Will you buy a diesel?

Really? Are you actually ready to get out your checkbook? The new “clean” diesels are almost here.

On the eve of the first press day of the New York auto show, Mercedes-Benz is holding an event (featuring Sheryl Crow) in connection with the arrival of its latest Bluetec diesels, models that are engineered to meet emissions standards for all 50 states (but at last check, had not been given final approval by the Air Resources Board in California). Audi and Volkswagen are soon to follow; BMW, Honda, Nissan and others are likely to be close behind.

But are Americans ready to buy them? After driving a diesel-equipped Chrysler in Europe recently, in which I got 50 percent better mileage than I did with its gasoline-burning counterpart in the U.S., I was nearly enthusiastic enough about them to buy one.

Then I did some research. Turns out even clean diesels have their dark sides.

Certainly, this coming generation of clean diesels is a significant, highly anticipated technological advance.

But a clean diesel will be a costly option – adding up to $2,000 to your new car’s window sticker.

And clean diesels still are just not as clean, from a smog-producing standpoint, as the best gasoline engines.

Csaba Csere, editor of Car and Driver, has been following the progress of clean diesels. He told me, “They are about as clean as a 50-state-legal gasoline engine.” But they are not capable of getting down to, say, ultra-clean SULEV emissions levels (a certification standard under California’s emissions regulations, which are the toughest to meet). “They are, however about 20 percent cleaner when it comes to carbon dioxide emissions.”

Diesels will generally deliver more torque. And they will get 25-50 percent better mileage. “Figure 30 percent as a good midrange measurement,” Mr. Csere said.

Diesel fuel isn’t as widely available as gasoline, of course, so you will have to plan your fill-ups more carefully around diesel availability. Will truck stops be an agreeable venue for you?

But more important, diesel fuel costs more in the United States. In fact, out here in California, diesel just zoomed past $4 a gallon. Even $4 a gallon is a bargain, relative to its price in Europe, where a gallon of diesel can cost $7. Gasoline is even more expensive there — up to $8 a gallon in Italy.

It may take quite a bit of driving — years’ worth, if ever — for the added fuel economy of a diesel to recoup the extra initial cost of the diesel engine.

“The cost advantage of a diesel is half what it once was”, Mr. Csere noted, compared with 25 years ago when Mercedes-Benz once sold 70 percent of its cars in the U.S. with diesel engines. Back then, diesel fuel was cheaper than gasoline. “And as diesel prices continue to rise, that advantage shrinks even further.”

Talk about a diminishing rate of return. My enthusiasm has been damped, to say the least. I still admire the technology, and think it is an important milestone on the road to more environmentally responsible vehicles. But is clean diesel-technology a panacea for America’s dependence on foreign oil that some people, especially the German automakers, seem to think it is?

Is a clean diesel-powered car still on your shopping list? It’s probably worth a little due diligence before taking the plunge.

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