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LiquidPiston unveils 70cc rotary gasoline engine prototype embodying HEHC; power dense, low-vibration

LiquidPiston X1 engine features three separate combustion chambers creating 3 power strokes per revolution for high power density. Source: LPI. Click to enlarge.

LiquidPiston, Inc. (LPI) the developer of engines based on its High Efficiency Hybrid Cycle (HEHC) (earlier post), has unveiled the alpha prototype of the X Mini—a power-dense, low-vibration, quiet, 70cc spark-ignited, non-Wankel rotary embodiment of the HEHC. Dr. Alexander Shkolnik, President and Co-Founder of LiquidPiston, is presenting the engine in a paper at the SAE International/JSAE 2014 Small Engine TechnologyConference today in Pisa, Italy.

The compact engine with a 4-lb (1.8 kg) core has only two primary moving parts—a rotor (the primary work-producing component) and an eccentric shaft—and fits in a 6.6" x 6.2" x 5.4" box. In prototype testing, the spark-ignited X Mini engine has shown high power density, producing 3.5 horsepower (indicated at 10,000 RPM). When mature, the engine is expected to weigh 3 pounds, produce more than 5 hp at up to 15,000 RPM, and be more than 30% smaller and lighter than comparable four-stroke piston engines.

X Mini_Two Angles
The X Mini 70cc rotary gasoline engine. Click to enlarge.

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).

The cycle has a theoretical efficiency of 75% using air-standard assumptions and first-law analysis. The rotary engine architecture shows a potential indicated efficiency of 60% and brake efficiency of >50%. As the engine does not have poppet valves and the gas is fully expanded before the exhaust stroke starts, the engine also has potential to be quiet. The cycle elements include:

  • Compression: For maximum efficiency, air is compressed to a high compression ratio, fuel is injected and compression ignited (CI-HEHC). The X Mini utilizes a spark-ignition (SI-HEHC) version of the cycle with a lower compression ratio standard for gasoline engines—and hence somewhat lower efficiency than the CI implementations, Shkolnik said.

  • A dwell near top-dead-center forces combustion to occur at nearly constant-volume conditions.

  • Combustion products are over-expanded using a larger expansion volume than compression volume, as in the Atkinson Cycle. (This is done by changing the locations of intake and exhaust ports asymmetrically which allows for the extraction of more energy during the expansion stroke.)

  • Cycle-skipping power modulation allows high efficiencies at low power settings while simultaneously cooling the engine’s walls internally and providing partial heat recovery.

  • Water may be injected to internally cool the engine. Some of this cooling energy is recuperated, as the water turns to steam, increasing the chamber pressure.

By combining HEHC with a rotary engine architecture, LiquidPiston is creating engines up to ten times lighter, quiet, and two to three times more efficient at part-load than conventional engines. LPI selected a rotary architecture because it offers more flexibility in optimizing each part of the cycle.

LiquidPiston is emphatic that its rotary engines are not Wankels; the X engine has a fundamentally different thermodynamic cycle, architecture and operation. The Wankel is characterized by a low compression ratio, no constant-volume combustion and no over-expansion. By contrast, the LPI X engine is characterized by high compression ratios, constant-volume combustion and over-expansion, the company says.

LiquidPiston earlier introduced the larger X1 (rotary) compression-ignition prototype (1370 cc, 70 hp). With the new X Mini, said Dr. Shkolnik:

What we’ve done is taken everything we’ve learned from the larger engine and, especially due to customer interest, focused on the very small engines. This one is spark-ignited, not compression ignition. It has a lower compression ratio, but it still has the constant-volume combustion. We don’t get the same efficiency as with the true compression ignition version, but we still get a significant efficiency improvement over gasoline engines.

—Alexander Shkolnik

In the paper being presented in Pisa, Shkolnik and his team explain that while the reduction in compression ratio (from 18:1 for the X1 to 9:1 for the X Mini) causes a reduction in efficiency compared to CI, the dwell in combustion volume near TDC results in higher peak pressure and efficiency than piston-engines operating with SI. This, the LPI team says, is related to the slower variation of displacement in proximity to TDC than piston engines.

Overexpansion further increases efficiency, similar to Atkinson cycle. The dwell in volume at TDC allows the engine to more closely achieve true constant-volume combustion (isochoric head addition), compared to a piston implementation of the Otto cycle.

Also in the Pisa paper, LPI notes that while the X1 has demonstrated 33% indicated efficiency at medium load at 1800rpm, diesel fueled, the early X Mini prototype is capable, at this stage of its development, of providing 10% indicated efficiency.

Those initial results indicate that the target HEHC efficiency of 60% is not yet achieved, but they support the feasibility of development of this engine architecture and the potential for rapid improvement. Future work and publications will focus on demonstrating efficiency and power density benefits, including running the engines at full load and in continuous (steady-state) operation over a wide range of engine speeds.

—Shkolnik et al.

The X Mini is meant to be a low-cost engine, really targeted especially towards the outdoor power equipment market, and especially in the hand held aren, Shkolnik said.

X Mini vs Honda Metropolitan 49cc Engine
49cc Honda Metropolitan moped engine (left) and 70cc LPI X Mini (right). Click to enlarge.

We’ve studied 60 rotary engine embodiments and patented dozens of rotary and pistons engines. This [X engine rotary] is by far the simplest strategy that there is. We really converged on this design and demonstrated that it’s working...so it looks like this is the one for us.

What we would like to do is to get it into production as quickly as possible. That’s why we’re speaking with so many customers behind the scenes. We really designed based on what we’ve heard, and we’re doing it in markets that don’t take a decade to get into production. After we get the X Mini into the market, we can go back to higher efficiency diesel engines.

—Alexander Shkolnik

The X Mini will enable many small engine applications to be smaller, lighter, and quieter, including handheld power equipment, lawn and garden equipment, portable generators, mopeds, unmanned aerial vehicles, robotics, range extenders for electric vehicles, and auxiliary power units for boats, aviation and other vehicles.

In addition to improving existing engine applications, the X Mini may enable entirely new applications not possible with current engine technology, LPI suggests. In early 2015, LiquidPiston will host an open call for ideas regarding these new applications. The company will award a cash prize for the most innovative submission.


  • Shkolnik, A., Littera, D., Nickerson, M., Shkolnik, N. et al. (2014) “Development of a Small Rotary SI/CI Combustion Engine,” SAE Technical Paper 2014-32-0104 doi: 10.4271/2014-32-0104



'The cycle has a theoretical efficiency of 75% using air-standard assumptions and first-law analysis. The rotary engine architecture shows a potential indicated efficiency of 60% and brake efficiency of >50%'

Would those who are far more educated in these matters than I comment on how this relates to the limits for combustion engine efficiency?

I had thought that that stood at around 45%


This can be the answer to the commuter battery operated car. While away at work start the generator and let it charge the car so you can get home. The weight being a key factor in making it usable. So you have a 70 miles per charge car, you can drive 50 miles to work, (not unusual in Chicago) park charge while at work and make it home.
Just wondering if it has to be air cooled?
It can also run while you drive, making it a range extender.


Range extender engines can have different characteristics from conventional gear/drive train types. Torque and horsepower that can do the job of driving the alternator under load is required.

I prefer reforming liquid fuels then using fuel cells as a range extender, but we are not there on cost yet. With fuel cells, there is no combustion, the CO2 from oxidation can be neutral if we use bio fuels. Cellulose E100 would be one candidate.



This is a heat engine, its efficiency must limited by the efficiency of the Carnot engine. No engine operating between two heat reservoirs can be more efficient than a Carnot engine operating between those same reservoirs. A practical efficiency limit of around 45% is about right.



That is what I thought.
In that case, is this just nonsense?

'The cycle has a theoretical efficiency of 75%'

If that is the case, it hardly seems worth looking at at all, unless they are talking about 75% of something totally different.


It is pretty dodgy remote controlling a combustion engine, which will quickly fill up a garage if that is where it is with lethal fumes.

There is no problem with fuel cells, as they don't combust the fuel and there are no harmful emissions.


Can you imaging the pollution from 1000s of these running for hours in parking lots?

We have a $200 penalty for leaving an ICE running in a vehicle for more than 6 minutes.



Theoretical efficiency is the efficiency you would get taking the ideal compression, ideal combustion, ideal expansion without any losses. Indicated efficiency is the efficiency that you would get taking the actual engine pressures so it would include compression, combustion, and expansion losses but would not include mechanical losses. What really matters is brake efficiency as this is the power measured with a dynamometer and compared with the theoretical power of the fuel consumed. However it is good to know something about theoretical efficiency and the indicated efficiency as that gives you some idea where your losses are occurring and what to strive for. You will not do better than Carnot efficiency but you can certainly get more than 45% as modern uniflow 2-stroke ship diesels get better than 50%.

One of the potential problems with the proposed engine is that they say nothing about pollution controls. If they have high pressure, high temperature combustion which leads to higher efficiency, it also leads to high NOX generation. However, it may still be better than the 2-stroke SI engines running a mixture of gas and oil used for leaf blowers, etc. Those should be banned.


Many thanks.
It is often helpful to keep a domesticated engineer to hand, providing they use the tradesman's entrance, of course! ;-)


Would its light weight, low noise, low vibration, compatness, higher efficiency et make it a very good candidate as a range extender?

A 20 Kw unit could be enough in most cases.


I'm a bit skeptical, but yet hopeful. Once a 20kw unit is produced, it can be a gm volt style range extender that charges the batteries while the vehicle is running. This will decrease by about half the kWh of battery storage required in a vehicle, and answer the range anxiety problem.


An FCEV can run the reformer\PEM in parking lots with no problem.

Nick Lyons

...while the X1 has demonstrated 33% indicated efficiency at medium load at 1800rpm, diesel fueled, the early X Mini prototype is capable, at this stage of its development, of providing 10% indicated efficiency.

Those initial results indicate that the target HEHC efficiency of 60% is not yet achieved...

No kidding.


This is starting to remind me of a certain alleged professor in Davis calfornia who has been touting the benefits of rotary engines, but 30 years later does not have one that a person can buy. His annual appeal for money usually comes in the form of "just another million dollars and we can build one."


Many here do not park in enclosed lots. We have a LOT of out door parking. So fumes were not my first thought but I understand the concern.

Problem with fuel cells is $$$ the tech is still way way out of reach. And what do you fuel them with compressed hydrogen? I wish I wasnt saying this, so far nothing is a good cheap and easily available as petrol.
I had hoped by now after dropping billions into research for years this was not the case.


Surely if you have the range to drive to work in an EV, you could arrange to charge it there from a power socket.
8 hours at 3kw would charge most small Evs (Tesla excluded).

The point of a range extender is that it can provide enough power to drive the car at 60-70 mph.

Roger Pham

Compact, yes.
Eficient? No.
Earlier 1370-cc, 70 hp Diesel version demonstrated 33% indicated efficiency, which may be translated to around 28-30% brake efficiency. The mini 70-cc spark-ignited version demonstrated only 10% indicated efficiency, or roughly 8-9% brake efficiency.

So, this would be great as a range extender engine for PHEV, but not for daily use.


Can I just say, as long as this engine burns fossil fuels, it will likely be subject to emission laws.

Meaning, Catalytic Converter, and thusly Lean-Rich cycling, with a rich warm up time. Without emission restrictions I am sure it would make a great alternative to fuel cells or small 4-stroke engines. But, to have all of that equipment on there would likely drain the technology of its potential.

Please know that (most)fossil fuel cars are large ecosystems of various systems, if you put a gasoline engine in anything you are going to need a fuel tank, pump and all of the supporting systems. Emission restrictions also apply to the fuel system.

I personally don't see this technology succeeding in the mainstream markets. Unless they ran on LNG or some other clean burning fuel.

I believe in BEVs for most simple commuter cars(ie. commutes under 150-100miles round trip)(hopefully most get to 300 mile round trip (in all circumstances, including a harsh winter) while being on a cost parity with Gasoline vehicles... I don't expect this but I like to think I'm hopeful).

I believe farther out in the future, H2 will supplant ICEs. BEVs would be insufficient in long haul trucks, or most of industry. They will also account for a large number of SUVs/Trucks in the passenger market

My justification in a mix solution is that we need an alternative to BEVs... if a long haul trucker had a BEV that could haul for 8hours and had to charge inside of 6-8 hours we'd probably melt power lines when more than a few have to charge. The trucking industry has to stay, we are way too reliant on it.(even with battery swapping, we still would need to charge at a certain rate per hour because of overall demand.)

Also infrastructure isn't there for a 100% BEV fleet. A mixed solution could mitigate most of this.


An impressive bit of engineering when compared to a conventional ICE. But compare it with this
and it becomes too complicated and expensive in production.
This solution, as achieved from DLR, is far more simple, cheaper to manufacture and less prone to mechanical failure.
The real problem, as I see it, is the involvement of Mercedes in this achievement. The unacceptable attitude of Mercedes is:
"Noli turbare circulos meos." (do not disturb my circles) to quote Archimedes.
To see this implemented in a mass produced BEV as a REX will more than likely take decades.


I think they are on the right path targeting this at scooters and mopeds. You can't get much more mainstream than that for most inhabitants of this planet.

The big question will be durability, especially as regards the apex seals. You either need to make them last the full life of the bike (at least a decade), or you need to sell them for pennies and make them easy to change with basic tools.

Roger Pham

>>>>"This solution, [free-piston engine] as achieved from DLR, is far more simple, cheaper to manufacture and less prone to mechanical failure."

Well, adding a crank shaft and crank case at a cost of perhaps $10 per hp in order to halve the size of electric motor and controller at $65 per hp would be a better solution to make PHEV more affordable, by shaving ~$5,000 off of the listed price. The high cost of PHEV is among factors that are holding back market penetration of PHEV's. The engine in a PHEV is used only 20% of the time, so the engine will never need service nor maintenance beside oil and filter change every so many years.

Mercedes-Benz's CEO states to the effect that EV's cost too much to make and they can't make money selling them. Anything to reduce cost would help!

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