Question

A motor cycle engine delivers a power of $10$ 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

• A. 5.5 kW
• B. 13.7 kW
• C. 11.2 kW
• D. 9.7 kW

Answer 1

The correct option is B 13.7 kW

Heat received (or produced by the burning of petrol) in one hour will be
$=(2.4\mathrm{kg}/\mathrm{hour})(35.5\mathrm{MJ}/\mathrm{kg})=85.2\times {10}^{6}J/\mathrm{hour}$
∴ The rate at which heat is received
$=\frac{85.2\times {10}^6}{\left(3600s\right)}=2.367\times {10}^{4}J/s=23.67\mathrm{kW}$
Evidently, the rate of heat rejection
= rate at which heat is produced - rate at which work is obtained = $23.67\mathrm{kW}-10\mathrm{kW}=13.67\mathrm{kW}\simeq 13.7\mathrm{kW}$