Military

LiquidPiston's "inside-out" rotary X-Engine wins Army research contract

LiquidPiston's "inside-out" ro...
The tiny X-Mini engine is a radically different, high-powered and very promising take on the rotary engine
The tiny X-Mini engine is a radically different, high-powered and very promising take on the rotary engine
View 8 Images
The tiny X-Mini engine is a radically different, high-powered and very promising take on the rotary engine
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The tiny X-Mini engine is a radically different, high-powered and very promising take on the rotary engine
The rotary X-Engine promises the performance of a trailer-mounted generator in a size that can be carried by two men
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The rotary X-Engine promises the performance of a trailer-mounted generator in a size that can be carried by two men
The Liquid Piston X-Engine with three spark plugs
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The Liquid Piston X-Engine with three spark plugs
A Honda 49cc single-cylinder piston engine (left) alongside a 70cc X-Mini
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A Honda 49cc single-cylinder piston engine (left) alongside a 70cc X-Mini
The X-Mini engine in a generator and a go-kart
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The X-Mini engine in a generator and a go-kart
The Compact Artillery Power System (CAPS) running a Howitzer
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The Compact Artillery Power System (CAPS) running a Howitzer
The Compact Artillery Power System (CAPS) generator unit powering the digital fire control system on an M777 Howitzer artillery piece
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The Compact Artillery Power System (CAPS) generator unit powering the digital fire control system on an M777 Howitzer artillery piece.
The Liquid Piston CAPS system is about the size of a gaming rig
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The Liquid Piston CAPS system is about the size of a gaming rig
View gallery - 8 images

Connecticut-based company LiquidPiston is developing a portable generator for the US Army that uses its X-Engine, a fresh and extremely powerful take on the rotary engine that'll deliver as much power as the Army's current-gen-set at one-fifth the size.

We've written a few times before about the fascinating LiquidPiston rotary engine. It's not a Wankel – indeed, it's closer to an inside-out Wankel – and with only two moving parts, it's able to deliver extraordinary power density at up to 1.5 horsepower per pound (0.45 kg).

According to co-founder and CEO Alec Schkolnik, the X Engine design combines the high compression ratio and direct injection of a diesel engine with the constant volume combustion process of an Otto cycle engine and the over-expansion abilities of an Atkinson cycle engine, while solving the lubrication and sealing issues of the Wankel rotary engine and delivering huge power and efficiency. Check out the design being used in a go-kart and an unmanned aircraft in the video below.

Liquidpiston engine in flight

"If you recall the Wankel," says Schkolnik, "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," he continues. "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."

The X-Engine's ability to eliminate bulk and weight is extraordinary; to give you an idea, the team pulled a 40-lb (18-kg), 6.5-hp engine out of the go-kart in the video above, and replaced it with a 4.5-lb (2-kg) X-Engine making 3 hp.

The Compact Artillery Power System (CAPS) generator unit powering the digital fire control system on an M777 Howitzer artillery piece
The Compact Artillery Power System (CAPS) generator unit powering the digital fire control system on an M777 Howitzer artillery piece.

LiquidPiston demonstrated the technology for the US Army by building a Compact Artillery Power System (CAPS) generator unit designed to power the digital fire control system on an M777 Howitzer artillery piece. It replaced a generator that needed a truck to move it around with something 20 percent the size: a 41-lb (18.6-kg), 1.5-cubic foot (28.3-L) box about the size of a gaming PC that can easily be carried by two men.

Smartly designed to work in conjunction with a battery in a hybrid system, the 2-kW CAPS generator impressed the Army enough that LiquidPiston has been awarded a Small Business Innovation Research contract to develop it further as a 2-5 kW Small Tactical Generator for a range of military use cases, running on diesel with compression ignition.

The company projects that a 5-kW version would weigh around 100 lb (45 kg) and take up between 4 to 6 cubic feet (113 to 170 L), replacing the currently fielded MEP-1030, at 764 lb (347 kg) and 30 cubic feet (850 L).

Could it come to the automotive world? Absolutely, says Shkolnik. "Long-term, we would definitely target the automotive world. It could be an excellent primary propulsion system for cars, or it could be part of a hybrid system. It'd work well in both conditions. But as you can appreciate, it takes a lot of time and money to develop an engine for the automotive world. When GM puts out an engine, it's got hundreds of millions of dollars worth of development behind it. And that's for one that's based on known technology. Here, we're changing the thermodynamic cycle, we're changing the architecture, we're changing everything. So we've made a business decision not to start with the automotive world. We want to go into a niche application first and prove it out. Then we can go into something like automotive."

It would certainly look attractive as an ultra-lightweight range extender for electric cars – or indeed an aircraft motor for the aviation world, in which every pound is an enemy. Electric aircraft could get a lot further by topping up their batteries on efficient, low-emission generators; even with powertrain efficiencies taken into account, says Shkolnik, fuel carries around 35 times more energy than today's batteries.

The Liquid Piston CAPS system is about the size of a gaming rig
The Liquid Piston CAPS system is about the size of a gaming rig

How is the engine going in durability testing? "We've been hyper-focused on proving the general operability, and showing that the engine works in these application demonstrators," says Shkolnik. "Now that it's pretty apparent that it's working, everyone wants to know how many hours can it run. We're working through that, it's part of what we're going to be doing over the next year. We're running engines for tens of hours, dozens of hours, we're not yet in the hundreds of hours where we want to be.

"We're not even running them long enough to think about things like seal replacement yet," he continues. "It's been a combination of little things we're addressing as we go along. We had some challenges with the gearing, for example; that's been addressed. Then it was some challenges with the bearings, and it looks like that's been addressed. So right now, I don't even know what the weak spot is in the engine. I need to build five or 10 of these engines, run them as long as I can, see what's breaking, and make adjustments until we break through the durability numbers we want to hit. There's nothing known right now that's a show-stopper for us. It's just time and engineering and resources."

It's certainly been interesting watching LiquidPiston's technology progress, and we look forward to the results of the reliability testing. Stay tuned.

Source: LiquidPiston

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21 comments
21 comments
NZRalphy
Good story, it is great to see people working on simple high tech. Thanks Loz
Eddy
A bit late in the day for a new type of IC engine but this one sounds radical enough with huge weight savings to make its development worthwhile. It seems it will have a future replacing many small appliances power source like generators which are huge in the RV industry alone. Small enough to be included in most electric cars to save that range anxiety. So many uses for an efficient small light motor that could be petrol or diesel. As it's low emission IC I hope it's not suppressed by the green brigade or auto industry spare parts empire for uses where electric is totally imprectical.
Raycon
I can see it has potential, but you'll come up against the same longevity, lubrication and sealing problems that both Wankel and Orbital came up against.
Tom Lee Mullins
I think the Liquid Piston rotary engine has lots of potential; since it is a flex fuel engine. I think it has a future even when there are those saying ICE doesn't.
Chris Coles
It would have been nice to see some fuel burn figures; some comparison to existing engines. Having said that, considering the size and weight differences; if they can prove long term reliability, they have a definite market to address that will serve them very well indeed. Well done!
Bob Stuart
Could we please have an animation of the process next time?
Sastry Dasigi
Good to see Wankel engine's successor, without Wankel's problems. Good luck to Liquid Piston. Wish them great success!
akarp
ICEs are going to disappear just like paper printing 'disappeared.' /sarcasm

Fossil fuels still provide a higher energy density and have an established infrastructure. I think we will add 'cleanup' operations to mitigate pollution before we shift to renewable energy. We are generally moving closer towards science/mathematic limits given advances in materials/engineering. Hence why 'old' ideas can become practical with new materials.
michael_dowling
Unless it is powered by biofuel,it is just another way to destroy the climate.
Kpar
Bob Stewart go ahead and google Liquid Piston (good stuff on Youtube). This story is several years old- the military has been looking into use of this engine to power drones, I'm glad to see they're still working on it.

This is a marvelously simple device, and I could see it very well making huge inroads into power generation, both for hybrid autos and portables.
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