I have a conceptual question regarding the Otto cycle. In a piston-cylinder when using the otto cycle at compression since the piston applies work on to the system (the gas), is the sign convention positive? And if so does this mean at expansion since the gas combusts and applies work on to the piston to push it down does this mean the work sign convention is negative?
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$\begingroup$ What is special about Otto's cycle? The same question doesn't apply to every compression/expansion? $\endgroup$– GiorgioP-DoomsdayClockIsAt-90Commented Jul 18, 2021 at 6:47
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1 Answer
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It depends on which version of the first law you are using. For $\Delta U=Q+W$, compression work is positive. For the more common version $\Delta U=Q-W$, compression work is negative. Reverse the signs for expansion work.
The key is compression work increases internal energy while expansion work decreases internal energy.
Hope this helps.
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$\begingroup$ thank you! much appreciated, I'll take note of this. $\endgroup$– PhenCommented Jul 18, 2021 at 10:34