# Thermodynamics exercise.

1. Jan 18, 2015

### itsjorge

1. The problem statement, all variables and given/known data
An engine works with an ideal diatomic gas (γ=1.40) according to a thermodynamic cycle consisting in four
processes that take the gas successively through the states 1, 2, 3 and 4. The processes 1 -> 2 and 3 -> 4 take
place at constant pressure and the two other processes are adiabatic. The cycle’s lowest pressure is 1 atm and
corresponds to the state 1.
a. Find an expression for the cycle’s efficiency exclusively in terms of the temperatures of the states 1,
2, 3 and 4.
b. If the efficiency is φ=0.40, what is the gas pressure in state 3?

2. The attempt at a solution

Ok, first part is easy (I guess) here's what I did http://imgur.com/hpBxIex if someone could give it a look I'd be very thankful. I'm sorry I cannot give you the P-V diagram because I don't know how to draw in this PC.
The second part is where it gets tougher because I only have 2 pieces of information and I really cannot relate them. If someone could sketch the solution I'd be very thankful.

2. Jan 18, 2015

### TSny

OK for part (a). For part (b), think of a way to relate T and P for the adiabatic processes.

Last edited: Jan 18, 2015
3. Jan 19, 2015

### itsjorge

Hello TSny, thx for replying, thing is my professor didn't give us a way to relate P and T however I found this formula in wikipedia but I'm not sure if it's correct: (It's valid iff the process is reversible) P1-γ Tγ=const. What do you think?
The statement doesn't specify if the process is reversible actually...

4. Jan 19, 2015

### TSny

Yes, that's the correct formula. For these types of questions you can generally assume that the entire cycle is reversible.