The potential of catalysed exhaust gas recirculation to improve high-load operation in spark ignition engines

Within the literature, there are a number of studies investigating the benefits of exhaust gas recirculation, and as a result it has become established as a promising technology for combustion control to allow engine downsizing technology to be advanced. Aside from the dilution effect of exhaust gas recirculation, some of the components, such as NO, CO, and hydrocarbons, can have significant chemical effects. The literature shows that the component within exhaust gas recirculation which has the largest chemical effect on combustion is NO, which can promote or inhibit the onset of autoignition causing reactions within the end gas at high load. The reduction in NOx gases in catalysed exhaust gas recirculation can increase the knock limit at high load, with some authors reporting up to a 5 degrees$ crank angle improvement in combustion phasing. There is conflicting evidence on whether this translates to an improvement in fuel consumption, with one study finding a decrease of up to 2% comparing to another finding an increase of 1.5%-3.5%. Crude calculations on the emissions of a 2.0-L direct injection spark ignition engine operating at high load show that in an extreme case the reduction in the calorific value of the inlet charge due to catalysis of the recirculated gases can be up to 4.5%. Despite the potential benefits, the literature on catalysed exhaust gas recirculation is fairly limited and the evidence seems so far inconclusive as to whether this technology may have the potential to further enhance the benefits of exhaust gas recirculation. This article uses current literature to ascertain the potential benefits of catalysed exhaust gas recirculation, compare to pre-catalyst exhaust gas recirculation, and investigates its individual components in more detail to explain how chemical interactions can either promote or inhibit ignition depending on their concentration and temperature.