Page 189 - Understanding Automotive Electronics

2735 | CH 5 Page 176 Tuesday, March 10, 1998 11:10 AM

5 THE BASICS OF ELECTRONIC ENGINE CONTROL

Figure 5.19
Simplified Intake
System

FPO

Each cylinder contributes to the pumping action every second crankshaft
revolution. For an N-cylinder engine, the frequency f , in cycles per second, of
p
the manifold pressure fluctuation for an engine running at a certain RPM is
given by

N × RPM
f = -----------------------
p
120
Figure 5.20 shows manifold pressure fluctuations as well as average MAP.
For a control system application, only average manifold pressure is
required. The torque produced by an engine at a constant RPM is
approximately proportional to the average value of MAP. The rapid
fluctuations in instantaneous MAP are not of interest to the engine controller.
Therefore, the manifold pressure measurement method should filter out the
pressure fluctuations at frequency f and measure only the average pressure.
p
One way to achieve this filtering is to connect the MAP sensor to the intake
manifold through a very small diameter tube. The rapid fluctuations in
pressure do not pass through this tube, but the average pressure does. The
MAP sensor output voltage then corresponds only to the average manifold
pressure.

Measuring Air Mass
A critically important aspect of fuel control is the requirement to measure
the mass of air that is drawn into the cylinder (i.e., the air charge). The amount
of fuel delivered can then be calculated such as to maintain the desired air/fuel
ratio. There is no practically feasible way of measuring the mass of air in the
cylinder directly. However, the air charge can be determined from the mass flow

176 UNDERSTANDING AUTOMOTIVE ELECTRONICS