1、UNIT 7 THE IGNITION SYSTEMUNIT 7 THE IGNITION SYSTEMThere are many different types of ignition systems.Most of these systems can be placed into one of three distinct groups:the conventional breaker point type ignition systems(in use since the early 1,900s);the electronic ignition systems(popular sin
2、ce the mid 70s);and the distributorless ignition system(introduced in the mid 80s).UNIT 7 THE IGNITION SYSTEMUNIT 7 THE IGNITION SYSTEMThe automotive ignition system has two basic functions:it must control the spark and timing of the spark plug firing to match varying engine requirements,and it must
3、 increase battery voltage to a point where it will overcome the resistance offered by the spark plug gap and fire the plug.UNIT 7 THE IGNITION SYSTEMUNIT 7 THE IGNITION SYSTEMAn automotive ignition system is divided into two electrical circuitsthe primary and secondary circuits(Figure 7-1).The prima
4、ry circuit carries low voltage.This circuit operates only on battery current and is controlled by the breaker points and the ignition switch.1.Point-Type Ignition SystemUNIT 7 THE IGNITION SYSTEMUNIT 7 THE IGNITION SYSTEM.Figure 7-1 A schematic of a typical conventional breaker-point ignition system
5、UNIT 7 THE IGNITION SYSTEMUNIT 7 THE IGNITION SYSTEMl The distributor is the controlling element of the system.l The distributor is a stationary housing surrounding a rotating shaft.l The shaft is driven at one-half engine speed by the engines camshaft through the distributor drive gears.l The cam o
6、perates the contact points,which are mounted on a plate within the distributor housingUNIT 7 THE IGNITION SYSTEMUNIT 7 THE IGNITION SYSTEMThe coil(Figure 7-2)is the heart of the ignition system.Essentially,it is nothing more than a transformer which takes the relatively low voltage(12 volts)availabl
7、e from the battery and increases it to a point where it will fire the spark plug as much as 40,000 volts.It is connected to the high-tension connection on top of the coil(the tower into which the coil wire from the distributor is plugged).UNIT 7 THE IGNITION SYSTEMUNIT 7 THE IGNITION SYSTEMFigure 7-
8、2 Cutaway view of a conventional coilUNIT 7 THE IGNITION SYSTEMUNIT 7 THE IGNITION SYSTEMEnergizing the coil primary circuit with battery voltage produces current flow through the primary windings,which induces a very large,intense magnetic field.This magnetic field remains as long as current flows
9、and the points remain closed.As the distributor cam rotates,the points are pushed apart,breaking the primary circuit and stopping the flow of current.UNIT 7 THE IGNITION SYSTEMUNIT 7 THE IGNITION SYSTEMThe voltage from the coil secondary windings flows through the coil high-tension lead to the cente
10、r of the distributor cap,where it is distributed by the rotor to one of the outer terminals in the cap.UNIT 7 THE IGNITION SYSTEMUNIT 7 THE IGNITION SYSTEMThe need for higher mileage,reduced emissions and greater reliability has led to the development of the electronic ignition systems.These systems
11、 generate a much stronger spark,which is needed to ignite leaner fuel mixtures.Electronic Ignition systems are not as complicated as they may first appear.In fact,they differ only slightly from conventional point ignition systems.2.Electronic Ignition SystemsUNIT 7 THE IGNITION SYSTEMUNIT 7 THE IGNI
12、TION SYSTEMFigure 7-3 Typical electronic ignition systemLike conventional ignition systems,electronic systems have two circuits:a primary circuit and a secondary circuit(Figure 7-3).UNIT 7 THE IGNITION SYSTEMUNIT 7 THE IGNITION SYSTEMElectronic ignition systems differ from conventional ignition syst
13、ems in the distributor component area.Instead of a distributor cam,breaker plate,points,and condenser,an electronic ignition system has an armature(called by various names such as a trigger wheel,reluctor,etc.),a pickup coil(stator,sensor,etc.),and an electronic control module.UNIT 7 THE IGNITION SY
14、STEMUNIT 7 THE IGNITION SYSTEMThe third type of ignition system is the distributorless ignition system.l The spark plugs are fired directly from the coils.l The spark timing is controlled by an Ignition Control Unit(ICU)and the Engine Control Unit(ECU).l The distributorless ignition system may have
15、one coil per cylinder,or one coil for each pair of cylinders.3.Distributorless Ignition Systems(DIS)UNIT 7 THE IGNITION SYSTEMUNIT 7 THE IGNITION SYSTEMFigure 7-4 Typical distributorless ignition schematicsIn this system,each cylinder is paired with the cylinder opposite it in the firing order(usual
16、ly 1-4,2-3 on 4-cylinder engines or 1-4,2-5,3-6 on V6 engines,Figure 7-4).UNIT 7 THE IGNITION SYSTEMUNIT 7 THE IGNITION SYSTEMThese two plugs are on companion cylinders,cylinders that are at Top Dead Center (TDC)at the same time.But,they are paired opposites,because they are always at opposing ends
17、of the 4-stroke engine cycle.The distributorless ignition system uses either a magnetic crankshaft sensor,camshaft position sensor,or both,to determine crankshaft position and engine speed.UNIT 7 THE IGNITION SYSTEMUNIT 7 THE IGNITION SYSTEM.The advantages of no distributor,in theory,are:(a)No timin
18、g adjustments;(b)No distributor cap and rotor;(c)No moving parts to wear out;(d)No distributor to accumulate moisture and cause starting problems;(e)No distributor to drive thus providing less engine drag.UNIT 7 THE IGNITION SYSTEMUNIT 7 THE IGNITION SYSTEMThe major components of a distributorless ignition are:(a)ECU or Engine Control Unit;(b)ICU or Ignition Control Unit;(c)Magnetic Triggering Device such as the Crankshaft Position Sensor and the Camshaft;(d)Position Sensor;(e)Coil Pack.
