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LiquidPiston To Unveil Compact High Efficiency Engine at 2010 SAE World Congress

In a paper to be presented at next week’s 2010 SAE World Congress in Detroit, LiquidPiston, Inc. will publicly reveal the architecture of its High Efficiency Hybrid Cycle (HEHC) engine. (Earlier post.) The paper describes LiquidPiston’s third-generation engine prototype, which was first test-fired in January 2010. It provides an air-standard analysis of efficiency, including a comparison to conventional engine technologies.

The thermodynamic efficiency of the HEHC cycle offers a 20% to 50% improvement over the best diesel engines, according to the company. In addition, the engine is ultra-compact, delivering one horsepower per pound, and operating quietly without a muffler.

“Imagine what you could do with a light weight, quiet, 20 HP diesel engine the size of a shoe box.”
—Edward Richards, CEO of LiquidPiston

The High Efficiency Hybrid Cycle (HEHC) is a thermodynamic cycle which borrows elements of Diesel, Otto and Atkinson cycles, including:

  • Air compression to a high ratio, followed by fuel injection and compression ignition (Diesel);
  • Constant volume combustion (Otto);
  • Over-expansion (Atkinson); and
  • Optionally, internal cooling heat recovery via steam generation (Rankine).

The main engine components consist of a rotor in pure rotation and two reciprocating gates directly driven by overhead cams. This combination separates the working mixture into three separate volumes. At a given rotor position each volume operates at a different part of the cycle. For instance, intake/compression, combustion, expansion/exhaust are occurring simultaneously in separate chambers.

As the rotor moves, the cavity formed by the side of the rotor, the retracting compressor gate, and the stationary housing is decreasing in volume, producing compression. The gate fully retracts, as the rotor passes beneath. The air is fully compressed into a combustion chamber within the housing and held at constant volume. Fuel is injected, and combustion occurs at relatively constant volume.

As the rotor continues its motion, the volume defined by the housing, expander-gate, and the rotor is increasing through the completion of the expansion stroke. Due to the geometry, a higher expansion ratio is achieved relative to the compression ratio. Predicted output is 143 hp/L and 30% thermal efficiency.

Combining the high peak pressures produced during constant volume combustion with Atkinson expansion, which continuously extracts energy as exhaust gases expand to atmospheric pressure, delivers unprecedented fuel efficiency. Also, using a ported design rather than conventional poppet valves makes for extraordinarily quiet operation.

—LiquidPiston co-founder and Chief Technology Officer Nikolay Shkolnik

Although the company is not pursuing the automotive primary propulsion market due to the long design-in times and prohibitive development costs, it believes HEHC engines for conventional passenger cars could deliver 100+ miles per gallon within a decade. In the near term, the company plans to produce and license flex-fuel engines in the 5 hp (3.7 kW) to 100 hp (75 kW) range for a variety of commercial, industrial, and military applications.

The prototype engine was brought from art to part in 8 months by a team of six full time employees. Since its founding in 2004, the company has used only a small amount of venture capital to achieve multiple design iterations and a working prototype.




How is 30% "high efficiency" compared with what's already on the road?


"Imagine what you could do with a light weight, quiet, 20 HP diesel engine the size of a shoe box.”

You could build an EV range extender, that is what.

If you didn't run out of fairy dust in the meantime.


Is it just me? It makes me think of a Wankel engine.

gotto besaid

seals, seals , seals this is the biggest killer of these types of engines, they just dont last. I see they are discussing using water sealing which sounds positive and if they mean high presure injection on the seal face it may just work . I hope so.
The other problem is the very low torque produced with this type of engine and torque is what turns the wheels, yes speed can be covered to torgue though gearing but this creats a large lot of energy loss.

Also the 30% effenancy needs to be at the work produced end not just the combustion process. A turbine has 80 to 90% effeciancy but uses huge amount of fuel to do the same thing as a diesel engine due to the high volume of air fuel mixture and the high loss of exhaust heat. The diesel on the other hand is only around 20% efficent at the drive end and about 60% effectent in the combustion process.

On the other hand most petrol engines have a final usable effenancy of less than 13% and the best are very slow speed high torque marine diesels, at about 35% so hopefully this engine has a 30% efficancy measured at the engine output doing work and is reliable and can be used in cars or trucks. It would certainly then produce huge egery savings.


I think "gotto besaid" is another one of those disinformationists, because he didn't give one figure that was remotely accurate. Small gas turbines (e.g. Capstone) have efficiencies in the mid-20% range, and even small diesels (Lister-type) exceed 30% at the crankshaft. GE's simple-cycle gas turbines have hit 46%, and large marine diesels (Wartsila-Sulzer) can exceed 50%.

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