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Pivotal Piston Two-Stroke Engine Eyeing Hydrogen ICE Market for Autos

Cross-section of Pivotal’s two-stroke engine. Click to enlarge.

A New Zealand company has redesigned the two-stroke combustion engine with the intention not only of overcoming the shortcomings of the conventional sliding-piston two-stroke engines, but also to produce a combustion engine that can better exploit hydrogen as a fuel.

The Pivotal engine replaces the conventional sliding piston with a pivoting piston, the pivot point of which is at the back of the piston. (See cutaway diagram to right.) The piston pin position connects to the crankshaft via a connecting rod. The side surface of the piston forms the inner wall of the combustion chamber when the unit pivots up in the compression cycle.

The result, according to the company, is the elimination of piston rock or slap, improved compression sealing and reduced lubrication requirements.

Cutaway of a water-cooled pivotal piston. Click to enlarge.

The engine is water-cooled via the center of the pivot shaft. This design provides an independent means of controlling piston temperature without the need to over-cool the engine.

With its high level of thermal control of the water-cooled piston and thermally smooth head surface, the Pivotal two-stroke engine is suited to running on a wide range of fuels including diesel, CNG, LPG and JP5—and, according to Pivotal, hydrogen.


  • Offers higher power density by weight than gasoline but lower power density by volume;

  • Is fast-burning, with a high combustion temperature and low ignition temperature;

  • Supports a broad range of air/fuel ratios with good ignition capability;

  • Has a high auto-ignition temperature; and

  • Offers no evaporative engine cooling and has no lubricating qualities.

Pivotal argues that its two-stroke engine is well-suited to exploit the potential of hydrogen-fueled internal combustion engines, particularly because it can avoid peak and localized high temperatures which cause pre-ignition or high NOx-forming conditions.

Pivotal won a 2005 AEI (Automotive Engineering International) Tech Award for the pivotal-piston engine. From the award statement:

The move to hydrogen fuel swings the balance in favor of a two-cycle engine, but only if the piston can be thermally controlled and required minimal lubrication. With the internal water cooling of the pivotal piston, a hydrogen-fueled IC engine can deliver higher power density than the modern gasoline automotive engine. The independent water cooling of the piston ensures uniform combustion chamber surface temperatures to control pre-ignition and excessive NOx emission.

The four-chamber, 2.1-liter two-stroke pivotal-piston engine.

Pivotal joined with Mace Engineering to develop a prototype 2.1-liter four-chamber gasoline pivotal-piston engine, which produced 170 kW of power while weighing 65 kg—a power density of 2.6 kW per kg of engine weight (1.5 hp per pound).

If optimized for hydrogen fuel at an air/fuel equivalency ratio of >.6, Pivotal estimates the output to be in the region of 120kW–130 kW. This hydrogen output represents about 1.8 kW per kg of engine weight—still an excellent level of power density when compared to one of the best gasoline, naturally aspirated, automotive engines: the V10 BMW at 1.55kW per kg.

Pivotal contends that this comparison suggests that it will be possible to develop hydrogen-fueled pivotal piston engines which are significantly smaller in size and lighter weight than current gasoline automotive engines.

In application, conventional two-stroke engines have proven to have a number of disadvantages—such as less efficient combustion, poor fuel efficiency and higher emissions—than modern four-stroke engines. Those limitations have largely limited two-stroke use to smaller displacement, off-road vehicles and scooters, where the higher power-to-weight ration and smaller size outweigh the problems.

Other engine-makers, notably Orbital in Australia, are applying modern technology such as direct injection to the two stroke, but cost-effectiveness remains an issue. Pivotal hopes that the combination of its new design and hydrogen fuel can alter the entire dynamic.




This engine design is interesting, but ultimately it will run into a piston seal oiling problem similar to the wankel engine. The water seals on those pistons could also be problematic as well.


I like this concept. I'm also concerned about the sealing, however Wankles with the new generation of apex seals have pretty much solved the issue of seals wearing out prematurely.

I don't understand why they are bringing air in via the crankcase other than to act as crankcase ventilation?

This engine seems to me that it would benefit greatly from (turbo)supercharging bu I don't think would work well while the intake manifold is the same as the crankcase :)

Reguardless, this is an exciting design imo.

Rafael Seidl

Replacing the usual crank-slider (i.e. infinite length pivot) four-bar linkage with a finite-length pivot-slider appears to be a deceptively small step - in truth, it's quite a leap. Note how one of the chamber walls is attached to the piston and the shape of the seals perpendicular to the pivot axle.

Internal water cooling of the piston is nice, but modern four-stroke engines already feature piston cooling by way of oil jets directed at the underside of the piston. Piston slap is still a problem for the engine acoustics, but in modern passenger cars the dominant source of exterior noise is the tire-road interface. Note that four-stroke engines can now be built small and simple enough for use in hand-held chainsaws:;/site=a4e/lng=en/do=show/alloc=3/id=2712

Which leaves the two-stroke aspect, promising higher specific power (though < 2x better!). The useful innovation here is that this pivot engine permits the oil to be applied in microscopic doses directly where it is needed. Compared to blending the oil with the fuel in 1:50 mixture, you get an order of magnitude lower oil consumption (and associated blue smoke in the exhaust).

The snag with two-stroke engines is that you have to contend with high variations in lamda values between combustion events, because the scavanging process is impossible to control precisely. Therefore, you have to operate at lamda > 1 which means you cannot use a three-way catalyst for this engine. Moreover, a two-stroke is very sensitive to back pressure in the exhaust. This means you can only deploy two-stroke engines where you need high specific power but face no emissions limits: light aircraft, motorcycles, off-road vehicles.

It might make sense to try this engine with compressed natural gas. Hydrogen is an expensive pipedream almost regardless of how you convert it into motive power.


Even the latest Wankels still require oil injection for seal lubrication. Its not a lot of oil that is consumed, but more the complexity of the system needed to deliver it that might cause problems or added cost.


I wonder why we all still fuss around with variants of mechanisms, sliding seals and lube when we all know the problems related thereunto. The gas bearing is highly developed and very successful in free piston stirling engines. It is easy to do with IC engines as well. Make an opposed piston gas bearing free piston IC, using all the known tricks about valve timing, fuel injection and such like, and take the power out with a variable nozzle area turbine. Then you get a fantastic power/weight ratio, and no lube oil whatsoever ( use close fit long piston seals). Right, you are scared of things that just vibrate and might have a mind of their own? Well, we know all about Newton's laws, don't we? And computers? ---So?


Wimbi, you're post seems unrelated the the topic at hand, but you've peaked my interest. Post some links to information if you can. Why would you bother with a piston engine if you're using a turbine to extract the power? Anyways, if you could explain what you're talking about more that would be nice.


Sure, Brett, glad to. I used to teach thermo, and always emphasized that a cycle was a cycle regardless of the mechanical detail of the hardware that did the compression, combustion, expansion and so on. Long time ago I invented a free piston IC that did part of the cycle with osclllating pistons floating on gas bearings, with the power taken out by a turbine expanding the hot gas, rather than a piston doing it. Mechanically much simpler and eliminates the lube problem. All this looked like a good way to go since modern valve actuation made a lot of things possible re timing and fuel injection. But somebody (name?) got there first and I dropped the invention and the lawyer's bills. I have heard nothing since and have spent my time on stirlings. You can find more by searching for internal combustion- free piston- turbine engines. There are lots of them.

By the way, I am not talking about a standard brayton cycle gas turbine, instead a piston does a lot of the cycle compression work, being more efficient than a rotating compressor in automotive sizes.


Brett, I am intrigued by the gas bearings supporting the pistons in a free piston machine. How does it work? Perhaps by injecting pressurized air?
Assuming that works fine, would one not still need a mechanical seal, piston rings, that are in contact with the cylinder wall and would need some amount of lubrication, as does the pivotal engine? The pivotal is even better as it has no piston cylinder contact and does not need a gas bearing or such at all.


Well, Floram, I am sure we all know how much pain it is to develop a new engine of any kind at all. And I sure don't know all the answers. But I can tell you that gas bearings work just great IF the piston is doing nothing but sliding back and forth in a cyl with no side forces. In stirlings it takes about 1% of power or less to pump the gas bearings, and you get friction free (almost) motion. No need for rings and lube since no contact. Free piston stirlings have run many tens of thousands of hours with gas bearings, no wear. Look at NASA space power stirling for lots of info. They are thinking of 14 year life, continuous operation.


That’s impressive wimbi. 14 years * 8760 hrs is over 120,000 hrs! I guess that the pressure of the ‘bearing’ gas needs to be higher then the gas pressure in the cylinder in order not to require rings for sealing. In an ICE, the peak gas pressure is likely much higher than in Stirlings, likely also requiring higher pressure gas and therefore more than 1% of output power. But I am guessing.
In case of the Pivotal Two-stroke, gas pressure should work there too without seals and lubrication. What’ your take on that?

Yes, it is difficult if not impossible to develop a new engine as an individual without having big money behind it. We are in the same boat. If you search the patent literature and you have something that is new, I would simply apply for a patent, regardless if someone else works on it. The other thing is to build an engine for proof of principle. However, that may be hard if you need a turbine for energy conversion. With a non-positive displacement apparatus you may have lower efficiency at low speeds. The new hybrid HDi engines, such as from Peugeot, display very high efficiencies.
For various mechanical linkage designs see and


Thanks, Floram, for all the good info. I gave up on IC engines because why work on things that many thousands of smart people are working on with megabuicks and big labs behind them? Stirlings in relation are totally simple and almost nobody is working on them, relatively speaking. BTW, hybrids allow the engine to run when it's best to do so, so at low power let the electric motor do the job. Neat thing about free pistons- you can run one or several pairs depending on power demand. But, enough- let those other guys have the fun. Back to stirlings.


Yes wimpi, I agree, engine ‘improvements’ even such as the combination of a free piston machine with turbine power, which seems a good thing when used within a hybrid, will hardly lead to any benefit for you. The big boys do what they want to do.

You would have to have a complete new concept, perhaps the Stirling, but be careful; would they not say the Stirling is known to us? It would have to beat prior design by a large margin. Of course the Stirling concept can be implemented in many ways. I hope you have a good one. Keep on developing, who knows, it may beat all the others. Regards.


Brett, regarding your above comment (March 13, 2006, 1st comment), I wonder why you feel the Pivotal Engine will ultimately run into sealing problems similar to the Wankel.

I see it quite differently. The piston action is similar and much closer to an RPE then to the Wankel and it should be compared to a two-stroke RPE. In the Wankel engine the housing is not round and piston vibration will hammer the apex seals into the trochoidal surface of the housing. In the Wankel nothing can be done to avoid the seal vibrations. To avoid damage extreme hard running surface with a strong underbody and extreme hard seals is the only way to overcome damage. Chatter marks are the result of the unavoidable piston/seal vibration.

None of this exists in the RPE, as piston vibrations, up and down, are parallel to the cylinder walls and cause no harm. The only point is piston slab on reversal where cylinder wear is highest.

In the Pivotal not even piston slab or piston rocking occurs as the pistons are held firmly without any side movement. The sealing angle is constant as in the RPE and there is no reason for the sealing not to be easier or even better than in an RPE. There is no equivalence to the Wankel. Of course, oil must be provided similar to a two stroke as a fuel additive. The ratio reported is 1:300 that makes sense, as the pistons themselves need no oil and piston seizure would not occurs.

Mike A

About 2 stroke emissions, the small two stokes can be easier to homolgate that their 4 stoke counterparts (50cc scooter). Go figure!

The Orbital injection works and works for gas fuels really well.?

I'm not sure I really see the piont of the pivotal engine though. Nothing than a traditional two stroke with the orbital system cannot do for less complexity and I would say, lower cost.


Hi Mike,
On your last point you are trading more complexity by pivoting the piston against a much lower lubrication need that is in the range of common four-cycle engines.

It now depends what is more valuable. In today’s environment, I would tend to think it might be the latter.

I was not aware those small 50 cc scooter engines are less polluting as two-cycle than in four-cycle version, as I understand it you were saying. Perhaps it is simply that twice the number of piston strokes and therewith twice the lubrication is required in the four-strokes? I would be pleased to know.

cyril mathew

i am very much interested in this i want to get more details about it please send it .


I might think that is true as the added complexity of say , an overhead cam 4 stroke 125cc engine will require far more oil than a simpler 50cc 2 stroke note also the simple point that a 125cc piston has a greater circumference and hance needs more oil to lubricate the barrel , some of this oil will be burnt off but with a larger exhaust outlet it may be allowed to escape partially unburnt, finally the gilera dna 50cc scooter has an interesting afterburner syatem which allows fresh air into the pipe just after it leaves the exhaust port hence allowing for better combustion of unburnt oil/fuel mix.

Jason Rogers

Ash said:
"The useful innovation here is that this pivot engine permits the oil to be applied in microscopic doses directly where it is needed. Compared to blending the oil with the fuel in 1:50 mixture, you get an order of magnitude lower oil consumption (and associated blue smoke in the exhaust)."

This "inovation" called SuperLube was used in the Suzuki motorcyle (and I think some outboard) engines as far back as the 60s. As this was more than 40 years ago any patents would be long expired and it should be a free for all. They used a system of injecting oil via sealed main bearings, then through tiny oil galleries drilled through the crankshaft to the big end bearings, then through another tiny oil gallery drilled through the conrod to the little end bearing.

I have a GT 750 Suzuki and I get the equivalent of about 100-200 to one fuel to oil consumption. That's better than 1000km per litre, better than many fourstroke motorcycles of the era that generally consumed around that amount of oil (without even counting oil that is consumed during an oil change!). It's particularly impressive when you consider that most twostroke motorcycles of that era used between 16 and 25 to 1 fuel to oil ratios. (the oils and metals were not as good then)

Additionally if the oil is not flushed away by incoming fuel (eg orbital injection as seen on aprillia Di-Tech) even leaner fuel to oil ratios are possible. The Aprilia only needs the oil tank refilling at major services. One might imagine that combining superlube with di-tech you would have an engine that used almost no oil at all.

Cheers Jason =:)


me a lil boi 14yrz old who have a aim to make his own car


hello, how are are very helpful to us.

manish hazela

hi...sir i am astudnt in and i am placing an injector in 2-s gasoline engine. so the problem is injector lubrication because i am doing DI. so if u can help me out of this problem. how the ECU works and its circuit dig.


I have a Mazda RX-7 with the typical Mazda wankel engine. I was wondering if you could help me out on possibly finding someone who knows anyhting about thses engines. there are a few leaks now. the gaskets are shot. but no one knows anything or will work on them. it'd be great if u could help me out a bit! thx.

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