This inside-out design solves most of the rotary engine’s problems

Rotary engines have an aura of cool. In games of Top Trumps, the V12 might have been king, but a rotary was a joker, a wild card. A lack of mainstream success no doubt contributes; there are reasons they were never commonplace, including their oil-burning apex seals, which created emissions and fuel-consumption headaches.

LiquidPiston thinks it has those problems solved, however, and in the process, it created a new internal combustion engine that's small and efficient. It has demonstrated its tech on the bench and in a go-kart, but also in uncrewed aerial vehicles for the US military.

While little about the rotary engine merits the word, in a "traditional" Wankel rotary, a triangle-shaped rotor turns within a chamber during its combustion cycle. Apex seals are fitted at the apices of the rotor, but they need constant lubrication with oil, plenty of which burns during combustion. So, a Wankel engine needs constant oil top-ups while dealing with the products of that burnt oil. And those apex seals wear down.

"[The Wankel] has this long, skinny, moving combustion chamber. It's probably the worst thing you can do for an engine," explained Alec Shkolnik, founder and CEO of LiquidPiston. "Combustion kind of develops like a ball and then when it hits the wall, the wall is cold, and it extinguishes. So imagine trying to grow a flame through a narrow corridor, and you see that you get really bad combustion," he told me.

The solution involves turning the engine inside out. Instead of an oval-shaped combustion chamber and a triangular rotor, now the combustion chamber is triangular and the rotor is an oval, which contains a pre-chamber.

"So instead of a long, skinny, moving combustion chamber, we now have a stationary combustion chamber inside of the housing," Shkolnik said. "What that means is we can make it smaller, and that drives a higher compression ratio. And because it's stationary, it's suitable for direct injection of fuel," he said. And since the seals are stationary, the oil problem should be fixed.

"We can directly interface the apex seals with the face seals. Now that there are no gaps anymore, the blow-by is significantly reduced, and... we can directly lubricate the seals by metering tiny amounts of oil... right to the ceiling surface," Shkolnik said.

It’s not just the engine design—it’s thermodynamics

LiquidPiston isn't just playing with engine shapes—in its own words, it's also trying to reinvent the thermodynamic cycle. "The HEHC [High Efficiency Hybrid Cycle] cycle is a four-stroke cycle. Basically, we are cherry-picking features of other cycles. We're trying to stretch the PV diagram in every direction to get more energy, more work extracted from a given amount of fuel," Shkolnik told me.

"What we're trying to do is capture the compression ratio of a diesel with a constant-volume combustion process of an Otto cycle, a true Otto cycle. And beyond that, we also want to over-expand; we want to have a larger expansion ratio compared to the compression ratio. Those are the three pieces of our cycle. So we just pick things we like, and we call it a high-efficiency hybrid cycle," Shkolnik said.

The HEHC engine turns out to be pretty adaptable. LiquidPiston has two- and four-stroke variants, for instance, and is currently concentrating on the former. "The difference is that in the four-stroke variant, one part of the rotor does combustion, and the other part does intake and exhaust. On the two-stroke variant, both sides of the rotor each do combustion. So every time the rotor spins around, we get six firing events. That's pretty remarkable in terms of power density," Shkolnik told Ars.

It has been tested with both compression- and spark ignition, too. "The advantage of spark ignition is that we can run a variety of different fuels. So in our small engine—the five horsepower engine—we've run on gasoline, kerosene, jet fuel, diesel, and even gaseous fuels like propane and most recently hydrogen," he explained.

Much of LiquidPiston's more recent work has been on developing a two-stroke, 25 hp (18 kW) variant, with intended use by the military. The startup started working with DARPA in 2016 and, in the last two years, has had contracts worth $55 million with the Department of Defense.

One application is part of a hybrid powertrain for a UAV, where the engine can charge the battery during flight. Take-off, landing, and up to 30 minutes of the UAV's flight can be on electric power alone. Another is for a portable 10 kW generator, which is 75 percent lighter and far more compact than the US Army's current Advanced Medium Mobile Power Source generator, as well as being about 8 percent more efficient.

But LiquidPiston has designs on applications outside of low-volume, hand-built drone engines or military generators that are exempt from emissions regulations. That's where a recent $30 million funding round comes in.

"That's a big reason why we are raising outside capital, to cross these productionization and emissions bridges so that we can get to the commercial market. I would estimate about two years to where we are hopefully delivering with the DoD and then maybe one or two years after that for broader commercial markets," Shkolnik said.