Environmental Engineering Reference
Air/fuel ratio (% stoichiometric)
Figure 8.9 A diagram of the mass of exhaust gas pollutants—carbon monoxide (CO), nitrogen oxide (NO),
and hydrocarbons (HC)—in a typical SI engine, as a function of the air/fuel ratio.
but a tiny fraction of the fuel is oxidized to CO 2 and H 2 O, reaching a state of thermochemical
equilibrium after releasing the chemical energy bound up in the fuel. However, a small amount
of the reactants do not reach this equilibrium state, but instead remain frozen into a metastable
form. The principal molecules of this type are NO, CO, and various kinds of hydrocarbon (HC)
molecules, all of which can contaminate the atmosphere into which the vehicle exhaust gas stream
is introduced. The purpose of vehicle emission control technology is to reduce the amounts of these
pollutants to such low values that the cumulative effects of many vehicles and other sources will
not be great enough to cause any damage to living systems, including humans.
In SI engines, the amount of each of these principal pollutants is sensitive to the air/fuel ratio of
the mixture inducted into the cylinder prior to ignition by the spark plug. The proportion of air to fuel
must not be too far from the stoichiometric value for the engine to function properly and efficiently.
If the mixture is fuel-rich (more fuel than can be completely oxidized by the available oxygen),
some CO will be formed and not all of the fuel's heating value will be released. If the mixture
is fuel-lean (excess, unused oxygen), the combustion product temperature and pressure will be
less, resulting in less engine work per cycle. These differences in the chemical and thermodynamic
state of the combustion gases influence the amounts of pollutants that leave the engine through the
Figure 8.9 shows how the mass of exhaust gas pollutants varies with the air/fuel ratio in a
SI engine. In rich mixtures, there is insufficient oxygen to oxidize completely the fuel molecules,
leaving some unburned HC or incompletely oxidized CO and H 2 , the more so as the oxygen
deficiency becomes larger. 21 When there is surplus oxygen (lean mixture), CO and HC diminish
21 The hydrogen, not shown in Figure 8.9, is formed through the reduction of H 2 O by CO at the end of the
combustion process. This hydrogen aids the catalytic reduction of NO in the exhaust converter.