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BMW Study on Rankine Cycle for Waste Heat Recovery Shows Potential Additional 10% Power Output at Highway Speeds

Energy utilization vs. complexity of different heat recovery systems. In this study, BMW focused on Rankine A (exhaust gas only) and Rankine B (exhaust gas and coolant). Adapted from Ringler et al. (2009) Click to enlarge.

BMW is exploring two pathways for waste heat recovery in vehicles: one thermoelectric, the other thermodynamic. In 2005, BMW Group Research and Engineering announced it was developing a steam-powered auxiliary drive—the Turbosteamer—to use the waste heat present in the exhaust gases and cooling system from a conventional gasoline engine as its source of power. The long-term development goal articulated at the time was to have a system capable of volume production within ten years.(Earlier post.)

At the recent SAE 2009 World Congress, BMW presented an analysis of two basic configurations of the Rankine cycle applied to a thermodynamic heat recovery system for a four-cylinder combustion engine. Based on bench test measurements, BMW has concluded that waste heat recovery can provide an additional power output of about 10% at typical highway cruising speeds.

Honda is also 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. Test results presented in 2008 showed that in 100 kph (62 miles/hour) constant-speed driving, the use of the Rankine cycle improved the thermal efficiency of the engine by 3.8%. In the US highway cycle, the Rankine cycle system regenerated three times as much energy as the vehicle’s regenerative braking system. (Earlier post.)

Researchers at Loughborough University and the University of Sussex, both in the UK, also have 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. (Earlier post.)

The basic principle of the Rankine cycle, said Andreas Obieglo, who presented the BMW paper at SAE World Congress, is to bring the working fluid to high pressure, feed it with heat, generate high energy dense steam, and convert it to mechanical energy. In the study presented, the BMW team restricted its evaluation to two basic single loop systems. System A used exhaust gas only as the heat source; System B used exhaust gas and coolant.

The working fluid, which is repeatedly vaporized, expanded, and re-condensed, plays a key role in the capability and cost-effectiveness of a Rankine steam cycle hear recovery system. To optimize the work output for a given temperature gradient, the authors noted, the evaporation enthalpy of the working fluid should be as high as possible. Water has the higher evaporation enthalpy (~2250 kJ/kg), followed by alcohols (methanol ~1100 kJ/kg, ethanol ~820 kJ/kg).

Based on the evaporation enthalpy, one would expect water to be the preferred working fluid for any heat recovery system based on the Rankine steam process. However, in most real world applications the utilization of waste heat is limited by technical restrictions (maximum and/or minimum pressure)

—Ringler et al. (2009)

Based on a quantitative analysis under constrained operating conditions, BMW determined that water delivered the highest thermal efficiency for system A, whereas ethanol is the preferable working fluid for system B (methanol was dismissed a priori due to health risks).

BMW developed a simulation model with the tool Dymola to evaluate the two alternative systems for different engine types. The process for both systems consists of the expander, the pump, the condenser and heat exchangers.

Based on the parametric analysis, BMW found that System B shows a higher potential at typical highway speeds (45-70 mph) for the engine type chosen (4-cylinder, stoichiometric combustion) and operating conditions. Nevertheless, the researchers cautioned, this cannot be interpreted as a general recommendation. Heat source parameters, which are deeply influenced ed by engine type and load profile, as well as operating parameters, which are limited by technical constraints (pressure level, ambient temperature), have significant effects on the net power output.

Based on bench testing, BMW concluded that System B could show additional power outputs between 0.7-2 kW.

This corresponds to an increase in engine performance in the range of 10% close to the road resistance curve for the top gear. Hence the operation of the Rankine cycle system presented leads to a remarkable increase in fuel efficiency. A further important step has been taken in the introduction of waste heat recover system in automotive applications.

—Ringler et al. (2009)


  • J. Ringler, M. Seifert, V. Guyotot and W. Hübner. (2009) Rankine Cycle for Waste Heat Recovery of IC Engines (SAE 2009-01-0174)



"..the Rankine cycle system regenerated three times as much energy as the vehicle’s regenerative braking system."

So a hybrid with regen can get even better mileage on the highway. I have been talking heat recovery for years, good to see it is happening.


Nice idea and direction.

Perhaps best suited to heavy trucks? Heat is heat even if it comes from diesel.

For light duty vehicles this approach is more likely to be useful if it can be located to improve the balance of the vehicle, which I assume will be a FWD hybrid or a luxury AWD hydrid.

FWD puts most of the weight on the front wheels. The weight of hybrid battery packs can offset that somewhat since battery location is less constrained than motor and drive train location.

We won't drive it soon. Perhaps it will be in a luxury car in five years.

i.e. cars that have it all, for people who want it all, and can pay for it.


I got a great idea: a plug-in hybrid electric Stanley Steamer. Burns any fuel, low NOx. Battery takes care of warm-up time.


"..the Rankine cycle system regenerated three times as much energy as the vehicle’s regenerative braking system."

"In the US highway cycle,"

And infinitely more is steady state cruising.

This technology is very intriguing – maybe it’s time has come.

Maybe not. As Ken asks – is it affordable?


I think it makes lots of sense in a long haul diesel truck tractor. Semi trucks run 120,000 miles per year on the U.S. highways hauling tons of freight and using 30,000 gallons of fuel each per year. If that 4 mpg can become 5 mpg, we could save a whole lot of fuel.


Not bad but I believe this has more potential as it can be added to any existing engine and produces greater efficiency gains.


Cogeneration is the only method of exceeding 60% efficiency. Hey toppatom, ever heard about mass-production? The first VCR was the size of a refrigerator and it cost $50k.

If only only we had the ability of miniaturizing while making technology less expensive... but that is impossible isn't it. toppatom is right, just keep building hummers because god forbid we try to improve anything.

Roger Pham

BMW and Honda's waste heat recycling improve efficiency about 15% max, at a great deal of complexity and cost that has prevented implementation commercially.

For much less complexity and cost, a mild hybrid (HEV) can improve efficiency 30%. Honda's IMA can improve efficiency 50% at a cost comparable with non-hybrid. An economy-optimized, clean-sheet hybrid design like the Prius can boost efficiency 100%.

Also, Ford eco-boost can do a better and much simpler.

Henry Gibson

If more than suficient steam were kept in an insulated tank, then both the car and the engine could be started with steam pressure most of the time. The Kitson-Still diesel locomotive used steam to move the locomotive and train at low speeds and then diesel started to be injected to run at higher speeds, and new steam was then heated by the cylinders and exhaust. Modern materials would allow a condenser to recycle the steam or some organic liquid. No gear shifting was needed and no diesel electric drive system.

Henry Gibson

@Dursun had a very good idea. Many people could use a car of that type.

Philips of Holland was working with a US car company to produce a car that would run a Stirling engine from the heat of molten salts. ..HG..


What they're using is called the Organic Rankine cycle and it's been around since the 1970s when it was used in low temp binary geothermal. Funny how it takes them over 3 1/2 decades to catch on.

Thermoelectric is 3% efficient and thermodynamic improves the thermal efficiency of the engine by 3.8% according to Honda... so that makes a petrol ICE equal in energy efficiency to a diesel..... why even bother???

An ICE will STILL waste well over half the energy it consumes... totally unacceptable.

Go EV and stop mucking around with heat engines!!! BMW and Honda would be better off spending their cubic billion dollar R&D budgets developing wheel motors so they can maximize brake regeneration... which is the ultimate in energy recycling... If they don't someone else will end up eating their lunch!


I'm with Paul. All this combustion efficiency nonsense is wasted just like the heat. Lots of complex systems just keep you paying and the service department rolling in cash.

EV systems are the best way to go. Simple, efficient, inexpensive (once the battery thing is figured out), long lived. They can be modular. Just install the latest battery/storage system and go. Spend the money on energy storage systems, not waste reduction.

Most of this is just PR to make the auto makers look good to their consumers.

EVs are easy to sell once you get people in them and driving. The EV smile says it all.

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