: Theoretical Otto cycle

Compression Stroke: During compression stroke, the piston moves from bottom dead centre
to the top dead centre, thus compressing air petrol mixture. Due to compression, the pressure
and temperature are increased and is shown by the line BC on the P- V diagram just before
the end of this stroke the spark - plug initiates a spark which ignites the mixture and
combustion takes place at constant volume as shown by the line CD. Both the inlet and
exhaust valves remain closed during this stroke.

Spark Plug
Exhaust Gas
Air + Fuel
Mixture

Piston

Cylinder

Connecting
Rod

Crank

(a) Suction (b) Compression (c) Working (d) Exhaust

Fig. 4: Four stroke petrol engine

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Working Stroke: The expansion of gases due to the heat of combustion exerts a pressure on
the piston. Under this impulse, the piston moves from top dead centre to bottom dead centre
and thus the work is obtained in this stroke. Both the inlet and exhaust valves remain closed
during this stroke. The expansion of the gas is shown by the curve DE.

Exhaust Stroke: During this stroke, the inlet valve remains closed and the exhaust valve
opens. The greater part of the burnt gases escapes because of their own expansion. The drop
in pressure at constant volume is represented by the line EB. The piston moves from bottom
dead centre to top dead centre and pushes the remaining gases to the atmosphere. When the
piston reaches the top dead centre the exhaust valve closes and cycle is completed. This
stroke is represented the line BA on the P-V diagram. The operations are repeated over and
over again in running the engine.

Four Stroke Diesel Engine:

The four stroke cycle diesel engine operates on diesel cycle or constant pressure cycle. Since
ignition in these engines is due to the temperature of the compressed air, they are also called
compression ignition engines. The construction and working of the four stroke diesel engine
is shown in fig.6, and fig.5 shows a theoretical diesel cycle.

The four strokes are as follows:

Suction Stroke: During suction stroke, the piston is moved from the top dead centre to the
bottom dead centre by the crank shaft. The crank shaft is revolved either by the momentum of
the flywheel or by the power generated by the electric starting motor. The inlet valve remains
open and the exhaust valve is closed during this stroke. The air is sucked into the cylinder due
to the downward movement of the piston. This operation is represented by the line AB on the
P-V diagram.

Compression Stroke: The air drawn at the atmospheric pressure during suction stroke is
compressed to high pressure and temperature as piston moves from the bottom dead centre to
top dead centre. This operation is represented by the curve BC on the P- V diagram. Just
before the end of this stroke, a metered quantity of fuel is injected into the hot compressed air
in the form of fine sprays by means of fuel injector. The fuel starts burning at constant

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pressure shown by the line CD. At point D, fuel supply is cut off. Both the inlet and exhaust
valves remain closed during this stroke.

C D
Pressure

A B

Volume
Fig. 5: Theoretical Diesel cycle

Fuel Injector
Exhaust Gas

Air fuel
mixture

Piston

Cylinder

Connecting
Rod

Crank

(a) Suction (b) Compression (c) Working (d) Exhaust

Fig. 6: Four stroke diesel engine

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Working Stroke: The expansion of gases due to the heat of combustion exerts a pressure on
the piston. Under this impulse, the piston moves from top dead centre to the bottom dead
centre and thus work is obtained in this stroke. Both the inlet and exhaust valves remains
closed during this stroke. The expansion of the gas is shown by the curve DE.

Exhaust Stroke: During this stroke, the inlet valve remains closed and the exhaust valve
opens. The greater part of the burnt gases escapes because of their own expansion. The drop
in pressure at constant volume is represented by the vertical line EB. The piston moves from
bottom dead centre to top dead centre and pushes the remaining gases to the atmosphere.
When the piston reaches the top dead centre the exhaust valve closes and the cycle is
completed. This stroke is represented by the line BA on the P- V diagram.

Two Stroke Petrol Engine:

The principle of two stroke cycle petrol engine is shown in Fig.7. Its two strokes are
described as follows:

Upward Stroke: During the upward stroke, the piston moves from bottom dead centre to top
dead centre, compressing the air-petrol mixture in the cylinder. The cylinder is connected to a
closed crank chamber. Due to upward movement of the piston, a partial vacuum is created in
the crankcase, and a new charge is drawn into the crankcase through the uncovered inlet port.
The exhaust port and transfer port are covered when the piston is at the top dead centre
position as shown in Fig.7. The compressed charge is ignited in the combustion chamber by a
spark provided by the spark plug.

Downward Stroke: As soon as the charge is ignited, the hot gases force the piston to move
downwards, rotating the crankshaft, thus doing the useful work. During this stroke the inlet
port is covered by the piston and the new charge is compressed in the crank case as shown in
the Fig.7c. Further downward movement of the piston uncovers first the exhaust port and
then the transfer port as shown in Fig.7d. The burnt gases escape through the exhaust port. As
soon as the transfer port opens, the compressed charge from the crankcase flows into the
cylinder. The charge is deflected upwards by the pump provided on the head of the piston and
pushes out most of the exhaust gases. It may be noted that the incoming air-petrol mixture
helps the removal of burnt gases from the engine cylinder, which is called as the scavenging

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process. If in case these exhaust gases do not leave the cylinder, the fresh charge gets diluted
and efficiency of the engine will decrease. The cycle of events is then repeated.

Exhaust port Spark Plug Transfer port

Inlet
port

Crank Case
(air tight)
(a) Compression (b) Ignition (c) Expansion (d) Intake

Upward Stroke Downward Stroke

Fig. 7 Two stroke cycle petrol engine

Two Stroke Diesel Engine:

In two stroke cycle diesel engine, only air is compressed inside the cylinder and the diesel is
injected by an injector. There is no spark plug in this engine. The remaining operations of the
two stroke cycle diesel engine are exactly the same as those of the two stroke cycle petrol
engine. The principle of two stroke cycle diesel engine is shown in Fig. 8. Its two strokes are
described as follows:

Upward Stroke: During the upward stroke, the piston moves from bottom dead centre to top
dead centre, compressing the air in the cylinder. The cylinder is connected to a closed crank
chamber. Due to upward movement of the piston, a partial vacuum is created in the
crankcase, and a new charge is drawn into the crankcase through the uncovered inlet port.
The exhaust port and transfer port are covered when the piston is at the top dead centre
position as shown in Fig.8. The compressed charge is mixed with the injected diesel and gets
auto-ignited in the combustion chamber because of the high pressure and temperature
present.

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Downward Stroke: As soon as the fuel is ignited, the hot gases force the piston to move
downwards, rotating the crankshaft, thus doing the useful work. During this stroke the inlet
port is covered by the piston and the new charge is compressed in the crank case as shown in
the Fig.8c. Further downward movement of the piston uncovers first the exhaust port and
then the transfer port as shown in Fig. 8d. The burnt gases escape through the exhaust port.
As soon as the transfer port opens, the compressed charge from the crankcase flows into the
cylinder. The charge is deflected upwards by the pump provided on the head of the piston and
pushes out most of the exhaust gases. It may be noted that the incoming air helps the removal
of burnt gases from the engine cylinder, which is called as scavenging process. If in case
these exhaust gases do not leave the cylinder, the fresh charge gets diluted and efficiency of
the engine will decrease. The cycle of events is then repeated.

Exhaust port Fuel Injector Transfer port

Inlet
port

Crank Case
(air tight)
(a) Compression (b) Ignition (c) Expansion (d) Intake

Upward Stroke Downward Stroke

Fig. 8 Two stroke Diesel engine

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Compare Petrol Engine and Diesel Engine:

Petrol Engine Diesel Engine

1. It works on Otto cycle. It works on diesel cycle.
Diesel is fed into the cylinder by fuel
2. Air and petrol are mixed in the carburetor
injection and is mixed with air inside the
before they enter into the cylinder.
cylinder.
It compresses only air and ignition is
3. It compresses a mixture of air and petrol and
accomplished by the heat of compression.
is ignited by an electric spark. (Spark Ignition).
(Compression Ignition).
4. Cylinder is fitted with a spark plug. Cylinder is fitted with a fuel injector.
5. Less thermal efficiency and more fuel More thermal efficiency and less fuel
consumption. consumption.
6. Compression ratio ranges from 4: 1 to 10: 1 Compression ratio ranges from 12:1 to 22:1
7. Less initial cost and more running cost. More initial cost and less running cost.
8. Light weight and occupies less space. Heavy and occupies more space.
Difficult to start in cold weather and
9. Easy to start even in cold weather.
requires heater plugs.
They run for longer periods between
10. Requires frequent overhauling.
overhauls.
11. Fuel (petrol) is costlier and more volatile. Fuel (diesel) is cheaper and less volatile.
12. Used in light vehicles like cars, motor Used in heavy duty vehicles like tractors,
cycle, scooters, etc. trucks, buses, locomotives, etc.

Compare Four Stroke and Two Stroke I.C. Engine:

Four Stroke Cycle Engine Two Stroke Cycle Engine

1. One Working stroke for every two One working stroke for each revolutions of
revolution of the Crank shaft. the crank shaft.
2. Turning moment on the crank shaft is not Turning moment on the crank shaft is more
even, hence heavier flywheel is required. even, hence lighter flywheel is required.
3. More output due to full fresh charge Less output due to mixing of fresh charge
intake and full burnt gases exhaust. with the burnt gases.
4. Less fuel consumption More fuel consumption.

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5. Higher thermal efficiency. Lower thermal efficiency

6. Engine design is complicated. Engine design is simple.
7. Lesser rate of wear and tear. Greater rate of wear and tear.
8. It has inlet and exhaust valves. It has inlet and exhaust ports.
9. Engine is heavy and bulky. For the same power, the engine is light and
compact.
10. It requires lesser cooling and It requires greater cooling and lubrication.
lubrication. (consumes more lubricating oil)
11. Complicated lubricating system. Simple lubricating system.
12. More initial cost. Less initial cost.
13. Less running noise. More running noise due to the sudden release
of the burnt gases.
14. Engine cannot run in either direction. Engine can be easily reverse if it is of valve-
less type.
15. Used in cars, buses, trucks, tractors, etc. Used in mopeds, motor cycles, scooters, etc.

Expression for the Indicated Power of I.C. Engine:

Indicated Power is the power produced inside the cylinder of an engine. It is calculated by
finding the actual mean effective pressure. The actual mean effective pressure is found as
follows.
Let a = Area of the actual indicator diagram, sq.cm
l = Base width of the indicator diagram, cm
s = Spring Value of the spring used in the indicator, N/m2/cm
Pmep = Actual Mean Effective Pressure, N/m2
sa
Pmep  N/m2
l
The indicated power of the four stroke and two stroke engines are found as follows:
Let Pmep = Mean Effective Pressure, N/m2
L = Length of Stroke, m
A = Area of Cross section of the Cylinder, sq m
N = RPM of the Crankshaft.
n = No. of cycles /min

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Work Produced by the piston / Mean Force Acting Piston displacement in

= X
stroke or / Cycle on the piston one stroke
= Pmep * L * A N-m

Work Produced by
Work Produced by the = X No. of Cycles / min
the piston / Cycle
piston / min

= Pmep * L * A * n N-m/min
i.e., Indicated Power IP = i* Pmep * L * A * n / 60000 kN-m/sec or kW
where i = no. of cylinders
n = N/2 for four-stroke engine
In four stroke I.C. engines, one cycle will be completed in two revolutions of the crank shaft.
Therefore the work will be produced in every alternate revolutions of the crankshaft. Thus
the number of cycles per minute will be equal to half the number of revolutions per minute.

Definitions:

Brake Power:

The power developed by the engine at the output shaft is called break power. Indicated power
produced inside the IC engine cylinder will be transmitted through the piston connecting rod
and crank. Therefore a certain fraction of the indicated power produced inside the cylinder
will be lost due to friction of the moving parts of the engine. Therefore net power available at
the crankshaft will be equal to the difference between the Indicated power produced inside
the engine cylinder and the power lost due to friction. The net power available at the
crankshaft is measured by applying the brake and is therefore called brake power. The
amount of the power lost in friction is called friction power. The friction power is the
difference between the indicated power and the brake power.
Friction Power = Indicated power - Brake Power
Brake power is calculated as follows:
W = Net load acting on the brake drum, kg
R = Radius of the brake drum, m
N = Revolutions per Minute of the crankshaft
T = Torque applied due to the net load W on the brake drum, N-m
= W*R kg-m
= 9.81*W*R N-m

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2NT
Brake Power = kW
60000

Mechanical Efficiency:

It is the efficiency of the moving parts of the mechanism transmitting the indicated power to
the crank shaft. Therefore it is defined as the ratio of the brake power and the indicated
power. It is expressed in percentage.
Mechanical Efficiency = ηmech = BP / IP * 100

Thermal Efficiency:

It is the efficiency of conversion of the heat energy produced by the actual combustion of the
fuel into the power output of the engine. Therefore it is defined as the ratio of the power
developed by the engine to the heat supplied by the fuel in the same interval of time. It is
expressed in percentage.
Power Output
Thermal Efficiency  *100
Heat Energy Supplied by theFuel

The power output to be used in the above equation may be brake power or indicated power
accordingly the thermal efficiency is called brake thermal efficiency or indicated thermal
efficiency.

The brake thermal efficiency is defined as the ratio of brake power to the heat supplied by the
fuel. It is expressed in percentage.
Brake Power
BrakeThermal Efficiency  *100
Heat Energy Supplied by theFuel

BP
 bth  *100
m f * CV

Where, mf = mass of the fuel supplied in kg/s

CV = Calorific Value of the fuel in kJ/kg

The indicated thermal efficiency is defined as the ratio of indicated power to the heat
supplied by the fuel. It is expressed in percentage.
Indicated Power
Indicated Thermal Efficiency  *100
Heat Energy Supplied by theFuel

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IP
 bth  *100
m f * CV

Specific Fuel Consumption (SFC):

It is the mass of fuel consumed per unit power in unit time. Here also there are two
parameters, depending on the power taken for calculation.
3600 * m f
SFC  kg/ KW - hr
Power Output
3600* m f
i.e. SFC on Brake power basis, SFC BP  kg/ KW - hr
BP
3600 * m f
and SFC on Indicated power basis, SFC IP  kg/ KW - hr
IP

Problem 1:
A four cylinder, four-stroke petrol engine 40 mm bore and 60 mm stroke was tested under a
constant speed of 600 rpm.
Diameter of belt dynamometer pulley = 1 m.
Tight side tension in the belt = 800 N
Slack side tension in the belt = 400 N
Determine the brake power and brake mean effective pressure.

Problem 2:
A four cylinder, four stroke internal combustion engine develops an indicated power of 50
kW at 3000 rpm. The cylinder diameter is 75mm. and the stroke is 90 mm. Find the mean
effective pressure in each cylinder. If the mechanical efficiency is 80 %, what effective brake
load would be required if the effective brake drum diameter is 0.6 m.

Problem 3:
A four cylinder four stroke petrol engine develops indicated power of 15kW at 1000 rpm.
The indicated mean effective pressure is 0.55 MPa. Calculate the bore and stroke of the
piston if the length of stroke is 1.5 times the bore.

Problem 4:
A single cylinder four stroke petrol engine develops indicated power 7.5 kW. The mean
effective pressure is 6.6bar and the piston diameter is 100mm. Calculate the average speed of
the piston.
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Problem 5:
A single cylinder, four stroke diesel engine develops indicated power of 30 kW at 3000 rpm.
The indicated mean effective pressure is 6.5 bar, and the piston speed is limited to 180
m/min. Determine the stroke and diameter of the cylinder. Also find the specific fuel
consumption on brake power basis, if the mechanical efficiency is 80% and the indicated
thermal efficiency is 30%. Take the calorific value of diesel as 40 MJ/kg.

Problem 6:
The following observations were obtained during a trial on a four stroke diesel engine.
Cylinder diameter = 25 cm
Stroke of the Piston = 40 cm
Crankshaft speed = 250 rpm
Brake load = 70 kg
Brake drum diameter = 2 m
Mean effective pressure = 6 bar
Diesel oil consumption = 100 cc/min
Specific gravity of diesel = 0.78
Calorific Value of diesel = 43900 kJ/kg
Find:
a) Brake Power
b) Indicated Power
c) Frictional Power
d) Mechanical Efficiency
e) Brake Thermal Efficiency
f) Indicated Thermal Efficiency

Problem 7:
The following observations were obtained during a trial on a 4 stroke diesel engine.
Cylinder (bore) diameter = 25 cm
Stroke of the Piston = 40 cm
Crankshaft speed = 250 rpm
Net load on the brake drum = 700N
Brake drum diameter = 2 m
Mean effective pressure = 6 bar

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Diesel oil consumption = 0.0013 Kg/sec

Specific gravity of diesel = 0.78
Calorific Value of diesel = 43,900 kJ/kg
Find: B.P, I.P, F.P, Mechanical Efficiency, Brake Thermal Efficiency & Indicated Thermal
Efficiency.

Problem 8:
The following observations refer to a trial on a single cylinder diesel engine. B.P = 75 kW,
ηbth = 35%, ηmech = 90%, Cv = 40 MJ/kg. Determine I.P, F.P, Fuel consumption per brake
power hour.

Problem 9:
The following data is collected from a 4 stroke single cylinder engine running at full speed.
Bore = 200 mm
Stroke = 280 mm
Speed = 300 rpm
Indicated Mean effective pressure = 5.6 bar
Torque on the brake drum = 250 N –m
Oil consumed = 4.2 Kg/hr
Calorific Value of diesel = 41,000 KJ/Kg
Determine:
Mechanical Efficiency
Brake Thermal Efficiency
Indicated Thermal Efficiency

Problem 10:
A 4 cylinder, 4 stroke I.C engine develops an I.P of 50 kW at 25 cycles/second. The stroke of
the engine is 90 mm and bore is 0.8 times the stroke.
 Find the mean effective pressure in each cylinder.
 If mechanical efficiency is 80%, what effective brake load would be required if
the effective brake drum circumference is 1m.

Problem 11:
The following data refers to a twin cylinder 4 stroke petrol engine.

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Cylinder diameter = 200 mm

Stroke of the Piston = 300 mm
Crankshaft speed = 300 rpm
Effective Brake load = 50 kg
Mean circumference of the brake drum = 4 m
Mean effective pressure = 6 bar
Calculate:
Brake Power
Indicated Power
Mechanical Efficiency

Problem 12:
A 4 cylinder, 2 stroke petrol engine develops an indicated power of 15kW at 1000 rpm. The
indicated mean effective pressure is 0.55 MPa. Calculate the bore and stroke of the piston if
the length of stroke is 1.5 times the bore.

Problem 13:
The following data refers to a test on a petrol engine: I.P = 40KW, B.P = 35KW, Calorific
value of fuel = 44,000 KJ/kg. Fuel consumption per brake power hour = 0.3kg. Calculate
Brake thermal efficiency.

Problem 14:
Calculate the brake power of a twin cylinder four stroke petrol engine, given:
Duration of the test = 1 hour, Diameter of brake drum = 600mm
Brake rope diameter = 3 cm, Dead weight = 24 kg
Spring balance reading = 4 kg
Total revolutions made = 27000

Problem 15:
A test on a single cylinder two stroke diesel engine gave the following while running at full
load. Area of the indicator diagram 300mm2, base length of indicator diagram 40mm, spring
constant 1000 bar/m, speed 400 rpm, diameter of the cylinder 16 cm and stroke 20 cm.
Calculate: Mean effective pressure & Indicated power.

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SIMPLE CARBURETOR

Function:

The main function of a carburetor is to vaporize & atomize the fuel & to mix thoroughly with
air at a fixed normal proportion.

Vaporization:

Change in state of fuel from liquid to vapour.

Atomization:

Breaking up of the fuel into small particles.

Working of a Simple Carburetor:

Float chamber

Petrol – Air mixture

to cylinder
Fig.9 Working principle of a simple carburetor
 As the level of fuel falls, the float comes down thereby needle valve opens the passage &
the petrol enters into the float chamber.
 Purpose of needle valve is to maintain the constant level of petrol in the float chamber.
 When the engine runs, the air is sucked in. As a result pressure at throat reduces. Due to
the – ve pressure the petrol issues out of the nozzle as a fine spray & is vaporized.

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 The mixture of air & petrol vapour in correct proportion passes through the throttle valve
& then to engine cylinder.
 The throttle valve is operated by the accelerator.

Lubrication

Lubrication is the process, or technique employed to reduce wear of one or both surfaces in
close proximity, and moving relative to each other, by interposing a substance called
lubricant between the surfaces to carry or to help carry the load (pressure generated) between
the opposing surfaces.

Purpose of lubrication:

 To reduce the friction between the surfaces of machine parts.

 To carry away the heat generated due to friction & to cool the parts.
 To clean the parts by washing away the deposition of carbon & metal particles caused by
wear.
 To seal the space between the piston & cylinder & to prevent the leakage of working
fluid.
 To cushion the parts against vibration & impact.

Types of lubricants

Type of lubricant Examples Application

Solid lubricants Wax, graphite, soap, Used where oil film cant be
graphite with grease maintained due to high
pressure

Liquid lubricants Mineral oils, vegetable Ordinary machinery, steam & I

oils, animal oils C engines.

Semi solid lubricants Grease Used where low speed &

high pressure exist.

Properties of lubricant:

1. Viscosity:
Viscosity of a good lubricant should not change with the varying operating temperature.

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 Internal Combustion Engines

2. Flash & fire points:

Flash point:
It is the lowest temperature at which the fumes of oil will not catch fire but the flash occurs
when the flame is brought into contact with it.
Fire point:
It is the lowest temperature at which the oil fumes catches fire & will continue to burn when
the flame is brought in contact with it.
So a good lubricant should posses a flash point temperature higher than the temperature at
which it is used.

3. Oiliness:
It is the ability of the lubricating oil to adhere to the rubbing surfaces.
When a thin film of oil is subjected to high pressure, the oil film will be squeezed out of the
lubricated surfaces.
So a good lubricant should adhere to the surfaces and maintain an oil film between the
rubbing surfaces.

4. Cloud & pour points:

Cloud point:
It is the temperature at which the wax & other substances in the oil separate out from the oil,
when the lubricant oil is cooled.
Pour or freezing point:
It is the temperature at which the oil stops to flow when cooled.
These two points will indicate the suitability of lubricants for use in cold conditions.

5. Carbon residue:
Lubricant oil contains high % of carbon in combined form.
At higher temperature, they decompose depositing a certain amount of carbon. The
deposition of carbon is highly objectionable.
A good lubricant oil should deposit a least amount of carbon while in use at higher
temperatures.

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6. Volatility:
When working temperatures are high, some oils vaporize leaving behind a residual oil having
different lubricating properties. A good lubricating oil should have low volatility.

Commonly used lubrication systems in I. C engines:

1. Splash lubrication:

The connecting rod is dipped into the oil of the crank case & at the time of rotation the oil is
splashed & it reaches to the different parts, requiring it.

Fig.10 Splash Lubrication

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