1、Lesson 8 Ignition System The purpose of the ignition system is to create a spark that will ignite the fuel-air mixture in the cylinder of an engine.It must do this at exactly the right instant and do it at the rate of up to several thousand times per minute for each cylinder in the engine.The igniti
2、on system sends an extremely high voltage to the spark plug in each cylinder when the piston is at the top of its compression stroke.The tip of each spark plug contains a gap that the voltage must jump across in order to reach ground.The voltage that is available to the spark plug is somewhere betwe
3、en 20,000 volts and 50,000 volts or better.The job of the ignition system is to produce that high voltage from a 12 volt source and get it to each cylinder in a specific order,at exactly the right time.The ignition system has two tasks to perform.First,it must create a voltage high enough(20,000+)to
4、 arc across the gap of a spark plug,thus creating a spark strong enough to ignite the air/fuel mixture for combustion.Second,it must control the timing of that the spark so it occurs at the exact right time and send it to the correct cylinder.The ignition system is divided into two sections,the prim
5、ary circuit and the secondary circuit.The low voltage primary circuit operates at battery voltage(12 to 14.5 volts)and is responsible for generating the signal to fire the spark plug at the exact right time and sending that signal to the ignition coil.The ignition coil is the component that converts
6、 the 12 volt signal into the high 20,000+volt charge.Once the voltage is stepped up,it goes to the secondary circuit which then directs the charge to the correct spark plug at the right time.Currently,there are three distinct types of ignition systems.The Mechanical Ignition System was used prior to
7、 1975.It was mechanical and electrical and used no electronics.By understanding these early systems,it will be easier to understand the new electronic and computer controlled ignition systems,so dont skip over it.The Electronic Ignition System started finding its way to production vehicles during th
8、e early 70s and became popular when better control and improved reliability became important with the advent of emission controls.Finally,the Distributorless Ignition System became available in the mid 80s.This system was always computer controlled and contained no moving parts,so reliability was gr
9、eatly improved.1 The Mechanical Ignition System The distributor is the nerve center of the mechanical ignition system and has two tasks to perform.First,it is responsible for triggering the ignition coil to generate a spark at the precise instant that it is required(which varies depending how fast t
10、he engine is turning and how much load it is under).Second,the distributor is responsible for directing that spark to the proper cylinder(which is why it is called a distributor).The circuit that powers the ignition system is simple and straight forward.When you insert the key in the ignition switch
11、 and turn the key to the Run position,you are sending current from the battery through a wire directly to the positive(+)side of the ignition coil.When current flows from the ignition switch,through the windings in the coil,then to ground,it builds a strong magnetic field inside the coil.The points
12、are made up of a fixed contact point that is fastened to a plate inside the distributor,and a movable contact point mounted on the end of a spring loaded arm.The movable point rides on a 4,6,or 8 lobe cam(depending on the number of cylinders in the engine)that is mounted on a rotating shaft inside t
13、he distributor.This distributor cam rotates in time with the engine,making one complete revolution for every two revolutions of the engine.As it rotates,the cam pushes the points open and closed.Every time the points open,the flow of current is interrupted through the coil,thereby collapsing the mag
14、netic field and releasing a high voltage surge through the secondary coil windings.This voltage surge goes out the top of the coil and through the high-tension coil wire.The coil wire goes from the coil directly to the center of the distributor cap.Under the cap is a rotor that is mounted on top of
15、the rotating shaft.The rotor has a metal strip on the top that is in constant contact with the center terminal of the distributor cap.It receives the high voltage surge from the coil wire and sends it to the other end of the rotor which rotates past each spark plug terminal inside the cap.As the rot
16、or turns on the shaft,it sends the voltage to the correct spark plug wire,which in turn sends it to the spark plug.The voltage enters the spark plug at the terminal at the top and travels down the core until it reaches the tip.It then jumps across the gap at the tip of the spark plug,creating a spar
17、k suitable to ignite the fuel-air mixture inside that cylinder.1.1 The Ignition Switch There are two separate circuits that go from the ignition switch to the coil.One circuit runs through a resistor in order to step down the voltage about 15%in order to protect the points from premature wear.The ot
18、her circuit sends full battery voltage to the coil.The only time this circuit is used is during cranking.1.2 The Distributor When you remove the distributor cap from the top of the distributor,you will see the points and condenser.The condenser is a simple capacitor that can store a small amount of
19、current.The points require periodic adjustments in order to keep the engine running at peek efficiency.1.3 Ignition Coil The ignition coil is nothing more that an electrical transformer.It contains both primary and secondary winding circuits.The coil primary winding contains 100 to 150 turns of heav
20、y copper wire.This wire must be insulated so that the voltage does not jump from loop to loop,shorting it out.The coil secondary winding circuit contains 15,000 to 30,000 turns of fine copper wire,which also must be insulated from each other.The secondary windings sit inside the loops of the primary
21、 windings.To further increase the coils magnetic field the windings are wrapped around a soft iron core.There are two mechanisms that allow the timing to change:Centrifugal Advance and Vacuum Advance.Centrifugal Advance changes the timing in relation to the speed(RPM)of the engine.It uses a pair of
22、weights that are connected to the spinning distributor shaft.These weights are hinged on one side to the lower part of the shaft and connected by a linkage to the upper shaft where the distributor cam is6.The weights are held close to the shaft be a pair of springs.Vacuum Advance works by changing t
23、he position of the points in relationship to the distributor body.An engine produces vacuum while it is running with the throttle closed.Vacuum advance uses a vacuum diaphragm connected to a link that can move the plate that the points are mounted on.Both Vacuum and Centrifugal advance systems worke
24、d together to extract the maximum efficiency from the engine.Once computer controls were able to directly control the engines timing,vacuum and centrifugal advance mechanisms were no longer necessary and were eliminated.1.4 Ignition Wires The job of the spark plug wires is to get that enormous power
25、 to the spark plug without leaking out.Spark plug wires are routed around the engine very carefully.For instance,a popular V8 engine firing order is 1,8,4,3,6,5,7,2.The cylinders are numbered from the front to the rear with cylinder#1 on the front-left of the engine.So the cylinders on the left side
26、 of the engine are numbered 1,3,5,7 while the right side are numbered 2,4,6,8.1.5 Spark Plugs The ignition systems sole reason for being is to service the spark plug.It must provide sufficient voltage to jump the gap at the tip of the spark plug and do it at the exact right time,reliably on the orde
27、r of thousands of times per minute for each spark plug in the engine.Most spark plugs contain a resistor to suppress radio interference.The gap on a spark plug is also important and must be set before the spark plug is installed in the engine.2 The Electronic Ignition System In the electronic igniti
28、on system,the points and condenser were replaced by electronics.On these systems,there were several methods used to replace the points and condenser in order to trigger the coil to fire.One method used a metal wheel with teeth,usually one for each cylinder.A magnetic pickup coil senses when a tooth
29、passes and sends a signal to the control module to fire the coil.Other systems used an electric eye with a shutter wheel to send a signal to the electronics that it was time to trigger the coil to fire.These systems still need to have the initial timing adjusted by rotating the distributor housing.Thanks for your presence!
