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.

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..

In response to the Army's needs, GM built a diesel hybrid Chevy Crew Cab to showcase its latest hybrid and fuel cell technology. The truck is powered by a 6.6L Duramax diesel and electric motors that can increase fuel mileage by 20 percent and also work as a generator, producing 30kw of electricity. The truck carries a fuel cell generator in the bed that can run when the engine is off to produce power more quietly than a gasoline or diesel generator while also reducing the heat signature so it's harder to detect with infrared optics.

Another interesting hybrid vehicle being tested by the military is the Shadow RST-V. The acronym stands for reconnaissance, surveillance, and targeting vehicle, and the U.S. Marines have been testing it for several years. The idea is to use the Shadow in some of the roles that have been previously filled by Humvees.

Powering the Shadow is a 2.5L turbocharged Detroit Diesel direct-injection inline four cylinder making 138hp. The engine is paired with a lithium ion battery to power four 50kw motors, one mounted on each hub. The diesel-electric hybrid system in the Shadow serves several purposes; it cuts fuel consumption by more than half when compared to a Humvee in the same role and it allows the Shadow to run in stealth mode. That's right, stealth. Ok, so it's not invisible, but the Shadow can run for more than 20 miles on electric power alone, allowing it to be quiet and also reduce its heat signature. Like the hybrid trucks, the Shadow can also use its engine to generate electric power, reducing the need to tow heavy, noisy generators.

The same capabilities that attracted the military's attention will likely make civilian diesel hybrids attractive to consumers. It will probably take years of ownership to offset the price premium that comes with the additional hybrid technology, and OEMs realize that few truck buyers are likely to pay the extra dough just for reduced emissions, so hybrids must offer other advantages. The biggest marketing strategy has been touting the hybrids as mobile generators. The idea of an onboard generator isn't just appealing to contractors but could be the ultimate tailgating accessory. Off-roaders could take along a welder on their trail ride, so selling that convenience will become a big part of making hybrid trucks appealing.

While the first consumer truck application of diesel hybrid power will almost certainly come from GM, Ford, or DaimlerChrysler, just imagine a civilian version of the Shadow tearing up logging roads and blasting through the desert. Don't laugh, four-wheel independent suspension on an off-road vehicle was a joke before the Humvee proved itself, so hopefully this military technology-in an affordable package-will trickle down to the civilian market

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