Some diesels may emit over 1,000 times regulated particulate emissions

Research by environmental group claims that even latest engine technology doesn’t stop troubling levels of harmful emissions

Diesel cars may be emitting as much as 1,000 times the legal limit of particulate emissions, according to research by environmental think tank Transport & Environment (T&E).

Particulate emissions are the tiny fragments of soot that are an inevitable by-product of burning fuel in an engine, and diesel cars are an especially egregious source. These tiny particles are carcinogenic, and have been linked by the World Health Organisation with such as lung cancer, brain cancer, cardiovascular disease, and respiratory illnesses. To combat them, car companies install complex filters in a vehicle's exhaust, designed to trap and contain such particles. It is these very filters which, according to T&E, are the very source of the problem.

In order for the exhaust filters to work properly without having to be either manually cleaned out or replaced, they are designed to self-clean, burning off the excess soot at temperatures of 550-degrees celsius or higher as part of a regeneration cycle. The problem, according to this latest research, is that during this cleaning process, the exhaust is effectively open and allowing particulates to flow through at rates way above the legislated limit.

Lab test

T&E commissioned the Ricardo Transport Laboratory in the UK to take two cars - a Nissan Qashqai 1.5 dCi and an Opel Astra 1.6 CDTI - and put them through several exhaust filter regeneration cycles.


During those cycles, the measured emissions of particulates of 23-nanometres size or larger, was found to be between 32 per cent and 115 per cent higher than the legislated limit of 6x1011/km.

T&E argues that the 23nm size limit is pointless, as medical research points to particulates of 10nm being the most dangerous to human health (that’s smaller than the size of most common viruses, so these particles can penetrate the walls of human cells). Take particles of that size into account, and the cars can exceed the legal emissions limit by as much as 184 per cent.

On top of which, these filters have a role to play in the control of emissions of nitrogen oxides (NOx), the harmful gas that’s been at the centre of the diesel scandal since 2015. During the regeneration cycle, NOx emissions increased significantly.

T&E’s research also showed that the two cars went into regeneration cycles much sooner than they were supposed to. Both the Qashqai and the Astra triggered their filter regeneration cycles at around 420km intervals – half the distance at which they are supposed to, according to T&E. On top of that, the cycles can be triggered in urban driving conditions, and last for up to 15km, causing spikes of emissions

T&E estimates that there are 45 million diesel-engined cars with exhaust particulate filters registered on EU roads. That adds up to 1.3 billion regeneration cycles per year, averaging out at one every two weeks for each diesel car.

In Irish terms, T&E estimates that from 526,000 registered diesel-engined cars equipped with a particulate filter, that means 14,931,000 regeneration cycles every year, putting us in 14th place overall in this new pollution index.

Anna Krajinska, emissions engineer at T&E, said: "These tests show that new diesels are still not clean. In fact they are spewing out highly-dangerous levels of particles in our towns and highways everyday. Carmakers are being given an easy ride but people's lungs are paying for it. Manufacturers should clean up their cars if they want to sell them."


There are limitations to T&E's research. For a start, only those two cars were examined. Speaking to The Irish Times, Ms. Krajinska said: "On vehicle selection, just these two vehicles were tested - it's a very expensive operation for an NGO. The Nissan Qashqai and the Opel/Vauxhall Astra were chosen as both were already available in the latest EU 6d-temp emission standard and both were top selling car models in Europe in 2018. They are two of the latest Euro 6d-temp cars with large market penetration. The DPF and other emission control components used on the two vehicles are typical of the kind of technology used on the latest EU 6d-temp cars. It should be noted that the two cars tested are from two different manufacturers with two different engine and emission control technologies, yet they both produce large amounts of particle pollution, suggesting that this could be a problem for many diesel cars on EU roads today - including the latest and cleanest Euro 6d-temp diesels."

The report could also cause confusion amongst those considering a diesel-engined car. Last year, an independent test by the German motoring group ADAC showed that for many cars the emissions of harmful NOx can be as low as zero mg/km (as recorded by the Mercedes-Benz C220d), 1mg/km (for the BMW 520d and the Opel Astra 1.6 CDTI), and 7mg/km (Citroen Berligo 1.5 HDI 130). The legal limit for NOx is 80mg/km. If the focus is now moving from NOx to particulate emissions, then what should those considering a diesel purchase do?

“T&E’s findings do not contradict ADAC results but add more data on different testing conditions and pollutants” Ms. Krajinska told The Irish Times. “These two diesels performed better than others for NOx emissions but spewed out dangerous levels of particles from filter cleaning. But it shouldn’t have to be a choice between the two, and zero-emissions cars mean it doesn’t have to be any longer.”

Neil Briscoe

Neil Briscoe

Neil Briscoe, a contributor to The Irish Times, specialises in motoring