Techsplanations: Diesel

"Diesel fuel" is one of those terms that divides drivers and vehicle owners. To some, it elicits "Cool," and "Fun." To others, the term brings out "Ugh!" But it is really all about knowing the facts, and understanding how diesel fuel works before deciding on whether you might want to own a car, truck or SUV that runs on diesel instead of gasoline.

What is it?

"Clean diesel" is a term for the latest and greatest technology that allows diesel engines to be as clean or even cleaner than their gasoline counterparts. We all know the stereotype of dirty diesel engines, an image many people formed back in the late 1970s and early 1980s, when diesel passenger cars first hit the market. Those diesels were known for smelly, black soot spewing from their exhaust pipes – it's no wonder they never caught on here in the United States. But clean diesels don't smell, they don't emit black clouds of soot from their tailpipes, and they produce no more emissions than cars that burn gasoline.

How does it work?

Clean diesel engines are made possible because of something called ultra-low sulfur diesel fuel (USLD). This is diesel fuel that's been refined to have just 15 parts per million of sulfur – as opposed to the 500 ppm that was in highway diesel fuel before the Environmental Protection Agency created the new standard. All diesel fuel sold in the U.S. since December 1, 2010 has been ULSD.

Removing the sulfur is a good thing because when it's burned it turns into sulfur dioxide, which causes acid rain. Sulfur is also responsible for the smell of diesel, so refining it out of the fuel means engines that burn USLD won't give off that foul, rotten egg smell.

But sulfur dioxide is just one of the pollutants that need to be eliminated to make diesel clean. In the past, engineers had a hard time removing enough of the harmful emissions from diesel exhaust, particularly something called nitrogen oxides, which are a key component of smog. One of the biggest stumbling blocks was that sulfur would clog up the emissions systems.

But with the sulfur banished, new emissions system designs are able to trap soot and other pollution and either burn it up or chemically convert it into something that's not harmful. The engines take advantage of modern computerized control systems to make sure this all happens as designed.

Why would I want it?

Diesel engines are inherently more efficient than gasoline engines, so they get better fuel economy. Diesel fuel has typically sold for about the same price as gasoline, so you could stand to save money at the pump.

Diesel engines also have characteristics that could be desirable if you want to use your vehicle for towing. Diesels typically develop lots of torque at low engine speeds, which means diesel vehicles can accelerate from a dead stop rather quickly – or get a heavy trailer moving with ease.

Diesel engines also tend to last a long time. They need to have more stout parts and robust build quality to withstand the extreme pressure generated in their cylinders, which can be twice as much as in gasoline engines. Plus diesel fuel has some natural lubricating properties, which helps reduce engine wear. This can translate into durable engines that can last for hundreds of thousands of miles.

Is there any downside?

Diesel engines have always been more expensive to build than gasoline engines, and particularly clean diesel engines with their extra emissions equipment will carry a significant cost premium. That cost, much like the cost of a hybrid, can sometimes be recouped through fuel savings.

Additionally, diesel fuel is not available at every fueling station. While there isn't any problem finding diesel pumps throughout the country (after all, semi trucks use diesel fuel), you might have to look twice to find the station that carries it.

What vehicles offer it?

Audi offers a diesel engine in the A3 and Q7.

BMW offers a diesel engine in the 3 Series and X5.

Mercedes-Benz offers a diesel engine in the E-Class, GL-Class, M-Class, R-Class and S-Class.

Volkswagen offers a diesel engine in its Golf, Jetta, Passat, and Touareg.

Bottom line

Clean diesel technology banishes diesel's bad reputation and gives drivers another choice for improved fuel economy and low emissions.

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