The simple solution is to strap a generator to a piston engine, as BMW did with the two-cylinder range extender in its i3 EV. But if the engine never turns a wheel, there’s no need for it to rotate anything. Why not cut out the middleman and use the piston’s reciprocating motion to generate electricity? That obviates camshafts and most other rotating parts, too.
The Free Piston Engine Linear Generator (FPEG). “Free” refers to the fact that the piston isn’t attached to a crankshaft; instead, as the piston is forced downward during its power stroke, it passes through windings in the cylinder to generate a burst of three-phase AC electricity. The FPEG operates like a two-stroke engine but adds direct gasoline injection and electrically operated valves. It can also be run like a diesel, using compression rather than a spark plug to ignite its fuel mixture.
It’s tested thermal-efficiency rating is admiringly 42 percent . Only the best, most complicated, and most expensive of today’s gas engines can come close to that number, and only in specific circumstances. Even better, a two-cylinder FPEG is inherently balanced and would measure roughly 8 inches around and 2 feet long. An engine of that size and type could generate 15 hp, enough to move a compact electric vehicle at highway speed after its main drive battery has been depleted.