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Here's a nicely integrated genset design:
http://www.greencarcongress.com/2008/09/modeling-a-free.html#more

"The free-piston engine offers a number of benefits, including optimization of the combustion process through variable compression ratios, making multifuel operation possible; reducing frictional losses because of fewer moving parts; and allowing homogeneous charge compression ignition (HCCI) to be more easily achieved. A linear alternator can use the linear piston force without requiring additional mechanical components necessary in a rotary configuration.
 

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Though it makes you wonder what happens if one side fails to ignite.
I think that may turn out to be the biggest technical challenge.

One idea is to have the generator/actuator control the position of the piston. If the piston doesn't fire, the engine controller can just move the piston to where it needs to be for the next firing. We have fast enough processors that the high sample required needn't be a problem. I think the real difficulty would be getting a good position sensor and an actuator that can respond fast enough.

One other possibility is that instead of using a two chamber, free piston, you just use a single combustion chamber with a spring pushing back on the other side. Again, you use the generator coil as the linear actuator. This is a simple second order, spring mass system and should be really easy to control. In addition, a misfire doesn't effect any of the other pistons and can be run independently.
 

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Thumper

One other possibility is that instead of using a two chamber, free piston, you just use a single combustion chamber with a spring pushing back on the other side. Again, you use the generator coil as the linear actuator. This is a simple second order, spring mass system and should be really easy to control. In addition, a misfire doesn't effect any of the other pistons and can be run independently.
Or instead of a spring you could use a flywheel and a traditional alternator. And tada.... you have a traditional single cylinder engine. The flywheel has the advantage of not robbing power when the engine is at a steady RPM.

It also has the advantage of already existing.

The two piston boxer style free piston is intriguing though.
 

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Or instead of a spring you could use a flywheel and a traditional alternator. And tada.... you have a traditional single cylinder engine. The flywheel has the advantage of not robbing power when the engine is at a steady RPM.

It also has the advantage of already existing.

The two piston boxer style free piston is intriguing though.
Maybe, but you lose the simplicity of the free piston.
 

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shouldn't the very air that the piston is compressing keep it from smashing into itself if you have a misfire?
 

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shouldn't the very air that the piston is compressing keep it from smashing into itself if you have a misfire?
That would depend on the speed at which the piston was moving, and the state of any system-controlled valves that might be present. Running it in a 2-cycle mode with fixed exhaust ports would be possible and would eliminate that valve-open piston crash possibility, but it would probably not meet emissions requirements.
 
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