Honda Investigating ‘Rankine Cycle’ to Power Hybrids

Honda is investigating ways of recapturing heat energy lost through a car’s exhaust to help generate electricity and power an electric motor for a new generation of ultra-efficient hybrid cars. Conventional internal combustion engines waste a lot of energy in creating heat, but by applying a ‘Rankine cycle,’ a process where hot exhaust gases are used to heat up a water reservoir and create steam to spin a turbine, electricity can be generated. This electricity can then be used to charge up an array of batteries and help power a hybrid-electric powertrain.

Engineers have been using a Honda Stream as their test vehicle, a compact crossover sold in Japan and parts of Europe powered by 2.0L direct-injection petrol four-cylinder. The car was fitted with a new cylinder head with insulated exhaust ports, an evaporator built into the catalytic converter, a high-pressure water reservoir plus a generator and condenser. Maximum power available from the volumetric expander was as much as 32kW and maximum thermal efficiency of the unit is rated at 13% at 23kW.

Initial tests show that a car travelling at a constant speed of 100km/h and can improve thermal efficiency of the engine by 3.8%. In the U.S. highway cycle, the Rankine cycle system regenerated three times as much energy as current regenerative braking units. At a recent hybrid vehicle press event in San Diego, Honda officials said the design would need to see higher efficiencies achieved if were ever to enter production.

 

Honda Researching Advanced Hybrid Drive with Rankine Cycle Co-Generation

14 February 2008

by Jack Rosebro

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.

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).

Resources

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

 

Seems like a good idea. I wonder how much that would add to the pricetag.

 

Sounds like a GREAT idea. I've always wondered why the tremendous HEAT generated by engines couldn't be used (except to heat the occupants ).

 

Sounds pretty impressive

 

Didn't BMW show something like that last year? Also, what happens to the water when the temperatures outside are freezing? If you use antifreeze, will that produce steam?

 

Yes BMW has already done something extremely similar to this about 18 months back if I recall correctly... at least in their research department.

 

Sounds good.

Any extra heat can be used as radiant heating (the exhaust pipe goes under the car right?)

 

That is a very good question, anti-freeze seems incompatible with a steam turbine. Maybe it's efficiency rating has taken into account a warmer using energy to prevent the water from freezing so the net gain is not quite there yet.

 

How much weight does this system add, and is it effective at all in city driving conditions?