After a big buildup, diesels are coming to America, but with today’s soaring fuel prices, they may land with a thud.

In Europe, better than 50% of the new car purchases are diesels. In France, Spain and Belgium, the share is between 70% and 80%.

Ward’s counts more than 20 diesel-powered cars and trucks coming to the U.S. over the next several years, starting this fall. On the surface, diesels sound like a good bet: We know how to build them, they work and cost less to make than full hybrid-electric-vehicle powertrains.

In October, Mercedes will start building diesel-powered cross/utility vehicles in Alabama. Volkswagen and its Audi luxury brand soon will offer 4-cyl. and 6-cyl. diesels. Honda promises a 4-cyl.diesel as well, first in its Acura division.

General Motors, Ford and Chrysler all say they will offer light-duty diesels for their big pickups and possibly SUVs in a year or two.

Because diesels offer 20% to 30% better fuel economy than gasoline engines, their success in North America should be guaranteed. Instead, their future looks cloudy.

The cost of diesel fuel is topping $5 a gallon in some areas of the U.S., nearly 25% more than gasoline. That wipes out the fuel-economy gains.

Some experts think diesels even have peaked in Europe, now that diesel fuel prices are catching up with gasoline.

In Europe, diesel historically has been taxed at a lower rate than gasoline, but its price lately has become equally outrageous. A friend of mine in France told me last month he was paying the equivalent of $8.50 a gallon for diesel.

Here in the U.S., federal taxes are comparatively low, but diesel taxes actually are higher than on gasoline.

U.S. emission standards also are hard on diesels. The new Mercedes Bluetec system, for example, stores 8 gallons (30 L) of urea liquid in the vehicle. The urea is injected into the diesel exhaust stream and enables it to meet the toughest 50-state Tier 2 Bin 5 emissions standards for oxides of nitrogen.

But the urea supply is expected to last just 10,000 miles (16,000 km) and has to be replenished at regular service intervals.

Thanks to Environmental Protection Agency regulations, if the owner allows the urea tank to get low, the car tells him to take care of it. At about 1 gallon, (3.8 L) the car gives him 20 starts to get more urea fluid. If he doesn’t snap to it, the car won’t start.

We’ll have to see how that goes over.

Diesels also cost more, $1,000 to $3,000 on most cars and sometimes far more on light trucks. With those issues, how fast can diesels really grow?

Turbocharger maker Honeywell estimates that by 2013, 16% of the vehicles sold in the U.S. will be turbocharged, and half of those will be diesels. That’s a huge jump, considering diesels held only about 3% of the U.S. market in 2007.

Everybody is working on HEVs, plug-in hybrids, turbocharging smaller gasoline engines – anything that can be done fast. Diesels could be part of the survival strategy.

But will Americans really be attracted to diesels if fuel costs 25% more than gasoline, the engine costs at least a grand extra and it stops working if you don’t refill the urea tank?

This is one tough call.

Jerry Flint is a columnist for, and former senior editor of, Forbes magazine.