Thermal Properties: Pressure, Volume, Temp question

  • Thread starter wcbryant87
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  • #1
wcbryant87
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Homework Statement



A Jaguar XK8 convertible has an eight-cylinder engine. At the beginning of its compression stroke, one of the cylinders contains 499 cm cubed of air at atmospheric pressure (1.01E5 Pa) and a temperature of 27.0 degrees C. At the end of the stroke, the air has been compressed to a volume of 46.2 cm cubed and the gauge pressure has increased to 2.72E6.

What is the final temperature

Homework Equations



P1*V1/T1 = P2*V2/T2



The Attempt at a Solution



I followed the equation that I posted above. Here is what it looked like.

(1.01E5 * 499 cm cubed)/ 27 = (2.72E6 * 46.2)/x

Solving for x, I got x = (2.72E6*46.2)/1866629.63 = 66.8 degrees C

Problem is, that answer is apparently wrong. Am I missing something here?

Thanks for the help!
 

Answers and Replies

  • #2
Lambduh
69
1
Try changing your temperature to kelvin :)
 
  • #3
wcbryant87
14
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well it wants the answer in celcius. Does that equation have to be done with Kelvin units?
 
  • #4
xxChrisxx
2,027
85
Were you given that equation? As you wont find T2 like that. Even if you use kelvin as the pv/pv is a ratio, it wont make any difference.

Hint: Expansion and compression is isentropic.
 
  • #5
wcbryant87
14
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No I wasn't given any equation. I thought that that one would work though because I was only missing one variable.

Do I use pv=nrt?
 
  • #6
xxChrisxx
2,027
85
If you've got a book, look up isentropic compression.

pv^gamma = constant.

where gamma is the polytropic constant you'll have to find the relationship between temperature and anther variable.
 
  • #7
Gorz
29
0
If you've got a book, look up isentropic compression.

pv^gamma = constant.

where gamma is the polytropic constant you'll have to find the relationship between temperature and anther variable.
Thats the Adiabatic process.

Calculate the mass of air in the cylinder from pv=mRT

Constants for air Cp=1005J/kgK, Cv=718J/kgK R=287J/kgK, calculate gamma from these.

Reaarange the equation for an adiabatic process to find T
 
  • #8
xxChrisxx
2,027
85
its both acutally, as isentropic is adiabatic and reversible. to find the temperatre after compression you dont need the mass in the cylinder as it comes out in the wash.

the relationships for isentropic processes for ideal gases are listed in any thermo textbook. but as this is homework help im not going to just give them out.
 
Last edited:

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