LV22 ignition systems (2)

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Student Workbook

LV22 Ignition Systems (2)

kap all phase 2 & 3 6/11/03 11:35 am Page 9

Student Workbook for Technical Certificates in

Light Vehicle Maintenance and Repair

MODULE LV22 IGNITION SYSTEMS (2)

Contents

Page Page

Introduction 3 Test Methods for Conventional

Vacuum and governor advancer tests 25

Ignition trigger signals – inductive 10

Exercise 2 10

Ignition trigger signals – Hall effect 12

Ignition triggers – optical 14

Dwell Period and Current Limiting 15

Electronic Ignition Systems 16

Computerised Ignition Systems 17

Introduction

Conventional ignition systems have now been replaced by electronic ignition systems The advantage that an electronic system has over a conventional system is that the mechanical operation of the contact breaker points, used to interrupt the primary circuit of the ignition coil, is now carried out by electronic components Transistors are used to interrupt the primary circuit on receiving

a trigger signal from the distributor

The signal generators are housed within the distributor and there are three types used at present; inductive (magnetic) trigger, Hall effect trigger and the optical trigger Each of these will be discussed during this phase of ignition systems As transistors are now used in place of contact breaker points, the metal-to-metal contact associated with the opening and closing of the point is eliminated This reduces the number of components that need to be checked during routine servicing and eliminates the secondary voltage drop associated

with a conventional contact breaker system

Secondary Voltage Requirements

When the ignition coil produces the secondary voltage, the voltage will travel

to earth via the path with the least resistance

In the secondary circuit, the earth path for the secondary voltage is via the ignition coil high-tension cord, (usually referred to as the “king lead”), through the distributor (via the rotor arm) to the spark plug and finally across the spark plug gap, creating the spark/flame to ignite the air/fuel mixture

If a path exists with less resistance than that of the ignition system

high-tension circuit, the secondary voltage will pass through it, bypassing the

spark, resulting in a misfire It is therefore important that no other path exists except that of the designated path to the spark plug

Additional resistances are normally fitted to high-tension cords, spark plugs etc to prevent electrical interference affecting electronic systems

Progress check 1

Answer the following questions:

1 What secondary voltage is necessary to overcome the rotor arm?

2 What secondary voltage is necessary to overcome the spark gap?

3 What will dictate the secondary voltage at the spark plug?

Cylinder Pressure and Mixture Strength

The lower the cylinder pressure, the lower the secondary voltage required to jump the spark plug gap

The higher the cylinder pressure, the higher the secondary voltage required to jump the spark plug gap

As with cylinder pressure, mixture strength also affects the strength of the secondary current needed to enable the air/fuel mixture to burn The richer the mixture the lower the secondary voltage needed to jump the spark gap The leaner the mixture the higher the secondary voltage required to jump the spark gap

Exercise 1

Ignition waveform

1 Identify the components of the secondary waveform and add a

description of each component:

2 Identify the components of the primary waveform and add a description

of each component:

The current limiting hump can be seen in the dwell section of the waveform The coil charge period is between the start of dwell and the current limiting hump

When the engine speed is low i.e engine at idle, the dwell angle is short

When the engine speed is increased, the start of the dwell period has moved

to the left, increasing the dwell angle with an increase in engine speed

The current limiting hump can be seen to move to the right as the engine speed is increased The ignition coil charge time becomes longer with an increase in engine speed

When the engine speed is increased further, the start of the dwell period moves further to the left

When the engine speed increases further, the current limiting hump can be seen to disappear, as the coil requires the full dwell period to charge fully The ignition timing does not alter with an increase in dwell as only the start of dwell changes With later engine management systems, the current limiting hump may not be seen in the waveform

Electronic Ignition System

Points – mechanical earth switch

Points – mechanical earth switch

Electronic earth switch

Electronic earth switch

Electronic earth switch

Trigger signal

Trigger signal

It has been seen that with a contact breaker ignition system, the ignition coil circuit is completed by a set of contact breaker points The contact breakers switch the primary circuit of the coil to earth The circuit can be referred to as

an earthed switched circuit

The contact breaker points are mechanically opened by a cam and closed by

a spring, which is rotated by the distributor shaft The rotation of the shaft/cam allows the points to open and close at the correct time in relation to the engine cylinder cycle

With an electronic ignition system, the ignition coil is switched to earth,

however an electronic switch replaces the mechanical switch The electronic switch is normally referred to as an ignition amplifier or in some applications

an igniter (Japanese manufacturers) The electronic ignition coil circuit can therefore still be referred to as an earth switched circuit The amplifier is in effect an ECU (Electronic Control Unit) Its sole purpose is to switch the coil

on and off at the correct time If it is to do this successfully, it needs to know where the pistons are in respect of the 4 stroke cycle at any given time

A sensor provides this information, and is generally referred to as the trigger mechanism The trigger is normally located in the distributor body, and the trigger signal passes directly to the ignition amplifier

There are typically three types of ignition trigger device that are used by

electronic ignition systems:

• inductive (magnetic) trigger (or sensor)

• Hall effect trigger (or sensor)

• optical trigger (or sensor)

On some earlier vehicles, the trigger was located next to the flywheel or crank pulley

Ignition trigger signals – inductive

AC output signal voltage

Pick-up coil Permanent magnet

Air gap

AC output signal voltage

Pick-up coil Permanent magnet

Air gap

The rotor (toothed wheel) is connected to the distributor shaft and therefore rotates as the engine rotates Fitted near to the rotor is an inductive trigger (sometimes referred to as a pickup) This is simply a coil of wire wrapped around a magnet When the rotor tip passes near to the coil, an AC current is produced which is sensed by the amplifier

Exercise 2

1 List two things that can affect the voltage produced by the trigger:

2 List three items of test equipment that can be used to check the signal

voltage from an inductive trigger:

When the rotor tip leaves the trigger, the tip reduces the strength of the

magnetic flux and produces a negative voltage from the trigger

The rotation of the rotor near to the trigger causes an AC current to be

produced

Ignition trigger signals – Hall effect

The Hall drum is connected to the distributor shaft When the distributor shaft rotates, the drum rotates The drum has cut-outs; the number of cut-outs correspond to the number of engine cylinders i.e four cut-outs are used on a

4 cylinder engine

+

O _

The Hall switch is an integrated circuit (IC), which is affected by magnetism The illustration shows an open/close switch which we will use to represent the Hall IC The Hall switch receives a power supply at the “+” terminal The “-” terminal is connected to earth (usually via the amplifier) The amplifier passes

a signal voltage to the “O” terminal (typically between 5-10 volts) The

amplifier uses any changes experienced in this voltage as a signal to switch the ignition coil on or off

High voltage

When the solid section of the drum passes between the magnet and the Hall switch, magnetism cannot influence the Hall switch If magnetism cannot influence the Hall switch, the Hall switch will be open When the switch is open, the signal current cannot flow to earth, therefore the signal voltage at Hall switch terminal “O” will be high (and also at the corresponding amplifier terminal)

Switch circuit voltage low – Earth circuit

Switch circuit voltage low – Earth circuit

When the cut-out section of the drum passes between the magnet and the Hall switch, magnetism can influence the Hall switch If magnetism influences the Hall switch, the Hall switch will close When the switch is closed, the signal current can now flow to earth The signal voltage at the Hall switch

Ignition triggers – optical

As with the previous two trigger systems, the optical trigger replaces the

conventional contact breaker point The basic system parts are the segmental trigger or chopper wheel, which is connected to the distributor cam, the infra-red light and the phototransistor The infra-red light, which is a gallium

arsenide cell, is kept at a constant intensity by a zener-diode which acts as a stabiliser Infra-red light is then collected by the phototransistor which is constructed of silicon and connected to a second transistor The two

transistors that are now connected together form a Darlington amplifier

Infra-red light is collected by the phototransistor causing it to be switched on, this signal is then sent to the ignition amplifier As the chopper wheel rotates,

it interrupts the flow of light to the phototransistor thus causing the

phototransistor to be switched on and off There are as many segments on the chopper wheel as there are cylinders and they are sized to give exactly 66% dwell

Dwell Period and Current Limiting

Ignition coil 0.5 ohms

Ignition coil 0.5 ohms

Current limiting is used with many electronic ignition systems Contact

breaker ignition systems fitted with a low resistance ignition coil (1.5 ohms) used a ballast resistor to limit the current in the ignition coil primary circuit

It does beg the question ‘Why not just use a high resistance coil?’

The use of a resistor enables us to bypass it during cranking to give a decent spark when the battery voltage is low (because we are cranking) and then reintroduce it during normal running to prevent coil damage when battery voltage is high (charging)

A low resistance coil has less windings on the primary side This has the added advantage of producing less counter electromotive force during coil charge/discharge resulting in a better spark A resistor has no windings and therefore produces no counter electromotive force

The current flow in an electronic ignition system can be limited to a

predetermined value by the ignition amplifier If a low resistance coil were to

be used (0.5 ohms), there could be 24 amps flowing in the ignition coil primary circuit This could damage the electrical components and wiring

Advantages of using current limiting ignition system:

• lower coil resistance (0.5 - 0.75 ohms)

• rapid charging of the ignition coil

• amplifier limits the current flow after predetermined value is reached

• no external ballast resistor is necessary

• variable dwell/ current limiting = “constant energy”

Electronic Ignition Systems

-Computerised Ignition Systems

Water temperature sensor

Water temperature sensor

Water temperature sensor

Manifold Absolute Pressure sensor

Manifold Absolute Pressure sensor

Manifold Absolute Pressure sensor

The next stage in the development of a modern car’s ignition system was to fully computerise the ignition system

The ignition amplifier has been incorporated with the Electronic Control Unit (ECU) The engine speed and engine position signal, usually provided by the distributor trigger, has typically been replaced by a crankshaft trigger/sensor The crankshaft sensor can provide an engine speed signal and the exact position of the crankshaft

We also supply the ECU with many other pieces of information such as

engine temperature and engine load The timing can be altered dependent on engine operating conditions, which can provide better performance and

improved fuel economy The timing can also be optimised for different fuel octane ratings

These computerised ignition systems are often linked to electronically

controlled fuel systems Two other versions of fully computerised ignition systems, are distributor-less or wasted spark ignition systems and direct

ignition systems

Distributor-less or wasted spark ignition systems use an individual coil for each of two cylinders, an ignition amplifier, (that in turn is controlled by the engine electronic control unit), controls the coil In some cases the coil is controlled directly by the engine’s electronic control unit The electronic

control unit sends a signal to the amplifier causing current to travel through the primary coil The length of time that the signal is sent is dependent on information received from the crankshaft and camshaft sensors These two sensors allow the electronic control unit to know the engine speed and piston position At the correct time the signal is switched causing secondary voltage

to be generated The secondary voltage/current is then sent to two plugs simultaneously, one being fired in the compression stroke and the other in the exhaust stroke This eliminates the need for a distributor

Direct ignition systems are now becoming commonplace with most

manufacturers The latest systems now place the ignition coils directly onto the spark plugs This eliminates not only the need for distributors but also high-tension cords By eliminating these two items the two main potential areas for current drop are removed, making the system more reliable This type of system also allows for greater control over when the spark is

delivered, as each spark plug is now operated individually

Testing Methods for Conventional Ignition System

Ignition coil tests

Ω

The primary coil can be checked for serviceability by the use of an ohmmeter The high-tension cord, together with any wires connected to the coil, must be removed The ohmmeter must then be connected between the positive and negative terminals of the coil The reading taken must be compared with the manufacturers’ specification

The final resistance check is carried out on the ballast resistor, if one is fitted

to the system The probes are this time connected across the ballast resistor The reading must be taken and compared with the manufacturers’

Voltage checks also need to be carried out as the coil will not operate if the voltage is to low If the voltage drops below approximately eight volts then insufficient current will travel through the primary coil causing secondary voltage to be low This will lead to engine misfire