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Chapter 10 Engine Management Unit

Section 1 System Description

Engine Control System generally consists of sensor, controller and actuator.

Sensor: convert the physical parameter of the device into electric signal (digital or analog) to monitor the

operating condition of the device and send these signals to the controller.

Controller: receive and process information sent from the sensor, and analyze the information to learn the

conditions of the device: use predefined control strategy and procedure, determine how to control the device

under current conditions; convert the decision into one or more orders and send them to the actuator. The

controller contains a microprocessor and stores pre-developed the programs or control software in the memory.

The controller can be regarded as the brain of the control system.

Actuator: receive orders from the controller, covert the electric signal into the action of the actuating ele-

ments (either action of electric element or mechanical movement). The actions of these elements will change

the operating conditions of the device and determine the operation and output of the device.

Large amount of information flows through the entire control system. Engine Control Unit learns the opera-

tion of the device from the information sent from the sensor, determines the approach and orders of the

control with the input and information stored by itself and ends up in sending the information about the orders

to the actuator.

I. Engine Fuel System Electronic Control System

Engine fuel system electronic control focuses on the quantified electronic control of the fuel, and actually is

the electronic control over the excessive air coefficient  .

ECU controls the  by controlling the fuel injection. When the engine is running, ECU receives the informa-

tion about the air flow from the sensor and determines the fuel injection by calculations to make the excessive

air coefficient of the mixed air reach the predefined value. The control order output by ECU is only a control

signal to trigger the injector. ECU output the pulse width and timing in this signal.

The pulse width of injection signal determines the fuel amount of the injection. The pulse is the function of the

following parameter. The input and output are modified into closed loop control.

ECU calculate the target - air fuel ratio.

Air flow value.

Other operating conditions of the engine, such as throttle position sensor,  exhaust manifold oxygen sensor.

 low feature of the injector.

Engine ignition system electronic control

The control of Engine Control Unit over the ignition includes ignition advance angle control, knock control and

ignition power control (dewell angle control).

The control over the ignition advance angle is the basic function of the ignition control. Ignition advance angle

= ignition advance angle basic value + correction.

The control over the ignition advance angle also relates to the operating conditions of the engine. The factor

varying with the operating conditions can be reflected by the correction factor.

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The control over the ignition advance angle is generally open loop control. But this open loop control is

different from the general open loop control, ECU test the knock signal with the knock sensor while control-

ling and outputting ignition advance angle orders in way of open loop. Once the knock occurs, it is converted

into closed loop control, and its feedback signal is knock sensor signal. Therefore, basically, it is also a closed

loop control.

Section 2  System Component and Working Principle

ECU receives information from the sensors and sends control signals to actuators after processing and

analyzing these information. It plays a role of nerve center in the electronic control system.

ECU hardware consists of input level, micro-computer and output level (see figure 153).

Figure 153

Ignition advance angle control

 Starting condition ¡ª fixed ignition advance angle

Running condition

Special control

advance angle basic value - determined by MAP

advance angle correction

instantaneous correction

stable idle correction

air fuel ratio feedback correction

engine overheat correction

knock control

maximum advance/delayed angle control

other correction

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ECU microcomputer consists of microprocessor, memory, clock generator, timer, I/O interface and input

level A/D converter that are integrated in a large scale integrated circuit chip, that is SCM (Single Chip

Microcomputer).

1. Input level

The input signals from the sensor are pre-processed by the input level. The input signals are sent to protected

circuit first, sometimes through the signal converter and amplifier, and then sent to microcomputer.

The sensor signal is separated into switch type and analog type. Ignition on/off signal, camshaft position

signal, vehicle speed signal and A/C signal are switch type; Signals such as battery voltage, engine temperature,

intake temperature, airflow, intake manifold absolute pressure, throttle opening, excessive air coefficient,

knock, A/C refrigerant pressure are analog type. Analogy signal can only be processed by the digital micro-

computer after being converted into digital signal by A/D converter. Hence, data collection shall be made

first for analog signal, and the sampling shall be maintained also.

Speed and crankshaft position reference signal from the sensor is processed in a dedicated circuit to restrain

the interference pulse.

2. Microcomputer

The microcomputer of the engine electronic control unit is integrated in a single chip microcomputer, consist-

ing of the following:

(1) Microprocessor, microprocessor is also called as central processing unit (CPU), consisting of the following

three parts:

Arithmetic logic unit.

Register group.

Controller.

(2) System assembly, The data transferred among the internal sections of the single chip microcomputer

(SCM) is performed on the internal bus, while that between the SCM and other components is performed

on the external bus. The external bus is also called system bus. It is separated into data bus, address bus

and control bus.

(3) Memory, the memory is designed to store the binary data. The primary components of the memory are:

Memory.

Data register.

Address register, address decoder.

a. Memory controller

(4) A/D converter

(5) I/O interface.

(6) Clock generator.

(7) Timer

(8) CAPCOM unit.

(9) Watchdog timer.

(10) Interrupt system.

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3. Output Level

ECU output level is designed to send control signal to the actuators of the electronic control system based on

the orders of the microprocessor in the micro-controller. The microprocessor control the output level by

controlling the occurrence position of the pulse, its length and duty ratio.

(1) Injection output level

The microprocessor controls the pulse length of the injection output level by making sure whether to

trigger to circuit or not in the injection output level.

(2) Ignition input level

The ignition output level is designed to amplify the ignition signal from the microprocessor into the primary

current of the ignition coil.

(3) Electric fuel pump output level

ECU microcontroller control the pump relay through electric fuel pump output level, and then control the

on and off of the electric fuel pump based on the current operating conditions.

(4) Other output levels

Control the output levels of the idle control valve, carbon canister solenoid valve, malfunction indicator

lamp. MR479Q, MR479QA, MR481QA engine idle control are closed loop control system, their injection

and ignition control are also closed loop control system.