The importance of periodic vehicle emission tests for air quality

DISCLAIMER: All opinions in this column reflect the views of the author(s), not of EURACTIV.COM Ltd.

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Many cities in Europe have problems because they exceed the current limits for air pollutants. Some of them are even considering banning diesel vehicles in general to batten down the hatches, writes Gerhard Müller.

Gerhard Müller is the Vice President – Technical Affairs at CITA, the international motor vehicle inspection committee.

In this context, the European legislation has a unique and very effective instrument for detecting high emitting vehicles in use: the periodic vehicle emission test.

This method identifies failures and/or manipulation of emission systems and forces the vehicle owner to perform repair or maintenance work. But unfortunately, the current periodic emission test is no longer applicable to modern diesel and petrol vehicles.

In fact, the current test procedure was developed over 25 years ago and was only slightly adapted for the development of modern vehicles. This means, for example, that a EURO 5 diesel vehicle would pass the periodic emission test even if the trap in question is removed and smoke emission increases by 500 times. In addition, important pollutants like NOx cannot be measured at all.

As a result, manipulation or even normal deterioration of the emission systems cannot be detected during the periodic emission test. In practice, this means that millions of vehicles in Europe are called big polluters, with a dramatic negative impact on air quality.

The European Commission estimates that only 5% of these vehicles are responsible for at least 25% of all emissions from combustion engine vehicles. If we would be able to detect gross polluting vehicles using a modern emission test, we could significantly reduce emissions.

The European wide CITA SET I study has shown that in the short term perspective it would be very helpful to introduce a mandatory tailpipe test and OBD reading in combination with more stringent thresholds.

This could be implemented with the current test procedures and equipment. Germany, for example, is going this way starting this year. The CITA SET II study is dealing with a measurement procedure for NOx emissions. The final report is expected in the coming month.

The type and amount of pollutants in new vehicles have changed significantly over the last years. For example, the size of particulate matters (PM) of diesel vehicles is becoming smaller and smaller, and a high efficiency engine (e.g. EURO 6) is producing much more NOx than a EURO 4 engine.

Therefore, very efficient but also expensive after-treatment systems are necessary to clean the emissions of modern vehicles. Additionally, vehicles with direct gasoline injection are emitting PM like diesel vehicles, which makes a particulate emission test significant for petrol vehicles as well.

First results of the CITA SET II study demonstrate that for a proper evaluation, especially of NOx emissions, a simple loaded test in combination with a comprehensive and standardized OBD reading is very promising. If we want to maintain the emission standards of modern vehicles at a high level throughout the whole life cycle, we need a modernized periodic emission test. As long as defects or manipulation are not detected nobody would repair their vehicle.

In a cost-benefit analysis, this new approach for an emission test will not automatically increase the costs for the car owner: Even if the measurement equipment would become more costly, the most expensive part of an emission test will always be the inspector’s working time. So if the test could be conducted faster than it is today, it could compensate for additional costs for the equipment.

If we really want to improve air quality in urban areas where, for many years to come, the combustion engine will be the most widespread propulsion system, we must conduct proper and valid emission tests. 

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