Page 245 - Understanding Automotive Electronics

2735 | CH 7 Page 232 Tuesday, March 10, 1998 1:15 PM

7 DIGITAL ENGINE CONTROL SYSTEM

Figure 7.4
Engine Control System Using the Speed-Density Method

air density d is computed from MAP and T , and the volume flow rate R of
a i v
combined air and EGR is computed from RPM and volumetric efficiency, the
latter being a function of MAP and RPM. The volume rate for air is found by
subtracting the EGR volume flow rate from the combined air and EGR.
Finally, the mass air flow rate is computed as the product of the volume flow
rate for air and the intake air density. Given the complexity of the speed-density
method it is easy to see why automobile manufacturers would choose the direct
mass air flow measurement once a cost-effective mass air flow sensor became
available.
The speed-density method can be implemented either by computation in
the engine control computer or via lookup tables. Figure 7.5 is an illustration of
the lookup table implementation. In this figure, three variables need to be
determined: volumetric efficiency (n ), intake density (d ), and EGR volume
v a

232 UNDERSTANDING AUTOMOTIVE ELECTRONICS