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Homework Help: Adiabatic compression in a diesel engine

  1. Jan 31, 2009 #1
    1. The problem statement, all variables and given/known data

    In a diesel engine, atmospheric air is quickly compressed to about 1/20 of its original volume. Estimate the temperature of the air after compression , and explain why a Diesel engine does not require spark plugs.


    2. Relevant equations

    f/2*ln(T_f/T_i)=-ln(V_f/V_i)

    3. The attempt at a solution

    I will assume the temperature inside a diesel engine is room temperature or 300 K.

    f=5 since air is a diatomic gas.
    T_i=300 K
    T_f=?
    V_i=V_i
    V_f=.05*V_i

    2.5*ln(T_f/(300K)=-ln(.05)

    ln(T_f/300 K)=3/2.5

    In(T_f/300)=1.2

    T_f/300=exp(1.2)

    T_f=300*exp(1.2)

    since this is an adiabatic compression , no heat escapes. I guess spark plugs are not required because they produced heat and heat production isn't required to compress air inside a diesel engine. OR . The compression must be so high that heat is produced and the engine ignites by itself that there is no need for spark plugs.
     
    Last edited: Jan 31, 2009
  2. jcsd
  3. Feb 1, 2009 #2
    Anybody not understand my question?
     
  4. Feb 1, 2009 #3

    Mapes

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Your answer is a touch off (<1%) due to rounding errors, but the approach is fine.
     
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