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 Water Management to conduct an In-Service Conformity programme for passenger cars in order 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. It is therefore 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 However, they can easily meet the high, outdated limits for smoke emissions, even without a particulate filter. For this reason, it is not possible to determine whether or not the particulate filter has been removed in the current MOT.

TNO was the first to develop a MOT particle test for diesel vehicles that detects defective or removed particulate filters. A particle concentration measurement determines within 10 to 30 seconds whether a particulate filter is filtering sufficiently or not. This test is expected to become the new global standard in the near future. As a result, TNO is not only reducing air pollution but has also achieved an international first.

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 the most important aspects of particle filters in accessible terms. It also gives details of the options for checking them in the MOT in order to see whether they are performing correctly.

This report is mainly based on four studies commissioned by the Ministry of Infrastructure and the Water Management and carried out by TNO. These 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.

  • At the request of the Ministry, the RDW and TNO also jointly tested approximately 400 vehicles which were required to undergo an MOT sample check in 2015. The main objective was to gain an impression of the proportion of defective or dismantled particulate filters. This study showed that there were greatly increased levels of particulate matter in around five to seven per cent of diesel passenger cars with an 'ex-factory' particulate filter, indicating that the filter had been removed or was defective.

  • 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 particulate 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. The launch of this new test procedure will likely be possible after the development of a dedicated, low-cost PTI-PN tester and the implementation of the procedure in vehicle regulations. The test procedure also showed that a limit on PN emissions is the robust requirement for a properly functioning particulate filter. A PN requirement has no direct relationship with air quality and, for this reason, it is not applied in America (for example). The particulate numbers measured in the air have a limited relationship with the particles and measurement principles which are suitable for diesel exhaust gas. The statutory particulate number (PN) requirement ensures that the total particulate matter from a modern diesel car is well below the statutory particulate matter (PM) requirement. Although the extra particulate matter emissions resulting from particulate filter regenerations are substantial compared to emissions at other times, the average emissions with a diesel particulate filter are still low compared to other vehicle technologies.

  • 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. The main attention was given to the PTI-PN limit values. Furthermore, the first investigations into gasoline vehicles with a particulate filter (GPF) showed specific PN emission behaviours. These results indicate that specific knowledge and further research is needed for a PTI-PN test procedure for gasoline vehicles.