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Study: Steam Hybrids Using Waste Heat Recovery Could Reduce Fuel Consumption Up To 31.7%

Results of the study indicate that a steam hybrid using exhaust waste heat could reduce fuel consumption by up to 31.7%, depending upon drive cycle and vehicle.

A study by researchers at Loughborough University and the University of Sussex, both in the UK, has concluded that using waste heat from light-duty vehicle engines in a steam power cycle could deliver fuel economy advantages of between 6.3% and 31.7%, depending upon drive cycle, and that high efficiencies can be achieved at practical operating pressures.

The basic concept of the “steam hybrid” system is that energy is recovered from the exhaust in the form of a steam/water mixture. Shaft work is produced as steam is expanded, and is used in one of three ways:

  • To add torque to the internal combustion engine (ICE) output, thereby reducing fuel consumption and emissions; or

  • To drive an auxiliary power unit (APU) if the energy is not required by the vehicle (e.g., during braking or idling). Excess energy can be used to generate electricity and charge an electric storage system; or

  • To provide all the required torque to the drive shaft resulting in zero-emissions driving in inner-city areas.

The study, which is part of a larger investigation of the controllability of energy recovery, modeled a steam hybrid vehicle using two toolboxes: the Quasi-Static Simulation Toolbox (QSS-TB), developed at ETH Zürich, and the Powertrain System Analysis Toolbox (PSAT), developed by Argonne National Laboratory.

For a base vehicle in QSS-TB, the researchers used a VW Golf with a 1.6 liter engine; for PSAT, they used a Honda Civic with a 1.8 liter engine. Both vehicle models were run against the New European Drive Cycle (NEDC); the US FTP-75 Urban Drive Cycle; and the US06 Highway Drive Cycle

Results showed an improvement for both vehicle across all three drive cycles, with the greatest gain in the FTP-75 cycle, and the least with the aggressive US06 cycle.

Subsequent work for the research will include optimized controls for different style of driving, and further work on optimizing the heat exchanger. The design of the heat exchanger will be evaluated in an experimental set-up in which the exhaust gas is generated by a 7.2-liter Caterpillar engine.

In further developing the control architecture, the researchers are using four objectives:

  1. Power/torque. The power or torque demanded from the vehicle driver needs to be met by the ICE, the steam expander, or a combination of the two.

  2. Fuel consumption. Fuel consumption needs to be kept at a minimum.

  3. Steam supply/reserve. The supply of water to the heat exchanger needs to be kept constant to ensure there is a constant steam supply, or at least a reserve of steam for the expander.

  4. Steam quality. The steam supplied to the expander is required to be superheated so that it contains no water droplets which could be potentially damaging to the expander. This will be achieved by raising the temperature of the cold side fluid beyond saturation and ensuring it is superheated at entry to the expander.

The work is supported by the UK Engineering and Physical Sciences Research Council.

BMW and Honda are both investigating the use of waste-heat powered steam systems to enhance fuel economy. BMW’s onboard water/steam-based cogeneration cycle is used to power the vehicle’s accessories, rather than a traction battery pack (earlier post).

Honda is actively exploring the use of a Rankine cycle co-generation unit to improve the overall efficiency of a hybrid vehicle by recapturing waste exhaust heat from the internal combustion engine and converting it to electricity to recharge the battery pack. (Earlier post.)

Canada-based Clean Power Technologies Inc. (CPTI) is developing a waste-heat powered steam hybrid system—CESAR, Clean Energy Storage and Recovery)—that it claims has shown an up to 40% reduction in vehicle fuel consumption in initial test results. The system is under development by a wholly-owned CPTI subsidiary, Clean Power Technologies Ltd. (CPTL), which is located in East Sussex, UK. The Clean Power Technologies system was developed by Fred Bayley, Professor Emeritus of the University of Sussex.

CESAR uses a heat exchanger to capture waste energy, which is then stored in the form of steam in an accumulator, for on-demand use either in the same primary engine, or in a secondary vapor engine. Power can be produced solely by the secondary vapor engine even after the primary combustion engine has shut down.

The test program using the system included a generic model of an accumulator, supplied by Clean Power’s collaborative partner Doosan Babcock (previously Mitsui Babcock). The CESAR system has been running in parallel with a Caterpillar C18 diesel engine within Clean Power’s test facility in Newhaven, East Sussex since mid-October 2007. Clean Power is now re-designing the second generation of the steam accumulator which will be lighter and more efficient.

In March, CPTI contracted with steam technology specialist Dampflokomotiv-und Maschinenfabrik DLM AG (DLM) to act as a consultant for the further development of the CESAR technology. DLM, a specialist in the development of modern steam traction systems, will provide consultancy, design engineering and stress test related services.

CPTI has entered into a first stage collaboration agreement with Safeway Corporation for the purpose of data collection and to undertake preliminary design work for the steam refrigeration units for the grocery trucks. CPTI has also entered into a collaboration agreement with Voith Turbo Gmbh & Co. KG of Germany to jointly develop a reefer engine.


  • Sandra Hounsham, Richard Stobart, Adam Cooke and Peter Childs (2008) Energy Recovery Systems for Engines (SAE 2008-01-0309)

  • M. Kadota and K. Yamamoto (2008) Advanced Transient Simulation on Hybrid Vehicle Using Rankine Cycle System (SAE 2008-01-0310)



Why not forget about the ICE and go steam-only?


If some poeple worry about exploding batteries, how safe is a steam accumulator?


How often would water need to be added to this sort of system to keep it operating?

Healthy Breaze

This is basically cogeneration. It works well in natural gas power plants where total system efficiency is very high. You would think that with the range of materials and machine tools we have, we could create a low-cost system to recover heat energy and store it as as electricity for PHEVs. Then, instead of a 15% fuel efficiency gain for in pure HEV-mode, we might see a 40-45% gain...which would get even better when plugging in to recharge as needed.


1 litre of water = 1 kg of water = 2.2 pounds ~= 1700 litres of water vapour at atmospheric pressure.

That and if intelligently designed will have a condenser to change the vapour back to a liquid state to reduce the need to keep recharging it.

This isn't a steam engine. It's an energy recovery system that uses water/steam.


This is what I had been taling about years before the turbosteamer announcement. Some day we might be able to use solid state heat to electric elements, but for now it is hard to beat this. BMW claims 15 hp just from an expander.

One of the big hurdles is the condenser. One of the engine companies uses a smaller centrifugal condenser and there are other methods. I think this could be really big in delivery and long haul trucks. Imagine how much waste heat that there is from a large turbo diesel engine.

As far as just using steam, Carnot pretty much has defined the efficiency there. But I have posted a link for a company that has made a brayton/rankine machine that is suppose to get 40% efficiency. Series hybrids can open the way for lots of innovation there.


Why add new hardware? I think Bruce Crower and his 6 Stroke engine are a good option. This is a low cost, simple and efficient way of capuring waste heat. I think this one has merit. Inject water to make a steam stroke in the same cylinder after the exhaust stroke. This captures the waste heat to create cylinder pressure and gets rid of the radiator at the same time for thermal and aerodynamic gains. Internal combustion engine and waste heat powered steam engine in one package.


It looks interesting, but the oil water situation is a problem. If the combustion chamber is 400f the exhaust gases are higher, maybe closer to 800f. As we know, the higher the difference between cold and hot side can make for higher efficiencies.

If you look at BMW's thermal images, you see that the exhaust headers and turbo are where the high temperatures are. If you can preheat the fluid with the cooling system, the oil system, the header heat and the turbo heat, you could really get it up there.

s dogood

Cyclone Power's WHE is being considered for powering big truck hotel loads. But then again, why not go steam all the way.


For an electric hybrid it might be wiser to combine high z thermocouples to charge the battery and run the electric motor off of the hot exhaust gas from the engine, this might be simpler then trying to fit a rankine cycle into the car and sacrificing the advantages of an electric hybrid (stop-start, braking regeneratio) to make space.

Harvey D

A neat approach to further increase the overall efficiency of hybrids such as the Prius, specially on long trips, where the Prius efficiency needs to be improved. The Volts could also benefit by with free battery (partial) recharge, specially on highways where it is realy needed.

Large long distance trucks could save a bundle in fuel each day.

Could turbo-prop airplanes use a similar approach (if not already done) to further increase their efficiency and reduce fuel consumption. Extra tons of fuel in the air is very expensive these days.


You could do both turbo alternator and steam turbine on a hybrid and really get some performance and mileage.


That might sound ridiculous but...

So they used steam engine as a heat the piston steam engine work .. well on one approach it used series of extenders, usually up to 2-4 pistons with progressively bigger diameter). The working fluid temperature and pressure decrees so in order to capture most of the energy from presure bigger piston diameter have to be used.
So why not add additional pistol to each cylinder that will act as a expander only.
The exhaust will act as working fluid.

By the way returning to moder steam engine with 35-40% efficiency (Cyclone Power) will not be bad idea at all.
Steam engine toque and power characteristics are much more suitable for modern car then ICE. You probably will get at list 34 time more millage then ICE.

Dennis Moore

I don't get the ICE/steam hybrid idea. With electric hybrid, you have the advantages of the electric motor and make up for the battery problems with the ICE.

But with steam you have some of those same advantages as electric (high torque, low pollution) but no big expensive battery. Not only can you use a liquid fuel (high energy density, quick re-fill) but you can burn any compatible fuel. So now you have the ultimate flex fuel vehicle that can burn the cheapest liquid fuel available.

We need to seriously consider a 21st century steam engine.

Brian P

@Anne, dogood;

The efficiency of a stand-alone Rankine cycle is poor by comparison to, say, a decent modern diesel engine. This is one of many reasons why steam-only is not a good plan (and why they disappeared from the automotive scene decades ago and have never come back). The maximum theoretical efficiency of a Rankine cycle can be calculated using charts in any common thermodynamics text. Hint; it's rather dismal. (@mki - 35% efficiency of a Rankine cycle is a pipe dream unless multiple stages of expansion, reheat, and feedwater preheat are used - but that's only practical on a utility-power-generation scale.) But if you are using it to do something with heat that you would otherwise be throwing away, that's where the attraction is.

@Tonychill and mulad,

A Rankine cycle of this type would most likely use a boiler with very small and thin-wall pipes with the working fluid inside the pipes (opposite of an old steam locomotive), to keep the "thermal inertia" low, and to maximize surface area for heat transfer relative to the amount of steel used to construct the boiler. An additional benefit is that the amount of fluid contained in the boiler becomes very small - so little that it is scarcely an explosion hazard. Also, note that I have been saying "fluid". Water is not a particularly good choice, in an engine that has to survive use in a below-freezing environment - most likely the working fluid would be something other than water. And that, in turn, means the working cycle (boiler, expander, condenser, feed pump) has to be hermetically sealed - like your average refrigerator. No fluid fill-ups needed.


Using a waste heat vapor turbine to drive an alternator to charge batteries and run the motor is a good way to go. The slides I have seen call it electric turbo compounding. Now get a turbo to drive an alternator too and you get more power to drive the motor and charge the batteries in addition to the waste heat vapor turbine. This would allow motor/engine hybrids to get better mileage on the highway as well as around town. The Clean Power people are using waste heat vapor turbines on large trucks, which is where I think the big gains can be made.

SJC - got a better link?

s dogood

I think Brian makes a good point. Of course a diesel could be used in a serial hybrid configuration as well. An advantage of an external combustion engine is that it can run on diesel, gasoline, or any high energy fuel. Kind of went off on a tangent with the steam only discussion. Good to see that companies are focusing on capturing previously untapped energy losses.


A better link? That is the link for the company sited here. What are you referring to?



The efficiency of a steam engine can be better than your 35% statement using flash steam with super critical water heated above 550C and recovery of wasted heat. I think Cyclone Power Technology has proposed an elegant approach to acheive that. Also as you can get rid of the transmission and gera box you save weight and friction making the overall efficiency better and for an engine that could be extremely clean without requiring a catalyst or after treatment exhaust.

The recovery of waste heat on an ICE is not a good idea, since it will result on a complex and heavy machinery on an already complex engine. On top of this the waste heat on a IC is scattered bewteen exhaust and liquid coolant making recovery difficult and inefficient, last but not least a steam engine need a stable high temperature heat source, the exhaust of an ICE sees strong temperature variation depending on laod then making the whole thing inefficient.

I agree if you think about steam then go steam for the whole thing since there is other way to improve efficiency of ICE, like Atkinson, Diesel Otto, Scuderi cycle, Variable compression engine, HCCI engine etc..

Rafael Seidl

There's a limit to how much exhaust waste heat can be used. For one thing, all exhaust gas aftertreatment systems only work above a certain light-off temperature. For another, the waste heat from a secondary steam cycle must be shed through a radiator, possibly creating a major packaging headache.

A better option is to simply apply turbocharging to a large number of engines - with high unit volume, even modest improvements add up to major gains. Variations include turbine gensets such as TIGERS or turbos with integrated motor/generator such as proposed by Honeywell for commercial vehicles.

Note that VTG turbines made from affordable materials can be applied to spark ignition engines if 20-30% low pressure EGR is applied (cp. Ricardo EGR-boost)


You can find more information on Electric Turbo Compounding by going to Google and entering "Electric Turbo Compounding" among the links there you will find:

It will download a PDF slide set. On slide 8 they refer to it. John Deere has a patent on this issued in 2007.


"The recovery of waste heat on an ICE is not a good idea, since it will result on a complex and heavy machinery on an already complex engine."

I do not agree with this assessment when it comes to large trucks. The truck is already very expensive and with $4+ fuel, the cost of operating it is getting higher.

I do not know what mileage 18 wheelers get, but we were talking about city buses and 3 mpg. I big rig with all the weight and lack of aerodynamics might get about that on the open highway at 60 mph. Filling 220 gallon tanks could cost $1000 and be used up in one day on a long haul trip.

So, even a 10% improvement in mileage could save the operator $10,000 to $20,000 per year on fuel. Obviously you go for the lowest cost best return improvements, but when so much goes out as waste heat, it might be worthwhile to recover some of it.


I remember reading on this site how Mercedes engineers put a little steam engine on one of their test cars that ran on waste heat. It was a closed system so the water never needed to be refilled (as it was recycled.) The improved fuel economy was significant but Mercedes decided not to bring it to market; I can't remember why.


BMW has the turbosteamer on a 1.8l I4 engine. The cost and complexity is a problem, but with big rig tractors and huge expensive turbo diesel engines, it might be real cost effective.

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