Page 250 - Understanding Automotive Electronics

2735 | CH 7 Page 237 Tuesday, March 10, 1998 1:15 PM

DIGITAL ENGINE CONTROL SYSTEM 7

economy are controlled. Enrichment of the air/fuel ratio to about 12:1 is
sometimes used.
Deceleration Leaning

Fuel flow is reduced dur- During periods of light engine load and high RPM such as during
ing deceleration with coasting or hard deceleration, the engine operates with a very lean air/fuel ratio
closed throttle. to reduce excess emissions of HC and CO. Deceleration is indicated by a
sudden decrease in throttle angle or by closure of a switch when the throttle is
closed (depending on the particular vehicle configuration). When these
conditions are detected by the control computer, it computes a decrease in the
pulse duration of the fuel injector signal. The fuel may even be turned off
completely for very heavy deceleration.
Idle Speed Control

When the throttle angle Idle speed control is used by some manufacturers to prevent engine stall
reaches its closed posi- during idle. The goal is to allow the engine to idle at as low an RPM as possible,
tion and engine RPM yet keep the engine from running rough and stalling when power-consuming
falls below a preset value, accessories, such as air conditioning compressors and alternators, turn on.
the controller switches The control mode selection logic switches to idle speed control when the
to idle speed control. A throttle angle reaches its zero (completely closed) position and engine RPM
stepping motor opens a falls below a minimum value, and when the vehicle is stationary. Idle speed is
valve, allowing a limited controlled by using an electronically controlled throttle bypass valve (Figure
amount of air to bypass 7.7a) that allows air to flow around the throttle plate and produces the same
the closed throttle plate. effect as if the throttle had been slightly opened.
There are various schemes for operating a valve to introduce bypass air for
idle control. One relatively common method for controlling the idle speed
bypass air uses a special type of motor called a stepper motor. A stepper motor
moves in fixed angular increments when activated by pulses on its two sets of
windings (i.e., open or close). Such a motor can be operated in either direction
by supplying pulses in the proper phase to the windings. This is advantageous
for idle speed control since the controller can very precisely position the idle
bypass valve by sending the proper number of pulses of the correct phasing.
The engine control computer can know precisely the position of the valve
in a number of ways. In one way the computer can send sufficient pulses to
completely close the valve when the ignition is first switched on. Then it can
send open pulses (phased to open the valve) to a specified (known) position.
A block diagram of a simplified idle speed control system is shown in
Figure 7.7b. Idle speed is detected by the RPM sensor, and the speed is
adjusted to maintain a constant idle RPM. The computer receives digital
on/off status inputs from several power-consuming devices attached to the
engine, such as the air conditioner clutch switch, park–neutral switch, and
the battery charge indicator. These inputs indicate the load that is applied to
the engine during idle.

UNDERSTANDING AUTOMOTIVE ELECTRONICS 237