Load Advance and Retard
Load Advance and Retard
When the diaphragm moves under engine vacuum and atmospheric pressure, the tie rod pulls, or pushes, the moving base plate so that it rotates on the fixed base plate. This moves the contact-breaker points in relation to the cam and alters the ignition-timing position. In a simple vacuum unit, the timing is advanced when the vacuum is greatest (lowest pressure about 500 mbar), for instance during idle, cruising and overrun. The advanced-ignition timing compensates for the slow ignition of the weaker mixture at these load conditions.
Partial Load Some vacuum- control units incorporate an advance and retard unit that gives improved exhaust emissions during changes in load conditions, for instance when moving from partial load
Dual-chamber vacuum unit
to full load during rapid acceleration.
Starting The circuit diagram for a basic contact-breaker ignition system is shown on this screen. This circuit requires a good battery voltage to produce a good-quality spark. During engine starting on a cold day, the available voltage to the ignition circuit drops by as much as 3 volts. This reduces the coil output at a time when the best-possible spark is required.
Ballast Resistor To overcome this problem, most distributor-ignition systems were fitted with a ballast resistor in the primary-circuit supply to the ignition coil. The ignition coil
was designed to operate on the
Ballast circuit
reduced voltage that the resistor produced, which was 6 to 9 volts for most systems. During starting, the supply to the coil bypasses the resistor and supplies the actual battery voltage, although this is reduced because of the large current draw to the starter motor.
Bypass Circuit The bypass supply originates from the starter-motor
solenoid. The actual voltage supplied to the coil is usually greater than the coil-rated voltage and the coil output is therefore boosted temporarily during engine starting. Coils for use Bypass operation with ballast resistors are not interchangeable with coils for battery-voltage systems.
Summary The purpose of an ignition system is to produce a spark in the correct cylinder at the right time. All ignition systems achieve the high voltage required by switching an ignition coil on and off.
State the purpose of a ballast resistor in an ignition system.
Look back over the previous section and write out a list of the key bullet points here:
SPARK PLUGS AND SECONDARY CIRCUIT
Spark Plugs The spark plugs are fitted into the combustion chambers at the top of each
Plugs in
cylinder. The electric current in cylinder head the secondary circuit is HT waveform sufficient to make the air gap
between the electrodes conductive, and a spark occurs.
A higher voltage is needed to initiate the spark than is needed to maintain it. The spark has sufficient heat and energy to ignite the fuel in the combustion chamber.
Combustion The conditions in the combustion chamber make the production of a spark difficult. There is a high pressure in the air charge following Damp-
compression and the fuel particles make that air proofing charge damp. Insulation of the secondary-circuit components must be able to prevent any premature
grounding of the high voltage. Damp-proofing is also required to prevent an alternative conductive pathway being formed.
Electrodes Spark plugs consist of a centre electrode in a ceramic insulator fitted inside a steel body, and an earth electrode, or electrodes, welded to the body of the plug. The spark-plug body is Cross-section of a spark threaded to fit the cylinder head where plug it protrudes into the combustion chamber. A gas-tight seal is made between the insulator and body of the plug, with sealing rings and crimping of the body to the insulator. Gas sealing at the cylinder head is made with a compressible gasket, or taper seat.
Matching Spark Plugs Spark plugs are matched to the engine to suit the cylinder-head design
Plug features
details and for normal driving
Spark-plug gap
conditions. They are selected from a range of varying types. The features of spark plugs are reach and thread diameter, gas- sealing method, heat range, and other design features. The gap between the electrodes is specified by the plug and engine manufacturers to ensure that the engine gives the best possible
performance.
Plug Reach and Diameter The reach, or length, of thread, and the thread diameter, refer to the threaded section of the plug and are used to match the
Plug reach and diameter
plug to the thread dimensions for screwing into the cylinder head.
Sealing Spark Plugs Two common types of gas sealing are used: A compressible copper gasket and a taper seat. These Washer
Taper-seat
are not interchangeable. The tightening sealing torque of the spark plug into the
sealing
cylinder head is different for each type of plug. Always refer to torque data and tighten appropriately because correct fitting affects the heat dissipation and, therefore, the performance and life of the plug.
Heat Range The heat range of spark plugs is determined by the ability of the plug to maintain a constant temperature at the plug tip in the combustion chamber. The normal temperature range at the plug Heat-loss paths tip should be at least 500°C, and at about 800°C for optimum performance. The lower temperature is sufficient to give a self-cleaning action at the electrodes and an extended life.
Self-Cleaning The self-cleaning action prevents the build-up of soot, oil and carbon deposits, which are burnt off above 500°C. Carbon deposits on the plug tip and insulator provide grounding paths Plug temperature for the secondary-circuit current and cause misfiring of the engine.
Pre-Ignition Exceeding the upper limit would make the spark tip glow and ignite the mixture before a spark is initiated. The result of any pre-ignition is
engine knocking heard as a ‘pinking’ sound, poor Overheated
plug!
engine performance and fuel economy, and the probability of severe engine damage.
Heat Paths The heat range of the plug is governed by the length of the heat-dissipation path of the central insulator. A
hot-type plug has a low thermal value and has a limited heat- transfer path. A cold plug has a high thermal value and transfers heat away more easily. Only in exceptional circumstances is it necessary to use plugs of a different heat range than the ones specified for the vehicle.
Plug Variations The other design features of plugs are the references to changes in the basic plug type. One development that has since become the norm is the extended tip, now being considered as the standard electrode position. The old design does not have the centre and earth electrodes extended from the plug tip.
Plug-Tip Designs There is a wide range of plug-tip designs for the wide range of engine applications. Many of these
Old tip design Two
are for non-vehicle uses such as lawn electrodes mowers, chain saws and marine engines.
A special range of plugs has more than New tip the one earth electrode used on
design
Four
standard plugs. These are referred to electrodes
by product name, or as ring-electrode types. The pressures and forces at the plug tip following ignition can lead to distortion of the earth electrode. To overcome this problem, reinforced earth electrodes can be specified.
Spark-Plug Cores The connection, from the plug terminal to the centre electrode tip, has been developed in a number of ways. An electrical- resistive core is used to provide radio
suppression. The electrodes of a Plug core standard plug are made from a nickel
alloy. Special-purpose and long-life plugs use other alloyed elements such as chrome, copper, silver and platinum. These materials may also be used to coat the tip or, in the case of copper,
be used as a core for the centre electrode.
Selection of the Correct Plug The range of design details that are included in spark-plug classification and identification are code letters,
Spark-plug
or numbers, for thread diameter, length of thread part numbers (reach), design features such as radio suppression,
taper seat and electrode type, the heat range, and the electrode gap. Special materials used in the construction of the plug are included in the manufacturer’s name for the range of plugs, or by a code letter in the identification classification.
Secondary Circuit The secondary circuit wires, or HT leads, are thickly insulated to ensure that
High-tension
the electric current reaches the leads
Lead details
spark plugs. Some plug wires have radio-suppression insulation to limit electromagnetic-wave
HT
transmissions. The wires, with a connections resistive core inside a thick outer insulation, limit and smooth the secondary current.
The connectors and terminals are also designed to provide radio suppression in some cases.
Cap and Rotor The distributor cap and rotor are made from a hard plastic material and have electrical conductors Rotor in place Interior of
for distribution of the high voltages of cap the secondary circuit from the ignition
coil to the spark plugs. The distributor cap is secured to the distributor body with spring clips, or screws, and located with a lug and slot to ensure correct positioning for number 1 cylinder and the engine-firing order. The rotor sits on top of the distributor spindle and is also located with a lug and slot.
Capacitor-Discharge Ignition One other type of ignition system that is used on some high-performance vehicles is a capacitor-discharge (CD) system. This
Capacitor discharge block
system produces a very high voltage in the
diagram
secondary circuit, but is of short duration.
A special coil is used of a tougher construction than a conventional coil. It is called a pulse transformer, although it is internally wound with primary and secondary windings like a conventional coil. It receives pulses of primary-circuit voltage at about 400 volts, and gives secondary outputs at about 40,000 volts.
Energy Charge The energy charge is very rapid and the primary- and secondary-electric pulses are so close together that no dwell control is needed. The secondary voltage is constant over a wide speed range. The system uses the charging of a capacitor to produce a high primary voltage. The charging is provided by an inverter, which changes battery voltage to about 400 volts.
Explain why a hot plug is fitted to a cold engine and a cold plug is fitted to a hot engine.
Look back over the previous section and write out a list of the key bullet points here:
ELECTRONICS AND SYSTEM OPERATION
Microelectronics The developments in microelectronics and solid-state
circuitry produced suitable
Amplifier Pulse
replacements for some of the ignition
generator
primary-circuit mechanical-switching components. The use of these reduced some of the wear problems.
Transistor Switching Transistor switching permits very low control currents and new types of sensors that send signal currents to the transistors. These have no mechanical contact and are wear-free. These developments have improved ignition-timing reliability, and further changes have improved the primary-circuit performance resulting in better spark generation.
Open Loop Microprocessor- ignition systems were originally open-loop systems using engine position, speed and load sensors to provide data. The
data were compared in the
Digital system
engine-control module to a pre- programmed map of ignition- timing positions for all conditions.
Engine-Control Module An engine-control module carries out all the functions of the primary circuit, including spark-
generation control, and the advance and
Digital system
retard of the ignition timing for speed and load conditions. Because there are no moving parts, the system is wear-free and has high levels of accuracy and reliability.
Electronic Control Unit Many modern ignition-control circuits
in the electronic control unit (ECU) are closed-loop systems using a knock sensor to provide feedback to the ECU, which adjusts the timing to a point
just a few degrees after the position where the engine
knocks. This provides the most effective ignition-timing position.
Integrated Systems The integration of fuel and ignition electronic-control microprocessors into one engine-management system is now common. The latest
developments have seen the connection of all vehicle electronic-control systems to form a power-train control module (PCM), or a vehicle control module (VCM).
Secondary Circuit The ignition secondary-circuit components
remained similar during the
Distributor cap
HT Leads
early developments of electronic primary-circuit controls. The traditional distributor was fitted with an electrically insulated cap, with turrets for each cylinder and the input from the ignition coil.
Rotor and Cap A rotor, fitted to the distributor spindle, ran
in contact with the centre input
Rotor in cap
contact in the cap, and aligned with the segment conductors for each cylinder in turn as the rotor rotated. The engine
firing order matches the
Secondary
rotation of the rotor and the
distributor
positions of the turrets on the distributor cap.
Secondary Distributor The secondary circuit-switching distributor retains the rotor and cap components but the primary-circuit control is fully electronic.
Distributorless Ignition
Distributorless ignition systems
(DIS) have a coil, or coils, that are
DIS coil and
System requiring
directly connected to the spark
leads
no distributor
plugs.
Spark Plugs Spark-plug design has developed with many changes in
the materials used in the