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I recently stumbled upon this video of a Stirling engine which includes a throttle mechanism. Unfortunately, there was no information about how the throttle worked.

I know that an internal combustion engine's speed is regulated by controlling the amount of air and/or fuel that enters the engine. But how would a throttle mechanism in a Stirling engine work?

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    $\begingroup$ Adjust the burner heat. That's what a car throttle does. $\endgroup$ – Mike Dunlavey Jul 5 '16 at 19:56
  • $\begingroup$ Thanks, that makes sense. However, that doesn't seem to be the mechanism that the engine in the link is using. $\endgroup$ – adamj537 Jul 5 '16 at 20:04
  • $\begingroup$ Right, it just looks like a valve in the pipe from one cylinder to the other. I said that before but I deleted it as just not a good way to do it. When you throttle something down, you want the fuel use to go down. That doesn't do it. $\endgroup$ – Mike Dunlavey Jul 5 '16 at 21:30
  • $\begingroup$ @MikeDunlavey, Re, "When you throttle something down, you want the fuel use to go down." Sometimes that's a requirement, yes, but not always. Sometimes it's just a "nice-to-have" feature, and sometimes, you might not even care at all. A stirling engine can run on waste heat from some other process. In that case, the "fuel" is free. In such an application, you literally would not care if reducing the power output also meant reducing the efficiency. $\endgroup$ – Solomon Slow Jul 5 '16 at 22:33
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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.

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If I were to throttle a Stirling engine I would use a pneumatic or electric regenerative brake on the flywheel. When you want to slow the motor down the electric generator or air compressor would draw a load and store it for when you want to speed it back up. A lot of the electric motors i work on are designed to start and stop quickly. They have a capacitor bank that stores energy to make sure they dont drift upon stopping, and allows them to start quickly without overheating.

A stirling powerplant on a motor vehicle would probably work best with an electric transmission anyway so this would not add any weight to the system. Doing this would could start the engine as well. Using a compressor to acheive this could slow it and speed it back up even more quickly. Using this kind of system would save much of the energy lost to pumping losses from throttleing an otto cycle engine.

Just my two cents, im not a an engineer by any means but this just reminded me of the systems I've seen in my experience. The electric motors i work on use a capacitor bank and when i used to work on aircraft the APUs were kick started by an airbottle. God forbid the pilot not having his switches right, then you were stuck pumping it back up with a breaker bar. Talk about a workout.

Edit: I just thought about how it would be difficult it be to keep its speed down for an extended period, or even speed it back up again quickly. This wouldnt be a huge issue if you use a throttled fuel like propane kerosene or solar, but a solid fuel like coal or wood would be difficult to throttle. Either bypass heat to the cold side or remove heat from the hot side, this would waste heat unless you could store it.

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The way I do it with my stirling engine. I change the dead volume to control the amount of working fluid being heated up and cooled down while keeping the same temperature gradient. This changes the power output instantly and at the same time keeping the efficiency relatively the same.

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