[tt] High-Efficiency Generators for Hybrid Vehicles

[Eugen Leitl]

http://www.technologyreview.com/Energy/21442/

Tuesday, September 30, 2008

High-Efficiency Generators for Hybrid Vehicles

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

By Kevin Bullis

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.

Credit: Peter Van Blarigan, Sandia National Laboratory

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.

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.