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Concept: A Rotary Engine Based on a New Thermodynamic Cycle

A sketch of the LiquidPiston Engine.

A father and son team—Dr. Nikolay Shkolnik, an entrepreneur and inventor, and his son Alexander, a PhD student at MIT—have developed an engine architecture they claim will achieve 50% fuel efficiency (compared to the ~30% of existing engines) and drastically reduce pollutant emissions.

The architecture, based on a patent-pending “High-Efficiency Hybrid Cycle” (HEHC) thermodynamic cycle, borrows elements from Otto, Diesel, Atkinson and Rankin cycles. LiquidPiston, Shkolnik’s company, is implementing the HEHC cycle in a rotary piston engine: the LiquidPiston Engine.

(In April, LiquidPiston was named one of the four finalists in the ECOnomics Environmental Business Plan Challenge presented by GE & Dow Jones. The ECOnomics winner will be announced this month and receive a $50,000 prize.)

The HEHC Cycle. The basic cycle uses a discrete compression chamber, isolated combustion chamber, and expander chamber.

Air (with no fuel) is compressed to a high ratio (> 18) in a compressor cylinder of the engine. The resulting compressed charge is directed into an isolated combustion chamber, where fuel is injected and auto ignites.

Combustion occurs under truly isochoric conditions (volume stays constant) and is allowed to complete until all fuel is fully combusted. The combustion products then expand into the expander cylinder, which has large volume than the intake volume.

Optionally, a small amount of water may be used to facilitate cooling, lubricating and sealing of combustion chamber and pistons.

A small amount of water (an optional component) may be used in the system. Water may facilitate the cooling, lubricating, and sealing of combustion chamber and pistons.

The Liquid Piston engine. A) full housing. B) Transparent housing showing principle components. Click to enlarge.

The Liquid Piston Engine (LPE). The HEHC cycle can be implemented in a variety of ways; LiquidPiston is developing an implementation that uses a separate rotary compressor, two isolated combustion chambers, and a separate rotary expander.

Each Combustion Chamber (CC) rotates at constant speed, and simultaneously acts like two valves, one of which regulates flow between the compression chamber and the combustion cavity (through the Compressed Air Port), and the other valve which regulates flow between the combustion cavity and the expansion chamber.

The operation of the four cylinders is such that all four strokes occur simultaneously within the engine.

The compressor begins on the right side, and moves counterclockwise, with center of rotation around the lower bearing. This motion induces the compression stroke in the left compression cavity, and intake stroke in the right compression cavity.

This is followed by combustion in the left combustion chamber, which occurs in complete isolation from both the compression and expansion cavities. After combustion completes, combustion products meet the Expander Piston (EP).

The EP is in the left most position, and moves counterclockwise, with center of rotation around the upper bearing. Combustion products from the left combustion chamber drive the EP, which induces the power stroke in the left expansion cavity, and exhaust in the right expansion cavity.

After 60 degrees of rotation, the pistons stop their motion and switch their centers of rotation. The engine, which is symmetric in its operation, now undergoes another cycle of 4 strokes just as described above, except that all roles of the cylinders and combustion chambers reversed.

The result is an engine with the following projected characteristics (with respect to Otto or Diesel engines of similar power specifications):

  • Significantly improved engine efficiency, reaching 50%

  • Reduced size and weight by 50%

  • Reduced parts count by 85%

  • Reduced NOx emissions by 70%

  • Reduced CO2 emissions by 50%

  • Low friction design leading to long life

  • Decreased maintenance requirements: no oil or spark plug changes required,

  • Less noise, due to low pressure exhaust and absence of poppet valves

Although it may appear similar to a Wankel, the rotary LPE is based on a different thermodynamic cycle, and avoids the sealing and combustion issues affecting the Wankel design.

LiquidPiston is currently seeking seed funding from investors and government grant funds to build an alpha-prototype to establish the feasibility of the proposed High Efficiency Hybrid Cycle Engine.

(A hat-tip to Shaun Mann!)




There have been many of these types things that have come up over the years which haven't worked for one reason or another. It sounds good on paper, but right now I am still skeptical.

allen zheng

I want to see a working prototype, or at lest a demo video. This could be game-changing! Everybody from Military to lawnmowers could benefit.




Emissions and fuel economy numbers without even a prototype built never mind running?

Hmm, surface to volume ratio = heat losses. Combustion at cosntant volume? So all that heat loss to the enviroment is going on while no work is being done?? Ever heard of the inefficiency of a pre-chamber combustion? Seems like lots of theory and little practical cycle thought.

Shock loading on the synch teeth (the single starting teeth on the outer rotor

Age old problem of wankel rotor sealing.

etc etc and that's just in a 30s coffee break glance!!


Some people think diesel engines can be boosted up to about 60% efficient. Marine diesel already >50%, VW road going diesels 43%.


Does this look like a marine diesel?


My point was that 50% thermal efficiency isn't all that earth shattering (ie bringing some comparison to the "other engines are only 30% efficient" bit). But I agree it's good that it's small and lightweight.


Smaller engines are generally less efficient. This engine due to heat losses alone is likely at first inspection significantly less efficient than even a normal road car engine.

Jay Tee

Look at the motion of the power 'piston'.


In an actual engine, that part might last about 3 seconds before it shatters into pieces....... what a joke.

The Star Rotor people have a concept that is much more likely to work.....


Smaller engines are not typically less efficient, as long as we're talking about a technical rule here. Other things do come into play, and there is a turnaround point where engines start to lose efficiency as downsizing continues (i.e. lawnmowers and other small 4 stroke applications) but in general, the rule is that efficiency increased by a degree of two while volume increases by a degree of three. In other words, in order to square your efficiency value, you have to cube your volume value. Once again, that's just theory and subject to plenty of other stuff, but it "generally" holds true.

Rafael Seidl

The reason reciprocal ICEs have been so successful is that they are alternately exposed to cold and hot gases. As a result, peak temperatures of 2500-3000 deg C can be tolerated in the flame front even though the materials used are just inexpensive aluminium and/or steel. High peak temperatures massively boost the Carnot efficiency, even if a fair amount is lost due to mechanical friction etc.

There have periodically been attempts to address some of these losses by segregating the compression, combustion and expansion portions of the thermodynamic cycle. Gas turbines are another, and very successful at large scales (>> 1000kW) provided the load changes quite slowly. The snag is that in continous combustion, the component that delivers the net positive shaft power (i.e. the turbine), the material is exposed to extremely high temperatures all the time rather than intermittently.

The workaround is to divert part of the compressed air through the shaft and out through nozzles in the leading edge of the vanes, effectively creating shrouds of relatively cool gas around them. At the small scales required for motor vehicles, that is no longer possible so those are constrained by the maximum temperatures that the (ceramic) material can sustain, plus fluid dynamic and heat radiation constraints. Currently, the limit is perhaps 1100 deg C using materials that raise the cost of the engine to 5-10x that of a reciprocating design of similar power.

This liquid piston idea uses valves of some type to permit isochoric combustion (as in the Otto cycle) rather than isobaric (as in the Rankine cycle). This is supposed to improve the thermodynamic efficiency. If there is an intercooler after the compressor, that would allow the combustion chamber(s) to be built from steel despite peak temperatures near 2000 deg C arising from the auto ignition (the fuel would have to be kerosene or diesel, btw). The chamber(s) will need poppet valves to achieve the sealing required to support the pressure differentials of 80+ bar between the chamber and the compressor/expander. The real Achilles heel, however, is that the expander is exposed only to high temperature gas. Atkinson-style overexpansion will reduce this a little, but the material used for this component will still be the limiting factor overall. See above under gas turbine.

Personally, I would not invest my money in the further development of the liquid piston design. It might work, more or less, but the improvement over regular turbodiesels will not be significant enough to displace this well-established technology.

My advice to would-be inventors: please focus on (a) incremental improvements to reciprocating ICEs and their combustion processes, (b) systems integration with boost and recuperation technologies and (c) affordable synthetic fuels with a viable migration path to biogenic feedstocks.


I agree with most of the above criticisms. If the expansion chamber is a constant volume, how are they getting any power from combustion? How much compression will they loose from the compression chamber to the combustion chamber? etc.

Moving the compressed gas into the combustion chamber will be a great efficiency loss, also, this is basically a wankle, but seperating the stages of combustion

Summary opinion: it looks like it will have all of the inefficiencies of a wankel blended with a 2 stroke, but with lss reliability than either.

However, I could be wrong :)

Barry R. Guthrie

The engine will also have unstable thermal expansion problems. All the connected gears, spinning chambers and cams need to be all precisely positioned like a Swiss watch. The clearances all these chambers need operate in very close tolerances to maintain the >18 compression ratio. I’m not sure this would be possible with the various thermal deformations that would be occurring. There is also too much large part mass movements that look unbalanced and will generate harsh momentum stresses and vibrations. You will also get a great deal of friction and wear on the large drive gear teeth.

Roger Pham

Agree with all above postings. The rotating cylindrical combustion chamber is pure fantasy. With all the heat and super high pressure involved, it will melt and stuck after a few engine rpm. So, make it out of ceramic, hence hi heat tolerance without need for lubrication? Still you will have high leak with super hot gases out of the minute clearance gap between the rotating combustion chamber and the engine casing that will melt and make a mess out of the rest of the engine external parts.
Forget about this pure-fantasy engine. Just use a piston engine running simulated Atkinson cycle, running on hydrogen, or hydrocarbon mixed with enough hydrogen to speed up combustion process so rapidly that combustion is already done before the piston has travel any significant distance downward. This will approximate the theoretical efficiency of this "constant-volume" extended-time combustion feature without all the complex and pure-fantasy hardware.


Could it be that the little bit of water injected into the system is added steam power? I think the comment about lubricating and sealing the engine is a bit of a smoke screen to distract people from where the gains are really coming from...



Smaller engines typically ARE less efficient. If we look at general data for a range of piston engines and we look at the best specific fuel consumption point then the best BSFC will tend to rise as capacity falls.

This is to do with heat losses through increasingly unfavourable surface to volume ratios as capacity falls so heat losses from the cycle increase. In addition, friction losses, a great deal of which also do not scale well, mean that for a 4 litre engine a best BSFC of 240g/kwh may be possible, by the time we get down to a 1l to 1.2l engine this best possible pointon the operating map has risen to greater than 280g/kwh.

A not insignifcant 16% increase.

The thing about engines is that you can talk theory until you're blue in the face: results are what matter. I see none here and the probs I highlighted above mean I doubt that we ever will.


The ASME paper page 8 is showing the sequnce of motion (pictures a to f). I would like to see the position between b and c e.g. on the compressor side where, I think, there is no positive force connection between the rollers and the cam. In other words, the force of the cam will not turn the rollers and pistons for a brief moment, thereafer, the roller next to the hub (shown on the bottom in figure c) will take all the load at a very short radius of action, creating very high loads between roller and cam. The contact between the two is by a line which further increases contact pressure and likely causes early wear.
The same really happens at the power or expander side.
Perhaps I see a problem where none exists? Too me, it is the mechanics that are not very 'sound'.
The transfer of working fluids between different chambers or compartments also causes various difficulties.
Am I too pessimistic? Let's see a working model please.

Jerry Wheeler

Uhhh... yea, right. What needs to be addressed is conservation of energy, I.E. heat from combustion. Why don't you get to the real problem of reusing the excess heat (heat exchanger, etc.) and applying it back into the process by a closed refrigeration type system with a turbine that "extracts" all the usuable heat differential between the combustion and ambient temperature. Using heat to expand air is stone-age tech. Think of the latest house furnaces that have PVC exhaust piping - all of the energy of combustion is being utilized. Take two and call me in the morning. Jerry



Extraction of energy from low temperatur heat is not that easy, I guess.
You mentioned AC units, fine, they just use more heat exchangers I think, heating up more air thereby cooling the exhaust.

However, in an engine application we need mechanical power not warm air and this is much more difficult to
do. So far there are only two ways I know of. One is a turbo driven off the exhaust gases, the other is the
approach by BMW to turn water into steam using the heat available and running the steam through an expander
that converts the energy into a turning shaft that can be coupled to the engine output shaft for added power.

Do you know of any other way?

Carlos Barrera

I inform aboute the new "Gearturbine" power by barr for; air, sea, land & generation aplication. With dextrogiro vs levogiro effect, an non parasitic looses sytem and over-unit engine. To see details please visit:

rob brown

who finished 1st 2nd & third? Ahead of Dr Shkolnik"s Liquid Piston ENGINE


the jury is still out.

It depends on the volume the public is going to buy when the engines are on the market.


There seems to be an infringement issue. There was a German, US & Intl 2001 patent (offered for $1.8M on ebay!! #7630926018) incorporating a "5 phase" combustion cycle, replete with a rotating combustion chamber, which appears to be substatially similar to Shkolik's but also seems to be prior art.
Also, it better addresses the cooling issues mentioned by Seidl above


Nobody has mentioned anything about the fact that there is no way to lubricate the necessary seals that are needed on every volumetric device. Constant high temperatures in the expansion side eliminate any chance for this to function.

Brian G

I dont see how they really expect to not need any lubrication? It states that it will not require oil? A question was raised about the intent of the water injection; they are planning to use it as steam generation to boost expansion cylinder pressures and to aid in cooling. This is being used because they have allready ignited the fuel in a closed rotating cylinder, next, that cylinder dumps the pure presure into the expander along with some water sprayed in. Again we have some great theory being blindly applied. Hey, how hard do you guys think it is to get these army grants? I have seen a few wacky engine concepts actually getting grants for further research.. Makes me want to apply and get my concept engine going.

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