Clean Diesel: A New Era of Green Cars

Together with their superior durability and fuel economy, clean diesel technology enables diesel cars to rival gas-electric hybrids as green cars.

clean diesel green cars

The universe of available green cars has expanded thanks to new clean diesel technology. 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.


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Whatever notions you have about diesel cars, forget them. Long gone are the days of smelly, black plumes of smoke, noisy engines, slow acceleration, and sometimes-finicky operation. The technology has evolved significantly in recent years. Diesel cars are now green cars and a strong green transportation option.

Modern diesel powertrains are quiet, clean, smooth, reliable, powerful, durable and economical. A diesel vehicle will usually cost more than a comparable gasoline vehicle, but the diesel engine’s more robust design means that, with proper maintenance, it should last considerably longer. Plus, some clean diesel cars qualify for a federal tax credit. (See our 2010 Clean Diesel Cars, Trucks and SUVs and 2012 Clean Diesel Cars, Trucks and SUVs charts for much more information on diesel vehicles.)

For generations, diesel power has been the best choice for work-intensive applications, with no other engine delivering as much stump-pulling power. What’s evolved is how the engine compresses and ignites the fuel to propel the vehicle — a change that has capitalized on diesel’s inherent advantages while virtually eliminating the traits that previously made diesel dirty.

Diesel enthusiasts now contend that a diesel car delivers fuel economy on par with that of a gasoline-electric hybrid, and also offers a better driving experience. (See Clean Diesel: More Efficient Than the Prius? to find out how clean diesel vehicles measure up with gasoline and hybrid vehicles in mpg, price, greenhouse gas emissions, annual fuel cost, and more.)

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 Makes Diesel Clean

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 aftertreatment 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 How Clean Diesel Works 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?

mike kiernan
1/30/2012 3:17:37 PM

Being up here in Northern Maine the really big question here is the diesel fuel freeze issue. I'd love to have a diesel, even one of the old M-35's, up here for both business and for the monthly grocery store run. Hey, when you're 35 miles from town in the snow for 6 months a throw, something has to give ! But every time I ask any truck dealer about the diesel fuel freeze problem and any ideas as how to solve it, all I get is a 1000 mile stare. Jeep was supposedly in the process of building the Jeep with a diesel and getting into the US market. So far, n-a-d-a !. Anyone have a fuel freeze solution, come up here and make money off it !!!!

3/5/2010 11:38:38 AM

Sorry for the double post, but it takes so long, it's hard to tell it went through. 4: Range issue? Seriously not much of an issue for most of us (not all of us as some commute way to far without the mass transit option) as we rarely, on a daily basis, drive further that an Electric would get us. An Electric vehicle or two for primary transportation, local trips and getting around town, and one diesel/Hybrid for the longer trips, or as a back up vehicle, would be huge on fuel savings for every family. BTW, my unmodified 1991 Ford F250 with the 7.3 "old tech" Diesel is capable of getting 18 city mpg and 24 highway mpg (at 50 to 55 mph). That is if I try real hard to use a light throttle and don't care that I'm seriously pissing off everyone behind me that is in a hurry and also shutting the engine down at long red lights! If driven normally to keep up with the other traffic and idling at red lights, it only gets 11 to 13 mpg city and 17 to 18 highway mpg (at 70 to 75 mph).

3/5/2010 11:02:52 AM

A few facts. 1: Emissions testing. Diesels emissions are tested at "per gallon" levels, but Gasoline emissions are "per mile", so better mileage makes it look cleaner. If Gasoline was tested at "per gallon" like diesels, the test and comparison would be fair, and diesels better efficiency would give it the edge. 2: Trucks idling. Yes, the whole "hotel load" aspect is there but they now have small generators that will supply that without the main engine idling. Also, diesel engines have a HUGE MASS and a HUGE COOLING SYSTEM,, and that takes a long time to warm up, and engines experience higher wear rates at less than operating temps, so they want to keep it warm. 3: Those in power want to continue to make money, and big oil is the power in the USA, so why would they want high mileage transportation? Proof? Ford, Chrysler, and GM way back in 1996 (THAT'S 14 YEARS AGO PEOPLE!) all had Diesel powered Hybrid cars ready for production. Other manufacturers were also were building them, but Toyota was the only one not to scrap their program and went to market with their Hybrid the next year in 1997, but only in Japan. WHY HAS NOBODY ASKED "WHY WAS THE PROGRAM SCRAPPED BACK IN 1996 IN THE USA?" Here is a link to an article from back in 1996.

todd kaho
2/5/2010 2:31:52 PM

Emissions regulations in the United States are among the most stringent in the world. The new generation clean diesel vehicles meet those standards in all 50 states. Please see the following article in Scientific American for more information on clean diesel particulate reduction. - CNG and LPG are great alternatives and very clean options, I agree we should expand their availability and use as a motor fuel.

jason hinton
2/5/2010 1:05:59 PM

Modern diesel engines do not have to be idled. I drive a 2003 VW diesel and have never had any issues with cold starts as long as I let the glow plugs cycle. The coldest I’ve started my car is -7F and it started with only two cycles of the glow plugs. I suspect owners of diesel trucks idle their engines because operators of tractor trailers do. Drivers of tractor trailers idle their engines to provide “hotel loads” (heat, A/C, power for TV’s, etc).

2/5/2010 12:06:42 PM

IT IS NOT TRUE what is mentioned in the article! The newest Diesel engines still exhaust soot. But with the soot filter it comes out in much more finer particles that penetrate the lungs deeper and do the same harm! Don't believe that diesel is the future. in Belgium 80% of our cars drive on diesel thanks to the support of our government (15% of the cost of the car tax reduction for "clean diesel"). The latest researches show that this "clean diesel" story isn't that clean at all. Our country is suffocating in fine dust from all those diesels! It's too crazy for words what I read here. I live 1 mile from the higway and I live 2 years less because of the soot. That's scientificaly prooven. You Americans are crazy to start with this... Buy cars on LPG now and start searching for more renewable energy.

2/5/2010 11:29:04 AM

Chrysler builds diesel Calibers in the US and SHIPS THEM TO EUROPE. Ford builds diesel Focuses (I've driven one) but only gas engines here (I've got one, but want a diesel). Somebody needs a kick in the rump. Obama? Congress? Car companies??

2/5/2010 10:29:32 AM

Just a note about my recent experience with new diesels... I was in Norway last fall and had two rental vehicles while there. The first was a very nice and peppy Toyota Yaris 5 dr diesel. My mileage was in excess of 48 mpg. A few days later I picked up a Volvo C30, which I drove from Oslo to Stavanger and back, over 1500 km over all types of roads, and was surprised to exceed 49 mpg! I have exact numbers if anyone doubts these calculations. After my return I visited my local Volvo and Toyota dealers to learn when I could expect to see these vehicles for sale locally. The Volvo dealer, who had a C30 in prize position on the showroom floor rated at 27 mpg, looked at me like I was from outer space. The Toyota salesman was sympathetic but clueless about any plans to import or build the Yaris diesel here. For my backwoods lifestyle I really would like a Rav4 diesel, but guess we won't see that until the world crumbles further. (Another mid-east war perhaps?)

2/5/2010 8:53:55 AM

I've owned a clean diesel 2009 Jetta TDI for over six months and I've never driven a better car. I'm AVERAGING 43 mpg per tank of fuel, considerably better than the EPA rating (and I use 5% bio diesel). Aside from the great fuel economy this car is FUN to drive. The considerably torque is available at the speeds you drive everyday so merging, passing, and accelerating are all done with a feeling of effortlessness. Because the engine is powerful, you have to learn to drive it a bit differently than a gasoline engine (light on the pedal). Oh, I believe big trucks idle because they carry refrigerated items.

2/4/2010 7:59:12 PM

What I'd like to understand about diesel cars and trucks is the apparent need to idle them. I understand that they can be hard to start in cold weather, but why do their drivers feel the apparent need to idle them year-round? I'd like to understand this; can anyone enlighten me?

2/4/2010 12:54:34 PM

As a member of the East Tennessee Clean Fuels Coalition and the North American TDI Club, I would like to let other readers know about two previous generations of TDIs that are available on the used market, are much more affordable than the current Clean Diesels and more biodiesel friendly. My car, a 2005 Golf TDI, belongs to the A4 generation immediately previous to the new Clean Diesels and I drive it on B20--20% biodiesel, which is widely available at pumps in our area. These cars have direct fuel injection, a slightly smaller engine, more torque (thanks to the direct injection),and great fuel economy, 50+ on the highway. The A3 generation previous to mine is the most compatible with 100% biodiesel. Chris Hill, a master TDI mechanic from Boston ("mrChill" on the TDI Club discussion boards) owns 13 of these cars and drives them all on ASTM comnmercial standard B100. He will only use dino diesel in an emergency. Ever since low sulphur diesel was introduced for air quality reasons there have been lubrication problems with diesel engines. The sulfur was there to lubricate the engine's cylinder walls. B5 biodiesel has therefore been specified by diesel auto makers because the small amount of biodiesel replaces the sulfur as a lubricant. Lastly, remember that these cars, especially the A4s and the new A5s, have sophisticated high pressure fuel injection systems that can choke on home-brewed biodiesel unless it is of the highest quality.

2/2/2010 5:59:13 PM

the new diesel cars sound great, i may even seriously consider one when i buy my next car. the one issue that is not mentioned in most articles about diesels is that you can only refine about half as much diesel from crude oil as you can gasoline. if a large segment of the cars on our roads were diesel powered this could actually increase oil imports. i'm not sure how much bio-diesel would change this situation.

1/30/2010 2:26:17 PM

I'm hoping to see a diesel AWD extended passenger van with strengthened roof pillars. Ford? Honda? Toyota?

1/20/2010 4:52:16 PM

I think it's great to see diesels coming back finally.Only thing I'd like to see from the automakers,and I am sure there are many out there who agree with me on this is to have a small pickup truck like a Nissan Frontier or Toyota Tundra or a Ford Ranger available in a small diesel model.