Technology Review: High-Efficiency Generators for Hybrid Vehicles

"An unconventional engine design is attracting attention as a potential alternative to hydrogen fuel cells or conventional engines in some hybrid vehicles. Called the free-piston engine, it could be used to generate electricity as efficiently as fuel cells yet cost less.

Free-piston engines aren't new: they were invented in the 1920s. But the increased recent focus on hybrid cars has led a growing number of research groups and automakers to start research programs to develop the technology. Unlike in conventional engines, there is no mechanical connection between the piston and a crankshaft (hence the name free-piston). Since the design allows for improved combustion and less friction, the engines could be far more efficient in generating electricity than either conventional generators or newer fuel-cell technology."