Question

A four-cylinder, four stroke petrol engine is to develop 40 kW at 40 rev/s, when it is designed for a volumetric compression ratio of 10.0 to 1. The ambient air- conditions are 1 bar and ${18}^{\circ }C$ , and the calorific value of the fuel is 44 MJ/kg. If the indicated overall efficiency is 50% of the corresponding air-standard Otto cycle, and the mechanical efficiency is 90%, then the specific fuel consumption ____ g/MJ of brake work. The specific heat capacity ratio for air is 1.4. (Correct upto one decimal place).

• A. 83.9

Answer 1

The correct option is A 83.9

The expression for the Otto cycle efficiency,

${\eta }_{otto}=1-\frac{1}{r^{\gamma -1}}$

Substitution of the values:

Compression ratio, r = 10.0, and

Ratio of heat capacities,$\gamma =1.4$ gives

${\eta }_{otto}=1-\frac{1}{{10.0}^{1.4-1}}=0.602$

$=60.2\%$

${\eta }_{brake}={\eta }_i\times {\eta }_{mech}=0.5\times {\eta }_{otto}\times {\eta }_{mech}$

$=0.5\times 0.602\times 0.9=0.271$

Equation can be used to calculate the specific fuel consumption (sfc)

$sfc=\frac{1}{({\eta }_{brake}\times CV)}=\frac{1}{(0.271\times 44\times {10}^6)}=83.9g/MJ$