A company called LiquidPiston has a new take on rotary engine technology, they have basically turned a Wankel engine inside out, which appears to have solved the apex seal problem while improving fuel economy.

It still has ports, instead of valves, so it’s also pretty simple:

Military and other operators prefer using kerosene, rather than gasoline, across ground and air platforms, but lightweight, reliable heavy-fuel engines for unmanned aircraft systems (UAS) have proved challenging to develop.

LiquidPiston, a startup developing a novel powerplant that is smaller and lighter than piston diesel engines and more efficient than gasoline engines, has been boosted by winning Sikorsky’s Entrepreneurial Challenge.

Developing multifuel rotary combustion engines based on its high-efficiency hybrid thermodynamic cycle (HEHC), the Bloomfield, Connecticut-based company has won $25,000 and the opportunity to explore applications for its X-engine on Sikorsky products.

“We are targeting our engine to be up to 10-15 times smaller and lighter than a piston diesel engine of similar power output, and up to 2-3 times more efficient than gasoline engines, especially at part-power,” says founder and CEO Alexander Shkolnik.

I think that the claims here are a bit much, but the shape of the combustion chamber is far less prone to the thermodynamic losses that bedevil the Wankel.

In a Wankel, apex seals on the triangular rotor move in and out at high speed during rotation. “The seals are impossible to lubricate, so they mix oil into the air, but 90% of the oil burns,” says Shkolnik. “In our engines, the seals are on the stationary housing and easier to lubricate.”

HEHC is a four-stroke cycle. The fuel/air mixture enters the X-engine through the rotor and is compressed and ignited. Constant-volume combustion increases efficiency. The combustion gases are then overexpanded before being exhausted through the rotor.

The overexpanded power stroke is similar to that used by the Atkinson Cycle engine used in the Prius to achieve higher fuel economy, though it appears that it does not share the rather low power density of the Prius engine (not an issue in a hybrid, as the electric motor supplies handles need for peak power).