Diesel Hybrid Electric Cars
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How come nobody sells a hybrid diesel car?
For those of you familiar only with the sooty smoke belching from older big-rig trucks or the foul smells from 1970s diesel cars, the question may come as a surprise. But modern diesel engine design coupled with the much-cleaner types of diesel fuel increasingly available (particularly "biodiesel") make diesel vehicles a surprisingly environmentally-friendly choice. Diesel-hybrid-electrics would be an obvious positive development. So why don't we see them?
I suppose the answer varies depending upon where you are. In the US, the diesel fuel available in most locations remains the old, dirty, high-sulfur variety, so a hybrid diesel actually wouldn't be a significant improvement in emissions; once low-sulfur regulations take effect in 2006, this may change. In Europe, where advanced-technology "clean" diesel autos are one-third to one-half of the auto market and growing, some diesel cars already get mileage roughly equivalent to hybrids, so I suspect there's simply less demand.
The irony is that diesel hybrids could be far more efficient and clean than anything now on the market, without any leaps in technology. The combination of modern clean diesel engines, Prius-style serial hybrid-electric systems, and biodiesel/vegetable oil fuels could provide amazing mileage, cleaner air, and vastly reduced petroleum dependency. Comfortable, powerful sedans could get upwards of 80 miles per gallon and be carbon-neutral.
It's certainly not that diesel hybrids are somehow impossible. Diesel-electric hybrid buses are available and have been rolled out in (among other places) Seattle, Washington and Apeldoorn, in the Netherlands. As for autos, Ford, GM, and Daimler-Chrysler each built prototype diesel hybrids a few years ago which got mileage in the 70-80 mpg range. But the automakers opted not to produce them, as the cars couldn't meet strict air pollution rules while running on the sulfur-laden American diesel.
Combining the power of diesel engines with the efficiency of hybrid technologies can have terrific payoffs. Last year, MIT's Laboratory for Energy and the Environment produced a study (PDF) comparing total lifecycle energy efficiency and greenhouse emissions (including use, production, fuel production, and eventual disposal) of idealized advanced internal combustion, hybrid, and fuel cell vehicles. Diesel hybrids turned out to be much better than gasoline/gasoline-hybrid cars, and highly competitive with the best hydrogen fuel cell systems (even assuming optimistic fuel cell vehicle development). But the best hydrogen fuel cell vehicles will require entirely new hydrogen production, storage, and fueling facilities; reformed-gasoline fuel cells (which are more likely to be used, as they would not require the wholesale replacement of fueling stations) fared much worse.
It's particularly notable that the results in the MIT study were based on the assumption that the diesel fuel would be petroleum-based. One of the compelling aspects of diesel engines is their ability to run on biodiesel, a fuel which does not actually contain any petroleum. Biodiesel is synthesized entirely from plants -- usually soy in the US, and canola/rapeseed in Europe -- and is therefore carbon-neutral: the soy/canola grown to create biodiesel pulls from the air the same amount of carbon the eventual combustion produces. Biodiesel is available both straight or in mixes with regular -- and, up until recently, somewhat less expensive -- oil-based diesel. These fuels are referred to by the percentage of biodiesel in the mix, from B5 (five percent bio) all the way to B100 (straight biodiesel); B20 is a relatively common mix. A variety of companies supply biodiesel around the world; adventurous types can even make biodiesel at home, using vegetable oil.
(Vegetable oil can be used to run diesel engines directly, although doing so can be somewhat risky. SVO, or "straight vegetable oil," can gum up the engine at lower temperatures, so there are SVO-conversion kits available for diesel vehicles which use a small amount of regular diesel or biodiesel to warm up the engine first. This is one way that hybrid technology could really show its strengths: a hybrid-SVO could warm up the SVO with an electric heater while running on batteries at lower speeds, much like the current Prius does. Such a system could easily use regular diesel and biodiesel, as well.)
European automakers are testing the water to see if American buyers, long ago turned off by earlier diesel technology, would be willing to give diesels another chance. The carmakers may want to wait for a couple of years; the 2006 change-over to clean diesel will make diesel vehicles much more attractive. The manufacturers should take advantage of the wait to license the Toyota hybrid system (as Ford did earlier this year) and integrate it with their advanced clean diesel engines. Stylish sedans coupling the power of a diesel engine with extreme fuel efficiency better than anything coming out of Japan -- dealers wouldn't be able to keep them in stock.