Efficient ‘LiquidPiston’ Engine Looks To Outdo Diesels and Other IC Engines

As automakers, the military, and industry take steps away from petroleum by experimenting in technologies including electrified and fuel cells, the internal combustion engine is still very much alive, and work is underway to maximize efficiency, while reducing pollution, complexity and cost.

One such endeavor claiming all these benefits in spades is the product of LiquidPiston, a 12-person company in Connecticut that’s had some success gaining money and attention for its unique, light, high-power-to-weight engines.

The company’s engine design(s) may burn diesel or other fuels, and borrow from a number of architectures, but mainly utilize principles of the Wankel rotary. The company says its unique innovation is “versatile and capable of outperforming conventional engines on all parameters.”

LiquidPiston is fine-tuning different variations of its new architecture that it has dubbed “High-Efficiency Hybrid Cycle” (HEHC), and says it can deliver 50-percent efficiency compared to perhaps 30-percent peak efficiency of traditional engines.

HEHC Cycle

The High Efficiency Hybrid Cycle’ (HEHC) attempts to combine the best features of numerous thermodynamic cycles including Otto, Diesel, Rankine, and Atkinson to create a highly efficient engine.

The design – notably a more recent “X1” variant – is said to provide an improved thermodynamic cycle that optimizes each process (stroke) of the engine operation, with the aim of maximizing fuel efficiency.

In a recent interview with the company’s CEO and President Alexander Shkolnik, GigaOm was told various versions of the for-now small diesel burning powerplant promise 10-times fewer parts than current diesel engines and are thus smaller and lighter.

The engine actually borrows elements not just from Felix Wankel, but also from Otto, Diesel, Atkinson, and Rankine cycles, but resembles a Wankel the most, albeit in reverse.

X2 with background

X2 version.

Instead of the rotor doing the sealing, the X1 lets this task go to the housing which utilize its “apex” seals, with the claim being oil sealing is improved over a traditional rotary.

It utilizes an 18:1 compression ratio, direct injection, maintains constant volume during ignition and burns fuel much longer than traditional engines essentially getting more buck out of the bang.

The design also does away with liquid cooling, as it uses the fuel so much more efficiently, and produces less heat as a byproduct of combustion. Alternately, if cooling is needed, the engine can be made to suck in and exhaust cooling air, or have water injected in to the combustion chamber.

Popular Mechanics notes these neat tricks provides a triple payback: the engine is cooled, NOx emissions are reduced, some of the water is converted to steam, so power is increased.


Here is an earlier generation of a 20-horsepower prototype engine able to use diesel and gasoline fuel.

Various experiments are ongoing to increase efficiency even as the company seeks investors, and potential contracts for commercial applications.

Alexander Shkolnik told GigaOm the engine would likely see service in military vehicle applications, plug-in range-extended vehicles, long-haul trucks, and other niche markets at first, and the startup has raised $12.3 million to date, and is aiming for another $20 million later this year.

The company’s Web site says the original idea was the product of Alexander’s father, Dr. Nikolay Shkolnik, during PhD studies in theoretical physics at the University of Connecticut. A couple decades later in the early 2000s he was working as program manager for a consulting firm, at which he had developed fuel cells, super-capacitors, batteries, and other energy technologies.

But, says his brief history, “he understood that a more efficient engine could compete with fuel cells and other technologies on well-to-wheel efficiency, while being cheaper to make and more compact.”

The actual first “Liquid Piston” engine was patented in 2003 by Nikolay, and in 2004 it won a $50,000 business plan competition at MIT, and today, having crossed other hurdles, the company led by his son has a 6,000-square-foot facility in Bloomfield, CT, and says it sees light at the end of the tunnel.

But, you may ask, why call it “Liquid Piston?” This we asked for you, and were told by Alexander – who also goes by Alec – the “Liquid” name has stuck around from earlier designs that actually did use liquid instead of metal in the combustion chamber.

“LiquidPiston has been exploring the scientific principles of the High Efficiency Hybrid Cycle for several years. There are many potential ways of embodying this new thermodynamic cycle,” said Alec. “Our first concept involved liquid in place of a metal piston, and gas would act on the liquid. It was a nice idea, but required a very large pump, so we moved towards a more ‘conventional’ (e.g. metal) solution. Over time, the design has continued to get more and more simple, while maintaining the cycle at the heart of the technology. The new X engine is the culmination of this effort. The new engine is incredibly simple, and still executes the original LiquidPiston’ HEHC cycle.”


LPI’s engine is architected to have the following unique properties:

• High fuel efficiency of 57-percent at peak, and 50 percent at part-load

• High power density of 1 hp / lb

• Simple design with low part count

• Multi-fuel capable: diesel, JP8, gasoline, natural gas, bio-fuels

• Low noise: the engine has low pressure exhaust and no poppet valves

LPI says its design outdoes traditional designs in applications where work loads can vary – such as during idling. Here. It says, traditional engines can drop as low as 1 percent efficient, and auto engines are typically only 15-percent efficient based on extensive bench testing and research.


Bigger is not always better.

In a best case scenario, it says, the “30-percent” efficiency of converting chemical energy to useful work also sees 40-percent of the fuel turned to heat, and 30-percent lost with exhaust gases.

Perhaps this is why 90-percent-efficient electric motors are gaining so much favor, but rather than digress, we’ll repeat the fact that the industry and society are still wedded to fuel burners, and there is a genuine need for them as other technologies are also being developed in the meantime.

The motor design and LiquidPiston were also briefly written about in June by the Society of Automotive Engineers, and for more info, you can visit the company’s Web site, or review a LiquidPiston SAE technical paper and another SAE technical paper as well.

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  • Modern Marvel Fan

    I think the biggest contribution is the “over expansion” combined with constant volume combustion. Most of the losses in the traditional ICE is mainly due to exhaust pressure and exhaust temperature. If the “over expansion” recovers both, then it just eliminated two of the biggest source of energy loss in ICE.

    Can’t wait to see it in real life. How much “torque” does it generate? I assume it is a “high RPM” engine….

  • Anonymous

    maybe you should sell your idea to ge

  • Roy_H

    They say it has no poppet valves. I could not find a detailed design and their animation did not work for me.

    If I understand the design correctly, each lobe acts like a cylinder in a conventional car with the rotary piston moving into and out of each lobe. A very interesting design, effectively 3 cylinders. But I do not see how this could work without valves and spark plugs on each of the 3 lobes. Maybe they have a different kind of valve?

    The picture shows a timing belt and I can’t imagine any different function other than to drive valves. Also poppet valves would seem to be the obvious choice, and would work well with this design.

  • Roy_H

    I think I just figured it out. One piston is for compression/power stroke and the other exhaust/intake. This way the air passages are through one of the double lobed piston. This arrangement would allow sleeve valves like a two-stroke, but is actually a four-stroke engine.

  • jamespatterson

    Hybrid cars do not use gasoline when they idle. They use both fuel and electric power and use less energy when compared to other fuel-generated vehicles. The batteries of hybrid cars are made of thousands of hundreds of cell. modded xbox 360 controller

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