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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 18C , 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).
  1. 83.9

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Solution

The correct option is A 83.9

The expression for the Otto cycle efficiency,

ηotto=11rγ1

Substitution of the values:

Compression ratio, r = 10.0, and

Ratio of heat capacities,γ=1.4 gives

ηotto=1110.01.41=0.602

=60.2%

ηbrake=ηi×ηmech=0.5×ηotto×ηmech

=0.5×0.602×0.9=0.271

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

sfc=1(ηbrake×CV)=1(0.271×44×106)=83.9g/MJ


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