TABLE OF CONTENTS In stoichiometric engine operation, emission levels heavily depend on how accurate the air-fuel ratio can be kept at ? = 1. Due to measurement and computational tolerances, sufficiently accurate stoichiometric operation requires a closed loop control.
In SI engines, the air-fuel ratio ? is either very lean at part load or stoichiometric at medium and high load. A stoichiometric ratio of ? = 1 should lead to an ideal combustion. Figure 5.1 shows the emissions at different air-fuel ratios. For ? = 1, the emissions of HC, CO and NO x are relatively low. Due to turbulence and local inhomogeneity of the gas mixture, real combustion actually produces HC, CO and NO x at the same time. By means of a catalytic converter, these raw emissions can be effectively reduced.
It can be seen in Figure 5.2 that the emission rates after the catalytic converter vary highly with the air-fuel ratio ?: A change of the average ? ? = 0.1% would already double the emission rates. Therefore, it is important to have an accurate closed loop lambda control to guarantee an average air-fuel ratio within...
