# Farm Tractor Questions and Answers – Calculation of Engine Efficiency

This set of Farm Tractor Multiple Choice Questions & Answers (MCQs) focuses on “Calculation of Engine Efficiency”.

1. How to calculate the mechanical efficiency of an engine?
a) ηm = B.P / I.P
b) ηm = F.P / B.P
c) ηm = I.P / F.P
d) ηm = F.P / B.P

Explanation: Mechanical efficiency is the performance of the IC engine and it is the effectiveness of an engine in converting the input to output. It is the ratio of brake power to the input power. It has no unit.

2. Find the mechanical efficiency of an engine having brake power 40 Hp and friction power 5 Hp.
a) 88%
b) 90%
c) 95%
d) 92%

Explanation: ηm = B.P / I.P
= B.P / B.P + F.P
= 40 / 40 + 5
= 0.888 * 100 = 88.88%

3. How to calculate the brake thermal efficiency?
a) ηbth = B.P / Volume of fuel
b) ηbth = B.P / Mass of fuel
c) ηbth = B.P / rate of heat supply
d) ηbth = F.P / B.P

Explanation: Brake thermal efficiency is a measure of performance of the engine and it has no unit. It is the ratio of output to the input i.e. ratio of the brake power to the rate of heat supply. Heat supply is the Mass flow rate and calorific value of fuel.

4. Find the brake thermal efficiency of a diesel engine having a Mass flow rate of fuel 10 kg / hr and brake power 50 Hp.
a) 29%
b) 25%
c) 26%
d) 27%

Explanation: ηbth = B.P / rate of heat supply = B.P / Mf * Cv
= 50 * 745.6 / 0.0027 * 45500
= 0.2949 = 29.49%
Where B.P – brake power, ηbth – brake thermal efficiency, Cv of diesel – 45500 KJ / kg

5. How to calculate the indicated thermal efficiency of an engine?
a) ηith = I.P / rate of heat supply
b) ηith = I.P / Volume of fuel
c) ηith = F.P / I.P
d) ηith = I.P / Mass of fuel

Explanation: The indicated thermal efficiency is a ratio of power produced in the cylinder of the engine to the amount of heat supplied. It has no unit. The indicated power is the output and heat supplied is the input.

6. Find the indicated thermal efficiency of a petrol engine having Mass flow rate of fuel 9 kg / hr and indicated power of 45 Hp.
a) 29%
b) 33%
c) 35%
d) 28%

Explanation: ηith = I.P / Mf * Cv
= 45 * 745.6 / 0.0025 * 45800     (1 Hp = 745.6 watts, Cv of petrol 45800 KJ / kg)
= 0.293 = 29.3%
Where ηith – indicated thermal efficiency, Mf – Mass flow rate of fuel, Cv – calorific value of fuel, I.P – indicated power

7. What is the volumetric efficiency of an engine?
a) ηvol = Ma / ρ * Vs
b) ηvol = Ma / ρ
c) ηvol = Ma / Vs
d) ηvol = Ma * Vs / ρ

Explanation: The volumetric efficiency is the ratio of the actual amount of air drawn into the cylinder during the suction stroke to the theoretical amount of air to be introduced or volume of the cylinder i.e. displacement volume. Ma – mass of air, ρ – density of air, Vs – stroke volume.

8. What is the scavenge efficiency of an engine?
a) ηscavenge = Ma / ρ * (Vs + Vc)
b) ηscavenge = Ma / (Vs + Vc)
c) ηscavenge = Ma / ρ
d) ηscavenge = Ma / ρ * (Vc)

Explanation: The scavenge efficiency is the effectiveness of the removal of burnt gases from the cylinder. It is the ratio of the amount of gases removed from the cylinder after combustion to the fresh air filled in the cylinder.

9. What is the relative efficiency?
a) ηrelative = ηth / ηideal cycle
b) ηrelative = ηth * ηideal cycle
c) ηrelative = ηth / ηscavenge
d) ηrelative = ηth / ηvol

Explanation: Relative efficiency of an engine is defined as the ratio of actual thermal efficiency to the ideal cycle efficiency. The cycle refers to the otto cycle or diesel cycle or dual combustion cycle.

10. Find the relative efficiency of the otto cycle engine having indicated thermal efficiency as 45% and air standard efficiency as 70%.
a) 64%
b) 66%
c) 68%
d) 63%

Explanation: ηrelative = ηith / ηotto
= 45 / 70
= 0.642 * 100 = 64.2 %
Where ηith – indicated thermal efficiency, ηotto – air standard efficiency

11. Find the brake thermal efficiency of an engine having relative efficiency as 65% and the diesel cycle efficiency as 60%.
a) 39%
b) 36%
c) 35%
d) 34%

Explanation: ηrelative = ηbth / ηdiesel
ηrelative * ηdiesel = ηbth
= 0.65 * 0.60 = 39%

Sanfoundry Global Education & Learning Series – Farm Tractor.

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