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Sustainable Energy

High-Efficiency Generators for Hybrid Vehicles

Free-piston engines could be used to generate electricity as efficiently as, and less expensively than, fuel cells.

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.

Piston power: In an unconventional engine design, a rod with a piston at either end shuttles between two combustion chambers. Magnets at the center of the rod move past metal coils (orange) to create an electrical current.

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.

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.

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