# Homework Help: Thermodynamics - engines question

1. Mar 14, 2015

### Jaydude

I'm stuck on a question, part d below:

My attempt:

I reckon that T4 = T3 and T2 = T1, otherwise I don't know how else you can get T4 and T2...?

Once I got first part of d, I reckon I could do efficiency using part c that I derived.

I don't see where I'm supposed to use compression ratios in this question, if someone can direct me to the right path please!

(If this is supposed to be in advanced physics section, please let me know for next time!)

Kind regards,

Jay

2. Mar 15, 2015

### TSny

Hello Jay. Welcome to PF!

You are given that $P_2V_2^\gamma = P_1V_1^\gamma$. See if you can use that along with the ideal gas law to express T2 in terms of T1, V1 and V2.

3. Mar 15, 2015

### Jaydude

Sweet! But now I'm not sure how to get t4? And how do I apply that to get work done?

I could use my equation (correct) in part b but I don't know what Cp and Cv are, how do I apply the constant 1.4 there?

(I'm gonna assume t2 is correct here...)

Thanks!

4. Mar 15, 2015

### TSny

Try a similar calculation to find T4 in terms of T3.

5. Mar 15, 2015

### Jaydude

But I don't have V3/V4 compression ratio :(

6. Mar 15, 2015

### TSny

How is V4 related to V1? How is V3 related to V2?

7. Mar 15, 2015

### Jaydude

I see what you did there, but I get the incorrect answer, T4 should be 1379k

What have I done wrong ?

8. Mar 15, 2015

### TSny

How do you justify taking V4 / V3 to be 16?

9. Mar 15, 2015

### Jaydude

If V4 = V1 and V3 = V2, and we know V1/V2 = 16, then V4/V3 = 16 too...?

10. Mar 15, 2015

### Jaydude

I see my problem.

V2 does not equal V3!

Can't solve this, its getting annoying as ive spent hours ripping my hair out on this!

Last edited: Mar 15, 2015
11. Mar 17, 2015

### Jaydude

Bump. Can Someone point me in the right direction here/helpout with the solution? This is only for my practise and is not part of any H/W or Cwk and all of my friends are struggling with this Q too.

Kind regards,
Jay

12. Mar 17, 2015

### TSny

V2 does not equal V3, but you can express V3 in terms of V2 using the ideal gas law.