Controlling the speed of a Stirling engine is problematic and poor throttle response is one of the key reasons this engine type has not seen use in automotive applications. In general, the Stirling engine has relatively large heat exchange surfaces and the engine's response to turning the heat input rate up or down is slow due to the need to change the temperature of all the engine internal parts and especially the temperature of all the heat transfer surfaces before the working gas temperatures and pressures change enough to effect engine output torque and rotational speed. So, other methods of controlling speed are employed.
One method is to simply bypass gasses from the compression to the expansion spaces lowering the highest pressure and raising the lowest pressure attained by the engine working gas during the cycle. This lowers the pressure difference achieved inside the engine and the torque output of the engine and thus the rotational speed. Venting hot gas into a cold space without doing work is obviously not an efficiency enhancing method of controlling engine speed, but then again neither is choking off an IC engine's fuel/air supply with a throttle plate butterfly valve and increasing the parasitic pumping losses in the process. It does have the advantage of being simple.
Another more complicated but more efficient method is to remove or to add working gas to the working cycle of the engine to slow it down or speed it up. More gas in the active parts of the engine means higher pressures and more gas molecules available to move energy through the engine, more work to be done by them, higher torque output, and generally higher engine speed.
So, bleed higher pressure gas out of the compression space and into an intermediate pressure tank and the engine slows down. Release gas from this intermediate pressure tank into the lower pressure expansion space and the engine speeds back up again. All this can happen relatively quickly and without changing the engine heat input rate or internal engine part temperatures quickly.
Engine part temperatures will change with time and input heat rate will need to be adjusted for long term operation at a different speed due to need to restrict the maximum internal temperatures engine parts are subjected to. However, reducing engine speed generally increases the high temperatures and reduces the low temperatures, it therefore widens the temperature difference the engine is operating from, and it increases the engine thermal conversion efficiency accordingly. This is therefore a better approach than simply adding a valve to bypass working gas from the hot to the cold section.