There are several parameters to indicate the performance of an IC engine, for example, indicated thermal efficiency (ηith), brake thermal efficiency (ηbth), mechanical efficiency (ηmech), volumetric efficiency (ηv), relative efficiency of efficiency ratio (ηrel), mean effective pressure (Pm), mean piston speed (Sp), specific power output (Ps), specific fuel consumption (sfc), and air fuel ratio (A/F).
Indicated Thermal Efficiency: It is ratio of energy in the indicated diagram (Ip) to the input fuel energy.
Brake Thermal Efficiency: It is ratio of energy in brake power (BP) to the input fuel energy. Brake power is obtained by subtraction of friction losses from indicated power.
Mechanical Efficiency: It is ratio of the brake power to the indicated power.
Volumetric Efficiency: It is ratio of the volume of air inducted at ambient conditions to the swept volume of the engine.
Relative Efficiency or Efficiency Ratio: It is ratio of the thermal efficiency of actual cycle and the ideal cycle.
Mean Effective Pressure: It is the average pressure inside the cylinder of an IC engine on the measured power output. For any particular engine operating at given speed and power output, there will be a specific indicated mean effective pressure and corresponding brake mean effective pressure. They can be expressed as
where
Mean Piston Speed: It is defined as Sp = 2lN, where l is stroke length in m and N is the rotational speed of the crankshaft in rpm.
Specific Power Output: It is defined as the power output per unit piston area.
where bp brake power in kJ and A is piston area in m2.
Specific Fuel Consumption: It is inversely proportional to thermal efficiency. It is ratio of fuel consumption per unit time and the power.
Air–fuel Ratio: This is ratio of mass of air and fuel.
Example 6.7: The following data were noted for a four-cylinder, four-stroke engine:
Diameter = 101 mm, stroke = 114 mm, speed = 1,600 rpm, fuel consumption = 0.204 kg/min, heating value of fuel = 41,800 kJ/kg. Difference in either side of the brake pulley = 378 N, brake circumference = 3.35 m. Assume mechanical efficiency = 83%. Calculate (i) brake thermal efficiency, (ii) indicated thermal efficiency, (iii) mean effective pressure of cylinder, and (iv) fuel consumption per brake power.
Solution:
Brake power, 
where N is speed in rotation per minute,
T is braking torque, R is radius, W is braking load.
Brake thermal efficiency, 
where wf is fuel consumption in kg/min and HV is heating value of fuel.
Indicated thermal efficiency, 
Indicated power, 
where Pi is indicated mean effective pressure, l is stroke length, A is internal cross-sectional area of cylinder.
SUMMARY
The chapter can be summarized as follows:
- The heat engine, in which the combustion takes place inside the cylinder or the product of combustion (flue gas) directly goes to the cylinder and the heat energy of the flue gas is converted into mechanical energy, is known as Internal Combustion Engine (IC Engine).
- In four strokes of a spark ignition engine, the cycle of operations is completed in four strokes of the piston or two revolutions of the crankshaft.
- The compression ratio of SI engines varies from 6 to 10 whereas in CI engines it ranges from 16 to 20.
- In a four stroke SI engine, there is one power stroke in two revolutions of crankshaft and two strokes, viz., suction and exhausts are non-productive.
- In two strokes of a spark ignition engine, the cycle of operations is completed in two strokes of the piston or one revolution of the crankshaft.
- SI engine is based on Otto cycle or constant volume heat addition and rejection cycle.
- CI engine is based on diesel cycle or constant pressure heat addition and constant volume heat rejection cycle.
- In Otto cycle, heat is added and rejected at constant volume.
- In diesel cycle, heat is added at constant pressure and is rejected at constant volume.
- Valve timing diagram is a graphical representation of valves opening and closing time with ignition time in terms of angle of crank revolution.
- For same compression ratio Otto cycle is more efficient but delivers less power than the diesel cycle. Therefore, in duel cycle partly heat is added at constant volume and partly at constant pressure.
- Indicated thermal efficiency is ratio of energy in the indicated diagram (Ip) to the input fuel energy.
- Brake thermal efficiency is ratio of energy in brake power (Bp) to the input fuel energy. Brake power is obtained by subtraction of friction losses from indicated power.
- Mechanical efficiency is ratio of the brake power to the indicated power.
- Volumetric efficiency is ratio of the volume of air inducted at ambient conditions to the swept volume of the engine.
- Relative efficiency or efficiency ratio is ratio of the thermal efficiency of actual cycle and the ideal cycle.
- Mean effective pressure is the average pressure inside the cylinder of an IC engine on the measured power output.
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