So it only can re-use the energy of slowing down?Every time you slow down, it makes more juice!
How does that help HIGHWAY mileage?
Obviously if you are on the highway, you do very little braking compared to city driving, and the benefit of regeneration of braking energy is de minimus. AFAIK hybrid vehicles attain much of their efficiency by using the electrical generator and battery to control the speed and power output of the gasoline engine to allow it to operate at the RPM and power at which it is most fuel-efficient.A primary disadvantage of the diesel engine is the relatively low RPM limit imposed by the massive pistons and connecting rods needed to withstand the "diesel knock" which occurs in normal operation. And although the diesel is so much more expensive and more efficient than the gasoline engine that it would be less necessary, I keep expecting someone to come up with a diesel/electric hybrid to minimize the RPM sensitivity of the diesel by holding RPM constant. That could be one response to the Obama efficiency mandate . . .
...The BMW 335d clean diesel sedan arrives in showrooms this month, followed by the BMW X5 clean diesel SUV in January. Before the public has had a chance to decide on the value equation of its first two luxury diesel vehicles in the $40,000 to $50,000 price range, BMW is now considering an even more expensive clean diesel 7-series sedan.
The European 730d is powered by a turbocharged six-cylinder diesel engine and yields 45 miles per gallon. Even with this impressive fuel efficiency, BMW is gambling on a much-improved economy, and the price of gasoline heading back up, by the time the diesel 7-series would arrive sometime in mid-2010.... http://www.hybridcars.com/buying-decision/bmws-7-series-diesel-possibly-headed-us-25304.html