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Emission Systems

Reducing Diesel Emissions

While new technologies such as electric cars and hydrogen engines will reduce tailpipe emissions to near zero levels, they will be slow to catch on in the mass market.


Even in 20 years’ time, the car park will be predominantly made up of highly efficient, low emission combustion engines.

Every few years, new Euro emissions standards set ever lower limits for tailpipe emissions of new vehicles. From September 2014, Euro 6 regulations come into force and will reduce diesel emissions to nearly the same level as petrol cars. As emission levels are reduced, new after treatment systems are developed and existing systems are enhanced.

Reducing Diesel Particulate Matter

In order to meet the particulate emissions of Euro 5, PSA Peugeot Citroën pioneered the development of the diesel particulate filter (DPF) – an efficient after treatment system which limits emissions of particulate matter. The DPF is so successful at reducing particulate matter that it is now standard fitment on new diesel cars. Upcoming emissions standards will require the on-board diagnostics system to check the effectiveness of the DPF with the help of soot sensors.

Every so often, a DPF must regenerate itself (i.e. burn off trapped soot particles) to prevent it becoming clogged. Timing of the regeneration process is critical to the life of the filter and associated components, and for achieving optimum fuel efficiency. Late regeneration means the filter becomes clogged, causing increased back pressure in the exhaust line; too early and it shortens the life of the filter. Soot sensors will ensure that regeneration only occurs when necessary thereby extending the life of the filter and increasing fuel efficiency by preventing back pressure.

The soot sensor uses resistive technology to measure the soot concentration in the exhaust line, downstream of the DPF. Two electrodes measure changes in resistance as soot particles are deposited on the sensor. A ceramic sensing element provides resistance to high temperature and will maintain its integrity despite the constant vibration and aggressive gases present in the exhaust line. A shield protects the sensing element from the odd large soot particle which would otherwise be deposited on the sensor and skew the measurements.

The requirements of the upcoming Euro 6 emissions standards mean that soot sensors will probably be fitted as standard to new diesel vehicles from 2014 onwards, making this new sensor one to watch.

Exhaust Gas Recirculation

Exhaust gas recirculation is a well-established way of reducing nitrogen oxide (NOx) emissions. Adding exhaust gas to the air-fuel mix reduces the oxygen content of the mixture which in turn lowers the combustion temperature. The lower the temperature, the fewer NOx particles are produced.

In its current form, a pipe branches off from the exhaust system upstream of the turbocharger and feeds part of the exhaust gas back into the intake manifold. This method has become known as high-pressure EGR. Achieving a further reduction of NOx using EGR means feeding more gas into the EGR system; this cannot be achieved in the high-pressure system because the turbocharger requires a large volume of exhaust gas in order to function. A number of vehicle manufacturers are investigating low-pressure EGR systems as a way of increasing the rate of EGR.

Low-pressure EGR technology recovers exhaust gases further downstream in the exhaust system, after the gases have passed through the DPF and the turbocharger. The gas is cooled by the intercooler to enable recirculation through the turbo, helping to boost turbo pressure. This cold loop enables the recirculation rate to be increased, while at the same time further lowering the temperature and intake pressure. Emissions of NOx are reduced more efficiently than is the case with a high-pressure EGR, and engine efficiency is improved. The combustion is of a higher quality and CO2 emissions are also reduced compared with a conventional EGR system.

Renault will be the first vehicle manufacturer to launch an engine in Europe featuring low-pressure EGR technology with the ‘ENERGY dCi 130’ diesel engine. This new Renault engine will first appear in the new generation of Scenic and Grand Scenic ranges, then across the entire Megane family.

Selective Catalytic Reduction (SCR)

Another method of reducing harmful NOx emissions is to convert the offending particles into nitrogen and water. This is achieved by injecting aqueous urea (AdBlue) into the exhaust gas downstream of the DPF. On injection into the exhaust system, the liquid urea is atomised into small particles which uniformly spread across the surface of a DeNOx catalyst where the NOx particles chemically react with the urea and are converted into nitrogen (N) and water (H2O). This method of reducing vehicle emissions has been used on large commercial vehicles for some time. Incorporating such a system into passenger cars and LCVs is more difficult because space is more limited. The SCR system also creates back pressure in the exhaust line and adds weight, both of which make these vehicles less fuel efficient. The requirements of Euro 6 make it likely that VMs will start to develop SCR for smaller vehicles. Audi, Mercedes-Benz and BMW are already using the SCR system in their American models in order to meet the requirements of strict American emissions standards. This technology is slowly filtering through to the UK and is already in series production for BMW’s X5 xDrive35d advanced diesel and the Mercedes-Benz E350 BlueTEC.

2nd Generation SCR

A number of premium brand German car makers have already begun developing second and third generation SCR systems. Second generation SCR systems integrate with the diesel particulate filter (DPF) to produce a catalytic-coated DPF which is smaller, lighter and less expensive than two separate systems. These catalytic-coated DPFs reach light off temperature much earlier than under body systems. Net efficiency is also greater as there is no need for heating measures.

3rd Generation SCR

In third generation SCR systems, gaseous ammonia supersedes liquid urea. While these 3rd generation SCR systems will soon be available for HGVs, it will be some time before they are ready for passenger cars and LCVs. Keeping abreast of these developments is a challenge in itself, however it is an essential business strategy for those in the independent aftermarket who want to continue as credible competition to the main dealer network.

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