Why diesel cars are spoiling the summer

Europeans aren’t just sweating through the long, hot summer. City dwellers may be coughing and wheezing more, too.

Diesel vehicles, which still command nearly half the market for new cars, are left with barely any pollution controls when temperatures soar above 35° Celsius (95° Fahrenheit), according to France’s Petroleum and New Energies Research Institute. That means smog-inducing nitrogen oxide emissions that were at the centre of the Volkswagen AG scandal spew into the environment unchecked.

“There are higher emissions of nitrogen oxides because the setups don’t work as well when it’s very hot,” said Gaetan Monnier, who heads the unit that led random government probes of cars after the VW scandal. “The atmosphere is also more reactive during a heatwave, making the level of cars’ emissions even more of an issue.”

Carmakers have been under scrutiny since the 2015 scandal that revealed VW had rigged the emissions setup in some 11 million diesel cars globally, the main emitters of nitrogen oxides. While sales have dropped as consumers fret over driving bans, diesels still made up 45 per cent of new vehicle sales in Western Europe last year. Excessive emissions of the pollutant linked to premature deaths and respiratory problems have prompted the European Union to sue France, Germany, UK, Romania and Hungary in May after failing for years to comply.

On-road tests have shown emissions clean-up systems start to reduce their effectiveness in cold temperatures as well as above 30° Celsius.

Paris, which plans to banish all petrol and diesel cars from the city center by 2030, stopped older cars from driving during five days because of heat-related air pollution this summer. The municipality also limited speeds and discounted the price of public transport. The cities of Strasbourg and Lyon restricted the most polluting cars from town centers as well.

The French capital cooled off on Thursday, after reaching 36° Celsius this week, while the heat endures in Berlin and Munich, home of BMW AG, with highs of 35° Celsius forecast, according to the Weather Channel. London, which is extending its ultra-low emissions zone, has seen top temperatures of 33° Celsius this summer.

Carmakers downplayed the concerns. While some diesel antipollution systems don’t perform as well in extreme temperature conditions, they still have an effect, a spokesman for Renault said. Since the VW scandal, automakers have faced criticism on their broad interpretation of rules that allow them reduce or turn off emissions systems to protect the engine.

To improve vehicles on the road and stave off growing public backlash, carmakers like Daimler AG and Ford Motor Co have agreed to recalls for fixes to improve emissions levels. Investigations into practices continue. As of September, the EU is switching to a new testing procedure to close the gap between emissions readings in test labs and real-world driving.