If I understand correctly, any heat produced by an internal combustion engine can be considered "waste heat"; energy from the fuel that did not get converted to mechanical energy. But if you burnt the same fuel outside of an internal combustion engine, 100% of the energy is released as waste heat. But once heat is created, you can't destroy it? So if an internal combustion engine is 50% efficient, then at the moment of ignition, the gas released from the burnt fuel is half the temperature that is would have been if it were outside the engine? How does this work in terms of physical laws?

How does the piston cool the gas?

  • $\begingroup$ It isn't wasted immediately after the energy is converted to heat. Some of the heat is first converted to mechanical energy, and the remainder is discharged through the exhaust and radiator as waste heat. $\endgroup$ Commented Oct 14, 2022 at 11:21

1 Answer 1


In terms of physical laws, the maximum efficiency of the engine is when the expansion of the hot gases is reversible plus adiabatic, which means isentropic. Assuming the hot gases right after ignition to be $T_0$ and the gases to expand from volume $V_0$ to $V_1$, the isentropic condition gives $$\Delta S_{01} = 0 = C_V\ln\frac{T_1}{T_0} + R \ln\frac{V_1}{V_0} $$ This fixes the temperature of the exhaust gases to $$T_1 = T_0\left(\frac{V_1}{V_0}\right)^{R/C_V}$$ This ensures maximum efficiency for given $T_0$, $V_0$ and $V_1$. At $T_1$ the exhaust gases are still quite hot. One may consider other ways of extracting more energy from them but whether these are technically feasible or not would depend on considerations that are beyond my expertise.


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