# IC Engine Questions and Answers – Engine Efficiencies

This set of IC Engines Multiple Choice Questions & Answers (MCQs) focuses on “Engine Efficiencies”.

1. The ratio of the heat equivalent to one kW hour to the heat in fuel per B.P. hour is termed as brake thermal efficiency.
a) True
b) False

Explanation: Brake thermal efficiency is defined as the ratio of the heat equivalent to one kW hour to the heat in fuel per B.P. hour.

2. The ratio of the work obtained at the crankshaft in a given time to the energy supplied during the same time is called ______________
a) mechanical efficiency
b) overall efficiency
c) indicated thermal efficiency
d) volumetric efficiency

Explanation: Overall efficiency is the ratio of the work obtained at the crankshaft in a given time to the energy supplied during the same time while mechanical efficiency is the ratio of the brake power to the indicated power and relative efficiency is the ratio of the indicated thermal efficiency to the air standard efficiency.

3. The ratio of the brake power to the indicated power is called ______________
a) mechanical efficiency
b) overall efficiency
c) indicated thermal efficiency
d) volumetric efficiency

Explanation: Mechanical efficiency is the ratio of the brake power to the indicated power and relative efficiency is the ratio of the indicated thermal efficiency to the air standard efficiency whereas overall efficiency is the ratio of the work obtained at the crankshaft in a given time to the energy supplied during the same time.

4. The ratio of the indicated thermal efficiency to the air standard efficiency is known as ______________
a) mechanical efficiency
b) overall efficiency
c) relative efficiency
d) volumetric efficiency

Explanation: Relative efficiency is the ratio of the indicated thermal efficiency to the air standard efficiency whereas overall efficiency is the ratio of the work obtained at the crankshaft in a given time to the energy supplied during the same time and mechanical efficiency is the ratio of the brake power to the indicated power.

5. Which is the correct sequence of the decreasing order of brake thermal efficiency of the three given basic types of engines?
a) four stroke C.I. engine, four stroke S.I. engine, two stroke S.I. engine
b) four stroke S.I. engine, four stroke C.I. engine, two stroke S.I. engine
c) four stroke C.I. engine, two stroke S.I. engine, four stroke S.I. engine
d) two stroke S.I. engine, four stroke S.I. engine, four stroke C.I. engine

Explanation: The brake thermal efficiency of the three given basic types of engines decreases in the order four stroke C.I. engine, four stroke S.I. engine, two stroke S.I. engine.
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6. The brake power of a diesel engine, keeping other parameters constant, can be increased by ______________
a) decreasing the density of intake air
b) increasing the temperature of intake air
c) increasing the pressure of intake air
d) decreasing the pressure of intake air

Explanation: By increasing the pressure of intake air, the brake power of a diesel engine, keeping other parameters constant, can be increased and by no other means.

7. Which one of the following event would reduce the volumetric efficiency of a vertical compression ignition engine?
a) inlet valve closing after bottom dead center
b) inlet valve closing before bottom dead center
c) inlet valve opening before top dead center
d) exhaust valve closing after top dead center

Explanation: Volumetric efficiency of a vertical compression ignition engine is reduced by closing inlet valve before bottom dead center.

8. As compared to air standard cycle, in actual working, the effect of variation in specific heats is to ______________
a) increase maximum pressure and maximum temperature
b) decrease maximum pressure and maximum temperature
c) increase maximum pressure and decrease maximum temperature
d) decrease maximum pressure and increase maximum temperature

Explanation: In actual working, as compared to air standard cycle, the effect of variation in specific heats is to decrease maximum pressure and maximum temperature.

9. The ratio of the volume of charge admitted at N.T.P. to the swept volume of the piston is called ______________
a) mechanical efficiency
b) overall efficiency
c) relative efficiency
d) volumetric efficiency

Explanation: Volumetric efficiency is the ratio of the volume of charge admitted at N.T.P. to the swept volume of the piston while mechanical efficiency is the ratio of the brake power to the indicated power and relative efficiency is the ratio of the indicated thermal efficiency to the air standard efficiency whereas overall efficiency is the ratio of the work obtained at the crankshaft in a given time to the energy supplied during the same time.

10. The overall efficiency is defined as the ratio of the work obtained at the crankshaft in a given time to the energy supplied during the same time.
a) True
b) False

Explanation: None.

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