28 Jun 2018


The US Defense Advanced Research Projects Agency (DARPA) has awarded LiquidPiston Inc., an advanced internal combustion engine technology company, an additional $2.5 million to continue development of its 30kW X4 rotary diesel engine prototype, bringing DARPA’s total funding of the engine technology to $6M.

LiquidPiston received this award after meeting the objectives for Phase I of the program, which had focused on the clean sheet design of the X4, and demonstrating the structural integrity of the new engine platform while operating under compression ignition of diesel fuel. LiquidPiston engineers presented a development update on the engine at the SAE International WCX World Congress Experience in Detroit in April this year.

LiquidPiston’s X Engines are non-Wankel rotary embodiments of the company’s High Efficiency Hybrid Cycle (HEHC). In contrast to other rotary engines, the X engine has a higher CR, and a stationary conical/spherical combustion chamber suitable for direct injection (DI) and compression ignition (CI). As with the Atkinson or Miller cycles, the X engine takes advantage of over-expansion. This is done by changing the locations of intake and exhaust ports asymmetrically which allows for the extraction of more energy during the expansion stroke.

LiquidPiston’s X engine (right) essentially inverts the Wankel engine (left). While a Wankel has a 3-sided rotor and a 2-lobed housing, the X engine has a 2-lobed rotor in a 3-sided housing. The X engine captures the main advantages of the Wankel (high power-to-weight ratio; simplicity; and inherent balance), but also addresses the design deficiencies of the older engine.

Because the combustion chamber is located in the stationary housing with most of the gas displaced during compression into this chamber, the X is uniquely suitable for high compression ratio operation with direct injection and compression ignition. The combustion chamber can take any geometry and be optimized for surface-to-volume ratio.

The apex seals of the X are located within the stationary housing. Because they do not move with the rotor, the seals do not experience centrifugal forces. Lubrication is simpler, with oil consumption lower.

Phase II also lays a foundation for future work. When development of the fully packaged engine is complete, the 30kW X4 engine is expected to weigh just 30 lbs (13.6 kg) and fit into a 10"x10"x10" box, while achieving 45% brake thermal efficiency—approximately an order of magnitude smaller and lighter than traditional piston diesel engines, and also 30% more efficient.


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