Toyota's free piston engine is brilliant

 Piston engine fundamentals haven't changed in 134 years—until now.

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. 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. That's the
future.