# Efficiency of engine taking diatomic ideal gas through cycles

1. Apr 8, 2013

### jensgt

1. The problem statement, all variables and given/known data

What is the thermal efficiency of an engine that operates by taking n moles of diatomic ideal gas through the cycle 1 -------> 2 -------> 3 -------> 4 ------> 1 shown in the figure ?

2. Relevant equations

e = W/Qh

3. The attempt at a solution

I know the answer is 10.5

I am trying to prep for a test today and this is the last thing from my homework that I don't understand....can someone explain to me how the answer is generated? I am sort of lost on it and I figure it has to be easier than it seems.

2. Apr 8, 2013

### mukundpa

There are two isochoric and two isobaric processes in the cycle. Find temperatures at all four points and calculate the heat absorbed from the source and the net work done.
The efficiency is always less than one thus it must be 10.5%.

3. Apr 8, 2013

### jensgt

How to find temp with no p or v numbers?

4. Apr 8, 2013

### mukundpa

It is P0V0/nR at 1.

5. Apr 8, 2013

### jensgt

I'm still confused...po is not a numerical value.

6. Apr 8, 2013

### mukundpa

say P0V0/nR = T0 then what are temperatures at 2, 3 and 4 ?

7. Apr 8, 2013

### mukundpa

these are 2T0, 4T0 and 2T0 respectively.
Now find the heat absorbed in isochoric and isobaric processes using Q = n Cv ΔT and Q = n Cp ΔT