Not too many years ago, the subject of this story would have been moot. There were no diesel-powered, light-duty trucks available on the U.S. market. That situation has changed, but a number of factors are keeping the availability of diesel-powered light trucks fluid.

In Europe and Asia, where fuel prices are several times those of the U.S., there is no such thing as a gasoline-powered light truck. Since the first diesel engines suitable for use in light trucks were offered in the late 1920s, European and Asian manufacturers leapt at the chance to use them.

The Diesel Edge

Why the rush toward diesels in those parts of the world, and the lackadaisical approach to them in the Western Hemisphere and the Middle East? The primary answer is fuel cost. The No. 1 advantage of a diesel engine over a conventional gasoline engine is that it consumes less fuel to get a given job done. The diesel combustion cycle, particularly when assisted by a supercharger or turbo-supercharger (turbo), is more efficient than the traditional Otto cycle used by gasoline engines.

In countries in which the cost of diesel fuel is approaching $1 per liter (equivalent to almost $4 per U.S. gallon), the diesel's ability to sip its fuel can result in some significant cost savings. Here, however, where the cost of diesel fuel is in the neighborhood of 30 cents per liter, the negative aspects of the diesel engine weigh more heavily into the equation.

The second advantage of the diesel is its ability to run practically forever, assuming normal maintenance. Whereas million-mile gasoline engines are a rarity, a million-mile service life is normal for a truck diesel engine. All a diesel engine asks to deliver this kind of long-term service is absolute adherence to the oil/filter change schedule.

Part of the reason for this long-term reliability is that diesel engines are built with greater precision than gasoline engines, and because of the nature of diesel combustion, they are more ruggedly built as well. That's fine, but the downside of that is that it costs more to build them that way, so there is a considerable premium involved when you check the "Diesel" box on the order form.

And the Bad News Is...

The downsides to the diesel, beyond the higher initial cost, come in the form of specific output, weight, noise, and emissions.

Diesel engines put out less horsepower for a given displacement in liters than a spark ignition (gasoline) engine of the same displacement. This is balanced by the fact that the diesel puts out far greater torque. So, if your application is one that uses horsepower, such as long, over-the-road trips at highway speeds, a gasoline engine will probably be more efficient. On the other hand, if your application includes short-run, stop-and-go driving while carrying heavy loads, the diesel will shine.

Diesel engines are heavier than their gasoline counterparts. For this reason, you will rarely find a diesel in a half-ton truck. The heavier suspension of three-quarter-ton and larger trucks is needed for the heavier diesel engine

Diesel engines are noisy. It's not a major consideration, but those outside the vehicles sometimes find it annoying. They clatter and rattle, particularly when starting up. The noise level goes down as the engine warms up, and modern trucks have cabs that filter out much of the noise from the driver. Developments are under way using electronic control of diesel fuel injection that promise to reduce or even eliminate the typical diesel clatter.

Particulates - the Real Problem

Diesel engines pollute. This is the big sticking point for the future, particularly after 2002. The state of California, the bellwether in emissions legislation, has considered issuing new rules that may effectively eliminate diesel engines as they are currently configured.

The pollution situation with the diesel is different from that of spark-ignition engines in that they put out a different form of pollution. The main ingredient of diesel pollution is smoke. Even the cleanest diesel, producing no visible smoke, emits particulates, which can possibly cause cancer when breathed for a sufficient period of time.

Auto manufacturers and diesel engine manufacturers are working to overcome the particulate problem. The diesel is regarded as the best answer in a hybrid propulsion system, where an internal-combustion engine shares duties with an electric motor. This is because in terms of the pollutants emitted by a spark-ignition engine, diesels pump out far less.

In the late 1980s, Mercedes-Ben offered a "trap oxidizer" on its Turbodiesel sedans that eliminated the particulates from the exhaust. Unfortunately, in actual service, it tends to plug up, and must be expensively replaced every few years.

Is the possibility of regulations feat will effectively ban diesel vehicles affecting how fleets look at them?

"Everything we put into California is gas," said Pete Silva, senior group manager, fleet operations, for Frito-Lay Inc. in Dallas, TX, who runs over 14,000 light trucks. "The GM 6.5L diesel won't pass the California emissions," he added. He also noted that although in the rest of the country he uses diesel vehicles because of the better fuel economy and lower operating costs, the higher resale value that diesels are beginning to show is not a factor. "From our standpoint, it's hard to say that the drivetrain drives the resale value, because in our rack-truck fleet, we hold them for 23 years." Silva added that after that time, there's little difference between what a gas or a diesel vehicle will bring at resale.

What's Happening Now

Despite the possibility of new diesel vehicles becoming unavailable in the future, diesel-powered light trucks look like a good investment now. According to the latest by the National Association of fleet Resale Dealers (NAFRD), the resale value of used diesel pickup trucks is on the increase. For example, in the 1997 survey, a two-year-old pickup with a diesel engine was worth on average $828 more than the same vehicle with a gasoline V-8. In 1998, a two-year-old pickup with a diesel engine was worth $1,175 more than the same vehicle with a gas engine.

When you combine the lower operating costs of the diesel truck with the significantly higher remarketing value, the premium for the diesel engine is covered in a relatively short time.

While the environmentalists are looking at diesels with jaundiced eyes, don't count Dr. Diesel's engine out yet. It can provide long-term benefits to many fleets.

Latest Developments May Change The Picture

Just as this issue of AF was going to press, Arco Products Co. held a news conference on Oct. 7. 1999 in Los Angeles announcing a demonstration program for its new EC (Emission Control) diesel fuel.

The new fuel follows in the path laid down by Arco's reformulated gasoline, which has been on the market for 10 years. According to the company, EC diesel reduces sulphur emissions by 100% and reduces particulates by more than 12%. In addition, it also cuts hydrocarbons by 12%, carbon monoxide by 7%, and oxides of nitrogen by 4%.

The elimination of sulphates from the exhaust means that particulate traps, such as the one Mercedes-Benz, used 10 years ago, will be more successful in the future.

The Diesel Difference

What are some of the main differences between a diesel engine and a normal gasoline engine?

1. Rudolf Diesel's invention was to use the heat of compression alone to ignite the fuel/air mixture in the cylinder.

2. Where a spark-ignition engine has a compression ratio of roughly 9:1. a diesel uses compression ratios of 20:1 or more.

3. Fuel is injected into the cylinder at the moment the compressed air reaches white heat, so that it instantly starts to burn.

Diesel fuel is a lower distillate than gasoline. It does not vaporize readily, so there is less danger of fires or explosions.