X-engine



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.

Design
This is geometrically an inverted Wankel engine that operates on the high-efficiency hybrid cycle. In the Wankel, 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 makes lubrication much easier. Common to both concepts is the use of ports to supply air and remove exhaust gas, thereby eliminating the need for valves.

Operating principle
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 being allowed to combust fully under constant volume conditions, which is how Otto cycle combustion is modelled. Finally, the combustion products are expanded to atmospheric pressure, utilizing the Atkinson cycle.

Feasibility
In order to use the Diesel cycle efficiently, high compression ratios are required: Typical engines use between 15:1 and 24:1. The LiquidPiston engine was demonstrated on the Diesel cycle with a compression ratio as high as 26:1. This would generally rule out the use of low-octane fuels like gasoline. Therefore a spark-ignited variant with lower compression was developed. This so-called "X-Mini" is a 70 cc air-cooled naturally aspirated four-stroke X-Engine, and has been demonstrated on gasoline, kerosene, Jet A fuel, propane and hydrogen.