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Controlling engine and vehicle emissions

The Integrated Approach

Catalyst and filters are part of a complete system, where all the elements must work together to achieve the greatest possible reduction in emissions. The fuel and fuel system, the engine and its combustion system, sensors and the design and location of the catalyst and filter combine with the electronic control system to give the maximum emissions reductions. AECC's members therefore work closely with the automotive manufacturers and their system suppliers to achieve these goals.

The sources of pollutants

If we could burn petrol or diesel perfectly in pure oxygen it would produce only carbon dioxide (CO2), water vapour (H2O), and energy. However in reality it is never possible to burn fuels perfectly, so there are always some emissions of unburned and partially burned fuel together with oxides of nitrogen (NOx) from the nitrogen in the air.

Fuel, whether it be petrol, diesel, natural gas (CNG) or LPG is made up of hydrocarbons. These can range from small, simple molecules such as methane, the main component of natural gas, to large complex molecules. In combustion these molecules get broken up. Ideally they break down completely to CO2 and water vapour, but some escape unchanged or partially degraded as hydrocarbons (HC) or as carbon monoxide (CO). For diesel engines especially some of the fuel ends up as particulate matter (PM). PM is mainly soot particles with volatile hydrocarbons and some sulfate and metallic residues from the fuel and engine lubricant. The other main pollutant in exhaust gas results from the fact that we burn the fuel in air, which is nearly 80% nitrogen, not in pure oxygen. At high temperatures, the nitrogen forms nitrogen oxides (NOx) in the combustion chamber. The more efficient the combustion, the higher the temperatures are likely to be and hence the higher the NOx emissions.

The role of catalysts, filters and adsorbers

Control of the fuelling and combustion processes offer a method of giving some reduction in engine-out emissions. Mechanical systems such as Exhaust Gas Recirculation (EGR) also offer the engine designer some opportunities to reduce specific emissions under appropriate operating conditions; for instance EGR re-uses some of the exhaust gas so as to lower the combustion temperature and reduce engine-out NOx emissions. Autocatalysts, traps and filters as part of a correctly-designed engine system can virtually eliminate pollutants under most driving conditions. Catalysts generally need to reach a suitable operating temperature, but with modern systems this is reached within seconds.

Fuel quality for catalyst, trap and filter performance

The quality of the fuel used can assist or degrade the performance of emission control systems. Lead has long been recognised as a catalyst poison as well as having impacts on human health, and is no longer permitted in European fuels. There are concerns over the use of other metallic additives, with suggestions that their use in gasoline may, under some driving conditions, lead to deposits on exhaust system components such as the oxygen sensor and catalyst. Metallic or other ash-forming materials in diesel fuel will add to the amount of ash captured by particulate filters and may require the system to be designed so as to allow for the additional ash. Detergent additives, on the other hand offer positive benefits. Their use helps keep the fuel injection system and combustion system clean, so helping to prolong optimum operating conditions for the emissions control technology. Sulfur in petrol and diesel fuel has a major negative impact on catalyst performance and in diesel also contributes to the mass of particulate matter (PM). The effect of sulfur on catalyst performance becomes more critical as lower tailpipe emissions are targeted and the loss of catalyst efficiency caused by sulfur in the fuel has a greater impact at the very low emissions levels required in future.

Sulfur strongly competes against pollutants for 'space' on the catalyst surface and this limits the efficiency of catalyst systems to convert pollutants at any sulfur concentration. The effect of sulfur as a competitor on the catalyst surface may be reversible but it can cause irreversible changes to the washcoat and some of the base metal components. The conversion of sulfur to a sulfate aerosol can cause net increases in particulate emissions. One tankful of high sulfur fuel will immediately degrade catalyst performance but this will normally be restored on reverting to a low sulfur fuel. The levels of sulfur in fuel are an important factor in the performance of most NOx catalysts and adsorbers. The lower the sulfur levels in fuels the better the catalyst performance that can be obtained. For this reason European legislation already limits road fuel sulfur content to 50 parts per million (ppm), with 10ppm (max.) having to be available since 2005 and to be fully introduced in 2009.