Finding the efficiency of an engine given energy input, energy wasted.

Join the discussion
Registration is free. Ask a follow-up in this thread, or start your own.
Sign up Log in
2 replies · 3K views
Dez1
Messages
2
Reaction score
0

Homework Statement


An inventor claims to have developed an engine which takes in 1.1x108J of heat energy at 400K, and rejects 5.0x107J of heat energy at 200K. Would you advise investing in this project? Explain your answer.


Homework Equations


W = Q1 – Q2
η = W/Q1 = 1 – Q2/Q1


The Attempt at a Solution


η=1-(Q2/Q1)
=1-(5*10^7/1.1*108)
=0.54

Which means the inventor is claiming 54% efficiency. In reality such an engine would be in the region of 30% efficiency or less.Therefore I would not suggest investing in this project."

I think my answer seems too short, which means I'm probably doing it wrong. Also, the temperatures probably have some relevance to the solution so I'm probably using the wrong equation(s).
 

Discussion

Physics news on Phys.org
Dez1 said:

Homework Statement


An inventor claims to have developed an engine which takes in 1.1x108J of heat energy at 400K, and rejects 5.0x107J of heat energy at 200K. Would you advise investing in this project? Explain your answer.


Homework Equations


W = Q1 – Q2
η = W/Q1 = 1 – Q2/Q1


The Attempt at a Solution


η=1-(Q2/Q1)
=1-(5*10^7/1.1*108)
=0.54

Which means the inventor is claiming 54% efficiency. In reality such an engine would be in the region of 30% efficiency or less.Therefore I would not suggest investing in this project."

I think my answer seems too short, which means I'm probably doing it wrong. Also, the temperatures probably have some relevance to the solution so I'm probably using the wrong equation(s).

Or simply consider the net entropy change of the system.