Question

A four-stroke petrol engine at full load deliveries 100 kW. It requires 10 kW to rotate it without load at same speed. What is the mechanical efficiency at half load?
1. 67.82%
2. 70.24%
3. 77.32%
4. 83.33%
5.

Answer 1

Correct Answer - Option 4 : 83.33%

Concept:

Mechanical Efficiency:

It is the ratio of Brake power to the Indicated power.

\(\eta_m=\frac{B.P}{I.P}\)

Indicated power is the sum of brake power and frictional power.

Note: During half load, the brake power gets halved while the friction power remains same.

Calculation:

Given:

Brake power at full load = 100 kW, FP = 10 kW

\(\eta_m=\frac{B.P}{I.P}\)

\(\eta_m=\frac{B.P/2}{B.P/2\;+\;F.P}\)

\(\eta_m=\frac{50}{50\;+\;10}=83.33\;\%\)