Diesel 101

Diesels are becoming increasingly popular passenger vehicles because of improved fuel economy and increased performance of their gasoline counterparts. Let's take the Volkswagen Jetta Wagon, the company's best-selling diesel model, as an example. Fuel economy in the diesel version is significantly better than in the gasoline model: it's rated at 36mpg city/47mpg highway compared to just 24 city/36 highway with the gasoline engine. Further, while the diesel version has less horsepower, it has oodles more torque: 177 pound-feet at 1,800rpm vs. 122 pound-feet at 2,600rpm in the 2.0-liter gasoline engine.

Diesel engines are also substantially less harmful to the environment today than they were in the past.

Of all the types of internal-combustion engines, the diesel engine is the most efficient; a given amount of diesel fuel can make more power than the same amount of gasoline. In order to tap into that greater potential energy, air is compressed to a much greater degree than it is in a gasoline engine. Most gasoline engines have a compression ratio of around 10 to 1 while a diesel's can be as high as 25 to 1. After compressing the air, small amounts of fuel are injected into the combustion chamber. This highly compressed mixture creates high temperatures, which in turn causes the diesel fuel to burn without a spark plug (a key component in a gasoline engine). The resulting energy is what gives diesel its edge in fuel economy--20 to 40 percent more than a comparably sized gasoline engine.

Diesel engines are also substantially less harmful to the environment today than they were in the past and are headed down the road to becoming even cleaner in the near future. When it comes to carbon dioxide, diesels are actually more environmentally friendly than gasoline, but pollutants such as nitrogen oxide and soot are a different story altogether. New federal standards, however, which go into effect in 2007, will result in a 77 percent cut in nitrogen-oxide emissions and an 88 percent drop in particulate emissions.

Unfortunately, the current emissions control systems that automakers are using don't work well with today's U.S. diesel fuel because it has a much higher sulfur content than Europe's diesel fuel. The EPA has mandated diesel fuel be produced with a lower sulfur content, but that change won't take full effect until 2006, and even then, the fuel won't be comparable to Europe's. Automakers and suppliers are working feverishly to develop systems that will reduce emissions to the new federal levels. While no one has developed a viable solution just yet, every automaker working on diesel technology has expressed confidence that a solution will be found.

More resources