Page 193 - Understanding Automotive Electronics

2735 | CH 5 Page 180 Tuesday, March 10, 1998 11:10 AM

5 THE BASICS OF ELECTRONIC ENGINE CONTROL

Unfortunately, the engine is not a perfect air pump. In fact, the actual
volume flow rate for an engine having displacement D and running at speed
RPM is given by

D
 RPM  
R = ------------ ---- n
v  60  2 v

where n is the volumetric efficiency.
v
Volumetric Efficiency
Volumetric efficiency The volumetric efficiency is a number between 0 and 1 that depends on
varies with MAP and intake manifold pressure (MAP) and RPM for all engine operating conditions.
engine speed. A table of For any given engine, the value of n can be measured for any set of operating
v
values representing volu- conditions. A table of values of n as a function of RPM and MAP can be
v
metric efficiency for prepared from this data. In a digital system, the table can be stored in memory
given speeds and MAP as a lookup table. By knowing the displacement of the engine, measuring the
values can be stored in RPM and MAP, and looking up the value of n for that RPM and MAP, the R
v
v
memory as a lookup can be computed using the previous equation.
table.
Including EGR
Exhaust gas recirculation Calculating R is relatively easy for a computer, but another factor must
v
also must be considered be taken into account. Exhaust gas recirculation requires that a certain portion
when calculating volume of the charge into the cylinders be exhaust gas. Because of this, a portion of the
flow rate. The true vol- displacement D is exhaust gas; therefore, the volume flow rate of EGR must be
ume flow rate of air is known. A valve-positioning sensor in the EGR valve can be calibrated to
calculated by subtracting provide the flow rate.
the volume flow rate of From this information, the true volume flow rate of air, R , can be
a
EGR from the total vol- determined by subtracting the volume flow rate of EGR (R EGR ) from R . The
v
ume flow rate. total cylinder air charge is thus given as follows:

R = R – R
a v EGR
The volume flow rate of EGR is known from the position of the EGR valve and
from engine operating conditions, as explained in Chapter 7.
Substituting the equation for R , the volume flow rate of air is
v
D
------------
---- n
R =   RPM   v – R EGR
a

60  2
Knowing R and the density d gives the mass flow rate of air R as follows:
m
a
a
R = R d
m a a
180 UNDERSTANDING AUTOMOTIVE ELECTRONICS