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LiquidPiston introduces latest X-series version of HEHC rotary engine; targeting 58% peak BTE, 2 hp/lb power density

Isuzu D201 35 hp diesel engine (left) and the LiquidPiston 40 hp X2 Prototype (right). Click to enlarge.

LiquidPiston, Inc. (LPI), the developer of engines based on its High Efficiency Hybrid Cycle (HEHC)(earlier post), introduced the latest version of its X-series of rotary engines embodying HEHC at the US Department of Energy’s (DOE) 2012 Directions in Engine-Efficiency and Emissions Research (DEER) Conference in Dearborn, Michigan.

The X2—which can be considered a beta version of the X1 alpha (referenced by Dr. Alexander Shkolnik, President and CEO, at the SAE High Efficiency IC Engine Symposium earlier this year, earlier post)—is a 40 hp (29 kW) multi-fuel capable rotary engine that requires no valves, cooling systems, radiators, mufflers, or other components. Expected realized brake efficiency for the X2 is 58% (peak) and 50% at partial load; power density is targeted to be around 2 hp/lb.

Other features of the X2 include:

  • Quiet operation: engine lacks poppet valves and over-expansion minimizes exhaust noise.
  • Power modulation by cycle skipping increases efficiency and recaptures heat.
  • Low heat signature from over-expansion and cycle skipping eliminates water cooling.

By combining HEHC with a rotary engine architecture, LiquidPiston has created an engine that is 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, Shkolnik had noted during his presentation at the SAE symposium.)

HEHC is an improved thermodynamic cycle optimized for fuel efficiency that combines features of four existing cycles:

  • High air compression ratio (Diesel).
  • Constant volume combustion (Otto)
  • Over-expansion to atmospheric pressure (Atkinson)
  • Internal cooling with air or water (Rankine)

HEHC thermodynamic advantages relative to the diesel and the Otto cycles. On these PV diagrams, the LPI gains on the upper portion of the curve correspond to constant volume combustion; the gains on the lower portion correspond to overexpansion. Efficiency is roughly proportional to the area within the curve. Click to enlarge.

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 an ideal thermodynamic efficiency of 74% at an 18:1 compression ratio.

Liquid Piston is married to the High Efficiency Hybrid Cycle—e.g. to the thermodynamics, not to a particular engine. Our engine architecture has evolved substantially over time, since 2007. We started with a “liquid piston”, moved to a spit-cycle rotary (compressor rotor with gate + combustion chamber + expander rotor with gate), collapsed the split-cycle rotary into a single deck with two gates (the M line), and finally we got rid of the gates and ancillary systems, leaving us with just a rotor and eccentric shaft (X line)—yet all of these engine designs executed our HEHC cycle.

We have explored and patented dozens of engine architectures capable of implementing the HEHC cycle. It took a lot of learning and refinement to figure out how to make such a simple engine as the X engine, which is 1) sealable, and 2) implements the HEHC cycle.

—Alexander Shkolnik

The X2, which is still a research-grade prototype, is very similar in architecture to X1, Shkolnik said, but is generally a simpler engine to manufacture and to assemble. It has a different eccentricity ratio for the rotor; the X2 rotor has a convex profile (the X1 rotor looks more like a figure-8), which helps with sealing; and the X2 engine is smaller (40 HP, instead of 70 hp for the X1). LPI has moved towards a wider rotor (axially), with shorter rotor radius, for sealing purposes.

Click to enlarge.

LPI has placed the “M” engine line on hold, for now, to focus on developing the X architecture.

The X1 engine—brought from a concept on paper to an engine on the dyno in less than one year—is firing reliably and consistently in the lab on synthetic diesel fuel with compression ignition, and producing positive power, Shkolnik said. Using a model calibrated to the data from the engine, LPI has shown that if it achieve sealing, heat transfer coefficients, and friction similar to today’s Mazda Wankel engine, it can achieve its efficiency goals.

LPI will continue to use the X1 for development work, while evaluating the performance differences after the X2 arrives. The company expects to transition to the X2 engine for development work probably around the end of the year. The X2 engine is less sensitive to prototyping/machining errors, and is very fast to assemble and disassemble, so ultimately it will improve our development time, Shkolnik said. It will also serve as a better platform for strategic partners in testing and development.

LPI has also designed in the ancillaries—including the fuel pump, oil pump, oil filter, starter, etc,—thereby bringing the X2 closer to a standalone engine capable of running outside the lab.

The X2 engine is planned to be available as a beta prototype for outside testing by the first quarter of 2013.



Looks like LPI has solved combustion. We can all go home and give up.

There is NO way they will get to 40% brake with this thing. Id love to see their "model"...


What an excellent mechanical experiment; I really enjoyed reading about the device. Too bad this comes way too late to cause any excitement in the automotive world...and about two decades behind the move to EVs.


Perfect as a range extender - run it on a biofuel preferable something made from waste, add about 24KWHs of some advance battery like Envia's and we knock out about 99% of our individual transport problems.


Now, if we can just get the thing to seal in the combustion...

Rotary engines ended up in a particular configuration for a very good reason. It works. There have been no other practical alternatives. The age old "gear engine" would have produced similar efficiency as quoted above, if we could only get the thing to seal...


Good points LAD.


It is not too late.

It is too little.

It is a research-grade prototype that competes with the Scuderi - for lack of progress.


It is really what we need for automobile electrification. GM have been considering Wankel for Chevy Volt as range extender. My prediction is that Ford and Mazda will be the first implementing rotary type range extender if any.


58% brake efficiency sounds somewhat (?) overoptimistic to me. One should also realize that the diesel cycle is more efficient at a given maximum cylinder pressure than the otto cycle. The geometric compression ratio for the diesel cycle has to be somewhat higher than the 18:1 CR mentioned for this engine to get equal efficiency as the otto cycle. Here is when we start to get outside conventional wisdom, since there is an optimum CR for highest efficiency and we know pretty well in what range that is for diesel engines. Higher CR increases a couple of losses that higher thermodynamic cycle efficiency cannot compensate for. If we, on the other hand, envision an otto cycle running at CR or 18:1, this is not possible with gasoline as fuel due to the “low” octane number of this fuel. However, using direct injection of methanol can give an “equivalent” octane number in the range of 150-200, as shown by MIT, so CR in the range of ~18:1 (or preferably, optimum CR if it would turn out to be slightly higher) could be used. This gives us an efficiency on pair with conventional diesel engines and maybe even marginally higher. However, we are still speaking of efficiency in the range of ~43% for engines of this size, or maybe a few per cent more if we are really optimistic. Overexpansion is a very nice feature but this can also be achieved to some extent in piston engines and it does not boost efficiency to those numbers. Even if the assumed “optimum” thermodynamic features and overexpansion are combined, I still find 58% brake efficiency very, very optimistic. I will leave friction, gas exchange and other losses for later discussion.


LPI has shown that if it achieve(s) sealing, heat transfer coefficients, and friction similar to today’s Mazda Wankel engine, it can achieve its efficiency goals.


If you are familiar with the analogies I hinted about, I just (indirectly) showed that they cannot achieve the efficiency goals. Do you want to bet? With the limitations regarding thermal efficiency I hinted about, the mechanical efficiency would have to be much higher than 100%. Do you think it could? Rotary engines do not have any inherent advantages regarding gas exchange and heat transfer compared to conventional piston engines. Instead, they have disadvantages in those areas.

I generally have an open mind about testing new ideas so, in essence, I am also positive about that this alternative engine is investigated. However, I do not like when inventors make unrealistic claims (or targets). A target of 50% break efficiency should be more realistic in this case, although even this level would be difficult to achieve.


I was just quoting the article.


How is this a new concept and how is it not a Wankel?. This is exactly one of the ideas Felix Wankel proposed,it was actually built and tested;The idea of a rotating outer ring was dumped after tests showed excessive heat warp of the outer ring. I guess these guys are just going through old patents and making prototypes to see what could/would work.Maybe that's what you do when you already get people to invest in a non existing concept.Correct me if I'm wrong, but I think this is the third different engine concept they already came out with,and non happen to be original to this company, just making prototype of other people's old ideas.

Account Deleted

99.9% sure this is another bogus story from a small company that is desperate to raise money like Envia. As said “Hollow drums make the most noise”. So forget about it.


I agree! This engine has much in common with the Wankel engine and potentially also share many of the problems. Felix Wankel’s original design made the engine fully balanced, which was a very nice feature. However, it turned out to be too complicated, so this design was abandoned by NSU (and others) in favor of the “wobbling” piston in all current designs.

I recall that Rolls Royce (division of diesel engines, not to be confused by the luxury cars) was working on a diesel Wankel engine. This was a two-stage design regarding compression and expansion, where a larger disk was used for the low-pressure stage and a smaller disk for the high-pressure stage. Thus, higher compression and expansion ratios than with a single-stage design could be achieved, which is essential for a diesel application. Connection between low and high-pressure stages was made via transfer ports. Besides the lack of the claimed overexpansion in the LPI engine, the RR design should have provided almost similar thermodynamic advantages as the LPI engine. Needless to say, however, the Rolls Royce concept did fail.

Do you or do you not agree with the quote? If you do not, why did you quote the article in the first place?


The only significant advantage over the RR design would be that compression and expansion ratios are not the same in the LPI case. The RR design could increase the total CR to higher levels than what would be practical with a single-stage Wankel engine. However, compression and expansion ratio would be similar, which is not the case for the LPI engine that utilize the Atkinson cycle. However, the LPI engine also has a significant drawback in this case, i.e. that the expansion ratio cannot be made variable in a simple way. Thus, a compromise has to be made, just as in, e.g. the Toyota Prius engine, which does not have fully-flexible valve timing (the BMW Valvetronic system could overcome this problem). For example, if LPI choose an optimum expansion ratio for full load, it would expand to sub atmospheric pressure, which would decrease efficiency. At low load, the optimum would be almost similar compression and expansion ratio, so we would end up with the RR design, if you like.


"LPI has shown that if it achieve(s) sealing, heat transfer coefficients, and friction similar to today’s Mazda Wankel engine, it can achieve its efficiency goals."

I have no basis to agree nor disagree, this is their statement. It says that if they achieve the Wankel performance then they can meet their efficiency goals.

I an not trying to pick a fight, I am trying to clarify their position. This is one of the reasons I do not comment here, the usual suspects make personal attacks.


Well, their statements are wrong and they have no experimental evidence to back it up, nor will they ever.
It is impossible to have an engine do what they say it will. It is insulting to those of us that do real engine research to claim that they are going to build the worlds most efficient engine based on faulty thermodynamics and an engine/combustion system design that is inherently less efficient than current slider crank engines.

But, I wish them luck but I find no compelling evidence that they will succeed.


I challenge the claim that this engine would approach ideal isochoric combustion. Of course, the combustion is supposed to take place in a confined space that should not change during the duration of the combustion. However, no expansion – or work – takes place in a confined space; i.e. one must also consider admission of the gas to the chamber where expansion is made. Admission to the expansion chamber is not instant and during this phase, the volume of the expansion chamber does indeed change, making the process non-ideal. In addition, the expansion chamber has a certain dead volume to start with, which must be considered. This is more or less equal to the heat release in an otto engine – or stem admission in a steam piston expander for that matter – where combustion or steam admission cannot be instant. Thus, from a thermodynamic point of view, the LPI engine cannot provide an ideal isochoric process. Geometry decide how fast the process can be but I would assume similar conditions as in an otto engine, i.e. that this process last for about 40 crank angle degrees, or – in the best of cases – is only slightly faster. It would be very interesting to see cylinder pressure traces from the expander, since this would show the “true” behavior of the engine. I would also like to see real experimental data on brake efficiency for this engine.

Why do you keep repeating their statements if you do not agree with them.

Kevin typepad

Audi is already playing with a rotary range extender.



Note that the Audi rotary has similar problems as all other rotary engines: notoriously bad efficiency.

A Wankel engine is the perfect compromise between all the drawbacks of conventional 2 and 4 stroke piston engines.

Roger Pham

I share the same concerns regarding the over-optimistic efficiency claim.
Purely isochoric combustion at CR of 18 will cause too high peak pressure and peak temperature. Losses due to heat transfer and leakage via the seals will negate most if not all of the gain via isochoric combuation.

Complete expansion of exhaust gases at high load (Over-expansion) will cause efficiency loss at part load or low load which is the cruise regime of most auto engines, as Peter XX mentioned, since this engine has no valves that their timing can be precisely controlled to avoid expansion to below atmospheric pressure.
For truck engines, turbo-compounding can be used to harness the residual exhaust pressure to achieve over-expansion without impacting the power density of the engine.

However, cycle-skipping in a rotary engine that has a separate compression chamber and a separate expansion chamber is an interesting way to reduce heat-transfer loss by recycling the heat transferred to the engine wall into heating the compressed air in the non-fueled, non-combustion cycle...or so they thought! But it ain't happening at CR of 18 in a single stage, adiabatically, since the temp of the compressed air will exceed the equivalence of cylinder head temperature. Aluminum engine can't have too high a cylinder head temperature.

What about Rankine recovery cycle akin to the Crower Six-stroke cycle? Won't pass thermodynamic muster, either...too inefficient, because water takes a lot of energy just to vaporize. More importantly, though, water and oil just don't mix...If the water mixes with the oil in the engine seal when some of it will condense on the expansion stroke, it will turn into mayonaise and messes up the engine. May be if only a little bit of water will be injected, perhaps on the compression stroke to help cool the compressed charge, it might work, but the efficiency won't go nowhere as high as what they're claiming. May be on order of 10% gain in efficiency the most, probably from 36-40% BTE to about 39-44% max.


Nice analysis! I have been thinking about the same problems, too, so I will add some comments.

Separation of compression and expansion might be good for volumetric efficiency, which appears to be a problem for conventional Wankel engines but the gas transfer between the chambers is an “unknown” area, so I would not expect any particular advantage over a piston engine after all regarding volumetric efficiency. As you indicated, heat will be a problem – I would say for any material, not only aluminum. One can read in the text (and between the lines) that they are aware of this problem. There are examples from piston engines (actually already Rudolf Diesel tried this), where water has been injected to cool the cylinder. If the amount of water is limited and the droplet size is small enough, they might be able to avoid condensation. If enough water could be used to manage the heat problem, I do not know. I suppose that they would avoid the oil sump, so this could help to avoid the problems of water-oil mixing experienced in 4-stroke engines. Careful control of water injection during cold start would have to be managed to avoid condensation directly on cold surfaces. The best solution would of course be to use water lubrication. This has been proposed for Rankine piston expanders but to my knowledge, nobody has been able solve the problems associated with tribology so far. If they would solve this problem then someone might immediately use the technology for a Rankine bottoming cycle expander on a conventional piston engine.

Regarding efficiency, it should be noted that water is “worse” than exhaust gases (and air) regarding the exponent “k” (according to their nomenclature), which would reduce efficiency. Furthermore, the heat of vaporization is high, so this loss would have to be compensated by reduced heat loss. I doubt that this could be accomplished, so (neglecting the gain from hyperexpansion to compare apples with apples) the result could even be reduced efficiency compared to a conventional engine.

I could agree that the potential for efficiency could be on pair with conventional automotive diesel engines, i.e. ~43%. With optimistic assumptions, why not aim at 50% as a future goal. However, the “moving targets”, i.e. conventional engines, also aim at this level.

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