Fuel quality for catalyst, adsorber and filter performance

The quality of the fuel used can assist or degrade the performance of emission control systems.

Fuel, whether it be gasoline, diesel, natural gas (CNG or LNG) 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.

Sulfur in gasoline 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. The loss of catalyst efficiency caused by sulfur in the fuel has a greater impact at the very low emissions levels required in the most advanced Euro stages.

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 structure of the catalytic coating 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 gasoline 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 catalysts and adsorbers. The lower the sulfur levels in fuels the better the catalyst performance that can be obtained. For this reason, European legislation has limited road and non-road fuels sulfur content to 10 ppm (max.) since 2009 and 2011 respectively.

Lead has long been recognised as a catalyst poison as well as having impacts on human health and is has not been permitted in European fuels since 2000. 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 (used to measure the exhaust gas concentration of oxygen and adjust air:fuel ratio as required to optimise the catalytic function) and the 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 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.