Highest possible efficiency of an engine

[Effect]

Homework Statement

The highest theoretical efficiency of an engine is

e=(T_H-T_C)/T_H

where T_H is the highest and T_C is the lowest temperature during the circuit process. In a test car with a stirling engine the highest temperature during the circuit process is 700 °C and the lowest temperature is 100 °C. Calculate the highest possible efficiency.

Homework Equations

e=(T_H-T_C)/T_H

The Attempt at a Solution

1 – 100/700 = 85,7%

But apparently the answer should be 61,7%. What other factors needs to be included here?

 

[billy_joule]

Do you know the SI unit for temperature?

 

[Effect]

Ah yes. Of course. Thank you!