What is the Thermal Efficiency of an Engine?

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SUMMARY

The discussion focuses on calculating the thermal efficiency of an engine using the equations U=Q+W and e=(Qh-Qc)/Qh. Participants emphasize the importance of understanding the isobaric, isochoric, and adiabatic processes, specifically noting the constants for pressure, volume, and heat. The lack of specific data points in the provided graph is highlighted as a challenge in deriving the correct values for Qc and Qh. Participants encourage sharing attempts at solutions to facilitate better guidance.

PREREQUISITES
  • Understanding of thermodynamic processes: isobaric, isochoric, and adiabatic
  • Familiarity with the first law of thermodynamics (U=Q+W)
  • Knowledge of thermal efficiency calculations (e=(Qh-Qc)/Qh)
  • Ability to interpret graphical data related to thermodynamic cycles
NEXT STEPS
  • Research the specific characteristics of isobaric, isochoric, and adiabatic processes in thermodynamics
  • Learn how to derive Qc and Qh from graphical data in thermodynamic cycles
  • Study examples of thermal efficiency calculations in real-world engine applications
  • Explore common mistakes in thermodynamic problem-solving and how to avoid them
USEFUL FOR

Students studying thermodynamics, engineers working on engine design, and anyone interested in understanding thermal efficiency calculations in mechanical systems.

yecko
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Homework Statement


http://i.imgur.com/SQ6jx94.png
SQ6jx94.png

(sorry for the low resolution~ it's already the best i can make it~)

Homework Equations


U=Q+W
the three variables of isobaric, isochoric, adiabatic process (can i save my effort from typing all of them out?)
e=(Qh-Qc)/Qh

The Attempt at a Solution


Substitute the relevant equations in order to get Qc & Qh
it always seems a lack of information to the three points
there should be a specific route for substituting the variables to the cycle and the few thought by me should be wrong
can anyone tell me the right way to get option "e" here?

Thank you
 
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Show your working.

yecko said:
it always seems a lack of information to the three points

I haven't attempted a solution but usually these come from the data provided on the graph combined with your knowledge about what is constant during an isobaric, isochoric, or adiabatic process.
 
CWatters said:
what is constant during an isobaric, isochoric, or adiabatic process.
isobaric=>p constant
isochoric=>v constant
adiabatic=>q constant
 
So of P, V, and T which are you missing at each of the three points and how might you work them out from the info provided?
 
yecko said:
isobaric=>p constant
isochoric=>v constant
adiabatic=>q constant
You must attempt the question yourself. Get as far as you can and then post that attempt here so that the homework helpers can see where your misunderstanding comes in.
 

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