Page 244 - Understanding Automotive Electronics

2735 | CH 7 Page 231 Tuesday, March 10, 1998 1:15 PM

DIGITAL ENGINE CONTROL SYSTEM 7

In this case, the mass of fuel delivered to each cylinder is

MAF
F = ------------------------------------
(
⁄
⁄
(
rN 2) AF ) d
This computation is carried out by the controller continuously so that the fuel
quantity can be varied quickly to accommodate rapid changes in engine
operating condition. The fuel injector pulse duration T corresponding to this
fuel quantity is computed using the known fuel injector delivery rate R :
f
F
T = ----
R f

This pulse width is known as the base pulse width. The actual pulse width used
is modified from this according to the mode of operation at any time, as will
presently be explained.
Open-Loop Control
After engine warm-up, For a warmed-up engine, the controller will operate in an open loop if
open-loop control is the closed-loop mode is not available for any reason. For example, the engine
used. The most popular may be warmed sufficiently but the EGO sensor may not provide a usable
method uses the mass signal. In any event, as soon as possible it is important to have a stoichiometric
density equation to cal- mixture to minimize exhaust emissions. The base pulse width T is computed
b
culate the amount of air as described above, except that the desired air/fuel ratio (A/F ) is 14.7
d
entering the intake man- (stoichiometry):
ifold.
MAF
T b = --------------------------------------- base pulse width
(
(
⁄
rN s ) 14.7)R f
Corrections of the base pulse width occur whenever anything affects the
accuracy of the fuel delivery. For example, low battery voltage might affect the
pressure in the fuel rail that delivers fuel to the fuel injectors. Corrections to the
base pulse width are then made using the actual battery voltage.
As explained in Chapter 5, an alternate method of computing mass air
flow rate is the speed-density method. Although the speed-density method has
essentially been replaced by direct mass air flow measurements, there will
continue to be a number of cars employing this method for years to come, so it
is arguably worthwhile to include a brief discussion in this chapter. This
method, which is illustrated in Figure 7.4, is based on measurements of
manifold absolute pressure (MAP), RPM, and intake air temperature T . The
i

UNDERSTANDING AUTOMOTIVE ELECTRONICS 231