So what exactly is a diesel hybrid? No, we're not talking about a diesel engine that runs on biodiesel, which we covered last month. Pay attention because there may be a test later.

A diesel hybrid is similar to a gasoline-electric vehicle such as the Ford Escape or Toyota Prius, but with a diesel engine rather than gasoline. The fuel mileage gains that can be achieved with diesel hybrids are even more impressive than gas-electric hybrids. Most are only prototypes now, but it's only a matter of time before a diesel hybrid is released in the United States.

If a hybrid drivetrain is designed with fuel economy in mind, it's only natural to pick a diesel engine instead of gasoline. Diesel engines are inherently more efficient for several reasons. First of all, diesel fuel contains more energy than gasoline, which only has 85 percent as much energy as diesel. Diesel is also much harder to ignite, so higher compression ratios are used. With nearly twice the compression ratio, diesels can achieve more torque than a gas engine of the same size. Diesel engines also use direct injection; fuel is introduced directly into the combustion chamber, resulting in better fuel atomization and more complete combustion. Finally, diesels burn about a third less fuel at idle than a comparable gasoline engine. Typically, gas-electric vehicles shut the engine off at idle to conserve fuel, so that aspect of fuel savings isn't realized but it still illustrates how diesels tend to sip fuel.

Of course, there are compromises with diesel engines. While many of the complaints that typically accompanied diesels have been resolved in the past 10 years, there are still a few key issues with consumers. The fuel is still harder to find for one, which could be a problem unless you live in an area that has a lot of big rig traffic. There is also the stigma of being a big polluter; everyone has seen big rigs and school busses pulling away from stoplights, leaving black clouds in their wake. While diesel emissions have improved significantly, they still emit about ten times as much particulate matter as a gasoline engine at the same load. Up until now, diesels have been given a break as far as emissions go, but that's about to end. EPA regulations for all engines will change in 2007, and diesel engines will not get any special provisions. They have to be just as clean as gasoline engines.