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.

| February/March 2010

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.


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.

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.

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