Perhaps the biggest drawback for diesel is the initial cost. The higher compression required in a diesel application requires tougher engine components from the block and heads to pistons, connecting rods, valves, and the crank. These heavy-duty parts cost more to build and they also increase weight. Add the additional cost of a hybrid powertrain and it's easy to imagine why manufacturers have been slow to adapt diesel hybrids to passenger car and truck applications.

GM, DaimlerChrysler, and BMW have partnered to work on a two-mode diesel hybrid system that is flexible enough that it can be scaled to fit FWD, RWD, or AWD applications. Two-mode means that power is applied in two different ways, depending on load, to increase mileage as much as 20 percent. So far, this hybrid technology has been used on the GMC Graphyte concept, a mid-sized SUV that might show hints of the next generation crossover.

The Graphyte's engine was gasoline, but the same technology was used on GMC's latest concept, an Opel Astra hatchback that features a hybrid diesel powerplant. What's remarkable about the Astra application is that the Astra wasn't designed as a hybrid, yet the system met the packaging requirements of a compact FWD platform. The Astra will be showing up in the States as the next generation Saturn ION, so a diesel hybrid could be on its way, too.

The best examples of diesel hybrid capability, at least as far as trucks go, come from military concepts. The U.S. military is constantly trying to improve its mobility and speed, which is where hybrids come in. Supplying fuel to vehicles on the battlefield is a tricky and expensive process. When all of the transportation costs are factored in, fuel delivered on the battlefield can cost more than $400 per gallon. If you thought $3 per gallon was motivating you to reduce your fuel consumption, imagine how badly Uncle Sam wants to conserve fuel..