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THE BASICS OF ELECTRONIC ENGINE CONTROL 5

Effect of Exhaust Gas Recirculation on Performance

Exhaust gas recirculation Up to this point in the discussion, only the traditional calibration
greatly reduces nitrous parameters of the engine (air/fuel ratio and spark timing) have been
oxide emissions. considered. However, by adding another calibration parameter, the
undesirable exhaust gas emission of NO can be significantly reduced while
x
maintaining a relatively high level of torque. This new parameter, exhaust gas
recirculation (EGR), consists of recirculating a precisely controlled amount of
exhaust gas into the intake. Figure 5.5 shows that exhaust gas recirculation is a
major subsystem of the overall control system. Its influence on emissions is
shown in Figures 5.10 and 5.11 as a function of the percentage of exhaust gas
in the intake. Figure 5.10 shows the dramatic reduction in NO emission
x
when plotted against air/fuel ratio, and Figure 5.11 shows the effect on
performance variables as the percentage of EGR is increased. Note that the
emission rate of NO is most strongly influenced by EGR and decreases as the
x
percentage of EGR increases. The HC emission rate increases with increasing
EGR; however, for relatively low EGR percentages, the HC rate changes only
slightly.
The mechanism by which EGR affects NO production is related to the
x
peak combustion temperature. Roughly speaking, the NO generation rate
x
increases with increasing peak combustion temperature if all other variables
remain fixed. Increasing EGR tends to lower this temperature; therefore, it
tends to lower NO generation.
x

Figure 5.10
NO Emission as a
x
Function of EGR at
Various Air/Fuel
Ratios

FPO

UNDERSTANDING AUTOMOTIVE ELECTRONICS 163