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Question

A four-stroke, four cylinder SI engine has a brake thermal efficiency of 30% and indicated power is 40kW at full load. At half load it has a mechanical efficiency of 65%. What is the indicated thermal efficiency at full load?

A
64.1%
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B
46.1%
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C
38.1%
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D
19.5%
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Solution

The correct option is C 38.1%At full load: bp+fp=40kW At half loadbp=0.5×bp at full load ηm=0.65=0.5×bp0.5bp×fp 0.5bp=0.65(0.5,bp+fp) 0.5bp=0.325bp+0.65×fp fp=0.1750.65×bp=0.27bp Put in equation (1) bp+0.27 bp=40 bp=31.5kW fp=8.5kW Mechanical efficiency at full laod,ηm=bpbp+fp=31.540 ηm=78.8% Indicated thermal efficiency at full load ηm=ηntnm=0.30.788=0.381=38.1%

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