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LiquidPiston raises $5M B Round for HEHC engine development

LPI’s HEHC cycle. Click to enlarge.

LiquidPiston, Inc., the developer of an efficient naturally aspirated rotary engine that can fire on both gasoline and diesel (earlier post) has closed a $5-million Series B investment round. Northwater Capital and Adams Capital Management were joined by unnamed angel investors.

The LiquidPiston engine is based on a thermodynamic cycle called the “High-Efficiency Hybrid Cycle” (HEHC), which borrows elements from Otto, Diesel, Atkinson, and Rankine cycles. The HEHC thermodynamic cycle has a 74% thermal efficiency limit, significantly higher than Otto or Diesel cycles on which most engines operate, according to the company. The company has progressed through a series of concept prototypes, and is on the M2.5 engine.

LPI engine. Click to enlarge.

LiquidPiston’s sub-50 horsepower designs are tailored for markets that are easier and quicker to enter, including portable gensets, Auxiliary Power Units (APUs), and Electric Vehicle (EV) range extenders.

Many unproven technologies with immature ecosystems are attracting attention right now, but it will be decades before combustion engines and ecosystems that support them surrender their prominent role in our economy. So much can be gained by making ICEs smaller, lighter, more fuel efficient, and hence more ecologically friendly. It would be a pity to falsely presume that combustion engines can’t be made even better.

—cofounder and inventor Nikolay Shkolnik

Elements of the High Efficiency Hybrid Cycle (HEHC) cycle include:

  • Air compression to a high ratio (>18), followed by fuel injection and compression ignition (Diesel).

  • Constant volume combustion (Otto. Fuel is injected into the combustion chamber and auto-ignites, also similar to the Diesel engine. However, instead of immediately expanding, the burning fuel and gas mixture is constrained so that combustion occurs under constant volume. The combustion is held under isochoric conditions, and the mixture burns for an extended duration of time before the expansion stroke begins.

  • Over-expansion (Atkinson). Combustion products are expanded to near-atmospheric pressure, which requires a larger expansion volume than compression volume.

  • Internal cooling via water injection (Rankine). A small amount of water may (optionally) be injected during combustion or expansion. Water may facilitate the cooling, lubricating, and sealing of combustion chamber and pistons. Water used for cooling from within the engine turns to steam, which in turn increases the cylinder pressure, allowing for some of the cooling losses to be recuperated.

The combination of high compression ratio, true constant volume combustion, expansion into a larger volume than intake, and (optionally) water turning to high pressure steam cumulatively add to the efficiency of the engine. An air-standard analysis predicts thermodynamic efficiency of 75%.

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

  • High fuel efficiency of 57% at peak, and 50% at part-load;
  • High power density of 1 Hp / lb;
  • Simple design with low part count;
  • Multi-fuel capable: diesel, JP8, gasoline, natural gas, biofuels; and
  • Low noise: the engine has low pressure exhaust and no poppet valves.




There is plenty of room for this type of high power to weight ratio higher efficiency ICE as range extender for PHEVs and for higher efficiency ICE machine for the next 20 years.

Hope that is goes main stream before it is too late for ICE.


Based on the assumption that we have quite sufficiently polluted our atmosphere and environment, we don't need further pollutants no matter how efficiently they carry on with pollution.
The simplicity, efficiency, torque capability, functionality, power to weight ratio, sustainability and reliability of an e-motor will never be reached from any type of ICE.



What exactly does "sufficiently polluted" mean?

We are but a few years at most before we can have a National holiday rejoicing in having cleansed our Air & Water across the USA, and shown the way for the rest of the World to follow. We are getting ready to declare Victory in the battle over toxic Air Pollution.

When we began the War on Air Pollution officially on Earth Day in 1970, standards were set that were the targets for 'Clean Air'. We have already exceeded them and have 'Clean Air', every where in the USA including current holdouts Houston and Los Angeles.

But the targets were changed, and we are tougher now than then. It will take only a few more years to meet today's definition of Clean Air. You should realize that but apparently don't, just how far we have come, and how little we have to go, yet.

As regard your completely fallacious statement about ICEs, it is obvious that you are no Engineer, or even conversant with the current achievements in technology or standards levels of air pollution. Modern ICEs have been certified as PZEVs or AT-PZEVs, levels thought to be unobtainable by the iCE.

Those ratings were created to measure the 'non-existant' pollution from pure EVs, yet modern ICEs can attain that cleanliness level now and do. As modern cars replace aging ones in the American fleet the numbers of such pollution free cars are climbing rapidly.

California's CARB, no friend of the ICE, had to admit in its filing to require all vehicles to meet the Zero pollution levels of the EV in the future, that 25% of California's cars already do meet that Zero Pollution level of cleanliness. And the EV is hardly measureable as a percentage of the California automotive fleet.

So please don't bray about 'facts' that are not correct.



@ExDemo Please come breath the air in Salt Lake City during an inversion and then tell me we have clean air. Almost all of our pollution is from ICEs.


Great claims. Their web site refuses video/information to individual emails.


My computations show a maximum theoretical thermodynamic efficiency of about 70% at an 18:1 compression ratio, with a power output of 260 J per cycle per liter displacement (swept volume), or about 26 kW (35 hp) per liter at 6000 rpm, assuming internal temperatures are kept low enough that the engine does not produce high levels of NOx emissions. An actual brake efficiency of 55% at 38 kW (48 hp) per liter would probably be achievable in practice, which sounds close to the performance they are claiming.

Account Deleted

excellent, water should not be optional but compulsory. Imagine conventional gases like nitrogen and CO2 being tasked to absorb the combustion heat, the specific heat capacity is just too low. With heat being wasted instead of being absorbed by these gases, gas expansion is actually minimum. Water should be the one to function as both cooling and expansion agent.

With water inside the cylinder, I have no doubt to believe that 75% efficiency is not impossible. Keep up the good work.


Since full transition to e-vehicles may take 20 to 30 years, cleaner running, more efficient ICE will be required for another 2 or 3 decades.

Together with electrified vehicles, more efficient ICE will help to reduce fuel consumption and imports.

E-energy storage units may reach 400+ Wh/Kg by 2020 but that may not be enough for practical, affordable, long range highway BEVs.

A major breakthrough in e-storage units is required to increase performance 3x to 5x and reduce Kwh cost to less than 1/4 current level. That may come sometime between 2020 and 2030?

Henry Gibson

This inventor made a mistake and built a prototype, what about the years of computer modeling, Scuderi style, while bringing in investors. ..HG..

Henry Gibson

Recycle CO2 with nuclear energy, and super tax large home and large automobiles. ..HG..


An interesting way to bring a Rankine cycle engine to production.
I think the Rankine cycle could be brought to relatively conventional piston engines with modern computer injection strategies as well.

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