You have surely had this argument with friends more than once. Which pollutes more, a diesel engine or a petrol engine? And I am also convinced that the discussion will have gone on for quite a while, because there is plenty of data supporting both cases. We have always been told that the diesel engine pollutes more than the petrol one, although its lower fuel consumption underpinned sales that kept on growing… until very recently. Diesel has now made it onto the blacklist of many cities and even countries, which are watching with concern as city air becomes unbreathable because of the presence of polluting gases. And everything points to diesels. Volvo has even launched the S60, its first model without a diesel engine.

Not long ago we went over how the diesel engine works and the question came up. A friend even sent us his doubts about a claim made in the article: the diesel fuel that powers the diesel engine emits more harmful residues, but modern filters — and even the engine itself — prevent most of those harmful elements from reaching the atmosphere. The petrol engine, on the other hand, emits more carbon dioxide, a gas linked to the greenhouse effect that can cause prolonged pollution once in the atmosphere.

Basically, we can say that both engines pollute at similar levels, but they do so with different residues. While the main problem with petrol cars is CO2, in the case of diesels it is nitrogen dioxide and particulate matter. These elements are not only harmful to the environment in general, they also directly affect the lungs. They are elements that enter the body and stay in the lungs, directly affecting the respiratory system, and that is why diesel is more harmful. It is difficult to remove from the body once inhaled and can lead to serious health problems.

Diesels, in the authorities’ sights
That is the problem that has led many cities to start talking about banning diesels. But let’s be fair. In most cases it is about restricting the circulation of diesel cars built before 2006 during high-pollution episodes. From that year onwards, with the entry into force of the Euro 4 standard, diesel technology began to improve and emissions have been progressively reduced. The Euro 4 standard began to apply on 1 January 2006, so that is the date from which diesel cars started taking emissions reduction seriously. Today, with the Euro 6 standard, emissions are far lower than back then and work is still under way to bring them down even further.

The correct answer to the initial question could be: both engines pollute, but the pollution from diesel engines is far more harmful to human beings (and animals, of course) in a direct way. Currently, all other things being equal, a diesel engine is always worse than a petrol one. The cause of this is the fuel used. Diesel is a heavier, less refined fuel than petrol. The combustion cycle also matters here, since the combustion mixture is leaner and leaves more unburnt elements, which pass into the exhaust and from there into the air we breathe.

As we have already mentioned, the filters and systems fitted to modern diesel cars reduce the impact of the NOx and particulate matter that can affect us. Although diesel engines are still highly polluting, the technology for removing those harmful gases before they reach the atmosphere secures the future of this type of engine, even if manufacturing, fitting and maintaining it costs considerably more. That leaves CO2, which both engines release into the atmosphere and which may pose a long-term problem, although the data on the evolution of the ozone hole is fairly positive, as it has shrunk considerably in recent years.