The First Law of Thermodynamics:
Thermodynamic Cycles
Most practical heat engines convert some combustion heat into mechanical
work, by taking a gas through a cycle in a piston-cylinder arrangement.
The cycle involves one or more expansion processes, during part of
which some heat enters the gas through the cylinder wall, and one or
more compression processes, during part of which a smaller amount of
heat is extracted from the gas. The difference between the larger
absorbed heat and the smaller liberated heat is the amount of net
mechanical work done by the gas in one complete cycle:
Wnet = Q = Qh - Qc.
As the gas
returns to its starting conditions, the Kelvin temperature and internal
energy U being proportional, DU = 0. According
to the First Law of Thermodynamics,
DU = Q - W,
so net heat equals net work. If the gas is of low density and not too
hot, it will also follow the Ideal Gas Law:
pV = nRT
so that it is possible to predict
what is happening to the temperature of the gas, based on how pressure
and volume are being changed.
If you have a Java-enabled web browser,
click here
to see a simulation of the Ideal Gas Law (follow the instructions for p-V
curves). Use the "back" arrow of your browser to return.
This diagram shows a thermodynamic cycle on a p-V diagram.
Net work done by the gas is positive when the p-V curves progress
around clockwise, and equal to the area enclosed
by the p-V curves for the cycle (positive work represented by more area
under the expansion curve(s)
than under the compression curve(s)). The diagram shows a three-process
cycle; the area that is shaded in yellow only is equal to the net
work done by the "engine" in one complete cycle. The
"cooling" happens at constant volume, which is why no
work is done by the gas during that part of the cycle. Heat may or
may not be exchanged with the gas during the other two processes, but
we can say for certain that, taken together, the other two processes
must involve net heat input in order that Wnet = Qin
- Qout for the cycle.
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