AQMD Board to Vote on Plug-in Hybrid Initiative
National Academies Report on Tires and Fuel Economy

Updating the Steam Engine: the Cyclone

An application of the Cyclone.

Cyclone Technologies LLP, developer of the Cyclone external combustion engine, received an Automotive Engineering International (SAE’s publication) Tech Award at the SAE World Congress in Detroit.

The Cyclone modifies the traditional Rankin cycle steam engine to deliver the use of super-critical pressure (3,200 psi) and super-heated steam (1,200° F)—normally found in high-efficiency electrical power plants—in an efficient, compact package suitable for a vehicle.

A sketch of the engine from the patent application.

The Cyclone. The engine, which uses water as both working fluid and lubricant,  consists primarily of a condenser, a steam generator and a main engine section having valves, cylinders, pistons, pushrods, a main bearing, cams and a camshaft.

Air pre-heated in a heat exchanger is mixed with fuel. An igniter burns the fuel charge as the flames and heat are directed in a centrifuge within the combustion chamber.

The combustion chamber heats the steam generator to produce high-pressure, super-heated steam.

A rocker and cam design which serves to open and close a needle type valve in the engine head controls the speed and torque of the engine. When the valve is opened, high-pressure, high-temperature steam is injected into the cylinder and allowed to expand as an explosion on the top of the piston.

Exhaust steam is directed through a condenser, in a centrifugal system of compressive condensation, consisting of a stacked arrangement of flat plates. Cooling air circulates through the flat plates, is heated in an exhaust heat exchanger and exits into the furnace. This reheat cycle of air greatly adds to the efficiency and compactness of the engine.

Efficiency. The efficiency of the Cyclone is based primary on three factors:

  • Heat Regeneration. Numerous design innovations significantly reduce the heat losses, cumulatively increasing the efficiency of the engine.

  • Super-Critical Liquid. Pressures in the range of 3,200 psi with temperatures of about 1,200° F cause super-critical vapor to act as a fluid. Maintaining the super-critical pressure, in the centrifuge process, eliminates the turbulence, backpressure events, and heat spikes that can occur during other less efficient types of super-critical processes. At these higher temperatures and pressures, the super-critical fluid carries more heat energy to the motor it powers. The Cyclone Engine is a piston engine with a special valve mechanism allowing it to operate at fluid pressures, thereby gaining multi-advantages: greater simplicity, reliability and enhanced power.

  • High compression. The Cyclone Engine is an external combustion/internal expansion engine, using variable timing, variable compression ratios and multiple heat exchangers to increases in engine efficiency. A reheat stage is included at lower compression ratios.

Cyclones can use almost any liquid or gaseous fuel. Power produced we be based on the energy content of the fuel in use. The design of the engine obviates the need for a separate cooling system.

The basic Cyclone engine is a compact, one-piece unit that can develop 3 hp per cubic inch of displacement, compared to a ballpark one horsepower per 1.5 cubic inches of displacement for a gasoline engine.

A Cyclone with three or more cylinders is self-starting—i.e., no flywheel needs to be turned by a starter.  Pulling the throttle lever activates flow through individual needle valves situated at each cylinder. From a dead stop, the engine attains its highest torque within less than one crankshaft revolution and transitions to engine horsepower at 5,250 rpm giving the Cyclone a remarkably robust power band.

The Cyclone Engine exhausts into the condenser, where the vapor is totally contained, not into the atmosphere. As a result, it requires no exhaust system muffler or catalytic converter.

Vibration is controlled because the crankshaft is counter weighted and the pulsations are balanced by equal offsetting pressures, thus requiring no harmonic dampener.

The Cyclone Engine is smaller than four-stroke internal combustion engines, with fewer parts. However, the materials handling the high operating temperatures and pressures are more expensive. At similar economies of scale, the two types of engines should be comparable in cost.

Applications. Harry Schoell, the inventor of the Cyclone, forsees a large range of applications for his engine, from small electrical generators in the 1 kW range up through medium-size cyclones for light-duty vehicles, large Cyclones of up to 1,000 hp for the heavy-duty diesel market and beyond to larger ships and generators.




any word about rough fuel efficiency ratings, warm-up period, life-cycle reliability, unloaded engine acceleration?


Whats next? back to horse?


I've been looking for a modern buggy.


It burns fuel (to produce steam) so how can there be no exhaust?

Tim Russell

It said it exhausts to the condenser so I guess it condenses the combustion vapors but then what happens to the resulting condensate?


I wonder if this could be used as a generator for electricity. Depending on noise and fuel efficiency, this might be a good solution for remote places without electrical lines or damamged lines.

Mark A

Interesting development. Steam engines were highly developed for trains at the beginning of the twentieth century, having power strokes on the up and down (or forward and back) strokes. Their limitations back then were their size, and that eras limited metallurgical knowledge. I always thought steam could have been a major player in the automotive world, with developed corresponding advances over the years, had it not been for cheap gasoline at the birth of the automotive age.

So it is now interesting to see these types of developments. I, too, would like to see efficiency numbers. But again, another potential option in a market that will ultimately determine its own destinity. Just give us more options.


their website is brief, but to the point. the want to target every power market between 20 and 4,000+ hp.

sounds like it is time to talk to a businessman and find 1 market to start with.

multifuel generators for serial hybrid H1s for the military, perhaps?

Rafael Seidl

There have been numerous attempts to revisit the steam cycle for automobiles. Its combustion is continous and can therefore be much cleaner than that of any reciprocating engine design. Note that this is only true if the fuel is free of contaminants such as sulphur and the burner designed for it. Steam power provides high low-end torque and a hyperbolic drop-off beyond a certain corner point - ideal for a vehicle.

Packaging has apparently been addressed in this design. However, overrall weight and, especially, crash safety are always issues - steam explosions were the reason Stirling, Benz and Daimler focussed on gas engines.

One serious downside is that it takes significant time to build up a head of steam. Therefore, a steam motor responds poorly to load changes (acceleration and deceleration demand from the driver). It is therefore best suited to powering vehicles traveling mostly at constant speed, e.g. ships, locomotives or large trucks on the freeway.

You can mask the lag with a hybrid system, which can also recuperate during braking and soak up the residual enthalpy in the boiler after you park the vehicle. As a result, the steam boiler and motor can be far smaller. However, the expense and weight of the entire system will be substantial.

tom deplume

The biggest problem facing an automotive steam engine is the condensor which these guy's claim to have solved. Now show me doing the job in a Pheonix summer and I will be impressed.


As per Rafael, external combustion can be great for controlling emissions (except for CO2). If they've solved the condenser problem, then the remaining major questions are efficiency and cost. High efficiency means materials exposed to sustained high temperatures which necessitates expensive, exotic metals or ceramics, etc. The beauty of the ICE is that exposure to very high temperature gases is alternated with exposure to cool gas, allowing the use of cheap materials in a relatively fuel-efficient engine.

Since limiting GHG will increasingly become the prime consideration, efficiency vis-a-vis comparable ICE engines should determine the viability of this approach.

Rafael Seidl

Nick -

pls provide a link to a steam motor or turbine that requires ceramics. The temperatures involved in handling steam are far lower than those in ICEs or gas turbines.

Mark A

Heres an interesting link:

As far as waiting to build up steam pressure, I am not sure that would be much of a problem, considering even the cheapest automatic coffemaker can build up steam pressure/hot water, in as little as 5-10 seconds. Plus this engine, I would think, would inheritently stay warm overnight except in the extremest of cases. Perhaps if the efficiencies are there, they could run overnight in generator mode to connect to the grid somehow. Would be interesting to see this developed, promoted in some fashion.


Mike Mulligan & (Mary Ann ) His Steam Shovel

----Has Anybody Read This Book?

I Loved this book when I was 5.



If this engine is light and efficient enough, it could be used to extend the range of EVs. An efficiency of 20% would enable to make the car competitive even on highways...
It is a great hope, but the security issue is stil there.


A steam-turbine powered hybride, maybe?


Although they don't post any hard numbers on thermal efficiency, they do provide a spreadsheet:

...which admits higher base engine efficiency for diesel, hybrid and fuel cell while claiming higher well-to-wheel efficiency for this engine. One guesses they may be assuming lower mechanical losses due to elimination of transmission or lower vehicle weight because of higher power density, perhaps? In any event, as they score their base engine efficiency 7 (10=best) while scoring diesel 8, hybrid 9 and fuel cell 10, it's hard to get too excited about using this invention to lower GHG emissions.

Roger Arnold

A steam explosion with this type of engine is not going to be a safety hazard. The old steam cars and locomotives--where explosions definitily were a big safety hazard--had large boilers for a hefty reserve of pressurized steam. That's because the control feedback loop was slow: "hey, Joe, pressures's dropping, better shovel that coal a little faster". New steam engine designs like this utilize "flash" boilers. Water is injected under high pressure into tubes where it it is immediately brought up to temperature. To minimize weight, the object is to minimize the volume of pressurized fluid. Ideally, the boiler tubes only hold enough for the next piston stroke.

This type of engine is not a good candidate for use in hybrids. Hybrids get the largest part of their efficiency advantage through elimination of idling and their ability to deliver high torque at low speeds. They compensate for weaknesses in gasoline engines. A steam engine, however, does not idle, and develops very good torque from zero RPM on up. So the added weight and cost of the electrical system would not pay off.

What's most attractive about this engine is that it would run well on "bio-oil"--the condensed product of flash pyrolysis of forest and agricultural wastes. Small-scale flash pyrolysis units can be built at low cost to serve individual farms. So this would be an ideal engine for powering tractors and other farm equipment in a post-oil world.

tom deplume

I would add a steam box to the system. A steam box is a high pressure insulated tank which can store energy for quick power bursts. It would cut fuel consumption considerably and allow for a smaller steam generator and condensor.

James Huang

I see the potential of marrying the steam engine with the normal gas engine. The gas engine generates lots of heat. The heat is lost via the exhaust pipe. The steam engine could utilize the heat to provide additional power. The likely scenario:

1. Generating electricity via the steam engine
2. Store the electricity in Li-ion batteries for later use (extra boost or electric only mode)

Roger Pham

So IF the Cyclone eng. is not (as per cyclone website)
1) any significantly more fuel-efficient from well to wheel in comparison to a hybrid (Prius),
2) not necessarily any cheaper because Cyclone requires expensive materials for its high temp and hi pressure requirement, plus its new unproven and exotic design which will require a lot of R & D investment in term of money and time. Tens of millions of dollars and decades of experimentation. Water used for lubrication? Oil has much higher film thickness and strength that can much better protect against wear than water (too thin). Even 5W20 oil will be much better than water.

3) not much lower pollution than the hybrid, since the Prius is 5 times less polluting than a comparable gasoline vehicle, then, WHAT'S THE POINT OF USING THE CYCLONE STEAM ENGINE TECHNOLOGY?
Multiple fuels capable? So can an ICE, which can run on alcohol, methane, gasoline, biodiesel, hydrogen etc...Biomass and coal can be gasified into syngas (to be used by ICE). Syngas is free of sulfur, mercury and other pollutants that are associated with direct combustion of coal or biomass. Even hydrogen produced by wind or solar generator can also be used in ICE very efficiently and very cleanly.
Please kindly educate me on WHAT IS THE POINT OF HAVING STEAM ENGINE? (which is an antiquated technology that has been superceded over a century ago with superior ICE technology)

Roger Arnold

Inventors, by their nature, are unreasonably optimistic about the prospects for their inventions. Which is a probably a good thing, else technology would never advance.

The brass ring that inspires efforts like this has to be better cost-performance, either for the engine system, the fuel, or both. The engine that is described would offer most of the advantages of a hybrid, in a potentially simpler and cheaper system. Whether that's a realistic expectation or not is hard to say, from our remove here in the peanut gallery. Probably not, but it's not off the wall.

The lubrication issue may be moot. There are now dry surface coatings that offer lower friction than oiled steel on steel. I don't think they're in commercial use as yet, so there are presumably still issues of durability and / or cost. But that's the sort of problem that tends to yield to nuts and bolts applied R&D. And while synthetic fuels burn very cleanly, producing them is capital intensive and not terribly efficient. FT synthesis is exothermic, and produces a lot of waste heat. A system that can run cleanly on cheaper fuels, or fuels that can be produced locally from crop wastes, would find a good market.

If it were a matter of betting my own money, I don't think I'd invest in this particular effort. Too much of a long shot, IMO. But it's not crazy. And it's certainly NOT your grandfather's antiquated steam engine. There's an outside chance that its inventors are on to something that will pan out.

Roger Pham

Appreciate your comments, Mr. Arnold. That dry low-friction coating is real interesting stuff. Agree with you that synthetic fuel is very expensive. Hitler tried to do it in WWII and lost the war. Ethanol production or bio-diesel is likewise not very energy-efficient. However, gasification of biomass to produce syngas is different, because the heat thus generated will be used to run a steam turbine to generate electricity to feed the plugged-in hybrid auto. Direct use of coal or biomass such as crop waste in a steam engine produces obnoxious pollution such as particles, sulfur, mercury, ash that requires labor-intensive cleanup etc and would likely be banned. Therefore, steam engine or IC engine, the various fuel sources must be cleaned up by a process such as gasification, turning the polluting raw fuel sources into a clean fuel such as syngas before combustion by the end user.

Dan Baker

The basic advantage of this engine is power per pound. Since the engine does not require a transmission, nor a radiator, and has about 2x the power density of an ICE, it can produce equivalent power output for much less weight. Because of steam/piston's high torque at low revs, it will have many of the advantages of a hybrid (near zero fuel use at 0mph, virtually silent operation, etc) without a hybrid's enormous materials cost or weight. It is also a very very clean engine.

The oilless lubrication is not tacked on, it is a requirement of the design. One of the weaknesses of traditional steam designs is oil contamination in the boiler. Oil will coat the inside of the heat exchanger, turn to carbon, lower the efficiency, and eventually destroy the steam generator tubes. No one has found a way to keep the oil from getting into the water stream. Cyclone eliminated the problem by removing the oil from the engine. This is done through the use of ceramic bearing surfaces at very close tolerances.

I think it is an excellent design and expect to see it turning up, at least in generator and similar (10-20 hp) applications fairly soon.


Please visit

Verify your Comment

Previewing your Comment

This is only a preview. Your comment has not yet been posted.

Your comment could not be posted. Error type:
Your comment has been posted. Post another comment

The letters and numbers you entered did not match the image. Please try again.

As a final step before posting your comment, enter the letters and numbers you see in the image below. This prevents automated programs from posting comments.

Having trouble reading this image? View an alternate.


Post a comment

Your Information

(Name is required. Email address will not be displayed with the comment.)