Automotive

"Inside-out Wankel" rotary engine delivers 5X the power of a diesel

View 2 Images
LiquidPiston's XTS-210 is an ultra-portable, multi-fuel capable rotary engine delivering as much power as diesel piston engines five times its size and weight
LiquidPiston
LiquidPiston's XTS-210 is an ultra-portable, multi-fuel capable rotary engine delivering as much power as diesel piston engines five times its size and weight
LiquidPiston
The XTS-210 is about the size of a basketball, and makes around 25 peak horsepower
LiquidPiston

LiquidPiston says its new XTS-210 solves the efficiency, lubrication and fuel type issues of Wankel rotary engines. This supercharged, liquid-cooled two-stroke claims 5X the power of an equivalent size or weight diesel engine, and 3X the torque.

Targeted at military, commercial and aerospace applications, the XTS-210 is about the size of a basketball, weighs in at 19 kg (42 lb), and displaces 210 cc. It'll run on multiple fuels, including diesel and kerosene/jet fuel. The company is shooting for about 20 kW (26.8 hp) and 29.4 Nm (21.7 lb-ft) of torque, both at 6,500 rpm.

These numbers compare favorably against the 18.8 kW (25.2 hp) and 63 Nm (46 b-ft) peak outputs of the Kohler KDW1003 E536A, says LiquidPiston, a diesel roughly five times the physical size of the XTS-210, and over four times the weight. And the XTS design uses just two primary moving parts: a rotor and a shaft. You can see a breakdown of an older version in the video below.

So how do they work? "If you recall the Wankel," LiquidPiston co-founder and CEO Alec Shkolnik explained to us in a 2020 interview, "they have a triangular rotor inside a peanut-shaped housing. 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.

"They have a long, skinny, moving combustion chamber, we have a stationary combustion chamber that's nice and round. You can drive it to a high compression, just by making the chamber smaller. And because it's stationary, we can directly inject fuel where the Wankel could not. So those are the two key advantages of the diesel: high compression ratio and direct injection.

"And then there's our apex seals, they're like our piston rings. In the Wankel engine, they're inside the rotor, again. They move at a high speed, and bounce around, they're very hard to lubricate. In our case, they're stationary, they don't bounce around, and you can lubricate them directly from the housing.

"So we basically solved the key challenges the old rotaries had with combustion and with oiling. Those oiling challenges caused both durability issues and emissions problems. By making those components stationary, we solve the challenges of the old rotary. And we also upgraded its cycle to give it much higher efficiency."

LiquidPiston has been working on these X-engines for nearly 20 years now, with numerous prototypes already tested in small planes (see above) and go-karts. Other prototypes have included naturally aspirated versions making up to 40 hp, and forced-induction engines up to 70 hp. They've run these things on diesel, gasoline, hydrogen and propane, and they're developing the XTS-210 at the moment on JP-8/Jet-A fuel due to its ubiquity in defense and aerospace.

The company is moving to commercialize the XTS-210 off the back of a US$9-million US Army contract, with a prototype expected for delivery in 2024. This brings LiquidPiston's military contract total over $30 million. Its immediate use cases are expected to be in portable generators and UAVs, although a separate project is evaluating it for use in a hybrid-electric VTOL UAV.

“There are essentially no diesel engines in the 25-horsepower power class today that are suitable for aerospace and mobile military applications, where size and weight parameters are especially critical," said Shkolnik in a press release. "The reduced weight, size, and heavy- or multi-fuel capability of the XTS-210 delivers significant end-system capability and utility benefits, especially for the military to reduce supply chain and logistical burdens in an era where ‘power on the move’ is increasingly important."

The XTS-210 is about the size of a basketball, and makes around 25 peak horsepower
LiquidPiston

As electrification takes off, it's a difficult time to be working on new combustion engine concepts – but the military certainly won't be fully electric any time soon, and the LiquidPiston design's portability and multi-fuel capability could certainly see it through to volume manufacturing if it proves efficient and durable across a wide range of conditions.

Source: LiquidPiston

  • Facebook
  • Twitter
  • Flipboard
  • LinkedIn
33 comments
Steve Jones
It would be nice to see some figures for efficiency.
Due to its compactness and low NVH, the Wankel engine is being explored as a BEV range-extender.
If this has added reliability and efficiency, it could be (figuratively) huge in that area.
GregC
I'd like to see something like this in a sports car. No, hear me out! I have often wondered why the range extender faded away: that class of vehicle that was basically battery driven but had an on-board generator (think Vauxhall Ampera, BMW i3 etc.). Pure EVs still have (at least perceived) range issues, or the batteries are ridiculously heavy. Hybrids are liquid-fuel-powered with a bit of a battery for traffic jams. I believe we need EVs with a modest battery plus highly efficient liquid-fuelled (Ethanol?) on board generator. A small, light, modestly powered engine like this could fit the bill nicely. Aim for 1000 miles a tank (nice round number) to start with and see how it goes. Cmon Lotus! ;)
stevendkaplan
So wait how does it run on all these different fuels? Do they have to adjust the engine each time they use a different fuel type? Or do they just drain the tank of the old fuel and then pour in the new stuff?
riczero-b
Interesting design but the rotors would be subject to cycles of rapid heating and cooling on the relatively thin walls , careful choice of material needed for longevity.
Brian M
Now if it could run off green synth fuels, be ideal as part of a car hybrid system with its low weight. There are many downsides to all electric vehicles powered by conventional type Li batteries.
Treon Verdery
30dB metamaterial polymer sound attenuators are published, tune those to make this engine much quieter
Catweazle
Interesting...
And I suspect that in the words of Mark Twain, "reports of the death of the death of the internal combustion engine are greatly exaggerated".
John Jacob
Actual rotary displacement is always twice the published value so not much more power than a comparable four stroke piston engine of equal displacement. Most gains probably come from having no power loss due to valve springs and perhaps better breathability due to port area vs valve area.
I'm skeptical of the seals. They're probably more reliable than a traditional Wankel style rotary but will never equal the reliability piston rings.
Gordien
Can hydrogen be a fuel? Jet fuel gives me a headache.
I have imagined a quick to start and stop engine for a chain saw - for work efficiency, and for safety (if it could be small and light enough).
Vehicle wise, that could be a solution for an electric vehicle to be used in an area with a lot of hills - power boost up, and battery only the rest of the time.
PAV
It would be great needful to see this used in a hybrid vehicle.