Diesels failing to meet air quality standards
If you’re thinking about buying a new car, make sure that you’ll be complying with the new air quality standards set by the European Union.
Although diesel engines are fuel-efficient, many do not conform to the new Euro 5 standard for air quality, even when fitted with a particulate filter. Particulate filters are designed to collect the particulates (otherwise known as soot) that are produced through the burning of diesel. Although the filters are effective, there are often problems when the particulate filter gets partially blocked.
To work effectively any filter needs to be emptied regularly – like the bag on a vacuum cleaner. With a diesel particulate filter (DPF) this is done through a process called DPF regeneration. The particulates are burnt at a high temperature and leave behind a small residue of ash.
There are two types of regeneration – passive and active. Passive regeneration takes place on longer journeys when the car is travelling at speed and the exhaust gets hot enough to burn off the particulates. However, with so many people taking short stop-start journeys, passive regeneration doesn’t get to take place, so car manufacturers have had to add in active regeneration, which is controlled by the car’s engine management computer (ECU).
This works by the ECU making adjustments to the fuel injection timing to increase the temperature in the exhaust and start regeneration. However, sometimes the journey does not continue long enough for the process to take place, and the filter becomes partially blocked. In order to clear the filter, you need to travel at speeds over 40mph for more than ten minutes.
If you tend to make short and slow journeys around town, it may be better to avoid getting a diesel and buy one of the many hybrid cars that are now available. Hybrid cars are fuel-efficient but also meet the Euro 5 air standard as they are low emission cars. Hybrids produce fewer emissions than conventional diesel or petrol cars.