Van Blarigan’s experiments suggest that these engines could be 50 percent efficient at generating electricity–close to the efficiency of hydrogen fuel cells and much more efficient than conventional generators. Free-piston engines are efficient in part because they have fewer moving parts than conventional engines do. The engine configuration also makes it practical to tune the engine so that the fuel in a combustion chamber burns very quickly. Faster combustion allows the engine to get more work out of a given amount of fuel, improving efficiency. It can also improve emissions.
The free-piston design can also allow the engine to be instantly optimized for different fuels, such as hydrogen, natural gas, ethanol, gasoline, and diesel. Ideally, drivers could use whatever fuel is cheap and readily available.
The development of free-piston engines, however, is still at an early stage. “The free-piston has some unique features–simplicity and variable compression–which make it intriguing,” says Gary Smyth, the science director of GM’s Powertrain Systems Research Lab. “But [they] also pose a number of challenges.”
Van Blarigan says that one major concern is the sound of the engines: the fast explosions are very loud and will be difficult to muffle. But perhaps the biggest issue is control. In a conventional engine, the movement of the pistons is constrained by the rods and crankshaft, which help even out any variations from cycle to cycle. The free-piston engine is more flexible. That allows for using different fuels, but it makes necessary some sort of active control mechanism to ensure that each cycle is the same: variations could cause poor performance and increased emissions. High-speed computers and the ability to electronically control piston movement in a free-piston generator (via the coils and magnets) could help engineers solve this problem.
Whether the engines will be significantly cheaper and more efficient than conventional engines is unclear, says John Heywood, a professor of mechanical engineering at MIT. “There’s been enough development to say that it works. But with very different engine geometries, it’s hard to work out just how good it is. Is it really better?” As research progresses, it will need to answer questions about efficiency, emissions, performance, and especially cost, Heywood says.
Meanwhile, conventional internal combustion engines keep getting better, which could make it difficult for the free-piston design to get a foothold.