The latest diesel engines are so full of filters and exhaust technology that they not only emit less, but could also make the air cleaner. That is the conclusion of the renowned German car magazine Auto Motor und Sport after extensive research.

The conclusions of the study are just as controversial as it is startling: the newest generation of Euro 6d-Temp diesels can take in polluted air from the inner cities and then make the engine come out cleaner. That is the most important conclusion of the study that is published in the latest issue of Auto, Motor & Sport (AMS): the most authoritative car magazine in Germany.

AMS called in the British company Emission Analytics for the investigation. To compare the emission results with the ambient air values, the researchers first measured the number of particles in the center of the German city of Stuttgart. On days with a high concentration of particulate matter, approximately 50,000 particles per cubic cm of air were found. On days with little contamination, the amount was approximately 10,000 particles per cm 3.

The test, which was conducted with a BMW 740d, a Citroën C5 Aircross BlueHDi, a Kia Optima Sportswagon CRDi and a VW Tiguan TDI, showed that the number of ultrafine dust particles in the exhaust gases remained completely the same, regardless of the amount of particles contained in the outside air. Even when the amount of particulate matter in the intake air was twenty times higher, no higher particulate matter concentration could be measured at the exhaust of these diesel engines, AMS reports.

Only in the so-called regeneration mode, that is, when the soot filter is full and must clean itself by raising the exhaust gas temperature and burning the collected particulate matter, did the emissions according to the car magazine increase for some time to 200,000 particles per cubic cm. During normal engine operation, that number dropped to less than 10,000 per cm3.

According to the magazine, the number of fine dust particles in inner cities can quickly rise to 50,000 particles per cm3 during busy days. When a modern diesel car emits around 15,000 particles / cm³ under an even load, this means in theory that the engine makes the air of inner cities cleaner at that time, concludes AMS, since engines suck a lot of air to run.

The necessary comments can be made about the investigation. Of course, diesels also continue to emit NOx (nitrogen oxides) and although these models fall below the legal threshold of 80 grams per kilometer, it remains an unhealthy form of emissions. Moreover, the question is how long the new diesel engines will continue to function so well. Nothing is reported about this in the study.

Gasoline engines have not been tested, it may be that they are even better at reducing particulate matter in polluted city centers. Electric cars and cars powered by hydrogen are of course the least polluting in inner cities, because they produce no emissions at all on the spot.