Wankel 2.0. The Return of the Rotary Engine.
Roopinder Tara posted on November 03, 2020 |
Nikolay Shkolnik and son Alexander turn the Wankel engine inside out.
The rotary engine reengineered. The Wankel on the left emerges as the X1 after a LiquidPiston makeover. (Picture courtesy of LiquidPiston.)
The rotary engine reengineered. The Wankel on the left emerges as the X1 after a LiquidPiston makeover. (Picture courtesy of LiquidPiston.)
Alec Shkolnik, founder and president of LiquidPiston.

Alec Shkolnik, founder and president of LiquidPiston.

Alec Shkolnik thinks the internal combustion engine (ICE) needs a complete makeover. Forget the conventional piston-driven engine that powers all our transportation. It is only capable of incremental gains after 150 years of service. Shkolnik blows the dust off a small but surprisingly potent internal combustion engine the world seems to have forgotten: the rotary engine. 

We are not immediately receptive. We’re on a mission to cover emerging technologies. We’re only too aware of the shift to electric power—hybrids, hydrogen fuel cells. The technologies of the future.

"Now, pay attention," schools Shkolnik. "Electric cars will not save us."

“There’s a lot of misconceptions about electric cars,” said Shkolnik. “MPGe (mile per gallon equivalent) used by the EPA only considers the energy as the electron leaves the battery and powers your wheels. Completely neglected is how the electron got into your battery. And that most people plug into the grid—which burns a lot of fossil fuel. The 1,200- or 1,400-pound battery takes an enormous amount of resources and enough CO2 goes into its production that it takes about 12 years for an electric car to catch up with a diesel car in terms of its CO2 footprint. An electric car starts in a CO2 hole. We think the right solution is a hybrid engine."

“You don’t need a 1,200-pound battery because you hardly ever need your full range,” Shkolnik continued. “Most trips are 5, 10, 15 miles. That’s 99 percent of your driving. So why not have a small battery? For the times you need more range, have a small, compact, efficient rotary engine available. It can be easily refueled in minutes at any gas station and gives you the range for those occasional long, long trips. It makes no sense to drive your battery around. Six gallons of gas weighs 36 pounds. That’s equivalent to a 1,200-pound battery pack.”

And besides, as he looks us in the eye to address our infatuation with electricity and hydrogen. “Our rotary engine operates very well on hydrogen. It doesn’t have glowing hot spots in the engine. We could have a zero-emission hydrogen-powered rotary engine. We have not done that yet, but it’s theoretically possible. And, in reality, you don’t perfectly burn the hydrogen. So, you might end up with a little bit of NOx emission. Still, it would be a zero-carbon emission.”

Mighty mite. LiquidPiston’s 70cc X Mini engine on the right compared to a 49cc Honda Metropolitan moped engine on the left (Picture courtesy of LiquidPiston.)
Mighty mite. LiquidPiston’s 70cc X Mini engine on the right compared to a 49cc Honda Metropolitan moped engine on the left (Picture courtesy of LiquidPiston.)

The rotary engine is 30 percent more efficient than a conventional ICE or diesel engine, according to Shkolnik. It has a much higher power-to-weight ratio. It can rev higher (up to 13,000 RPM) and, without reciprocating pistons, it can run smoother. It has far fewer parts, so it is more reliable—and cheaper to produce.

Shkolnik formed LiquidPiston along with his father, Nikolay Shkolnik, PhD, to educate the public and promote variations in internal combustion engines, hybrid engines and a redesigned rotary engine—the X1, which reimagines the Wankel. Shkolnik has PhD from MIT in AI and a bachelor’s and a master’s in computer science and neuroscience from the Georgia Institute of Technology. Father and son are admittedly not the typical Detroit motorheads who like to say they have gasoline in their veins. They live in Bloomfield, Conn.

“My father is a physicist, and my background is in modeling and optimization." Shkolnik says unapologetically. “We came at it from outside of the engine field. We approached the problem through a physics and optimization perspective.”

The father has a background in innovation consulting using a technique called TRIZ, explains Shkolnik. He worked with companies to develop their technologies in a systematic way. He has patents in fuel cells, supercapacitors—all sorts of particularly clean energy technologies. And it always ended up coming back to the rotary engine. The rotary engine is hard to beat for its simplicity, its price and its capability.

How Did We Get Here?

The Wankel KKM motorcycle: The “A” marks one of the three apices of the rotor. The “B” marks the eccentric shaft, and the white area is the lobe of the eccentric shaft. The shaft turns three times for each rotation of the rotor around the lobe and once for each orbital revolution around the eccentric shaft. (Picture courtesy of Wikipedia.)
The Wankel KKM motorcycle: The “A” marks one of the three apices of the rotor. The “B” marks the eccentric shaft, and the white area is the lobe of the eccentric shaft. The shaft turns three times for each rotation of the rotor around the lobe and once for each orbital revolution around the eccentric shaft. (Picture courtesy of Wikipedia.)

Indeed, next to the elegant motion of a rotary engine, the ICE is a Rube Goldberg contraption of reciprocating pistons, connecting rods, crankshafts, valves, camshafts, and springs. While we are swept up with engineering pride for creating the machinery that allows a ton of steel to rocket down the road by igniting gasoline, have we fallen in love with our creation, a fire-breathing monster that roars under the hood and belches smelly gas? Did it require da-Vinci-level genius to turn the up and down motion of pistons into the rotary motion of the driving wheels? Or would the Renaissance inventor have created the rotary engine—presuming he had a supply of gasoline for which he needed to find purpose.

The rotary engine, AKA the Wankel engine—named after German engineer Felix Wankel, who patented it in 1929—has practically disappeared from the public view. Mazda received accolades for its rotary engine powered RX-7 sports car (Motor Trend’s Car of the Year in 1986) but abandoned the rotary engine in 2002, and to this day remains the only major car company to have used a rotary engine.

The beating heart of the rotary engine is what looks like a triangle with bulging sides as if it were overinflated, called the Reuleaux triangle, which in your father’s rotary engine was connected almost directly to the drive shaft. Like in the piston engine, fuel is ignited in the rotary engine and the pressure from gases after ignition forces the movement of the rotor toward a bigger volume. With a rotary engine, the rotor’s eccentric rotation is transferred to concentric rotation of a driveshaft by planetary gearing. Piston engines, in contrast, have linear motion and require a complex mechanism (connecting rods and a crankshaft) to convert linear motion into rotary motion.

A point on the corner of the Reuleaux triangle will trace a peanut shape path as gases move in and out of different sized chambers, compressing, expanding, exhausting.

The redesigned rotary engine. LiquidPiston turns the Wankel inside out. See a video of engine parts coming together and motion here: https://bigfish.egnyte.com/dl/Qv1nuIUHsk/
The redesigned rotary engine. LiquidPiston turns the Wankel inside out. See a video of engine parts coming together and motion here: https://bigfish.egnyte.com/dl/Qv1nuIUHsk/

We ask call up Shkolnik to hear more, partly from nostalgia (full disclosure: the author loved his 1986 RX7) but also to find out why, in this day and age, we should be paying attention to the latest incarnation of the Wankel?

Why did Big Auto give up on the rotary engine?

The old rotary engine struggled in a number of core areas. First, let’s look at the combustion chamber shape—it’s long and skinny like a corridor—and moving. When you ignite the fuel, it wants to develop like a sphere. When it hits the wall, it extinguishes. Growing a sphere inside of a narrow corridor doesn’t work very well.

It’s really hard to get complete combustion inside of a Wankel engine because of the shape. We turned it inside out and instead of the chamber being long, skinny and moving, our chamber is stationary. That means we can get a much higher compression ratio and we can shape the chamber to be spherical. It is also suitable for direct injection of fuel right in the combustion chamber. All this adds up to better combustion characteristics than the Wankel.

But the Achilles’ heel for the Wankel is sealing. Mazda never really solved the seal problem. There’s kind of two problems with the seals. The first is lots of blow by. You exert a lot of effort to compress your gas and add heat through combustion only to lose much of that energy as the gas leaks past the seals. The Wankel design has apex seals that interface with the face seals axially and in the corners. They really can’t seal very well so there’s button seals, but with tolerances for thermal expansion, you end up with big leakage pathways for gas to escape.

Also, Wankel engines struggle with lubrication. You had to add oil periodically to your rotary engine. Moving apex seals are very hard to lubricate and there was a durability issue. Mazda injected oil into the intake, hoping that some of it [would get] to the seals. But most of the oil ended up being burned. That results in very high emissions because oil doesn’t burn well. The Wankel was known for poor emissions, poor fuel consumption, and relatively poor durability.

The emission was enough to knock the Wankel engine out of the consumer market. That’s why Mazda stopped production of the RX-7. On the RX-8, they could have switched to a heavier oil to take care of the durability issue, but then they couldn’t meet emissions standards. Or they meet emissions standards but fail on durability but they couldn’t meet both at the same time. So, in the end, it was the emissions problem that killed the Wankel.

What makes the rotary engine different this time around?

In our design, the apex seals are stationary in the housing, making the sealing challenge much more manageable. We’ve reduced blow fivefold compared to a Wankel engine. That also allows us to run higher pressure and have higher efficiency compared to the Wankel.

We’ve redesigned the rotary engine, so oiling the seals is no longer required. Where the old rotary engine was the triangle (rotor) inside the peanut shape, ours is the opposite. We’ve turned the rotary engine inside out, you could say.

What is the big advantage of a rotary engine over a piston engine?
Where the rotary engine really shines is in the power-to-weight ratio. It blows the piston engines out of the water in this regard. That’s what makes it attractive for a sports car and for aircraft. It’s a more elegant design [with] fewer parts. Without the oscillating motions, you can go to higher speeds; with higher speeds, you can go to higher power because power is a function RPM. If you’re trying to get power, the rotary does better than a piston engine.

You created a new rotary engine from a new thermodynamic cycle?
Thermodynamic cycles haven’t changed in over a hundred years. We needed a new one. We call it the high efficiency hybrid cycle, HEHC.

Higher efficiency is possible with high compression ratios. We want to restrain the combustion so that it’s occurring under approximately constant volume conditions. And then we want to overexpand like the Atkinson cycle. Put this together and it gives you about a 30 percent improvement in efficiency over a diesel engine.

Then came the new rotary engine. Instead of a peanut shaped rotor inside of a three lobed housing of the Wankel engine, we have made the opposite—an inside out, or inverted Wankel engine.

An automotive engine only converts about 15 to 20 percent of the fuel into useful work. How to get more out of your engine? This made us pursue a new thermodynamic cycle. We are investigating dozens of different types of engines to see if any could embody the new cycle. We’ve been investigating thermodynamic cycles for over 10 years. We’ve built four generations of engine architectures and finally converged on what we call the X-Engine. It was an evolution.

The rotary engine, with reciprocating masses, is prone to vibration. Less so the rotary engine?

It is not a good thing to have your piston constantly coming to a stop and then accelerating dramatically and then stopping again. That is the source of NVH, noise and vibration and harshness. It causes durability issues. If you need to get a million miles from a truck engine and you look at how it ultimately fails to do so, it’s because the piston rings have to reverse direction. You lose the oil film at the reversal of motion.

Do you have a patent for the new rotary engine?

We have 41 patents, and 23 pending.


Update: Since this interview, LiquidPiston received a small business innovation research contract to develop its X-Engine for a hybrid-electric propulsion system for unmanned aircraft. The company was also one of 12 winners in the U.S. Army’s xTechSearch 3.0 and will configure the X-Engine as an auxiliary power unit (APU).

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