Editor's choice
26 Aug 19

Newest diesel engines clean the air

Previous independent tests already showed that the latest diesels stay well below the NOx limits - contrary to their Euro 5 and Euro 6b predecessors. Specialists at Emissions Analytics have now found that particulate matter (PM) emissions are also a fraction of what they used to be. In fact, Euro 6d-Temp diesels can even absorb PM from the air.

That is the key conclusion of a study that published in the latest issue of Auto, Motor & Sport. To compare the PM emission results with the values of the ambient air, the researchers first measured the number of particles in the centre of test-city Stuttgart. On days with a high concentration of particulate matter, some 50,000 particles per cm³ of air were found. On low-pollution days, the amount was about 10,000 particles/cm³.

The test, that was carried out with a BMW 740d, a Citroën C5 Aircross BlueHDi, a Kia Optima Sportswagon CRDi and a VW Tiguan TDI, showed that the number of PM in the exhaust fumes was completely independent of the concentration of particulate matter in the ambient air. Even when the amount of PM in the intake air was twenty times higher, there was no higher concentration of particulate matter in the exhaust.

Warning: regeneration mode

There is a but. In regeneration mode, i.e. when the diesel particulate filter is full and needs to clean itself by increasing the exhaust temperature and burning the accumulated PM, emissions increased to 200,000 particles per cm³. During normal engine operation, that number dropped to less than 10,000 per cm.

However, if you know that the car is also used on days with high concentrations of particulate matter (50,000 particles/cm³) and emits around 15,000 particles/cm³ at an even load, this actually means that the engine is purifying the air of particles at that moment.

The researchers’ conclusion is that on days with little particulate matter in the air, a new diesel only purifies the air under the most favourable conditions, but that on days with high concentrations of particulate matter it almost always does.

New diesels compensate for old ones

The latest diesel engines ensure - even if higher regeneration emissions are taken into account - that the air that passes through the engine and the exhaust line contains less particles than the ambient air. What older diesels emit is eliminated by the new ones.

Add to that the findings that the introducion of the WLTP and RDE have effectively reduced NOx limits to well below the 80mg/km threshold and there seems no real rational argument left to ban the latest diesels from city centres - on the contrary.

Authored by: Stijn Blanckaert