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The X-engine is a type of pistonless rotary engine manufactured by LiquidPiston of Bloomfield, Connecticut.

The X engine is a hybrid cycle rotary engine.

In the Wankel engine, the only successful pistonless rotary engine to date, a figure-eight-like epitrochoid housing surrounds a curved sided triangular rotor. The rotor revolves around a fixed gear in a hula-hoop motion. The output shaft revolves at three times the speed of the rotor. One power pulse is delivered in one revolution of the output shaft. For comparison a single cylinder four-stroke piston engine delivers one power pulse for two rotations of the output shaft. The three operating chambers of the Wankel formed by the triangular rotor are separated by seals installed on the three apexes of the rotor. They require elaborate lubrication, and in older designs have been a weak point for the engine's durability.

The LiquidPiston design reverses the shapes: a figure-eight-shaped rotor moves within a triangular housing. The required seals (both face and apex) are mounted on the stationary housing, which simplifies lubrication. This makes it geometrically an inverted Wankel that operates on the high-efficiency hybrid cycle.^[1][2] Common to both concepts is the use of ports to supply air and remove exhaust gas, without the need for valves.

This cycle consists of compressing air (with no fuel) to a very high ratio, as is typical in the Diesel cycle. The air is then isolated in a constant volume chamber. Fuel is directly injected and allowed to combust fully under constant volume conditions, similarly to Otto cycle combustion. Finally, the combustion products are expanded to atmospheric pressure, utilizing the Atkinson cycle.

The X-Engine uses the Diesel cycle. Efficiency requires high compression ratios: typical diesel engines use between 15:1 and 24:1. The LiquidPiston engine was demonstrated with a compression ratio as high as 26:1.^[3] This generally rules out the use of lower-octane fuels like gasoline.

The X-Mini is a spark-ignited variant with lower compression. It is a 70 cc air-cooled, naturally aspirated, four-stroke X-Engine, and has been demonstrated using gasoline, kerosene, Jet A fuel, propane, and hydrogen.^[4]

• LiquidPiston, Inc. official site
• Integza tour of the engine
• Warped See Through Operation
• D4A Review

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