Question

What is the maximum efficiency of an ic engine?

Answer 1

I basically agree with Chase's answer - with some additions. But let's see thoroughly what is behind that sentence in Wikipedia ("Most steel engines have a thermodynamic limit of 37%").

Internal combustion engines are modeled by the Otto cycle rather than by the Carnot cycle. Looking at this link for the Otto cycle (or this great link from MIT), you may observe that in this cycle there are four temperatures involved, and the efficiency is given by

ϵO=1−T4−T1T3−T2.ϵO=1−T4−T1T3−T2.

However, in an ideal Otto cycle we have that T4/T1=T3/T2T4/T1=T3/T2, and we get an "effective Carnot form" for the efficiency:

ϵO=1−T1T2.ϵO=1−T1T2.

Now, using the isentropic equations of ideal gases, we obtain the following simple expression

ϵO=1−1(V1/V2)γ−1,ϵO=1−1(V1/V2)γ−1,

where γγ is the specific heat ratio (cp/cvcp/cv)

At the end of the Wikipedia sentence that you quote, there is a reference to a course by the University of Washington (see link here). There it is stated that most current auto engines have compression ratio V1/V2=10V1/V2=10 and the mixture of air, gasoline vapor, CO22, CO and H22O has an effective specific heat ratio of γ=1.27γ=1.27. Plugging this into our formula we get ηO=0.46=46%ηO=0.46=46%, which is pretty close to what Chase said. I would call this the theoretical limit for the efficiency of the internal combustion engine!