• Card 13 / 35: A reciprocating, spark ignition engine takes in an air-fuel mixture at 20°C. It has a compression ratio of 12. The air-to-fuel ratio is 14, and the heating value of the fuel is 70,000 kJ/kg. For an air standard cycle analysis, what is the highest temperature reached in °K? You may assume Cv is approximately 0.7 kJ/kg K and that k=Cp/Cv = 1.4.
    A) 7934°K
    B) 791°K
    C) 5934°K
    D) 591°K
    E) 1243°K

    Answer:
    A) 7934°K

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Explanation:

The key to this analysis is understanding what constitutes the air standard cycle analysis. There are really two key features: air is the working fluid (that is, we ignore the presence of fuel or combustion products except as an energy source) and all expansions or compressions are adiabatic and reversible (isentropic). For this latter case, we may write TV(k-1) = constant, where k = Cp/Cvis 1.4 for air. By memorizing these two factors, you will be able to perform air standard analyses. For this question, we first perform an isentropic compression by a factor of 12, and then we add the heat of combustion at constant volume. The final temperature is then T = (293)12 0.4 + (70,000/14) (1/0.7) = 7934°K.

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Thermal-Fluid Systems ME303

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Attribution:  Dr. Steve Gibbs. Thermal-Fluid Systems. The Saylor Academy 2014, http://www.saylor.org/courses/me303/
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