Toyota recently showed a prototype engine that does just that. It’s called 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.
Toyota says this mechanically simple engine achieves a claimed thermal-efficiency rating of 42 percent in continuous use. Only the best, most complicated, and most expensive of today’s gas engines can come close to that number, and only in specific circumstances.