Green NCAP, the consumer programme for greener cars, said the Mercedes C220d 4Matic (4WD) scored maximum points for the clean air index, the same as electric vehicles, in its second round of results announced on 11 July.

Five cars were rated on their performance for clean air and energy efficiency. The Daimler model and the Renault Scenic dCi 150, both diesels, achieved three stars each, the Audi A4 Avant g-tron, the first CNG car assessed, also gained three stars, the Opel/Vauxhall Corsa achieved a "creditable" four stars in one-litre petrol form and the Nissan Leaf, the only pure electric vehicle in the group, achieved the maximum full five stars.

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"In recent years, diesel has been stigmatised as a dirty fuel but [the Mercedes] shows it needn't be," the organisation said in a statement.

"While electric vehicles produce no pollutant emissions, those of the Mercedes were below Green NCAP's 'high-performance threshold meaning it scored maximum points.

"The car's three-star rating was defined by its performance in energy efficiency, less impressive than in clean air, but still a reasonable rating for such a car.

"It is clear new diesel-engine cars, with proper calibration and effective after-treatment, can deliver extremely low pollutant emissions."

 The three star-rated Audi A4 Avant g-tron CNG is currently compliant with Euro 6b emissions standards. Such cars can still be sold in the EU but must soon be upgraded to meet Euro 6d-temp requirements.

While the g-tron operates almost exclusively on CNG, it also has a small 'emergency' petrol reserve, to get it to the nearest fuel station. Both fuel modes were tested, the score and star rating being based on the CNG results with penalties if performance on petrol was significantly worse.

The fifth-generation GM Corsa received four stars though the car will be replaced later this year with a PSA platform-based redesigned model.

"Four stars is a creditable result for a petrol engine vehicle, and the model performs especially well in its control of pollutant emissions," said.

"Even the Scenic performed well, with impressive control of pollutant emissions and missed out on a four-star rating by only a small margin in energy efficiency."

Nissan's second-generation Leaf, the third electric vehicle tested by Green NCAP, achieved the full five green stars. Currently, the tests are 'tank-to-wheel and don't account of the energy involved in generating electricity or producing fuel. On this basis, electric cars currently offer the cleanest, most efficient motoring. Green NCAP plans later to assess 'well-to-wheel' emissions and, ultimately, the whole life-cycle of vehicles to give an even more realistic view.