What is Clean Diesel?

How Diesels Are Different

Aside from using diesel fuel rather than gasoline, diesel engines operate in a fundamentally different way than gasoline engines. A diesel engine doesn’t have a spark plug to ignite the fuel/air mixture in the engine. Instead, a diesel engine ignites its fuel via the heat created when the fuel/air mixture inside the cylinder is compressed sufficiently. These high-compression engines are controlled by the timing and duration of the fuel injection events rather than by the firing of spark plugs, as with gas engines.

It used to be that you could tell the difference between a gasoline engine and a diesel engine just by the sound. But the latest clean diesel cars — even those with four-cylinder engines — are quiet and smooth, with only a hint of the old telltale diesel clatter when they’re driven at certain speeds or under certain conditions. Luxury diesel models are nearly silent.

Diesel automakers have used two technologies to deliver better, sleeker performance: turbocharging and direct fuel injection. Audi and Volkswagen’s clean diesel designation, TDI, stands for turbocharged direct injection. Modern injectors meter fuel quite precisely, injecting diesel fuel into the combustion chamber as many as seven times for each power stroke of the engine. By adding injections at different times — rather than injecting the fuel all at once, as was formerly the case — diesel engines run smoothly and quietly. This also increases fuel economy and lowers tailpipe emissions.

Another rub against older diesel cars was that they were slow, with lethargic acceleration that sometimes made freeway merges challenging. That’s no longer true. New diesel cars have a bit less horsepower than their gasoline counterparts, but they make up for that with more torque. Think of torque as the get-up or pulling power of the engine, which impacts how the vehicle accelerates. Diesel engines deliver significant power at low revolutions per minute (rpm), whereas most gasoline engines deliver their best power at higher rpm. So with a diesel vehicle, abundant torque is available right off of idle. Torque is partly why modern diesels are not only powerful, but also fun to drive.

What is Clean Diesel?

What triggered diesel’s recent evolution was the 2006 federal mandate that all highway-grade diesel fuel sold in the United States have sulfur content of no more than 15 parts per million. Ultra-low sulfur diesel fuel allows automakers to incorporate more sophisticated after treatment devices in the cars’ exhaust systems. Previously, high levels of sulfur in diesel fuel would have poisoned the advanced catalytic converter needed to scrub out pollutants. Modern diesels that use these devices are truly “clean” diesels — meeting even California’s emissions standards, which are the strictest in the country.

To publicly demonstrate just how clean and soot-free modern diesel cars are, auto industry representatives hold white handkerchiefs to the tailpipe while the engine is running. Minutes later, the white hanky is still white — not a trace of soot or other emissions. This is made possible by catalytic converters designed to reduce emissions, a particulate filter and, in larger engines, a final catalyst that uses a small amount of ammonia from an injection of urea solution to minimize nitrogen oxide emissions. The diesel particulate filter is an innovative device — it literally traps harmful particulates, then burns them off, producing carbon dioxide and water vapor. See our diagram for a visualization of this technology.

The Advantages of Diesel

Fuel economy is the primary reason many folks consider diesel. Compared with a similar-sized gasoline engine, a diesel engine delivers about 30 percent better fuel economy, which also means roughly 30 percent less carbon dioxide emissions. While diesel fuel is more expensive than regular gasoline, diesel’s better mileage saves money in the long run. (See Clean Diesel: More Efficient Than the Prius? for an in-depth look at how clean diesel vehicles measure up with gasoline and hybrid vehicles in mpg, price, greenhouse gas emissions, annual fuel cost, and more.)

Another plus of such superior fuel economy is the increased range between fill-ups. Compared with a gas car, a diesel car requires up to one-third fewer stops at the pump throughout the life of the vehicle.

With the exception of a few poorly designed diesel engines produced by General Motors in the late 1970s and early ’80s (which were basically V8 gasoline engines that had been converted to diesel), diesels generally have a long service life. If properly cared for, it’s not uncommon for a diesel car to be reliable for more than 300,000 miles.

The increasing desire to wean America off foreign oil also plays in diesel’s favor. True, diesel fuel is still a petroleum product, but using 30 percent less of it has a considerable impact on how much we buy. Diesel engines are also compatible with biofuels, which can be derived from domestic sources. Most new diesel cars are certified for use with B5, a mix of 5 percent biodiesel and 95 percent petrodiesel. B20 (20 percent biodiesel) is an option in some diesel cars that are more than a few years old. While automakers would like to allow the use of higher blends of biodiesel, there is currently no universal standard for biodiesel that would allow sophisticated engines to be tuned to accept different blends.

Clean Diesel Options

Chrysler, Ford and General Motors have diesels available in full-size, heavy-duty pickup trucks. Over the past few decades, the rising popularity of these trucks has made diesel fuel widely available such that it’s no longer necessary to go to a truck stop to find a diesel pump.

As for clean diesel cars and SUVs, the pool of options is growing, with German automakers leading the charge. Volkswagen currently offers a clean diesel version of its Jetta model (30 city mpg, 42 highway mpg), which is available as a sedan or station wagon. (See “The Volkswagen Jetta: How Does 58 MPG Sound?” below.) The Volkswagen Touareg SUV is rated at 18 city mpg and 25 highway mpg. Volkswagen has also reintroduced a diesel version of its Golf hatchback (30 city mpg, 42 highway mpg).

The Q7 SUV from Audi (17 city mpg, 25 highway mpg) is now available in the United States. Also from Audi is the smaller, superefficient A3 hatchback (30 city mpg, 42 highway mpg), which was recently named the 2010 Green Car of the Year by Green Car Journal.

Both BMW and Mercedes-Benz now offer clean diesel technology in their upscale sedans and SUVs. BMW has the sporty 335d sedan (23 city mpg, 36 highway mpg) and X5 SUV (19 city mpg, 26 highway mpg), while Mercedes-Benz is offering its BlueTEC clean diesel engines in its GL350, ML350 and R350 SUVs. (See our 2010 Clean Diesel Cars, Trucks and SUVs and 2012 Clean Diesel Cars, Trucks and SUVs charts for much more information on these and other diesel vehicles.)

Asian automakers Honda and Nissan are eyeing the American diesel market as well. Expect Honda to unveil a clean diesel version of the Accord in the future.

Most major automakers already sell diesel models around the world. In Europe, where fuel prices have been much higher for many years, diesels are generally more popular than gasoline cars. See What Drives the Demand for Diesel? for more on the Europe/United States diesel divide.

Clean diesel cars are likely to grow in popularity, particularly if gasoline prices remain unstable. While they can be a bit more expensive upfront, a diesel will offer a higher resale value when it’s time to trade. Best of all, diesel cars offer superior fuel economy and durability. Not only is clean diesel here to stay, it enriches green car options for everyone.

The Volkswagen Jetta: How Does 58 MPG Sound?

Volkswagen didn’t like the way its 2009 Jetta TDI performed in the EPA mileage tests (29 mpg in the city and 40 mpg on the highway). So the automaker had an independent lab test the Jetta’s fuel economy, and it calculated a rating of 38 city mpg and 44 highway mpg.

But to really illustrate just how efficient the Jetta TDI can be, Volkswagen asked the husband-and-wife driving team of John and Helen Taylor to use their fuel-efficient driving techniques to set a Guinness world record for mileage. The Taylors drove a course that covered the lower 48 states, totaling more than 9,000 miles. They averaged 58.82 mpg — an amazing 7 mpg better than the old record.

When I put numerous tanks of ultra-low sulfur diesel through a 2009 model, with a conservative driving style, I was able to average an mpg in the high 40s to low 50s. My best effort yielded nearly 55 mpg. Even if you drive this car without regard for fuel economy, mileage rarely drops below 40 mpg. Plus, with the Jetta’s 14.5-gallon fuel tank, it’s possible to push 600 miles of steady freeway cruising before refueling.

But fuel economy isn’t the Jetta TDI’s only strength. This car is a roomy, five-passenger sedan with a large trunk. It’s fun to drive, too, thanks to tight, responsive handling.

The 2010 Jetta TDI is priced at $24,510 (including destination fee), and qualifies for a federal tax credit of up to $1,300.

Read more: See how the 2012 Volkswagen Jetta TDI stacks up against the competition in Clean Diesel: More Efficient Than the Prius?

Originally published in February/March 2010 issue of MOTHER EARTH NEWS and regularly vetted for accuracy.