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First Law for a Heat Engine

Net Energy Transferred by
Heat for the Cycle

Net Energy Transferred by Work for the Cycle

A cycle diagram for a heat engine or power cycle. The system gets heat from the hot reservoir, produces a net amount of work and rejects some heat to the cold reservoir.

Energy Balance for the Cycle

Lesson
4F Blog

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Lesson 4F: page 5 of 14

The form of the first law that we derived for a generic cycle applies for our power cycle or heat engine, that is, Wcycle = Qcycle.
The tricky part of using these tie-fighter diagrams is that the heat and work flows that are labeled on them ARE ALL POSITIVE. They do NOT use a sign convention.
This can be confusing.
WHE is a positive quantity and it flows in the same direction as our sign convention.
So, Wcycle = WHE.
The problem is that QC and QH are positive quantities in our tie-fighter diagram.
Qcycle is the net heat transferred INTO the system because of our sign convention.
So, Qcycle = QH – QC. Does that make sense ?
If you are ever in doubt, use some common sense. Do a reality check.
Remember, all the energy flows on a tie-fighter diagram are positive.
Would it make any sense to say that the net amount of heat transferred into the system is QH PLUS QC ? No, it would not.
So, how can we COMPARE the performance of one heat engine to the performance of another ?
Flip the page and let’s see.

First Law for a Heat Engine

Ch 4, Lesson F, Page 5 - First Law for a Heat Engine