A giant step backward for diesel vehicles: Why we are suspending Volkswagen diesel Green Scores on greenercars.org

September 25, 2015

On September 18th, the Environmental Protection Agency (EPA) slapped the Volkswagen Group with a notice of violation of the Clean Air Act for circumventing EPA emission standards for nitrogen oxides (NOx). Volkswagen has now acknowledged using so-called “defeat devices” on its diesel cars that turn on full emissions control technologies while in testing mode but allow the vehicles to emit nitrogen oxide levels up to 40 times the certified level during normal operation. In the United States, Volkswagen will be forced to recall half a million vehicles, including the diesel versions of the popular Jetta, Jetta Sportwagen, Beetle, Beetle Convertible, Golf, Golf Sportwagen, Passat, and Audi A3 models for model years 2009 to 2015.

In response to these events, greenercars.org has suspended Green Scores for the affected vehicles until further notice, since these scores are no longer reasonable estimations of the environmental impact of the Volkswagen diesels. Volkswagen’s diesel cars have performed well on ACEEE’s annual rankings since 2009, hovering just below our list of the top twelve “Greenest” vehicles with Green Scores in the high 40s. However, a 40-fold increase in on-road NOx would mean that these vehicles did not deserve those high Green Scores.

Diesel vehicles have made strides in the United States in recent years, largely due to the development of sophisticated emissions control technologies that can limit the emission of noxious combustion by-products while maintaining diesels’ substantial natural fuel economy advantage over conventional gasoline vehicles. Nitrogen oxide emissions, in particular, are substantially higher in diesels than traditional gasoline vehicles. Most commonly, light-duty diesel vehicles in the United States use NOx traps and a urea-based solution to catalytically reduce smog-causing nitrous oxide to its harmless components: nitrogen and water.

However, automotive experts recognize that the application of these emissions control technologies can involve a tradeoff with vehicle performance (fuel economy, acceleration and power). Volkswagen appeared to have threaded this needle, producing a successful line-up of diesel vehicles over several model years that maintained their performance excellence while qualifying as “clean diesels.” EPA’s recent findings spell the unraveling of this narrative.

There’s no evidence at this point that other diesel car manufacturers have resorted to similar workarounds. The technology to enable diesels to meet US emissions standards is available. In particular, the International Council on Clean Transportation’s road testing that led to the Volkswagen revelations included a BMW X5 xDrive 35d that met the standards with room to spare. Nonetheless, Volkswagen’s actions not only violate consumer trust but inevitably will set back diesel cars’ bid for environmental bragging rights around the world. Volkswagen’s response to the crisis will be critical to understanding the future of diesel cars in the United States, as will the in-use testing that will likely be carried out on vehicles from other manufacturers.