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There is probably no better chronicler into the full depth of American ingenuity than YouTube. Here one finds not just computer models for all manner of esoteric combustion engine designs, but actual working prototypes of them, often built by individuals. Big companies can also innovate here sometimes. A new free piston engine linear generator (FPEG) from Toyota Central in Maine is a case in point.
The piston is called “free” because there is no crankshaft. On its power stroke, the piston dumps its kinetic energy into the fixed windings which surround it, generating a shot of three-phase AC electricity. It can be run sparkless through a diesel cycle or run on standard gasoline. What has folks excited is the claimed thermal efficiency for the device — at 42% it blows away the engines used in cars today. Toyota’s demo engine, just 8 inches around and 2 feet long, was able to generate 15 hp. A two-cylinder model would be self-balancing and have much reduced vibration.
Not surprisingly, the valves are electrically operated and can therefore be better used to fine-tune the power delivery through the full range of the stroke. Speaking of strokes, the video indicates a two-stroke design, which might present a few problems for a road-worthy design. For one thing, emissions would be suspect. Nonetheless Toyota imagines that a twin unit design pumping out 20 kW could power a light electric vehicle at a cruise speed of 120 kph (75 mph).
Linear generators and linear combustion engines are nothing new. Shake-to-charge “Faraday” flashlights, smartphones, and even energy-harvesting backpacks are all standard fare, while single-acting direct power pistons have also seen action in applications as intriguing as power-assist boots for the Russian military. The trick is to get the two working efficiently in unison and that is the beauty of what Toyota appears has done. Considering that the piston is decelerated and re-accelerated at each end of the stroke, any mismatch between combustive power input and electromagnetic power extraction needs to be absorbed somewhere. Mechanical or air springs can help although there is still likely to be some efficiency loss.
At the risk of adding some confusion, the device is technically an alternator as it generates AC. As (most) electric cars use 3-phase AC induction or “AC-like” 3-phase brushless DC motors, they could potentially run directly from the output of this device, perhaps save for some intermediary voltage and current conditioning. However, like standard car alternators, there will likely be DC conversion to charge the battery pack — unless Toyota has also secretly perfected the AC battery. There is still plenty of room to innovate here. Linear alternators are similar in design to linear motors, but one does not simply reverse the cycle to swap one into the other — there are certain control functions that need to be imposed on how the coils are energized in a motor. However that does not mean a multipurpose linear electric power device could not be constructed.