Article taken from: Fierce Energy

by William Pentland

ACEEE, a nonprofit organization based in Washington D.C. that advocates for energy-efficiency policies, said the decision to remove VW's diesel vehicles is an effort to ensure the emissions scandal did not undermine the public's confidence in the integrity of its green ratings system.

On September 18, the U.S. Environmental Protection Agency (EPA) issued a notice of violation of the Clean Air Act that accused the Volkswagen Group of circumventing federal emission standards for nitrogen oxides (NOx).

Diesel vehicles have a significant advantage over conventional gasoline vehicles in terms of fuel economy. The trouble is that NOx emissions are substantially higher for diesel vehicles than NOx emissions for traditional gasoline vehicles. In recent years, the development of sophisticated emissions control technologies like NOx traps and urea-based solutions has allowed diesel manufacturers to reduce NOx emissions from diesel vehicles significantly.

The engineering challenge for automobile manufacturers has been applying these emissions control technologies without adversely affecting vehicle performance. In particular, advanced NOx control technologies can reduce acceleration and increase fuel consumption.

Volkswagen has admitted 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. As a result, Greenercars.org said it would remove the affected VW vehicles from the Green Scores rankings until further notice.

"These scores are no longer reasonable estimations of the environmental impact of the Volkswagen diesels," wrote Shruti Vaidyanathan, a senior transportation researcher at ACEEE, in a statement explaining the ACEEE's decision to remove VW's diesel vehicles from the rankings. "Volkswagen's diesel cars have performed well on ACEEE's annual rankings since 2009, hovering just below our list of the top 12 '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."

In the United States, federal authorities have instructed Volkswagen to recall about half a million vehicles.

In 2015, ACEEE named the Smart Fortwo electric drive vehicle made by Mercedes-Benz the world's greenest car. The Chevrolet Spark and Fiat's 500E tied for second place on the list of greenest cars. Nissan's Leaf and the Toyota Prius C hybrid tied for the third spot. Volkswagen's E-Golf electric vehicle took the fourth top spot on the list, which is shared with BMW's i3 Rex electric vehicle.