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.