S AMERICA SEARCHES for a solution to harmful exhaust emissions and its dependence on foreign oil, diesel proponents claim the answer lies in diesel-powered passenger vehicles. The combination of cleaner diesel fuel and significant advances in diesel engine technology over the past two decades has resulted in dramatic improvements in diesel emissions and fuel efficiency, as well as engine reliability and durability.

Perhaps the greatest benefit offered by diesel engines is the fuel efficiency. Because the diesel combustion process is more efficient than gasoline, and because diesel fuel contains more energy per unit volume, diesel engines usually deliver 45 to 60 percent better fuel economy than gasoline engines. For example, the diesel-powered Volkswagen Jetta TDI station wagon, equipped with a five-speed manual transmission, achieves 50 miles per gallon on the highway and 42 mpg in the city. The gasoline version of the same vehicle achieves 31 mpg in the city. According to the Department of Energy, three of the top five highest fuel economy rated passenger cars were diesel-powered in model year 2001.

The fuel economy benefits and reliability offered by diesel-powered passenger cars has earned them a sizable group of devoted owners. "Once you've had one, to go back to a car that gets 15 to 20 miles per gallon is depressing," says Sam Johnson, a mass communications professor at St. Cloud State University in Minnesota and owner of a 2002 Jetta TDI wagon.

A recent study conducted by research firm M Cubed of Davis, CA, reports that gradually increasing the use of clean diesel technology in passenger vehicles to the levels currently seen in Europe could save the state of California 110 million gallons per year by 2030.

"Clean diesel technology is a proven, efficient and readily available solution for California's interest in reducing petroleum consumption," said Allen Schaeffer, executive director of the diesel Technology Forum (DTF), a sponsor of the study. "Other petroleum reduction strategies such as fuel cell-powered cars are not commercially available and may take 15, 20 or even 30 years of research and development to reach the market. And even then these other technologies would not be as cost effective as diesel or even use less net energy to produce."

Concerns over the effects of carbon-based greenhouse gases on the environment has prompted the Environmental Protection Agency (EPA) to mandate stricter and stricter emissions standards over the years. Because diesel engines offer greater overall efficiency and improved fuel economy when compared to gasoline engines, they emit 30-35 percent fewer carbon emissions. In addition, largely due to cleaner, lower sulfur diesel fuel and improved engine technology, total oxides of nitrogen (NOx) from on-road diesels have dropped 25 percent in the past 10-15 years, while sulfur dioxide (SO2) has dropped 76 percent, coarse particulate matter (PM-2.5) has dropped 35 percent in the last 10-15 years.

While Europe and Japan have recognized and embraced the benefits of diesel-powered passenger vehicles, the United States has the lowest percentage of diesel-powered passenger vehicles of any industrialised country. Diesel-powered passenger vehicles account for only about one-third of one percent of all U.S. car sales, or about 50,000 of the 16 million vehicles sold last year, while more than one-third of all new vehicle sales in Europe are diesels.

Diesel proponents realize there is great potential for diesel-powered passenger vehicles to thrive in the United States. Diesel-powered light trucks and SUVs are already increasing their presence in the U.S. market, with 435,000 diesel-powered light trucks manufactured for the North American market in 1999.

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