Technology Articles
Diesel - Overview
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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 green alternative to hybrids?
When it comes to fuel efficiency, the answer is yes. The Volkswagen Jetta Diesel, with a combined highway/city fuel rating of 35 mpg, is in the same league as the most fuel-efficient hybrids. When considering the other benefits of a diesel vehicle—like great performance and long life—modern clean diesel vehicles really give hybrids a run for their money. (Keep in mind: high fuel efficiency means low carbon emissions.)
The diesel Volkswagen Jetta, now available in all 50 states, slips right behind the Toyota Prius and Honda Civic Hybrid to become the third most efficient vehicle in America. Official EPA numbers peg the manual 6-speed at 30 mpg in the city and 41 on the highway, and the automatic at 29 city and 40 highway. That’s a huge jump over the gas versions of the Jetta sedan and wagon, which are rated at 21/29.
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 sprayed into the cylinder, mixed with air, 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 percent more energy than a gallon of gasoline. These two factors help modern direct-injection diesels achieve roughly 50 percent higher fuel economy than their gasoline counterparts.
Diesel Emissions
Modern diesels require something of an environmental tradeoff. While generating fewer greenhouse gas emissions, 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 unpleasant.
- NOx (oxides of nitrogen) 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. For a diesel passenger car to be sold in all 50 states, it must meet new tougher emissions requirements established by the State of California. Only a handful of diesel vehicles—from Volkswagen, Mercedes and BMW—pass that test.
The Diesel Dilemma
| 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 is often more expensive than unleaded gasoline. (In Europe, diesel is taxed less heavily than in the U.S., 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. |
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.) |
Power: Diesels provide greater torque, which can be important for drivers who carry heavy loads or tow trailers. | Price: Adding a diesel engine to a Volkswagen Jetta adds more than $1,000 to the car’s price, and in medium-duty pickups the increased cost of a diesel engine can exceed $5,000. |
Incentives: Clean diesels are eligible for the same types of tax benefits that hybrid vehicles receive. Buyers of the Mercedes GL 320 Bluetec qualify for a tax credit of $1,800. | 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, Volkswagen, Toyota, Ford, Nissan and Peugeot Citroën (among others) have shown diesel-hybrid prototypes—which are capable of getting as much as 70 miles per gallon. Unfortunately, the combined added expense of a diesel engine and a hybrid system are too costly. Most industry analysts predict that diesel-hybrids will be slow to market, and will never rise above a niche status.
