Computing Cylinder Pressure from Temperature Trace

[tangodirt]

I'm just trying to put together a very basic engine model with extremely limited combustion. Basically, I am modeling isentropic compression, combustion (using LHV to calculate the delta T), and isentropic expansion.

At the moment, I am calculating a peak cylinder temperature under motoring of ~400 C and a peak cylinder temperature under firing (5 deg BTDC) of ~2800 C. This puts my exhaust gas temperature around ~1200 C, which all seems to make sense. I am using a compression ratio of 8:1 and air standard conditions.

However, when I try to convert these numbers to a cylinder pressure, my ~2800 C point is exploding to ~3700 bar! Obviously, this is not correct. I am using an isentropic expansion equation (P2/P1 = (T2/T1)^(g/g-1)).

Any ideas? Can anyone double check my reasoning and let me know what you get?

 

[Randy Beikmann]

Part of the discrepancy might be that in a real engine you don't have isentropic compression or expansion, where PV^k=constant, k being 1.40. Considering heat losses, it's polytropic expansion and compression, so PV^n=constant, where n is about 1.30.