Two Paths to Clean Diesel

As more clean diesel vehicles begin to hit the market, consumers should be aware that diesel car manufacturers are taking different technology paths. The two main paths mean different responsibilities for car owners, although the goal is the same—to reduce emissions of oxides of nitrogen (NOx), a key contributor to smog. A great deal of sophisticated technology is used to reduce emissions, but we’ll focus on the chief differences between the two systems.

1Trap and Burn

First, let’s consider the Volkswagen Jetta TDI, which uses a device to trap and store NOx. The system captures the pollutant and periodically changes the engine’s air-to-fuel mixture to burn off the NOx.

This reduces NOx emissions by up to 90 percent compared to dirty old diesel engines. And it requires nothing at all from the owner.

A particulate filter further reduces emissions. In the Jetta application, the 16-valve engine produces 140 horsepower and 235 foot pounds of torque. It features a common rail fuel system with high-pressure injectors. The engine delivers “real world” fuel economy improvements of 30 percent compared to a similar gasoline model.

2Spray with Urea

Meanwhile, the Mercedes clean diesel vehicles requires customers to return to a dealership every 10,000 miles—this could be done during regular maintenance visits—to refill a special tank holding urea. The urea solution is a chemical synthesized from natural gas—not urine.

An instrument panel light will warn the driver if the tank falls below one gallon of fluid. If the urea level gets too low, a counter will appear on the dashboard informing the driver that 20 restarts remain. If the warnings continue to be ignored, the vehicle will not start until the urea fluid is added.

The Mercedes-Benz Bluetec vehicles—including the ML320, GL320, and RL320 models—feature improved combustion from a third-generation common rail system that uses piezo-electric injectors. A tiny squirt of urea solution—which Mercedes calls “AdBlue”—is injected into the exhaust mix before reaching the tailpipe. The average consumption of AdBlue is around 0.025 gallons per 60 miles. The solution is held in a seven-gallon tank in the space usually occupied by a spare tire—so the vehicles will have run-flat tires instead of a spare.

Urea for Larger Engines, NOx Traps for Smaller

The NOx trap approach is more expensive to manufacture than the urea-based system. Both engines meet the most stringent diesel emissions requirements in the world—California’s Tier II/Bin 5 (or Ultra Low Emission Vehicle) standards. Those standards had effectively outlawed diesel engines in California (and other states following California’s standards) for a generation.

The different approaches demonstrated by Mercedes and VW are likely to be used in future engines, with larger engines taking the urea approach, and smaller four-cylinders going without urea.


  • Ben

    I would like to add that using the “trap and burn” approach has a major limitation…you can’t use biodiesel with it.

    Though VW claims that B5 is acceptable, but higher blends will cause engine oil dillution issues. I was skeptical of this claim at first, but it has been verified by TDI enthusiasts and some biodiesel producers recently. I own an 09 VW TDI and have followed this discussion closely. I’m yet to hear of any aftermarket technologies that can overcome this limitation…maybe we’ll see some down the road.

    Does the urea approach have a similar limitation with regards to biodiesel?

  • Bryce

    This is really interesting. Whatever gets us using less fuel. Can’t wait until a hybrid diesel application comes out. : )

  • gok

    How much does urea cost?
    If you only use 4+ gallons every 10,000 miles I’m sure it wouldn’t hurt the cost per mile.

    There might be a special oil out there, someday, to allow you to run biodiesel. I just read in my owners manual I’m suppose to be using a special oil blend in my E85 vehicle.

  • wxman

    There was an interesting article in the September 2008 issue of DIESEL PROGRESS regarding Cummins’ reasoning for changing from its original plan to use NOx storage catalyst (NSC) to SCR-urea on its heavy-duty 2010 diesel engines.

    According to the article, a new SCR catalyst has been developed (copper zeolite – ref. http://www.anl.gov/Media_Center/News/2007/CMT070427.html ) that is so effective in removing NOx from the exhaust (95%-100%), that Cummins is able to optimize for fuel efficiency (which tends to “create” more engine-out NOx). They’re expecting 5%-10% improvement in thermal efficiency compared to using NSC, according to the article.

    Assuming this technology can be applied to light-duty diesel engines, fuel economy penalty from emissions after-treatment could be essentially eliminated.

  • thomatt12

    The Jetta TDI Sportwagen looks really good plus it’s fuel efficient too. Awesome!

  • Bill

    Are these Diesels noisy like most diesels cars?

  • Bryce

    I saw one the other day, and they are not particularly loud at all. Only could tell it was a diesel cuz I saw the TDI on the back, otherwise, I would have never known.

  • ex-EV1 driver

    Why not just make diesel serial plug-in hybrids that get 50 miles or more of pure electric range. The diesel would run so seldom for most people that it won’t have to be cleaned up. Even if it does need to be cleaned up, clearly the NOx trap would work since there would be no need for big diesel engines in passenger vehicles if an electric motor does the heavy lifting.

  • Jakub Kudlacz

    Cost! and Diesel Stations! are two main reasons why Diesel Hybrid cars are not going to enter US market anytime soon.

    On average Hybrid system costs around 2,000$ more and Diesel Engine around 3,000$ now when you add those two its 5,000$ more than same car using just Gasoline engine.

    Diesel Engines last longer than Gasoline Engines and offer your car a better resale value, so that is a big plus. Hybrid technology is on a rise, but companies are still experimenting with different types of batteries.

    Future looks bright, but car companies will start thinking outside a box only if OIL prices continue going up, right now with recession and OIL prices down (it will not last but it may take a year or two before they go back up) many car companies will invest sporadicly instead of heavily into new technologies.

    All that aside, I would love to see more Diesel powered cars in US as well as Diesel Hybrid in a year or two. Possibly Golf Diesel Hybridhttp://blog.wired.com/cars/2008/03/vws-golf-diesel.html

  • Bryce

    Series hybrid sounds good to me. Better than a parallel drivetrain.

  • thomatt12

    I agree a hybrid Jetta TDI Sportwagen will definitely be a good idea…

  • Kenyon

    I took delivery of a Jetta TDI Sportwagen Friday and it is great. Styling is modern and sleek. Engine is as quit as a gas engine, and power is impressive. I have a TDI ’06 Beatle and regularly get 40+ mpg on the highway. I expect at least as good mileage in the Jetta. This same car was used to set a record of 58 mpg across the US.

    I haven’t had time to determine any mileage values yet, but I know that the Beatle took 10-15 k miles to break in.

    There are 2 small down sides so far. They are very hard to get. I ordered mine in the summer and just got it. The other is the stupid iPod adapter that they are including an almost all cars being built. It only seems to fit iPods over 2 years old and the user interface is extremely bad. When you get the iPod adapter you lose the aux-in jack. Really a dumb accessory.

  • Cesar M. Gonzalez

    There will be a diesel hybrid car which is the Aptera……( See Aptera dot com ) CMG……….

  • Cesar M. Gonzalez

    There is a diesel hybrid…….Go to Aptera dot com………………..CG…………

  • Adrian

    Two innovative ways to reduce NOX emissions. I think diesels have a bright future. They are more efficient. However, they release a lot of particulate pollutants. Hoping to have clean diesels soon.

  • tapra1

    injected into the exhaust mix before reaching the tailpipe. The average consumption of AdBlue is around 0.025 gallons per 60 miles. The solution is held in a seven-gallon tank in the space usually occupied by a spare tire.Tech News