Flex-Fuel Rotary Engine Concept Gets Simpler

LiquidPiston Inc. has unveiled a new, simpler rotary engine that demonstrates its "high efficiency hybrid cycle" (HEHC).

The Bloomfield, Conn.-based startup's X2 40-hp prototype requires no valves, cooling system or muffler and is roughly one-tenth the size and weight of a comparable piston engine. LPI says the new design also is easier to produce, seal and assemble.

The prototype X2 weighs 80 lbs, but the company says the engine's weight could probably be cut to about 50 lbs in production form. LPI notes that a comparable 40-hp diesel engine would weigh about 400 lbs.

LPI emphasizes that its mission is to develop the thermodynamics of HEHC rather than to lock the technology into a specific engine design.

HEHC converts heat to work by combining elements of four well-established combustion cycles: Diesel (high compression ratio), Otto (constant-volume combustion), Atkinson (over-expansion to atmospheric pressure) and Rankine (internal air or water cooling).

LPI reported its latest performance results last week at the U.S. Dept. of Energy's 2012 DEER Conference in Dearborn, Mich. The company says the X2 engine has a brake efficiency of 50% at partial load at least twice that of a piston engine and a theoretical thermodynamic efficiency of 74%. Power density is about two horsepower per pound.

Internally, the prototype powerplant is resembles a Wankel rotary engine, but with an eccentrically rotating oblong rather than triangular rotor. The new design is similar to the company's X1 prototype but with a wider rotor and shorter rotor radius for improved sealing.

Unlike a Wankel engine, the LPI unit performs intake/compression, combustion and expansion/exhaust simultaneously in three separate chambers instead of sequentially in one chamber.

LPI says it expects to have copies of the X2 engine available for independent evaluation early next year.