Diesel Filters make Pollution Worse

Greg Archer, UK Director Transport and Environment

C armakers can no longer claim new diesels are clean. Recent research published by Transport and Environment has exposed that even the most modern diesel cars belch out huge numbers of hazardous fine particles. (1)

For a decade new diesel cars and vans have been fitted with a diesel particle filter (DPF) to collect particles in the exhaust. The DPF was thought to have solved the diesel particle problem, but diesels produce so much soot the filters quickly clog and need regular cleaning - typically every 300 miles. This regeneration of the filter, which typically lasts around 25 minutes, releases huge numbers of the smallest, most hazardous particles.

T&E commissioned Ricardo Engineering to conduct the tests on the road and in the lab. We tested 2 cars a Vauxhall Astra and Nissan Qashqai but would have found very similar results on every diesel car, van and urban buses and delivery trucks too. The tests measured a massive spike in the particle numbers during the time the filter was cleaned. The levels were so high the vehicles breached the particle number limit – by 32 to 115% - but in standard testing the car is not considered illegal because tests are invalidated and redone when a regeneration occurs. In effect between 60 and 99% of all the particles emitted from the vehicle occur during the regeneration and are ignored.

The legal limit for particle number is 6x10 11 . Most diesels in routine driving produce emissions substantially lower than this. The Nissan measured levels above the legal limit during the regeneration on both occasions, 7.9 and 8.5 x10 11.. The Astra was much worse seeing 1.3x10 12 particles being released during the regeneration per km - around 5 times the legal limit.

The research shows diesels are an even more important source of particulates than previously thought and demonstrate that it is nonsense to suggest new diesel cars are clean. All diesel cars should be banned from low emission zones - as Bristol has done.

Figure 1.Diagram showing particulate emission spikes of different cars tested during filter cleaning.

T&E also found that many of the particles released are too small to be measured during a standard road test. Only solid particles larger than 23nm in diameter are measured in the test, but T&E measured solid particles as small as 10nm and this increased the total number of particles by between 11% and 184% compared to when only regulated particles were measured. Nearly half the particles may be being missed in some tests. This omission is especially worrying because it is the smallest particles that have the greatest health concerns.

There are 7.5 million dirty diesel cars and vans on the UK’s roads fitted with DPFs. Given their typical mileage and the interval between filter cleaning there will be around 230 million regenerations every year. Not all of these will occur in urban areas – but most will.

Even those occurring on highways will emit particles that once airborne will add to the background pollution levels. In addition diesel cars emit ammonia through the process of using urea to control NOx emissions. The ammonia is quickly converted in the air into secondary particles as will the NOx emissions. So what should be done? Firstly, as part of the Environment Bill we must set an objective to achieve the WHO PM2.5 guideline value of 10um/m 3 by 2030.

Secondly, we need to rapidly phase out of new Internal Combustion Engine (ICE) cars and vans by 2030. In doing so the UK will become a leader in Electric Vehicle (EV) sales attracting manufacturing of vehicles and cells into the UK. (2) We cannot achieve net zero emissions by 2050 unless we end sales of ICE cars as soon as possible. The budget must also ensure that grants for zero emission vehicles are retained as they are scheduled to end in March 2020. These grants could be funded through reform of first year vehicle excise duty. (3)

Thirdly, we need to improve the tests of particle number and limits should be met when a DPF regeneration occurs. The EU is developing a new Euro Standard and we should follow their example to apply stricter rules. Until diesel vehicles are clean – and this work shows definitively they are not – they should be excluded from all low emission zones.

Finally, we need a new Clean Air Act. The toxic air we are daily forced to breathe must be cleaned up. The Environment Bill

will not tackle the main causes of air pollution hotspots, 95% of which are caused exclusively by traffic. The first Clean Air Act resolved the 1956 Smog – now we need a new Act focused on today's principal source of pollution: traffic.