W
T2 – T1
—
=
———
H
T2
where W is the amount of work done by an engine supplied with a quantity of heat, H, and T2 is the temperature of the heated gases which expand doing work, and are thereby cooled to the temperature T1, at which they are exhausted.
It is therefore evident, that to make an engine work perfectly efficiently we must obtain an amount of work from it exactly equivalent to the heat put in. That is to say, W must equal H in the above equation. We therefore have the efficiency of such a perfect engine
T2 – T1