Page 243 - Understanding Automotive Electronics

2735 | CH 7 Page 230 Tuesday, March 10, 1998 1:15 PM

7 DIGITAL ENGINE CONTROL SYSTEM

Figure 7.3
Illustration of
Lookup Table for
Desired Air/Fuel
Ratio

warm-up mode to an open-loop (warmed-up engine) mode after a sufficient
time by means of an internal timer.
It is worthwhile at this point to explain how the quantity of fuel to be
injected is determined. This method is implemented in essentially all operating
modes and is described here as a generic method, even though each engine
control scheme may vary somewhat from the following. The quantity of fuel to
be injected during the intake stroke of any given cylinder (which we call F) is
determined by the mass of air (A) drawn into that cylinder (i.e., the air charge)
during that intake stroke. That quantity of fuel is given by the air charge
divided by the desired air/fuel ratio:

A
F = -----------------
( AF ) d
⁄
The quantity of air drawn into the cylinder, A, is computed from the mass air
flow rate and the RPM. The mass air flow rate (MAF) will be given in kg/sec. If
the engine speed in revolutions/minute is RPM, then the number of
revolutions/second (which we call r) is

RPM
r = ------------
60

Then, the mass air flow is distributed approximately uniformly to half the
cylinders during each revolution. If the number of cylinders is N then the air
charge (mass) in each cylinder during one revolution is

MAF
A = ------------------
⁄
rN 2)
(

230 UNDERSTANDING AUTOMOTIVE ELECTRONICS