FIGURE 11.1 Effect of air/fuel ratio on automotive gasoline engine emissions and dependence of the simultaneous conversions of NOj, CO, and HC on the air-fuel ratio in a three-way catalyst. (Adapted from Church, M.L., Cooper, B.J., and Willson, P.J., Catalyst Formulations 1960 to Present, SAE 890815, Society of Automotive Engineers, Warrendale, PA, 1989; McCabe, R.W. and Kisenyi, J.M., Chem. Ind. (London), 15, 605, 1995. With permission.
depends on two closely related factors which predominate in the formation of the three primary pollutants (CO, NO,, and HCs) — air-to-fuel ratio (A/F, denoted by A,) and temperature — these, in turn, are determined by driving conditions [12, 13]. The exact composition at any give time thus largely depends on these two factors. Figure 11.1 shows the variation in the production of NO,, CO, and HCs as the A/F ratio varies. A value of A = 1 indicates the point where the ratio is stoichiometrically balanced, i.e., when the amount of oxygen (air) present is exactly that require to fully combust all of the fuel present (to CO2 and H2O). This corresponds to an A/F ratio of 14.7. Combustion mixtures with low A/F ratios (A < 1) are said to be fuel rich (net reducing), and thus form the highest amounts of CO and HC. Their levels of production decrease through the stoichiometrically balanced ratio (A = 1) and into the fuel-lean situation (A > 1, net oxidizing), where most of the fuel is consumed by the available O2, resulting in production of small amounts of CO and HCs. At high values of A, the output of HCs begins to increase because combustion becomes inefficient.
For NO,, the situation is slightly more complicated. In fuel-rich conditions, little NO, is formed due to a lack of availability of O2, which is largely consumed in combustion. The amount of NO, formed continues to increase as the A/F ratio increases through stoichiometry (A = 1) into fuel-rich stoichiometries (A > 1). The amount formed then goes through a maximum and starts to decrease. This is because the temperature of combustion falls, as the heat produced from the fuel combustion is required to heat up an increasing amount of air. As outlined above, the NO, produced in an internal-combustion engine is mainly thermal in origin, and the drop in temperature results in a decrease in its production. Another temperature-related feature is the situation immediately after start-up, when the engine temperature has x not reached its normal operating level. In this period, the lowest concentrations of NO are formed, while CO and HCs levels are high.
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