Hybrid vehicles make great use of regenerative braking and the efficiencies of electric motors for lower speeds and stop-and-go driving. Some hybrids are so good that they have better MPG ratings in the city than they do on the highway. But out on the highway, a hybrid doesn’t fare any better than a vehicle with standard gas engine. Regenerative braking is the way hybrids recover energy during stop-and-go driving, but at highway speeds, there is nothing helping feed back energy into the system, and hybrids are at par with non-hybrids.
Honda has now come out with a demonstration vehicle (using a Japan-only Honda Stream hybrid) that recaptures waste heat from the gasoline engine and runs a small Rankine cycle generator from that to recharge the battery pack.
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.
BMW has reportedly also been investigating co-generation energy recovery, but their system has only been used for running the vehicle accessories, rather than recharging a drive battery.
At present, the system is not cost effective for implementation in production cars. But with higher fuel prices in the future as well as improvements in the technology, this will likely find its way into production hybrid vehicles in the future.