Petrol cars pollute more than diesel ones, according to a study

The particulate filters fitted to diesel vehicles are one of the main reasons for their lower pollution compared with petrol cars.

The high level of pollution from diesel vehicles has been one of the most debated issues since the scandal involving Volkswagen. Governments in several countries are taking steps to restrict their access to cities, but what about petrol cars? Surely these don’t pollute? According to a study, they not only pollute, but do so in greater proportions than diesels.

The research has been published in Scientific Reports and was led by several researchers from Switzerland, Norway, Italy, France and the United States, but the main author of the study has been Patrick Hayes, a scientist at the University of Montreal (Canada). To reach the conclusion that diesel vehicles emit less than petrol ones, Hayes analysed carbonaceous particles (PM) from traffic in Los Angeles.

“Since diesel is now much cleaner than before, environmental regulators must increasingly shift their focus to dirtier petrol cars and other sources of air pollution,” says the scientist from the University of Montreal. “Diesel has a bad reputation because you can see the pollution, but in reality the worst is the invisible pollution coming from petrol cars,” Hayes adds.

The results obtained in the laboratory (at the Paul Scherrer Institute, near Zurich in Switzerland) show that petrol cars emitted on average 10 times more PM at 22 ºC and 62 times more at -7 °C compared with diesel cars.

“The increase in emissions at lower temperatures is linked to a more pronounced cold-start effect”, when a petrol engine is less efficient because it has not yet warmed up and its catalytic converter has not yet lit up.

In recent years, new-generation diesel cars have been legally required to be fitted with particulate filters (DPF), a component that significantly reduces the emission of harmful gases and which petrol cars do not have.

“These results challenge the existing paradigm that diesel cars are generally associated with much higher PM emission rates, and reflect the effectiveness” of engine add-ons such as diesel particulate filters (DPFs) in stopping pollution,” says Hayes.

For this reason, the European Commission has approved a series of measures aimed at mitigating the environmental impact of petrol cars, which will come into force next September. At that point the transition from the NEDC to the WLTP homologation cycle will begin. With this new method, in addition to particulate emission tests being carried out under real driving conditions, everything suggests that manufacturers will be obliged to fit a particulate filter to petrol engines, just as is already done with diesels.