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LiquidPiston X-engine can run on propane

The LiquidPiston (LPI) (earlier post) team has adapted and demonstrated the X-Mini engine running on propane fuel. A propane tank was connected to the engine through an EFI propane injector, and the engine ignition timing was retarded in the ECU. With a little adjustment to injector pulse settings, the engine fired right up, the company reported.


LPI X-Mini rotary engine.

This marks the first operation of an X-engine on gaseous fuel, and further demonstrates the diversity of the platform which has been shown to operate on gasoline, kerosene, jet fuels, and now propane, simply by changing the ignition control timing and the fuel metering mechanism.

This testing was preliminary in nature. While the company priority continues to be on optimizing the engines for jet and diesel fuels, this testing provides support for a future multi-fuel engine platform, the company said.

LiquidPiston develops multi-fuel-capable rotary combustion engines that are theoretically scalable from 1HP to more than 1000 HP. LiquidPiston has comprehensive patent coverage that includes the novel HEHC thermodynamic cycle, engines that embody the cycle, and enabling technologies.

Although the LiquidPiston X engine is a rotary engine, it is not a Wankel. The X engine essentially “inverts” the Wankel engine. While a Wankel engine has a 3-sided triangular rotor, within a 2-lobed oval housing, the X engine has a 2-lobed oval rotor in a 3-sided housing.

The X engine captures the main advantages of the Wankel, including 1) high power-to-weight ratio [a one rotor X engine behaves like a 3-cylinder 4-stroke]; 2) simplicity—having only 2 moving parts: a rotor, and a shaft; and 3) like the Wankel, the X engine is inherently balanced with no oscillating components, therefore having minimal vibration.


Unlike the Wankel however, there are several key differentiators which address the bulk of the older Wankel’s design deficiencies:

The combustion chamber in the X engine is located in the stationary housing, with most of the gas displaced during compression into this stationary combustion chamber. This makes the X engine uniquely suitable for high compression ratio operation with Direct Injection and Compression Ignition (which is not possible in the Wankel without boosting or a second compression rotor).

Additionally, the combustion chamber can take any geometry, and can be approximately spherical, optimized for surface to volume ratio, thereby improving combustion efficiency and reducing heat transfer.

The apex seals of the X engine are located within the stationary housing, and do not move with the rotor. The seals do not experience centrifugal forces, and can be lubricated directly by metering small amounts of oil directly to the sealing surface through the housings, which means that oil consumption can be reduced to levels potentially comparable to that of a 4-stroke piston engine (essentially negligible).

The unique sealing geometry of the X engine has 3-5 times less blowby than the Wankel rotary. This is mainly because 1) the Wankel requires clearance at the corners between its side/face seals and its apex seals, while the X engine does not; and 2) the Wankel seals traverse across holes that contain spark plug(s), whereas the X engine does not.


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