The U.S. Air Force (USAF) granted a contract to a developer of advanced rotary internal combustion engines, LiquidPiston, to evaluate the use of the company’s X-Engine in unmanned aerial systems (UAS) and eVTOL aircraft as a means to help overcome limits of current battery technologies.

Awarded on March 4 through the USAF’s AFWERX Agility Prime program, the Phase 1 $150,000 Small Business Technology Transfer contract covers the adaptation of the X-Engine to power UAS and eVTOLs, referred to as “orbs” by the Air Force, through a hybrid-electric propulsion system. Agility Prime is exploring the use of eVTOL and other emerging vehicles for military missions.

The X-Engine is a lightweight rotary engine that can run on JP-8, jet-A, diesel, and other heavy fuels. Coupled with a generator, the engine can be configured to keep a battery charged during flight, significantly extending the range of the propulsion system. It can also be configured in parallel with an electric drive using the engine to produce thrust or lift.

“Today’s solutions for power and energy are held back by a lack of technology innovation; gasoline engines are inefficient, diesel engines are big and heavy, and while the world wants to go electric, batteries lack significantly compared to the energy density of fuel,” said LiquidPiston CEO and co-founder Alec Shkolnik CEO and co-founder. “The X-Engine solves these challenges.”

The lightweight engines incorporate a modular design that can scale in size. LiquidPiston said the engine is 30 percent more fuel-efficient than a diesel engine but up to five to ten times smaller and lighter. It fits a comparable envelope and weight of a small turbine but is up to two to four times more efficient, the Bloomfield, Connecticut company added.

Stressing that the X Engine is not the traditional Wankle-designed rotary engine, LiquidPiston explains its engine has a fundamentally different thermodynamic cycle, architecture, and operation. Aside from ancillary parts such as injectors, fuel pumps, and oil pumps, the only moving parts are the rotor and an eccentric shaft.

The engine is powered by the company’s patented “High Efficiency Hybrid Cycle (HEHC)” thermodynamic cycle that company founders invented in 2003. Since then the U.S. company has grown through a series of investments, along with DARPA and other military awards, including multiple U.S. Army contracts exploring hybrid-propulsion possibilities.

In 2019, LiquidPiston demonstrated its hybrid propulsion capability using jet-A on a 55-pound UAV.

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