But what about a diesel hybrid? It sounds like the Holy Grail for fuel economy, combining two great fuel-saving technologies into one package. Indeed, Volvo's promising to put it into practice as soon as 2012. BMW, though, sees some temptation and lots of dollar signs in combining diesel engines and hybrid motors.
Putting the two together is an "interesting combination," says Wolfgang Mattes, who's in charge of getting BMW's diesels ready for sale in the United States. A diesel hybrid could boost fuel economy over a gas-only engine by 10 to 15 percent.
However, BMW sees more potential by boosting the efficiency of its diesel engines alone. Even from the current state of the art, they "can optimize another 15 to 20 percent, just from existing diesels," Mattes says, through eight-speed transmissions, reductions in friction and the like.
The essential problem with combining the two, he notes, is cost. Hybrids and diesels have similar cost disadvantages to plain old gas-powered engines--and putting them together might only work on a high-end vehicle where the cost would be a smaller fraction of the purchase price.
While BMW expands both hybrids and diesels across its lineup, "all options are on the table," says Mattes; even a Z4 powered by diesel isn't out of the question.
Though its lineup is canted toward diesels--nearly half its sales in Europe are oil-burners--BMW is pursuing hybrids at the same time. "There is a segment where hybrids make sense and a bigger segment where diesels make sense," Mattes says.
Interestingly, BMW's hybrid technology is getting help from its BMW-Sauber F1 team, says Markus Duesmann, Director of Powertrain Development for BMW-Sauber. The race team's new KERS--for "kinetic energy recovery system"--uses F1's astonishing braking forces to charge batteries that can be discharged up to once per race lap for an instant burst of power, one that can net critical seconds overall in a Formula 1 race. The team will use it in its 2009 season cars.
Sauber in the past helped BMW engineer the carbon-fiber roof found on the current M3. Could the race team's hybrid tech ever find its way into roadgoing BMWs? "It would be ideal for an M car," Duesmann says.