Performance and Testing of Internal Combustion Engines.ppt

1. Performance and Testing of Internal
Combustion Engines

2. Performance testing of IC engines
• Engine performance is an indication of the degree of success with which it is
doing its assigned job, i.e., conversion of chemical energy into useful
mechanical work.
• In the evaluation of engine performance certain basic parameters are chosen
and the effect of various operating conditions is studied.
The different parameters considered are;
(i) Power, Thermal efficiency and mechanical efficiency
(ii) Volumetric efficiency
(iii) Specific fuel consumption
(iv) Mean effective pressure and torque
(v) Specific output
(vi) Fuel air ratio
(vii) Heat balance sheet

3. • Power, Thermal efficiency and Mechanical efficiency:
 IP
 BP
 FP
• The engine performance is indicated by the term efficiency. The
different efficiencies are:
 IT eff- Indicated Thermal efficiency
 BT efficiency
 Air-standard efficiency
 Mechanical efficiency
 Relative efficiency

5. Specific fuel consumption

6. • Mean effective pressure (Pm):
Where IP = Indicated power in kW, L = stroke length in meter, A = area of the piston in m2. N = rpm,
Ne= N/2 for four stroke and N for two stroke engines. i = number of cylinders.
Another way of specifying the indicated mean effective pressure is from the data obtained by
indicator attachment, which gives the indicated area.
Where spring constant is the constant in N / mm2 – mm.
Mean piston speed: It is the average speed with which piston reciprocates inside the cylinder.

7. Specific power output
• The specific power output of an engine is defined as the power output per
unit piston area. This shows the engine designers capacity to use the
available piston area to obtain power regardless of the size.
• For a given i, L and A the specific power output depends upon Pbm and N.
• Increasing the speed involves increasing the mechanical stress of various
components.
• For increasing the Pbm better heat release from the fuel is required which
results in higher thermal load on engine cylinder.

8. Fuel - air (F/A) or Air - fuel ratio:
• The relative proportions of fuel and air in an engine - important from the
standpoint of combustion and efficiency of the engine.
• Ratio of the mass of the fuel to that of air or vice versa.
• In SI engine fuel air ratio practically remains constant over a wide range of
operation.
• In the CI engine at a given revolution the airflow does not vary with load, it is
the fuel flow that varies with load. Therefore the term fuel air ratio is
generally used instead of fuel-air ratio.
• Chemically correct or Stoichiometric fuel air ratio
• A mixture having more fuel than that in a chemically correct mixture is
termed as rich mixture and a mixture that contains less fuel (or excess air) is
called as lean mixture

9.  If Φ = 1 it means that mixture is chemically correct, Φ < 1 means that lean mixture and
Φ > 1 means that rich mixture.
Heating Value: Heating value of a fuel is the energy released per unit quantity of the fuel when the
combustible mixture is burned and the products of the combustion are cooled back to the initial
temperature of the mixture.
The ratio of actual fuel air ratio to that of chemically correct fuel air ratio is called as
relative fuel air ratio.
Φ =

10. • Heat Balance Sheet:
The brake thermal efficiency of a heat engine gives an overall knowledge
of its ability in converting the heat supplied into useful mechanical work.
The rest of the energy is called as unavailable energy or lost energy, which
is degraded to the surroundings. It is the purpose of the heat balance
sheet to trace the distribution of the heat energy supplied to the engine
into its various forms.
Heat balance sheet is a credit and debit account of heat energy. The
energy supplied to the engine is in the form of fuel energy and useful
energy is the brake power. It gives the opportunity to improve the brake
thermal efficiency of the engine by minimizing the losses to the maximum
possible extent
The energy which is not available includes;
(1) Heat energy carried away by cooling water. (2) Heat energy lost by
exhaust gasses. (3) Radiation and other unaccounted losses.

11. Torque and its measurement: A device that measures torque of an
engine is known as dynamometer.
Types, (1)Absorption dynamometers (2) Transmission dynamometers.
• Absorption dynamometer: These dynamometers absorb the power
developed by the engine, measure and dissipate it as heat. Ex: Rope
brake, Prony brake, Hydraulic and Eddy current dynamometers
• Rope brake dynamometer: It is a simple device to measure power
output of an engine. It is more economical and simple design, easy to
fabricate.

12. Where D = diameter of flywheel, d = diameter of rope, W = Load or weight, S = spring balance reading
 It is a simplest possible device to measure power output of an
engine and most economical root.
 A rope is wound around the brake drum attached to the crank
shaft.
 One end of the rope is connected to the spring balance(S) and
other end to a loading device (weights).
1. Absorption dynamometer:
(a) Rope brake Dynamometer :
 The power developed by the engine is absorbed due to friction between the rope and the
drum and results in increase in temperature of drum, hence cooling is required.

13. b. Prony Brake Dynamometer
Torque,T=W*R
1. A flywheel is rigidly attached to the engine drive
shaft with an adjustable friction band.
2. An arm rigidly attached to the friction band is free
to move through a limited arc and rests on a
weighing scale.
3. As drive shaft and attached flywheel rotate, the
friction between the surrounding band and the
wheel tends to rotate the arm.
4. These results in an application of force on the
weighing scale and rotating force of the engine can
be evaluated.
5. If W is the load applied and R is the distance
between the flywheel center and the load acting,
then

14. 1. It consists of a rotor and a stator.
2. Rotor or impeller is coupled to the engine
crankshaft.
3. The stator is free to rotate in the trunion
bearings.
4. A torque arm extending from the side of the
stator is attached with a loading platform.
5. When the engine is rotating, impeller also
rotates in a casing filled with fluid (water).
c. Hydraulic Dynamometer
6. As the rotor rotates the centrifugal force is developed, tends to rotate the stator.
7. Application of suitable loading on the loading platform prevents the rotation of the stator.
8. This sets a torque reaction, which is equal to the torque developed by the engine.
9. If W = Load applied , R = Distance between the dynamometer Centre and load Centre then
Torque = W * R,

15. d. Eddy Current dynamometer
 It consists of a stator fitted with a
number of electromagnets and a
rotor disc made of copper and steel.
 The rotor which is coupled to the
driveshaft of the engine when
rotates produces eddy current in
the stator.
 The eddy current set up by the
magnetic flux opposes the rotor
motion which loads the engine.
 An arm carrying the load on the
loading platform opposes this
rotation. This sets a torque reaction
which is equal to the torque
generated by the engine

16. 2. Transmission Dynamometers
1. In these dynamometers power is transmitted to
the load coupled to the engine.
2. The stress developed due to load transmission
is measured by the strain gauges.
3. A set of strain gauges fitted on the driveshaft
measure the torque by angular deformation of
the shaft.
4. The effect of axial or traverse load on the strain gauges is avoided by arranging them in pairs.
5. A four arm bridge reduces effect of temperature to minimum. These dynamometers are very accurate
and used to measure continuous load transmission.

17. Fuel consumption measurement
• Simple burette method
Fuelflowrate= m3/sec
Flow meter

18. Measurement of air consumption: Air Box Method
• Volume of the air box is 500 to 600 times
of the swept volume of the engine in case
of single cylinder engine and it is little less
in case of multi cylinder engine
• The velocity across the orifice is given as
• V =
• Since the differential head is measured by
water column, it must be converted into
equivalent air column.
• ha = hw *

20. • Measurement of heat lost to cooling water
• Qw = mw Cpw (Two – Twi) kJ/min.
• where mw = mass of water(kg/min), Twi = Inlet temperature of
water(oC), Two = Outlet temperature of water(oC), Cpw = Specific heat
of water= 4.187 kJ/kg K.
• Measurement of heat lost to exhaust gases
• mg * Cpg * (Tgi – Tgo) = mw * Cpw (Two – Twi)

21. Heat Balance Sheet- A heat balance sheet is an account of heat supplied and heat
utilized in various ways in the system. Necessary information concerning the
performance of the engine is obtained from the heat balance
• Qs = mf * CV kJ/min
• Heat equivalent of BP = BP kW = BP kJ/sec. = BP * 60 kJ/min
• Qw = mw * Cpw * (Two – Twi) kJ/min.
• Qg = mg Cpg (Tge – Ta) (kJ/min.) or (kJ/sec)

22. Measurement of Friction Power
 This method is also known as fuel rate extrapolation
method.
In this method a graph of fuel consumption (vertical axis)
versus brake power (horizontal axis) is drawn and it is
extrapolated on the negative axis of brake power.
The intercept of the negative axis is taken as the friction
power of the engine at that speed.
As shown in the figure, in most of the power range the
relation between the fuel consumption and brake power is
linear when speed of the engine is held constant and this
permits extrapolation.
Further when the engine does not develop power, i.e.
brake power = 0, it consumes a certain amount of fuel
• Willian’s line method
• From the measurement of indicated power and brake power
• Motoring test
 1. Willian’s line method

23. From the Measurement of Indicated Power and Brake Power
• This is an ideal method by which friction power is obtained by computing
the difference between the indicated power and brake power.
• The indicated power is obtained from an indicator diagram and brake
power is obtained by a brake dynamometer.
• This method requires elaborate equipment to obtain accurate indicator
diagrams at high speeds.
• Indicated power of a multi-cylinder engine can be obtained by Morse test
also

24. Measurement of Indicated Power - Morse Test
This is applicable to petrol as well as for diesel engines.

25. 3. Motoring Test
In this test, the engine is run to the desired speed till it attains the steady state.
Measurement of output is done with suitable dynamometer.
Now the fuel supply is cut off from the engine
By suitable arrangement dynamometer is converted as motor and engine is made to
run by this motor for the same constant speed
The power supply to this motor is a measure of frictional power of the engine

26. Combustion in SI and CI engines

27. Ignition limits
 These corresponds to those mixture ratios, at the lean and rich ends of the scale where
heat released by the spark plug is no longer sufficient to initiate the combustion in the
neighbouring unburnt mixture.
 Flame propagates only if the temperature of the burnt gases exceeds 1500K for
hydrocarbons. At room temperature it will be reached only when the relative fuel air ratio
is in between 0.5-2.1.
 Stoichiometric Air- Fuel ratio means chemically correct air fuel ratio.
 Ignition limits are wider at higher temperature because of better reaction rates at higher
temperature.

28. Combustion in S I Engine
• In case of SI engine, a momentary spark is sufficient enough to ignite the homogeneous
mixture of fuel and air.
• A thin thread of flame thus initiated spreads the surrounding envelops of mixture at a rapid
rate.
Stages of combustion
• I Stage – Ignition Lag or Initiation of Flame
• The first stage is a preparatory phase i.e. AB as shown only about 1.5% percent of the
whole mixture is burnt during this period releasing about 5% of total heat released. Flame
travels only about 5% across the combustion chamber.
• II Stage or Flame Propagation as represented by BC on the p - θ diagram
Flame travels about 95% of the combustion chamber space.
• III Stage – After Burning : - Even though maximum pressure is reached at point C,
combustion is not complete at this point and continues further. The flame travels only 5%
of the combustion space.

30. Combustion in C I Engine
• The combustion process in CI engines is different from that of SI engines
because instead of fuel - air mixture, air alone is compressed using a high
compression ratio (12-22) raising its temperature and pressure.
• At the end of compression fuel is injected at a high pressure (110-220bar)
into combustion chamber where the pressure is about 30-40 bar and high
temperature air about 723-823 K.
• In CI engines fuel injection takes place over a range of 20-400 CA travel and
starts at about 15-250 CA bTDc.
• Stages of combustion
• I Stage - Delay period: Physical delay, Chemical Delay
• II Stage – Rapid / Uncontrolled Combustion
• III Stage – Controlled Combustion
• IV Stage – After Burning