Amid renewed criticism that they are selling cars with low fuel economy, many auto manufacturers are once again expressing interest in diesel engines.
But are consumers really ready to buy diesels again, even in the face of sharply higher gasoline prices this summer?
The answer is unclear, but some analysts think they will.
"Conventional wisdom today is that we'll probably see diesels in the US. It's a question of when," says Dan Margosian, a spokesman for Amoco.
Craig Dana, vice president and technical director of the Association of International Automobile Manufacturers, concurs. "With the pressure [to improve] light truck fuel economy, people are going to be looking at diesels," he says.
The rising interest in diesels can be seen in several places.
*The Partnership for a New Generation of Vehicles, a cooperative effort between the government and the Big Three, is looking at diesels as one possibility for a full- sized vehicle that would get 80 miles per gallon and still meet consumers' current expectations for performance, comfort, size, and price by 2004.
*Ford and Chrysler are developing Diesel-electric hybrids as part of the PNGV program. Chrysler exhibited one such model at the Detroit auto show this year. While the car is only for show, some of the technology it uses will likely trickle down to production cars in the near future, says Steve Barnhart of Chrysler's advanced technology group. "We're excited about diesels because of their efficiency," Mr. Barnhart says. "Diesels are the most efficient engines available today."
*At least one automaker is banking heavily on Americans' interest in that efficiency: Volkswagen is planning on offering three diesel models here in the fall. One of those is at dealerships now. Mercedes-Benz is the only other carmaker currently selling a diesel model in the US.
The main advantage of diesels has always been their fuel economy. That advantage caused Americans to flock to diesels, at least temporarily, as a respite from high gas prices in the early 1980s. But interest slipped, and Barnhart says it could take a $3 to $4-per-gallon hike to get Americans to change their buying habits. In Europe, where fuel prices are as much as three times those in the US, diesels account for more than 50 percent of vehicles in use.
Thermally, diesels are 40 to 45 percent efficient, compared with 30 to 35 percent for gas engines, Barnhart says. (Thermal efficiency is a measure of an engine's power output relative to the potential energy in the fuel it burns. The residual energy becomes heat and is wasted.)
Also, because they have no electrical ignition system and must be built stronger to withstand higher internal pressures, diesels have a reputation for reliability and longevity surpassing that of gasoline engines.
There is another environmental plus to diesels as well, says Tony Fouladpour, a spokesman for Volkswagen US. Diesels emit 20 percent less greenhouse-warming carbon dioxide than equivalent gasoline engines, and diesel fuel requires less energy to refine, he says.
The diesel's fuel economy is a draw with consumers. "The mileage issue is not just economic, but one of convenience," Mr. Fouladpour says. "A lot of customers ... don't like to stop at filling stations so often."
Diesels must still overcome their reputation for being noisy and sluggish before they can gain consumer acceptance. But engineers are working out these problems.
Volkswagen's new turbocharged direct injection (TDI) engine is the first of its kind in a passenger car. Direct injection has usually been rejected as too noisy and rough for passenger-cars. But the technology improves efficiency and emissions. And a new computer-controlled fuel injection system has largely tamed this harsh vibration in the Passat TDI. While the car still sounds like a diesel, in a week of testing it proved much quieter and more powerful than diesels of yore.
Another problem with diesels, as anyone who has ever followed one knows, is pollution. While Fouladpour is correct that Diesels produce 20 percent less of the greenhouse-warming CO2, that is based on their better fuel economy. Yet they have trouble meeting standards for nitrogen oxides, which form smog.
As a result, says Jim Spearot, head of General Motors' Fuels and Lubricants Research and Development department, "nothing is going to happen with diesels in the US, at least on GM's part, in the near future," mainly because of emissions regulations. "As far as the US is concerned, we would be looking for advances in pollution control to meet emissions laws after the turn of the century," he says.
Those emissions regulations are largely coming from the state of California. While the state's regulations no longer restrict emissions from individual models, they do limit the overall emissions of a manufacturer's entire fleet, says Allan Hirsch, a spokesman for the California Air Resources Board. And to meet these ever-tightening standards, carmakers will likely have to begin selling cars that qualify as low emission vehicles (LEVs) by the year 2000, Mr. Hirsch says. "I'm not aware of any technology that would allow diesels to meet LEV standards," he says.
But there too, engineers have made progress in cleaning up diesel emissions.
Mercedes-Benz hopes its own direct injection diesel, to be introduced in 1998, will meet LEV standards. The company's current E300 Diesel is the only diesel in the world that uses four valves per cylinder to increase airflow and promote cleaner burning, says Greg Goolsby, Mercedes's E-class product manager. The car shows no sign of diesel smoke or smell.
Fuel improvements may also help clean up diesel exhaust. Three years ago, diesels were banned from California because none met the state's emissions requirements. But in 1993, the state mandated new low-sulphur diesel fuel, which allowed Mercedes and VW cars to pass certification. Now oil and auto companies are working on improving fuel technology to make diesels pass the stricter future standards.