Our work

Emissions of particulate matter from diesel cars

Clean air is vital to health. Air quality can be improved by cutting road-vehicle emissions. TNO has been commissioned by the Ministry of Infrastructure and the Environment to conduct an In-Service Conformity programme for passenger cars, to obtain a better understanding of their emissions. As part of that programme, TNO is carrying out real-world measurements to gather objective data on passenger-car emissions. The emission of particulate matter is one of the programme’s focus areas.

Diesel particulate filters sharply reduce particulate emissions

The combustion of diesel fuel in a vehicle’s engine produces particulates, which are often referred to as ‘particulate matter’ or ‘PM’. These particulates are emitted to the environment through the exhaust. Particulates have a negative impact on human health, because they enter the human body through the lungs.

To reduce the emissions of particulate matter, modern diesel passenger cars and vans are fitted with a closed diesel particulate filter (DPF). In everyday practice, the introduction of diesel particulate filters has cut particulate emissions from these road vehicles by 95% to 99%.

However, particulate filters are sometimes removed, for various reasons, and this has a major adverse impact on air quality. Thus, it is important to ensure that particulate filters remain in place throughout the operational life of the vehicle, and that they continue to function properly.

Current MOT unsuitable for checking particulate filters

In the current MOT, the Periodic Technical Inspection for passenger cars and vans, the emissions of modern diesel vehicles fitted with particulate filters are checked by retrieving data from the On-Board Diagnostics (OBD) system. However, when the particulate filter is physically removed, the vehicle’s software is also modified in order to prevent fault codes from occurring. For this reason, the OBD check will not show whether the particulate filter has been removed.

Older diesel vehicles (up to and including Euro 4) are subjected to a smoke measurement. Such measurements have been used for many decades. Only those cars that produce very high emissions fail this test. Modern diesel vehicles (Euro 5 and Euro 6), fitted with a DPF, undergo a smoke measurement if they fail the OBD test. However, they can easily meet the outdated, high limits for smoke emissions, even without a particulate filter. For this reason, in the current MOT, it is not possible to determine whether or not the particulate filter has been removed.

TNO reports on particulate filters and options for checking these in the mot

The report entitled ‘Diesel particulate filters for light-duty vehicles: operation, maintenance, repair, and inspection’, at the bottom of this page, describes, in accessible terms, the most important aspects of particle filters. It also gives details of the options for checking them in the MOT, to see whether they are performing properly.

This report is mainly based on four studies commissioned by the Ministry of Infrastructure and the Environment and carried out by TNO. Those studies (the reports of which can also be found on this page) are briefly discussed below.

  • In 2013, TNO carried out a preliminary study (on behalf of the Ministry) into testing methods and procedures for the assessment of closed particulate filters. These procedures involved the use of various types of smoke meter (also known as opacity meters). The results of this study showed that the current free acceleration test (which uses a modern, enhanced opacity meter) may be suitable for use in the MOT test for vehicles with closed particulate filters.
  • In 2015, at the request of the Ministry, the Netherlands Vehicle Authority (RDW) carried out supplementary tests on about 400 vehicles that were required to undergo an MOT test. The main objective was to get an idea of the percentage of vehicles that have a defective particulate filter or none at all. This study showed that in approximately 5% to 7% of diesel passenger cars the particulate filter was either defective or it had been removed entirely.
  • In early 2016, TNO conducted a follow-up study for the Ministry into how future MOTs might be able to check that vehicles are fitted with fully functional particulate filters. In addition to examining modifications to the current measurement method (the smoke test), this study also explored new methods of measurement. TNO measured the particulate emissions of 213 diesel vehicles, using two different types of smoke meter. In addition, a particle counter was used to measure the number of particles in the vehicles’ exhaust gases. The study highlighted two suitable options for an MOT particle filter test.
  • In 2016 and 2017, TNO performed an assessment of the current PTI smoke emission test procedure for the Ministry. This smoke test yields rather poor test results, mainly due to the fact that the test equipment is insensitive and its signals are filtered. As a consequence, the test results do not correlate well with the real-world PM or PN emissions. Furthermore, a new simple PTI test procedure was developed, based on a PN measurement at low idle speed. Launching this new test procedure is likely to be possible after the development of a dedicated, low-cost PTI PN tester and implementation of the procedure in vehicle regulations.
  • In 2018 and 2019, TNO performed additional research activities for the Ministry and defined the final PTI-PN test procedure for diesel vehicles with a DPF. Main attention was given to the PTI-PN limit values. Furthermore first investigations with gasoline vehicles with a particulate filter (GPF) showed specific PN emission behaviour. These results indicate that specific knowledge and further research is needed for a PTI-PN test procedure for gasoline vehicles.



Gerrit Kadijk BSc

  • biofuels
  • vehicles
  • emissions
  • emission factors
  • DPF


Stay up to date with our latest news, activities and vacancies

TNO.nl collects and processes data in accordance with the applicable privacy regulations for an optimal user experience and marketing practices.
This data can easily be removed from your temporary profile page at any time.
You can also view our privacy statement or cookie statement.