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Honda Researching Advanced Hybrid Drive with Rankine Cycle Co-Generation

by Jack Rosebro

Outline of Honda’s Rankine cycle waste heat recovery system. Click to enlarge.

Honda is 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. Honda engineer Kensaku Yamamoto presented an overview of the work in a paper at the 2008 SAE Hybrid Vehicle Technology Symposium in San Diego.

Test results 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.

Layout of the system components in the test vehicle. Click to enlarge.

The Rankine cycle is a simple closed thermodynamic cycle that converts heat from an external source into work. Variants of the Rankine cycle have been explored by others as a mechanism for waste heat recovery. Cummins, for example, is exploring the use of a Rankine Bottoming Cycle system to boost the performance of its heavy-duty diesel engines. (Earlier post.)

Honda also looked at the possibility of incorporating a gas turbine or a Stirling engine before settling on the Rankine cycle system as the best solution. The temperature range of an internal combustion engine’s exhaust corresponds favorably with a Rankine cycle.

Honda’s test vehicle was a hybridized version of the Honda Stream compact crossover vehicle, which uses a 2.0L gasoline direct injection engine. The Stream is sold in Japan as well as parts of Europe. Elements of the Honda Rankine cycle system include:

  • A modified cylinder head with insulated exhaust ports;

  • Evaporator built into the catalytic converter;

  • High-pressure water unit (water is the working fluid for the Rankine system);

  • Expander/generator; and

  • Condenser

The high-pressure water pump forces water into the evaporator, which converts the water into steam using the reaction heat of the catalyst. The steam is then passed to a volumetric expander that uses the steam to rotate an electric generator, which produces a current that is utilized to charge the vehicle’s battery pack.

The volumetric expander is an axial piston swash plate type, which is similar in construction to some automotive air conditioning compressors. The steam is then routed from the volumetric expander to a condenser mounted in the air stream at the front of the car. The condenser returns the steam to a liquid state before passing it along the high-pressure water pump.

Honda developed an automatic steam’s control system for the Rankine unit to keep the steam in a target range of 400-500° C and at a pressure ranging from 7-9 MPa, depending upon the load on the engine. The control system allows optimized use of the Rankine cycle in transient driving conditions.

Maximum power available from the volumetric expander is as much as 32kW (43hp), and maximum thermal efficiency of the unit is 13% at 23kW (30hp). In comments following the presentation, Yamamoto indicated that Honda would need to see higher efficiencies achieved with the system if it is going to be considered for production. A paper on the system will be presented at SAE World Congress 2008.

BMW has also developed an onboard water/steam-based cogeneration cycle in a research vehicle, but that system is used to power the vehicle’s accessories, rather than a traction battery pack (earlier post).


  • Advanced Transient Simulation on Hybrid Vehicle Using Rankine CYcle System (SAE 2008-01-0310, not yet published)


Healthy Breeze

Tip of the hat to them for doing it. Given how much energy is lost in combustion, it's a pity 3.8% is all that gets easily recovered.



Seems like energy costs are rising to a level, were previously unthinkable improvements of efficiency are suddenly possible. Exhaust gas heat is a significant, yet completely wasted source of energy. Since all these concepts of recapturing waste heat are limited by the carnot cycle limts (approx 30-40% at the temperature/pressure ranges possible in a mobile application, achiving 13% in an early prototype is significant (unless this is 13% of the theoretical maximum of 30-40%...).

I'll take three more generations of cars at least, before we can have a commercial version; plus crude at 150-200 USD, I guess...


3.8%? The Cyclone engine that uses engine heat to generate another stroke should be able to achieve more. Am I right?

Rafael Seidl

@ HealthyBreeze -

Unfortunately, the secondary steam cycle components are somewhat bulky and heavy, which explains why the gain is not greater. Given that the peak effective thermodynamic efficiency of a conventional gasoline engine is ~33%, so a ~4% in *that* actually translates to a best-case fuel economy gain of roughly

1/33 - 1/37
----------- = ~11%

In stop-and-go traffic, the secondary steam cycle will exhibit relatively poor dynamics and much lower efficiency. Still, the same is true for the ICE, so the relative fuel economy gain may be similar. In terms of engine power, the parasitic load of the alternator and perhaps even the A/C compressor can be eliminated.

Of course, you can get ~15% better fuel economy for substantially less by applying a regular turbo and ~25% downsizing. It seems unlikely that anything involving a secondary steam cycle will come to market anytime soon. See also: BMW Turbosteamer.


Marginal efficiency gain and probably at high cost both initially and in maintenance... I won't hold my breath for waste heat recovery anytime soon.


Way too complex. Downsized/turbocharged/micro-hybrid has got to be a more cost-effective way to go.


Nifty idea, and a credit to Honda. But... seems like a very complicated solution with modest benefits, as several, particularly Rafael, have pointed out. Reminiscent of the Accord Hybrid, with more engine than it needed, a borderline silly cylinder deactivation system on that excessively big engine, even sillier active engine mounts to counteract the imbalance of the V3 mode, the electric motor to keep the AC compressor running at idle shutoff, etc. etc. The Accord Hybrid was a marketing failure, for those who may have missed it.

I am still puzzled why we are not seeing cars that are basically electric cars with an onboard generator, which could run at a steady state, efficiently, and be less than 1.0L displacement, maybe a lot less. This is not a brand new idea - diesel locomotives have had a similar architecture for at least 50 years. It doesn't even require hub motors, you could have a single electric motor running into a differential, using lots of off the shelf parts, and you could have a modest battery pack - marginally bigger than what's now in hybrids - to provide limited PHEV operation and enough power for good acceleration.


Why not instead just use an exhaust gas turbo generator (similar, but simpler than an aircrafts APU) to directly generate the electrical energy ?


Interesting but totally useless, recovery of waste heat on ICE can't be efficient since the waste heat are scattered between liquid coolant and exhaust and a steam engine needs multiple heat recovery to be efficient so make it very complicated. Better to switch to a full steam engine like Cyclone technology, simpler an cleaner.

s dogood

Gary took the words right out of my "mouth".



I agree, it is not hard to imagine a series hybrid that would cost very much. Add batteries to make it a plug hybrid as the prices come down.

On this heat recovery story, I think that they can get the efficiency up and I like using it for electricity in a hybrid instead of the mechanical drive of the turbo steamer.



Re: serial hybrid with small battery: How much efficiency is lost in the power conversions (engine->generator->motor) from mechanical to electrical and back? With a small battery you're going to spend most of your time with that genset running. (You can benefit from idle-stop and regen braking, of course.)

I think serial hybrids start to make a lot of sense with a BIG battery, as a plug-in. Then you're getting low cost grid power, which can come from green sources as well.


Is the exshast pipe hot enough to attach a small stirlling engine to it, then recharge the battiers from that ?

Mad Max

The principle is interesting, but why not start from something that is already bigger and heavyer than a car, such as a Diesel rail locomotive? Then the sysstem may be scaled down to trucks and finally to cars.


"Is the exshast pipe hot enough to attach a small stirlling engine to it, then recharge the battiers from that ?"

the article states Honda considered this, but rejected it, probably for reasons of size/cost/weight.

John Taylor

The extra parts are going to add enough cost, weight, and repairs to negate any gains in operating costs. Few will buy it.

But... the system would be great indoors as a part of your home heating system. Making your own electricity while you heat your home would help out total grid electricity in the winter.

Alex Kovnat

My suggestion: Avoid generating as much waste heat to begin with. How? By using totally variable valve timing, and controlling the intake valve timing so that the expansion stroke is substantially longer than the compression stroke. That way, the exhaust will be cooler. Voila: You won't have so much waste heat going out the exhaust.

To the extent that there is waste heat to be recovered, turbocompounding may be less effective but at the same time, simpler than using a steam engine. Those of us who value our nation's aviation heritage no doubt remember turbocompound engines were used on the old Lockheed Super Constellation and Douglas DC-7 piston engine airliners of the 1950's.

Rafael Seidl

@ Gary -

@ Treehugger -

turbochargers are actually fairly efficient if they are designed to operate in a narrow range. This is the case for heavy-duty trucks, diesel locomotives and marine diesels. Passenger car turbos are less efficient because the engines operate in part load much of the time.

The heat lost to the coolant - approx. 1/3 of total input in gasoline engines - is at a low temperature, roughly the boiling point of water. That means it is low-grade, i.e. its specific exergy is low. In stationary applications, it can be used to heat buildings, greenhouses, swimming pools, support indoor aquafarming of tropical fish or shellfish, support indoor agriculture of certain types of mushrooms etc. It can also be used to drive a secondary absorption chiller. In mobile applications, coolant heat is currently exploited only for cabin heating, though several researchers in Germany are working on vibration-tolerant compact chiller systems powered by low-grade heat.

In theory, you could use part of the engine coolant as the working medium of the secondary cycle. However, the boiler device would have to evaporate not just the water but also the anti-freeze components. Otherwise, you might end up with ice formation in the secondary cycle if you leave the vehicle parked outside overnight in severe cold after driving it.

You could even use the engine itself as a steam generator by sharply reducing coolant mass flow. Unfortunately, heat transfer to a gas is orders of magnitude less effective than to a liquid, so critical components - e.g. the thin section of metal separating the exhaust valves in the cylinder head might not receive sufficient cooling and melt. The issue becomes more manageable if the coolant mass flow through the head is controlled separately from that through the block.

@ Mad Max -

co-generation on board ships, locomotives (except yard shunters) or 18-wheelers would indeed make more sense as these vehicles tend to cruise for long distances. That makes tuning the whole system for optimum efficiency much simpler. There is also more space for the additional components and radiators. After all, any heat not lost via the exhaust gases must be transferred to the ambient air in a heat exchanger.

The downside is that increased complexity also implies reduced reliability. Container ships run on giant two-stroke diesel engines to avoid camshafts, valves and a transmission. What you don't have cannot break.

tony d

Anyone know what the delta T needs to be for a reasonably efficient Stirling engine?
I thought a cheap way to use waste heat might look like this...
replace starter battery with 4 firefly (48V).
run everything on them. ac, ps, waterpump, all electrics.
remove alternator, replace radiator with stirling to trickle charge the fireflies.
Don't know how high the water temp can go on ice without major changes in materials.


John, home heating systems have almost no heat loss.
My gas-powered home heating has an efficiency of 106% (because gaseous natural gas is converted to CO2 and H2O, which is condensed to liquid H2O).
I would rather see such heat-recovery systems in large power plants or waste incinerators.


Did I miss something? Water still frezes at 0 C / 32 F right?

richard schumacher

On the home heat recovery tangent: in warm climates one would get a much faster payoff rejecting heat from the air conditioner compressor into a potable water pre-heater which then supplies the nominal water heater. Roughly comparable amounts of heat are involved in cooling a typical house and that house's hot water requirement, so during the cooling season one can get almost free hot water and a reduced power requirement for the A/C system (because it is rejecting heat into incoming utility water, which can be 20'C cooler than the outside air).


Honda and BMW should not be using water to steam as the working fluid for either application.

They should use either propane or butane to allow boiling at a much lower temperature (say 50oC) to allow collection of the majority of the waste heat from the engine coolant circuit as well as an exhaust heat exchanger.

Such an approach would also lead to smaller radiator openings, which would improve aerodynamic Cd too.

With a line wrapped around the catalytic converter pumping propane, Hope there's no leaks!


They can not take too much heat from the cat converter, it works with heat. BMW used ethanol as one of the working fluids. There are engineered fluids that freeze below -100F and boil below 212F and are non flammable.

To recover heat from the cooling system in a car it might take an Organic Rankine Cycle machine. They are called organic, because the early ones used organic fluids that boil at lower temperatures.

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