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);
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)