# Homework Help: Exam revision question. Help

1. Apr 25, 2005

### bruce999

Exam revision question. Help!!

Hello.
I have a thermodynamics exam tomorrow. This is a past exam question and i'm not doing very well with it. If anyone has any ideas please help!!

A charge enters a spark ignition engine at 330K and 1 bar, and is isentropically compressed through a ratio of 7:1. Estimate the temp and pressure at the end of the compression, taking the charge to be:
a) pure air with constant specific heat.
b) a stoichiometric mixture of air and octane(C8H18) with variable specific heats (neglect residual gases).

2. Apr 25, 2005

### minger

For part a: you will use the constant specific heat relationships. They are something like P1/P2 = (V2/V1)^k-1 note: That equation is probably not true, but the three formulas are in that same format. Knowing that compression ratio is V1/V2, you can easily plug that number into the formula given initial T and P to find final T and P.

for b: hmmmm.....partial pressures and stuff...wow, I have no idea. I guess one would find the stoichiometric ratio, I believe it's ~14:1, then go from there. That's about all the help I got for that part.