ENGINE MANAGEMENT ELECTRONICS

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ENGINE MANAGEMENTENGINE MANAGEMENT

ELECTRONICSELECTRONICSAlmost every diesel engine today is controlled by computer. Anyengine certified for on-highway use since 1998 has been computercontrolled. Onboard vehicle computers are referred to asengine/electronic control modules (ECM) or engine/electroniccontrol units (ECU). We are going to use the acronym ECM in thisbook; however, be aware that some manufacturers use ECU andother terms.


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Engine Controller

The ECM is the engine controller.The ECM contains a microprocessor, computer memory, and usually

an output or switching apparatus.

Most chassis systems are computer controlled. They are also

networked to each other by means of a data bus.

Every controller with an address on the data bus can communicate

with other modules on the bus.

Data Processing

A vehicle computer may be very simple such as that required to

manage a pulse wiper circuit.

On the other hand, it may be more complicated if it manages the manysystems required to run a diesel engine.

In a typical computer, information is managed in three distinct stages:

1. Data input

2. Data processing

3. Outputs


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INPUT CIRCUIT

ECM inputs can be divided into sensor inputs and switched inputs.

Inputs to the ECM can be divided into:

monitoring

command

Monitoring sensors are responsible for watching over circuit

conditions such as: temperature

pressure

speed

Command sensors and switches are used by the operator to tell

the management electronics what to do.

The oil pressure signal sent to the ECM is an example of amonitoring input. The throttle position sensor is an example of a

command input.


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Sensors

Anything that signals input data to a computer system can be

described as a sensor.

Sensors may be simple switches that an operator toggles open orclosed. Or they may be devices that are supplied with a reference

voltage, and then send a portion of that back to the ECM.

ReferenceVoltage

The term reference voltage is almost always referred to as V-Ref.

Actually V-Ref is an ECM output Regardless of manufacturer, V-Refis specified as 5VDC.

More correctly, it is a voltage as close to 5 V-DC as the ECM canmaintain through conditions that vary, such as temperature and

battery voltage (V-Bat).Because the ECM knows exactly what the V-Ref is at any given

moment of operation, it knows how to interpret the signals thatare returned to it.


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Sensors UsingV-Ref

There are four types: (1) thermistors, (2) pressure sensors, (3) positionsensors, and (4) induction pulse generators.

Thermistors.

Thermistors precisely measure temperature. To do this they use a

variable resistor.The resistance varies according to temperature.The voltage supplied to the thermistor is V-Ref. Thermistors are two-

wire devices:1.V-Ref

2. signal

The two types of thermistors are classified by whether resistance

increases or decreases when a temperature rise occurs:1. NTC (negative temperature coefficient): temperature goes up,

resistance goes down2. PTC (positive temperature coefficient): temperature goes up,

resistance goes up


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Some examples of thermistors used on diesel engines are: coolant temperature sensor

ambient temperature sensor

oil temperature sensor boost air temperature sensorAlmost without exception, NTC-type temperature sensors are

used on diesel engine systems. This means that as temperatureincreases, resistance decreases, and therefore signal voltage (to the

ECM) increases. In summary, we can say that in an NTC

thermistor:

As temperature rises, the output voltage signal rises

proportionally.


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Pressure Sensors.

Most pressure sensors used in engine management electronics operate on avariable capacitance principle.

These are three-wire sensors that are supplied with V-Ref. In addition to V-Ref,they also have ground and signal terminals.

The device consists of a ceramic disc behind which is a flat conductive spring

and steel disc. Pressure acts on the ceramic disc and move sit either closer or

farther away from the steel disc.This varies the capacitance of the device and therefore that portion of V-Ref that

is returned as a signal to the ECM.Variable capacitance pressure sensors are used as:

oil pressure sensors

turbo-boost pressure sensors

barometric pressure sensors

fuel pressure sensors

mass airflow sensors (in pairs)Piezo-resistive sensors may be used to signal pressure values. This type of

pressure sensor is often used to measure and signal turbo-boost pressure. They

provide greater accuracy than an equivalent variable capacitance sensor.

Piezo-resistive pressure sensors are sometimes referred to as Wheatstone

bridge sensors,a reference to the digital chip that outputs the signal.


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Operation of a variable capacitance pressure sensor.


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Position Sensors.

A potentiometer is commonly used to signal position to the ECM. They

are three-wire devices:1.V-Ref

2. ground

3. signal

The output signal portion of the potentiometer is proportional to the

movement of a mechanical device.

Potentiometers are voltage dividers. The moving mechanical device movesa contact wiper over a variable resistor. As the wiper is moved over the

variable resistor, the resistance path changes.

This means that V-Ref is divided between the signal (sent to the ECM) and

ground.

A disadvantage of a potentiometer-type TPS is the mechanical contactbetween the wiper (moved with accelerator pedal) and the variable

resistor.


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Induction Pulse Generator.

An induction pulse generator does not require a V-Ref input to function.

Instead it generates its own signal using the same principles as those ofan electric motor. Induction pulse generators are used to signal the speed and sometimes

position of anything that rotates.These types of sensors are used in many no engine systems for

functions such as wheel speed signaling.

A toothed disc known as a pulse wheel, tone wheel, or chopper wheel is

connected to the rotating component. The pulse wheel is manufactured

with evenly spaced teeth. As the wheel rotates, it is driven through a

stationary magnetic field.This induces an alternating current (AC) voltage in the signal circuit as

the magnetic field builds and collapses.The analog AC voltage pulses thatare generated are signaled to the ECM.

The ECM is not so much interested in the actual voltage value as the

frequency. The faster the pulse wheel rotates, the higher the frequency

of the wave signal produced. The frequency is used to identify a specificrotational speed.


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An accelerator position sensor of the potentiometer-type shown at

zero, mid, and full travel positions.


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SIGNALING POSITION

Inductive pulse generators can also be used to signal position. Simply byadding a tooth in one position on the pulse wheel, a specific position

can be signaled by the change in frequency. An example would be on acamshaft position sensor. By adding a tooth to the pulse at the #1 TDC

position, the ECM is able to reference this for fuel injection timing

purposes.

Hall-Effect Sensors.

Hall-effect sensors generate a digital signal and may used to signal both

speed and position. They may be used to accurately signal both linear

and rotational position. A rotary Hall-effect sensor has a rotating discmachined with timing windows or vanes. The rotating disc is positioned

to pass through a magnetic field by alternately blocking and opening amagnetic field. This produces an on/off effect. In a rotary Hall-effect

sensor, the rotating disc is known as a pulse wheel or tone wheel.

Because these terms are also used to describe the rotating disc of an

induction pulse generator, care should be taken to avoid confusion. We

will use the term tone wheel here. The frequency and width of the

signal provides the ECM with speed and position data. To signal positiondata, the tone wheel uses a single narrow window or vane at one

specific position.


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POWER-UP

A Hall-effect circuit has to be powered-up.

Some OEMs power-up a Hall-effect circuit usingV-Ref.Others power-up a Hall-effect circuit using either V-Bat or an ECMmodulated voltage such as 8 V-DC.

A Hall-effect sensor outputs a digital square wave signal as opposed tothe analog signal of the inductive pulse (AC) generator.

NoncontactTPS.

Hall-effect sensors can also be used as throttle position sensors.

Their advantage over a potentiometer TPS is that there is no mechanical

contact between the moving components.The result is that they tend tobe more reliable.

In a Hall-effect TPS, a noncontact sliding shutter alternately blocks andexposes the Hall-effect magnetic field to the signaling semiconductor

sensor.


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MAF Sensors.

Mass airflow (MAF) sensors are required on all current diesel engines. They have been used for decades in automobile applications.

Their objective is to provide the ECM with accurate measurement ofthe weight of air delivered to the engine cylinders.

MAF sensors may be classified as (a) hot wire, (b) vortex flow and

(c) critical flow venturi (CFV)

In current diesel engines, the most common type of MAF sensor isthe CFV.

Critical FlowVenturi.

A critical flow venturi (CFV) MAF sensor is also known as apressure differential flow sensor. It works based on the relationship

between inlet pressure and flow rate through a venturi.

A venturi is a throat-like device located in a flow passage that isdesigned to accelerate gas flow and decrease pressure.

It uses the same flow principle that made a carburetor operate.

When the pressure is known at the entry point to the venturi andat its exit point, the differential pressures can be used by the ECM

to calculate the weight of the airflow. Some manufacturers refer to

these as delta pressure sensors.


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Criticalflow venturi, pressuredifferential MAFsensor.


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Grounding-typeswitch operation.(Courtesy of Navistar)


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Manual Electromechanical Switches.

Manual electromechanical switches control electrical circuit activity by

opening and closing circuits.There are many examples of electromechanical switches on the dash ofa vehicle.

They are used by the operator to control vehicle functions. Someexamples would be the ignition key, engine retarder mode switches, and

the cruise control switches. Switches that are controlled by the driver

are sometimes called command switches.

Smart Switches.

Smart switches use digital signals to indicate a change in status.The signal produced by a smart switch may be automatically generated

by a change in status condition or be generated by a mechanical action

such as an operator toggling a switch.The advantage of smart switches is that they are responsible for sending

a message rather than changing electrical status.

The messages are sent to the ECM, which then computes how thosemessages will be used.

Smart switches can significantly reduce the number of wires required by

an electronic management system.


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THE ECM

The electronic/engine control module (ECM) has the following functions:1.filter and prepare input signals for processing

2.house the data memory required by the system3.house the processing hardware

4.convert the results of processing into action using drivers

In most cases, the ECM is located on the engine.

This is not always the case but it does make sense to keep it close tothe systems it is monitoring and controlling.

Central Processing Unit

The "brain" of the processing cycle is the central processing unit (CPU).

It manages the processing cycle and performs all the program

instructions and high-speed calculations required by the system.

The instructions the CPU relies on are retained in memory banks in thesame way that computer retains memory on chips and a hard drive.Memory in an ECM can be classified as (a) nonvolatile: permanent and

semi-permanent memory and (b) volatile: electronic memory thatrequires an electrically active circuit only functional in real time.

When an ECM is booted by a wake-up signal or ignition key, the CPU has

to transfer data from nonvolatile to electronic memory.


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The primary functions of the CPU are:

1.manage the processing cycle

2.organize incoming input data in main memory3.fetch and carry data from memory

4.process data to produce outcomes5.generate switching commands to output drivers

A schematic showing some of the functions of a typical diesel engine ECM.Theoutput drivers are shown as transistors: these enable a very small signal

current from the processor to control the high-current circuits required to

manage the ECM output circuit.

ENGINE MANAGEMENT ELECTRONICS

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