This Tiny ‘Inside-Out Wankel’ Rotary Engine is the Size of a Basketball

A modernized version of the rotary engine is being commercialized, and this one won’t chew up apex seals.

byRob Stumpf|
LiquidPiston's tiny rotary engine.
via LiquidPiston
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The rotary engine is perhaps one of the most polarizing motors to have ever been commercialized. From its checkered past to its intent to live on as a range extender, the small spinning triangle has amassed a niche following, including a few companies that believe the powerplant is still the future of combustion engines.

One of those companies is LiquidPiston, which has been developing a tiny rotary as part of a multi-million military research contract. Now, the manufacturer has finalized its basketball-sized engine and has announced its intent to commercialize the product, complete with a few modernizations that address the shortcomings of yesteryear's rotary—and yes, that includes apex seals.

via LiquidPiston

LiquidPiston calls its engine the XTS-210. With just 25 horsepower, it's not like this exact configuration will be putting the power down to the wheels in a future Mazda-branded sports car, but it is still cool nonetheless. To reach that power figure, the 210cc rotary is supercharged, providing around one bar (14.5 PSI) of boost and six combustion events per revolution of the rotor. It also features some modernization through direct injection and can be powered by both spark-ignited fuels or by using compression-based ignition (like diesel).

The manufacturer says that it reduced the size and weight of a modern diesel engine by around 80%, which puts this small 42-pound motor on par with the size of a basketball. That puts this modern take on the rotary at around five times the power-to-weight and three times the torque-to-weight compared to a traditional piston-based diesel engine.

LiquidPiston says that it achieved the feat by combining the best features of the Otto, Atkinson, and Diesel cycle combustion engines, then packaging them into a modified rotary structure.

"If you recall the Wankel, they have a triangular rotor inside a peanut-shaped housing," LiquidPiston's CEO Alec Shkolnik told New Atlas in 2020. "We have the opposite, a peanut-shaped rotor in a tri-lobed housing. So take everything you know about the Wankel and turn it literally inside out.

Moreover, Shkolnik says the company solved the age-old, meme-worthy problem surrounding the rotary engine: its apex seals. LiquidPiston says that its apex seals are stationary, so rather than bouncing around like a Wankel's, it stays in place and can be better lubricated directly from the motor's housing.

Last year, LiquidPiston won a $9 million development contract from the U.S. Army to help evolve its system for military and commercial applications. Specifically, the Army became interested in the product for its Unmanned Aircraft Systems, which presently have engines that "are not designed for military fuels" and have experienced reliability problems.

We've seen claims of new rotary engines before, even from Mazda, but none have lived up to the hype of what a true road-going rotary could have been. And with combustion-based tech taking a backseat to electrification, it's unlikely that LiquidPiston's tech will ever make it into a future mainstream vehicle. But it's still fun to dream. For now, let's just take solace in knowing that a company still sees a place for the rotary in commercial applications.

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