First slide

Heat engine

Question

A motor cycle engine delivers a power of l0 kW, by consuming petrol at the rate of 2.4 kg/hour. If the calorific value of petrol is 35.5 MJ/kg, the rate of heat rejection by the exhaust is

Moderate

A

5.5 kW
B

13.7 kW
C

11.2 kW
D

9.7 kW

Solution

Heat received (or produced by the burning of petrol) in one hour will be
= (2.4 kg/hour)(3.5 MJ/kg)
= 85.2 $\times 10^{6} J / hour$
$∴ The rate at which heat is received$
= $\frac{85.2 \times 10^{6} J}{(3600 s)} = 2.37 \times 10^{4} J / s = 23.7 kW$
Evidentaly, the rate of heat rejection = rate at which heat is produced - rate at which work is obtained = 23.7 kW - 10kW - 13.7 kW

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