Page 190 - Understanding Automotive Electronics

2735 | CH 5 Page 177 Tuesday, March 10, 1998 11:10 AM

THE BASICS OF ELECTRONIC ENGINE CONTROL 5

Figure 5.20
Intake Manifold
Pressure
Fluctuations

FPO

rate of air into the engine intake since all of this air eventually is distributed to
the cylinders (ideally uniformly).
There are two methods of determining the mass flow rate of air into the
engine. One method uses a single sensor that directly measures mass air flow
rate. The operation of this sensor is explained in Chapter 6. The other method
uses a number of sensors that provide data from which mass flow rate can be
computed. This method is known as the speed-density method.
Speed-Density Method
The concept for this method is based on the mass density of air as
illustrated in Figure 5.21a. For a given volume of air (V ) at a specific pressure
(p) and temperature (T ), the density of the air (d ) is the ratio of the mass of air
a
in that volume (M ) divided by V:
a
M a
d = -------
a
V
Another way of looking at this is that the mass of air in the volume V is the
product of its density and volume:

M = d V
a a

UNDERSTANDING AUTOMOTIVE ELECTRONICS 177