One of the most widely-read WorldChanging articles we've ever done (at least based on ongoing Google hits) is Diesel Hybrid Electric Cars Now!. Clean diesel engines in Europe get very good mileage; a clean diesel hybrid could easily get upwards of 70-80 miles per gallon, and probably more. But despite a few test vehicles shown back around 2000 and the growing use of diesel hybrid buses, nobody seems to be making diesel hybrids for the consumer market.
That may soon change. Green Car Congress (of course) has had a couple of posts in the last few days about the "ECO TARGET" diesel hybrid powertrain shown at a recent "Engine & Environment" conference in Austria. It's a "mild hybrid" -- the electric motor is used for power assist, not as an alternative source of motive power -- but it is designed to be an "add on" to existing diesel engine designs. According to GCC, the designers claim a "30% improvement in fuel economy and reduced emissions against the baseline conventional 2-liter diesel."
AVI, the primary design company, is explicit about the rationale for the diesel hybrid system:
The main issues around which compliance with future mobility requirements revolve are of an ecological and economic nature: fuel consumption should be reduced through a variety of measures, in the interests both of the environment and people’s wallets. The constraints are already defined: in July 1998, the European Automobile Manufacturers Association (ACEA) committed to reducing fleet CO2 emissions for new vehicles to 140 g/km by 2008. By 2012, this parameter should be at the 120 g/km level. The Kyoto conference stipulated a fleet CO2 emission of 90 g/km for 2010.
AVI claims that their diesel hybrid powertrain will have CO2 emissions in the 90-100 g/km range. GCC reports that the ECO-TARGET design still has a few kinks yet to work out, but could be available to automakers in another two years or so, just in time for European automakers to meet the Kyoto fleet emission deadline.