The New 'inside out' Wankel engine packs a punch despite its size

LiquidPiston's latest XTS-210 solves, so the company claims, the problems associated with older Wankel engines by turning them inside out.
Christopher McFadden
XTS-210 engine core compared to 25 hp Kohler KDW1003.


One company, LiquidPiston, claims they have solved many of the issues Wankel engines suffered from with their XTS-210 "inside-out Wankel" rotary engine. They claim that its two-stroke supercharged liquid-cooled engine boasts three times the horsepower and five times the power of a diesel engine of comparable size or weight.

Its development, and planned commercialization, have been boosted by a recent $9 million U.S. Army contract.

Felix Wankel, a German engineer, came up with the Wankel engine in the 1950s. In a Wankel engine, fuel and air are compressed using a rotor that spins around a central driveshaft inside an oval-shaped chamber.

This is different from traditional piston engines, which obviously use pistons. Three curved sides on the rotor serve as pistons and compress the air and fuel as it rotates inside the chamber. They were renowned for their small size, smooth operation, and high power-to-weight ratio. Still, they used a lot of gas and caused a lot of pollution, which caused them to lose popularity. The XTS-210, however, has been built to solve some of the critical issues with the Wankel engine by turning it inside out.

"[The XTS-200] has a long, skinny, moving combustion chamber, we have a stationary combustion chamber that's nice and round. You can drive it to 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," LiquidPiston co-founder and CEO Alec Schkolnik explained.

"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," Schkolnik said.

"So we solved the key challenges the old rotaries had with combustion and 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," he added.

Aimed at military, industrial, and aerospace uses, the XTS-210 is roughly the size of a basketball, weighs 42 lbs (19 kg), and has a displacement of 210 ccs. It can operate on various fuels, such as diesel and kerosene/jet fuel, and at 6,500 rpm, the firm is aiming for about 26.8 hp (20 kW) and 29.4 Nm (21.7 lbs/ft) of torque.

According to LiquidPiston, these figures are competitive with the 25.2 hp (18.8 kW) and 46 lbs/ft (63 Nm) peak outputs of the Kohler KDW1003 E536A.

With the growth of electric vehicles, LiquidPiston is in an interesting position with regards to developing a new combustion engine. However, the XTS-210's portability and ability to use multiple fuels could help it reach mass production if it proves effective and durable in various situations.

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