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.
Having a cheap and efficient way to generate electricity is becoming more important as automakers develop electric vehicles with onboard generators for recharging the battery pack and extending range. Such vehicles, called series plug-in hybrids or extended-range electric vehicles, are to be sold starting in late 2010. (Click here for a comparison of different hybrid and electric vehicle types.) The first will use generators based on conventional engines. But later models could incorporate fuel cells or other unconventional generators, such as free-piston engines.
The potential high efficiency of free-piston engines gives them an advantage over conventional generators, and their ability to use a variety of fuels is an advantage over hydrogen fuel cells. What’s more, free-piston engines don’t require expensive materials such as the platinum catalysts needed in fuel cells, so they could be cheaper too.
Automakers such as GM, Lotus, and Volvo have started to investigate the possibility of using such engines in future vehicles. Meanwhile, in the past couple of years, an increasing number of academic research teams have started developing the engines. So far, most have focused on computer simulations. An exception is a research group at Sandia National Laboratory led by Sandia researcher Peter Van Blarigan that has been testing physical components of free-piston engines. He is assembling a complete free-piston engine prototype, a project that he expects to complete within a year.
In conventional internal combustion engines, multiple pistons are connected via rods to a crankshaft that, via the transmission, drives the wheels. Free-piston engines do away with the crankshaft: the pistons aren’t connected to anything. Instead, two opposing pistons just shuttle back and forth inside a chamber. To generate electricity, the pistons could be equipped with rows of magnets that shuttle past metal coils to create an electrical current.