Automotive free-piston engine

Automotive application is that studied by most current free-piston engine developers. The free-piston engine generator is a very attractive candidate as a prime mover in series hybrid vehicles or as a range extender in plug-in hybrids. It can provide high thermal efficiency (including high part load efficiency), low emissions, fuel flexibility, and high power to weight ratio through a compact design and a flexible engine layout.

While conventional internal combustion engine technology is highly developed, there exists a significant potential for improvement in the conversion efficiency compared to the theoretical limitations. Key limitations include: engine mechanical losses (friction); mechanical limitations (bearing loads and materials stress) at high compression ratios due to high in-cylinder gas pressures; and heat transfer losses and the formation of temperature-dependent emissions at high gas temperatures. The free-piston engine can provide substantial advantages in this respect, with:

• extremely low friction: all frictional losses associated with the crank system are eliminated and piston friction is reduced due to no side forces acting on the piston;
• no load-carrying bearings, allowing operation at high compression ratios; and
• very high piston acceleration around top dead centre, reducing the time spent in the high-temperature parts of the cycle and thereby reducing heat transfer losses and emissions formation.

The group is currently carrying out a design study for a automotive free-piston engine power unit incorporating innovative technology and utilising the group's developments within advanced control strategies for such engines. The system is based around standard dual piston free-piston engine configurations modified to run on a four stroke engine cycle in order to minimise emissions in pre-mixed, spark ignited engines.