Army Funding Advances LiquidPiston’s Jet-Fuel Rotary Engine

Unmanned aircraft has flown with parallel hybrid-electric propulsion powered by LiquidPiston’s rotary X-Engine.
Credit: LiquidPiston

Development of compact, efficient rotary internal-combustion engines able to run on jet fuel has been boosted by U.S. Army contracts to advance the technology for unmanned-aircraft propulsion and rotorcraft auxiliary-power applications.

Startup LiquidPiston has received a small business innovation research contract to develop its X-Engine as a hybrid-electric propulsion system for unmanned aircraft. The company has also won a prize competition to develop a version of the X-Engine as an auxiliary power unit (APU). Both contracts are aimed at supporting the Army’s Future Vertical Lift (FVL) initiative.

The X-Engine resembles a Wankel rotary turned inside out. Instead of a triangular rotor inside an oval housing, it has an oval rotor in a triangular housing. In a Wankel, apex seals on the rotor move in and out at high speed during rotation and are difficult to lubricate. In the X-Engine, both apex and face seals are mounted on the stationary housing and are lubricated directly, improving durability and reliability.

“When we reimagined the rotary engine with the X-Engine, we knew it could be used to improve power generation in a number of different ways, given its high power-to-weight, efficiency and ability to run on heavy fuel including Jet-a/JP8 fuel,” CEO Alec Shkolnik said in a statement.

Bloomfield, Connecticut-based LiquidPiston is aiming the X-Engine at powering the vertical-takeoff-and-landing Future Tactical Unmanned Aircraft System being developed under FVL, as well as providing auxiliary and supplementary power in manned fixed- and rotary-wing aircraft.

Scalable from 1-1,000 hp, the X-Engine has a high power density of up to 1/5 hp/lb. The design is 30% smaller and lighter and 50% more fuel efficient than spark-ignition gasoline engines, with reductions of up to 80% and 30%, respectively, over compression-ignition diesel engines, according to LiquidPiston.

The company built its first working diesel X-Engine in 2012, demonstrating the new architecture and thermodynamic cycle, called the high-efficiency hybrid cycle. In January 2019, the company demonstrated a 40-hp diesel X-Engine core under a $6 million DARPA contract.

In July 2019, LiquidPiston delivered a hybrid JP8 generator to the Army under a Rapid Innovation Fund award and, in October last year, the company flew an unmanned aircraft fitted with a parallel hybrid-electric propulsion system powered by the X-Engine running on jet fuel.

The X-Engine is suited to hybrid propulsion because of its compact size and fuel efficiency, the company said. Configured as a parallel hybrid—where both engine and battery drive the propeller—the X-Engine can be shut down, allowing the UAV to operate in electric-only quiet mode, and then restarted in flight.

“The ability to do this on jet fuel has not previously been demonstrated in other vehicles currently fielded,” LiquidPiston said in a statement.

In auxiliary power applications, the X-Engine approaches the power density and packaging of a gas-turbine APU, but with the greater efficiency of a diesel engine, according to LiquidPiston. The X-Engine APU is designed for use on the ground and in flight to offload the main engines and provide supplementary power for systems, avionics, cooling and weapons.

Graham Warwick

Graham leads Aviation Week's coverage of technology, focusing on engineering and technology across the aerospace industry, with a special focus on identifying technologies of strategic importance to aviation, aerospace and defense.