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No Cranks Allowed

Can three Australians produce a free-piston engine efficient, clean and powerful enough to power a series hybrid vehicle?The reply to an earlier e-mail from a powertrain engineer at one of the Big Three was to the point: “Interesting technology.

Can three Australians produce a free-piston engine efficient, clean and powerful enough to power a series hybrid vehicle?

The reply to an earlier e-mail from a powertrain engineer at one of the Big Three was to the point: “Interesting technology. They came to see us almost two years ago, and the general impression was that their estimates were questionable. However, that doesn’t mean there isn’t a lot of potential in the technology. At one point there was even talk of investing in them.”

After looking at the design for the Free Piston Power Pack (a.k.a. “FP3”) from Pempek Systems (Sydney, Australia; www.freepistonpower.com) you can understand the reticence of the engineer quoted above. The FP3 has no crankshafts, connecting rods, or flywheel because its pistons don’t move up and down to create reciprocating motion. Its pistons are paired together, slide back and forth, and create electrical power when the permanent magnets attached to them move through fixed coils wrapped around the cylinders. This self-contained internal combustion electric generator is proposed for a series hybrid with 50-kW motors at each wheel.

According to Bert van der Broek, managing director of Pempek Systems, the project was begun on Valentine’s Day 2001, by Ed Wechner. “He came up with the idea of putting the valves in the crown of the piston, which made it possible to make a very compact design,” he says. These intake valves–they draw air into the combustion chamber through ports around the piston’s inner wall–are passive in that they have no drive mechanism, and open when cylinder pressure drops after the electro-pneumatic exhaust valves open. “The intake air scavenges the exhaust gases, and the exhaust valve’s variable valve timing and lift lets us choose how much exhaust gas we retain, keep the intake charge from escaping, and vary the compression ratio dynamically.”

Rather than use a sensor to determine the location of the horizontal piston pair, Pempek determines its location based on information from the coils. “At start-up,” says van der Broek, “we fire the coils in sequence to get the pistons going, and can determine its whereabouts to about one-quarter of a millimeter.” The spark ignition prototype is a one-quarter slice of the proposed 100-kW output unit, and has been under test in Australia since late 2004. Unfortunately, it has fallen short of the expected 25-kW output thus far. “We’re getting a consistent 15 to 18-kW out of it right now,” says van der Broek, “but inefficiencies in the generator and problems with the scavenging process have kept us from getting more out of it. So the three of us [Pempek Systems is a three-man company] are redesigning parts of it for better performance.”

Despite these shortfalls, the trio is sufficiently encouraged by the performance to have started development of a diesel version of the design. This common-rail injection diesel features four passive intake and four electro-pneumatic exhaust valves per piston, and runs at the same 1,800 cycles per minute as the gasoline version. “With the benefit of hindsight,” laments van der Broek, “we should have started with the diesel because it is easier to control, and would have made our learning curve less steep.” Nevertheless, Pempek hopes to bring the diesel up to its 300-kW output potential, and has plans to eventually produce a 450-kW engine that is 1/7 the size of a conventional diesel. “If we’re successful, it could basically sit in the crankcase of a diesel V8 with the same output,” says van der Broek.

Hurdles remain, including the design and development of a compact intake manifold, and emission and exhaust systems. Van der Broek feels the engine’s constant speed will make pollution control relatively simple, while the intake and exhaust are minor worries as the design evolves. The biggest hurdle he sees is convincing OEMs the promise is real.