Formula (4.2) for the indicated cyclic work of an ideal Stirling engine immediately suggests that output can be increased by charging the workspace with more working gas, keeping everything else the same. This is the motivation behind pressurizing or supercharging an engine. What matters in the end, of course, is whether shaft output improves, and this is a matter of mechanical efficiency.

An easy case to understand at this point is that of an ideal Stirling engine having a constant mechanism effectiveness and optimum buffer pressure. Its mean workspace pressure would be proportional to m, as Formula (3.9) explicitly shows. The Maximum Shaft Work Theorem (4.4) thus implies that if the engine has the charge of its working gas increased by a certain factor, and its buffer pressure adjusted to be optimal for the new charge (in fact, it will need to be increased by exactly the same factor, as Formula (3.4) shows), the shaft output will increase by the same factor. Hence, pressurizing an optimal ideal Stirling in this way will increase output in direct proportion to the charge factor. This kind of pressurization, called system charging, where the workspace and buffer pressure are charged together uniformly by the same factor, produces the same best possible results in many engine and buffer pressure combinations.