ABSTRACT
The development of the internal combustion engine, in the form of the gasoline spark ignition engine used in automobiles, has provided society with tremendous mobility over recent decades. Indeed, the benefits to the individual are so strong that in many countries it can be said the car is a key component of modern society. The desired internal combustion reaction is oxidation of hydrocarbon fuel to carbon dioxide and water, according to reaction (4.1):
4HmCn þ ðmþ 4nÞO2 ! 2mH2Oþ 4nCO2 ð4:1Þ
In practice, however, combustion is not completely efficient, and for this reason, as well as other physical effects, unburned hydrocarbons and partially combusted hydrocarbon oxygenates, such as aldehydes, may be present at varying levels in raw engine exhaust gas. In the pollution control arena these species are referred to differently in different parts of the world. The ‘‘hydrocarbons’’ are usually designated the abbreviation HC, but in Europe total hydrocarbons (THC), including the most difficult to oxidize, methane, are measured and reported. Partially oxidized oxygenates are not measured. In America methane is excluded, but oxygenates are included in nonmethane organic gases (NMOG) analysis. Carbon monoxide is also present as a partial oxidation product which is formed according to reaction (4.2). Moreover, under the conditions of high pressure and temperature during the power stroke, nitrogen and oxygen react in the engine cylinder and establish an equilibrium with nitric oxide, as in reaction (4.3). At some stage, as the product gases expand and cool rapidly en route into the exhaust system, this equilibrium is frozen, and although thermodynamically unstable at low temperatures, appreciable amounts (e.g., up to 3500 ppm) of nitric oxide can be present in the exhaust gas from an engine.
