Diesel vehicles now account for nearly half of all new vehicle sales in Europe. In some European countries (such as France), diesel vehicles account for as much as 70 percent of new car sales. Are diesel vehicles a viable alternative to hybrids?

The Diesel Difference

Diesels are also known as compression ignition engines, and have a different combustion cycle than gasoline engines.

  • In a gasoline engine, fuel is mixed with air, drawn into the cylinder, and ignited by a spark from the spark plug.
  • In a diesel, air is drawn into the cylinder and compressed first without fuel present. This compression heats the air to such a high temperature that when fuel is then injected into the cylinder, it combusts.

By using higher compression ratios and higher combustion temperatures, diesels operate more efficiently. As a result, diesel vehicles attain better fuel economy than their gasoline counterparts. This fuel economy advantage is enhanced by the fact that a gallon of diesel fuel contains about 10% more energy than a gallon of gasoline. These two factors help modern direct-injection diesels achieve roughly 50% higher fuel economy than their gasoline counterparts. For example, a European model Honda Accord with a 2.2 liter i-CTDi diesel engine is rated at 43.3 MPG, 49% higher than the rating of a Honda Accord with a 2.4 liter gasoline engine. The Camry Hybrid is rated at 39 MPG, 10 percent lower than the diesel Accord.

Diesel Emissions

Modern diesels require something of an environmental tradeoff. While generating fewer greenhouse gas emissions (due to greater fuel efficiency), diesels emit larger amounts of two other pollutants:

  • Particulate matter is the black cloud that trails many older diesel vehicles. Diesel particulates are harmful to human health as well as aesthetically unpleasing.
  • NOx, while less visible, is a key ingredient in the formation of urban smog, and also can contribute to the formation of acid rain.

Higher emissions of these pollutants are diesels’ greatest drawback. There has been an ongoing split in diesel emissions regulations in the US between those required by the Environmental Protection Agency, and those required by the California Air Resources Board.

Pros Cons
Fuel Economy: 81% of U.S. diesel buyers say they bought a diesel engine for higher fuel economy. U.S. buyers should be aware, however, that in the United States diesel could often be more expensive than unleaded gasoline. (In Europe, diesel is taxed less heavily in Europe, and therefore can be substantially cheaper than gasoline.) Availability: Until cleaner fuel and advanced emissions controls arrive here, availability of diesel models will be limited. New diesels are already absent from five states (California, Massachusetts, New York, Vermont, or Maine) that have stricter air quality standards. At the end of 2006, federal pollution rules will tighten, pushing cleaner diesel models out of the entire U.S. market.
Longevity: Diesel engines tend to last longer than gasoline engines, leading to higher resale values for many diesel-equipped models.

Emissions: Particulate and NOx emissions are higher than those of comparable gasoline vehicles. (Most diesel engines can use biodiesel without any modification.)

Diesels provide greater torque, which can be important for drivers who carry heavy loads or tow trailers.

Adding a diesel engine to a Volkswagen Jetta adds over $1000 to the car’s price, and in medium-duty pickups the increased cost of a diesel engine can exceed $5000.

Future clean diesels will be eligible for the same types of tax benefits that hybrid vehicles receive. Buyers of the Mercedes E320 BLUETEC, for example, qualify for $1500 off of their tax bill.

Availability of Fuel:
Diesel owners must also cope with a refueling network that is more limited than that of gasoline, although their vehicles’ longer range means they have more time to find a station that sells diesel.

Not an Either-Or Situation

It’s technically possible to use a hybrid drivetrain with a diesel engine. In fact, PSA Peugeot Citroën recently showed a diesel-hybrid prototype: the 307 CC Hybride HDi, a compact convertible that gets 70 miles per gallon, about 30 percent better fuel economy than the existing diesel version. No one makes diesel hybrids yet, mainly because they are expensive. The added benefits come at a double expense—more for the hybrid system and more for the diesel engine. PSA Peugeot Citroën is planning to introduce a diesel hybrid to the European market in 2011. But no promises yet for the North American buyers.