Honda testing Rankine cycle hybrid to recapture energy

Back in my engine design class in college one of our lab experiments involved determining where the energy in gasoline goes in a running engine. Using an engine running on a dynamometer, we measured the amount of fuel going in, the power at the flywheel, and the temperature of the exhaust and coolant. We used the increase in the exhaust and coolant temperature as it flows through the engine to determine that over two thirds of the energy in the fuel was dissipated through the exhaust and cooling systems. That lost heat energy is one of the reasons that engineers spent a lot of time looking at ceramic materials for engines during the eighties and nineties because of their ability to survive at higher temperatures. This would have allowed engines to run at higher temperatures sending more of the energy to the flywheel instead of the radiator. Problems with other material properties ultimately scuttled the idea.

The ever-innovative engineers at Honda are still looking for ways to use some that wasted energy. The most common way that exhaust energy is used today is drive turbochargers. Honda's engineers have devised a system with a Rankine cycle generator that uses steam produced by heating water with the hot exhaust to drive an electrical generator. The system has been adapted to a hybridized Honda Stream (a Japanese domestic-market model) to provide more electrical power generation at higher speeds. At highway speeds the Rankine cycle generator was able to produce three times the energy of the regenerative braking system and increased the overall thermal efficiency of the powertrain by 3.8 percent. Nonetheless the Rankine system only had a peak of 13.8 percent thermal efficiency at 30hp output, something that Honda's Kensaku Yamamoto said that would have to improve before they would consider producing such a system. Perhaps if they harnessed the engine coolant as well, they could close off more of the front air intakes to improve aerodynamics. Honda will publish a paper on their research at the SAE World Congress in April.

[Source: GreenCarCongress via EcoGeek]

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