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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.