How clean are new diesel cars?

The latest diesel-engined cars from motoring giants BMW, Mercedes and Vauxhall produce “almost no nitrogen oxides [NOx]”, a study has found.

Researchers at the German automobile club (Adac) discovered that most new diesel models “dramatically undercut” the 168mg/km legal limit currently imposed by the Euro 6d engine rules, Auto Express reports.

Using real driving emissions (RDE) tests to assess the cleanliness of new petrol and diesel cars, Adac found that the Mercedes-Benz C220d emitted “no NOx whatsoever”, while BMW’s 520d and Vauxhall (badged Opel in mainland Europe) cars equipped with its 1.6-litre diesel motor produced just 1mg/km of NOx, the motoring mag says.

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The tests also found that most of the diesel cars examined produced less NOx than today’s crop of petrol-powered vehicles.

Nitrogen oxide gases have been the centre of attention in the motoring world since the Volkswagen emissions scandal erupted in 2015, when it emerged that the company used cheat devices to cover up the amount of NOx its diesel cars produce.

While diesels had been “hailed” for their low carbon dioxide (CO2) emissions and impressive fuel economy compared to petrol cars, NOx gases produced by them “have been linked to ill health”, resulting in a massive shift in public perception, says Car magazine.

This has pushed the market towards petrol cars, which led to average CO2 emissions for new vehicles sold in the UK to rise to a five-year high last November.

Adac’s research should come as a relief to carmakers that have decided to stick with diesels despite their decline in popularity.

From January 2020, all new diesel cars will be limited to a maximum 80mg/km of NOx gases produced, says Auto Express.

While the European Commission had initially allowed manufacturers to produce double that limit to “allow for inaccuracies” in the equipment used to measure the gases, the European Court of Justice ruled against “those conformity factors” and the strict limit of 80mg/km was set.

But with diesel cars seemingly cleaner than ever, next year’s NOx limits won’t be an issue for most manufacturers.