Isostatic piston behavior if filled with an ideal gas.   In a regular piston engine in your car or truck, the source of the heat is the combustion of the vaporized gasoline mixed with air, inside the piston: internal combustion.  In Detroit, they have many engineers and scientists working hard to understand this combustion process, and though it is not an ideal gas, it is still helpful to them to use the ideal gas model to get their initial hold on the process, after which they refine their model with more specific dynamical information about the gases.  In addition, the piston in an engine experiences haugely variable forces as the engine cranks through each cycle, so isostatic does not apply for very long!