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LiquidPiston signs $1M agreement with DARPA to advance development of power-dense X-Mini rotary engine for JP-8

LiquidPiston, Inc. (LPI), the developer of engines based on its High Efficiency Hybrid Cycle (HEHC) (earlier post), has entered into an agreement totaling $991,557 with the US Defense Advanced Research Projects Agency (DARPA) to advance the development of its efficient, power-dense rotary internal combustion engine X-Mini (earlier post) for portable and small-engine applications.

Under this agreement, LiquidPiston’s primary objective is to demonstrate a pathway towards a rotary JP-8 fueled engine that has the potential to reduce fuel consumption by 50% and to increase power density by threefold compared to today’s conventional heavy-fuel piston engines. JP-8, or Jet Propellant 8, is a kerosene-based jet fuel used widely by the US military.

Today’s diesel/JP-8 engines and generators are extremely heavy. For example, a typical 3kW heavy-fuel generator weighs over 300 pounds, requiring six people to move it around. LiquidPiston’s engine technology may enable a JP-8 generator of similar output weighing less than 30 pounds that could fit in a backpack.

—Dr. Nikolay Shkolnik, LiquidPiston’s Founder and CTO, and Co-Principal Investigator of the DARPA project

The ultimate goal of the funded effort is to demonstrate a pathway to a heavy-fueled engine that could deliver above 50% average brake efficiency, 57% peak brake efficiency and high power density (>1 hp/lb), using a test-bed environment.

Such efficiency would reduce fuel consumption by approximately one half compared to today’s conventional piston engines. The effort will demonstrate key enabling components of the engine technology, as well as initial experiments with JP-8 fuel. As part of the agreement, LiquidPiston is investing 40% of the project costs.

Early LiquidPiston prototypes have validated the principles and confirmed compression ignition of diesel and JP-8 fuels. LiquidPiston has built 70-horsepower and 40-horsepower compression ignition heavy-fuel engine alpha prototypes, and recently unveiled a 70-cubic centimeter (3- to 5-horsepower) gasoline powered rotary four-stroke engine prototype, which would serve as the test platform for the work with DARPA.

HEHC. HEHC is an improved thermodynamic cycle optimized for fuel efficiency that combines features of four existing cycles: high compression ratio (Diesel); constant volume (isochoric) combustion (Otto); over-expansion to atmospheric pressure (Atkinson); and internal cooling with air or water (Rankine).

LiquidPiston’s engines, have only two primary moving parts—a shaft and rotor—resulting in compact size and low-vibration operation. Although they are rotary engines, LiquidPiston’s engines are not Wankel engines; they are uniquely configured to adopt the company’s patented thermodynamic cycle and its associated efficiency and low-noise benefits.



I still recall driving a fast, smooth Mazda rotary across country in the 1970's and a even much better RX-8 in the 2002.

Maybe, between a more oil 'sealable' design and today's 3D printing, a/this? rotary engine will go mass production - even if it's initially scaled to a unmanned drone or weed-eater.

Nick Lyons

Looks like the perfect light aircraft engine--simple, light weight. If their efficiency claims are for real and it doesn't have sealing problems (a big if), this could really be something. I wonder about its emissions profile...


This actually looks very very impressive. It seems that it can do multiple strokes at the same time... very power dense.

The use of kerosene could possibly help sealing issues, it has a high lubricity. But the pressures could be a bit higher than what we would find in a wankle.
1 rotor = 40hp with an average efficiency more than double typical ICEs.

a great application would be a range extender for an EV.

I bet this responds to boost well too (assuming they fix the apex seals). Emissions are controllable with existing technology (if it doesn't leak oil everywhere).


Nice, but these rotary engines all have the sale problem when it come to lubricate, cool and insure the hermeticity of the all things, there is a lot of surfaces frictioning, as a results power is never as great as claim as well as efficiency...umnfortunately, I see more promise in Achates or Ecomotor approaches


Maybe it lacks low rpm torque and also oil consumption. Im almost sure it has a big flaw somewhere, a lot of illuminates over the years tried to invent a better engine than actual spreaded ice gasoline/diesel engines. Also if it were good why not associate with Toyota for more resources in the developing stage, that's a proof that it has major bugs.

Brent Jatko

@ gor: $1M is a minor expense, Department of Defense-wise, and good ideas have often come from outside the mainstream. If it takes off, Toyota may buy the technology.

As it is, they get research without having to spend that $1 million themselves.


The ideal PHEV would have:

1) a modular (1 to 4 or more) 4 to 6 kWh plug-in (under the floor..a la Tesla) rechargeable modules.

2) a clean running range extender (good for 500 to 800 Km) such as an H2 FC or an aluminum battery.

3) People (the majority) with few long drives would run most of the time on the rechargeable batteries and would not use the (more expensive aluminum; about the same as gasoline price) range extender unit very often.

4) People with frequent long drives could use the lower cost FC range extender option

5. People who can wait till 2025 would buy 800+ Km range BEVs.


Powerpoint engineering. How many designs has LiquidPiston gone through already? They already tried (or at least rendered, lmao) a version of the Wankel engine, predictably we haven't heard from that in some time.

Henry Gibson

Cooling will be an issue for large machines. Efficiency is questionable at small sizes. Bladon jets are also a competitor for weight(see Jaguar)with lower efficiency? Unit can be incorporated into generators. ..HG..

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