4.1 Exhaust pollutants
The most important chemical reaction in a petrol engine – that is, the one that provides the energy to drive the vehicle – is the combustion of fuel in air. In an ‘ideal’ system, combustion would be complete so that the only exhaust products would be carbon dioxide and steam. In practice, the complete oxidation of the fuel depends on a number of factors: first, there must be sufficient oxygen present; second, there must be adequate mixing of the petrol and air; and finally, there must be sufficient time for the mixture to react at high temperature before the gases are cooled. In internal combustion engines, the time available for combustion is limited by the engine’s cycle to just a few milliseconds. There is incomplete combustion of the fuel and this leads to emissions of the partial oxidation product, carbon monoxide (CO), and a wide range of volatile organic compounds (VOC), including hydrocarbons (HC), aromatics and oxygenated species. These emissions are particularly high during both idling and deceleration, when insufficient air is taken in for complete combustion to occur.
Another important result of the combustion process, particularly during acceleration, is the production of the oxides of nitrogen – nitric oxide (nitrogen monoxide, NO) and nitrogen dioxide (NO2). Conventionally, these two oxides of nitrogen are considered together and represented as NOx. At the high temperatures involved (in excess of 1 500 °C) nitrogen and oxygen in the air drawn in with the fuel may combine together to form NO. On leaving the engine, this monoxide cools down and is oxidized by oxidants in the atmosphere to form the dioxide. Although the ‘fixing’ of nitrogen from the air is the major source of NOx, it may also arise from the oxidation of any nitrogeneous components in the fuel.
Primary pollutants are defined as those gases emitted directly from the exhaust of a vehicle. None of these is a desirable addition to the atmosphere, but perhaps the most notorious consequence of exhaust emissions is their role in the formation of photochemical smog – a mixture of ozone, nitrogen dioxide, other secondary products and small particulates. These secondary pollutants can cause severe damage to human health.
The role of an emission control catalyst is to simultaneously remove the primary pollutants CO, VOCs and NOx by catalyzing their conversion to carbon dioxide (CO2), steam (H2O) and nitrogen (N2).