It is A system for igniting air-fuel mixture.
High Voltage Is Necessary.
Jumping a spark plug gap requires thousands of volts. However, the vehicle`s battery can produce only 12 Volts. Since this is not enough voltage to jump across the electrodes of a spark plug in the combustion
chamber, away to raise the voltage must be found. In modern vehicles, the voltage needed at the spark plugs can exceed 60,000 volts. This means that the battery`s 12 volts is increased many times. However, if you touch a plug and are shocked, there is little real danger involved. Current, which is the part of electricity that does the actual damage, is very low in an automotive ignition system.
The ignition system is divided into two separate circuits, which are called the primary and secondary circuits. The next figure will show two types of ignition system the first one is conventional contact or breaker point type, and the other figure shows an electronic ignition system which incorporates a breaker less style distributor .
OrinPrimary Circuit.
The primary circuit consists of the battery, ignition switch, resistor (where used), ignition module or contact points, and coil primary wiring. they are covered in the order that electricity flows through them. The primary circuit voltage is low, operating on the battery`s 12 volts. The wiring in this circuit is covered with a thin layer of insulation to prevent short circuits.
Battery.
To better understand the operation of the ignition system`s primary circuits, we will start at the battery and trace the flow of electricity through the system. The battery is the source of electrical energy needed to operate the ignition system. The battery stores and produces electricity through chemical action. When it is being charged, it converts electricity into chemical energy. When it is discharging (producing current), the battery converts chemical energy into electricity.
Ignition Switch.
The primary circuit starts at the battery and flows to the ignition switch. The ignition switch is operated by the ignition key. It controls the flow of electricity across the terminals. The ignition switch may have additional terminals that supple electricity to other vehicle system when the key is turned on. Most ignition switches are installed on the steering column.
Resistors.
Some ignition system include a resistor in their primary circuits. Electricity flows from the ignition switch to the resistor. The resistor controls the amount of current reaching the coil. The resistor may be either the calibrated resistance wire or ballast type.
Most resistor simple consist of a calibrated resistance wire built into the wiring harness between the ignition switch and coil. The resistance wire lowers battery voltage to around 9.5 volts during normal engine operation. However, when the engine is started, the coil receives full battery voltage from a bypass wire. The bypass wire supplies the coil with full battery voltage from the ignition switch or starter solenoid while the engine is cranking. When the key is released, the circuit receives its power through the resistance wire.
The ballast resistor, which is used in some vehicles, is a temperature sensitive, variable resistance unit. A ballast resistor is designed to heat up at low engine speed as more current attempts to flow through the coil. As it heats up, its resistance value increases, causing lower voltage to pass into the coil, As engine speed increases, the duration of current flow lessens. This causes a lowering of temperature. As the temperature drops, the resistor allows the voltage to the coil to increase. At high speeds, when a hotter spark is needed, the coil receives full battery voltage. The ballast resistor is a coil of nickel-chrome or nichrome wire. The nichrome wire`s properties tend to increase or decrease the voltage in direct proportion to the heat of the wire, Some early transistor ignition systems use two ballast resistors to control coil voltage. From the resistor, the current travels to the coil. Most modern vehicles with electronic ignition don`t use a resistor in the ignition circuit. The majority of modern electronic ignition systems use full battery voltage at all time.
Ignition Coil.
The primary circuit leads from the ignition switch or resistor to the ignition coil. An ignition coil is actually a transformer that is capable of increasing battery voltage to as much as 100,00 volts, although most coils produce about 50,000-60,000 volts. Coils vary size and shape to meet the demands of different vehicles.
Coil Construction
The coil is constructed with a special laminated iron core. Around this central core, many thousands of turns of very line copper wire are wound. this fine wire is covered by a thin coating of high temperature insulating varnish. One end of the fine wire is connected to the primary circuit wire within the coil. All these turns of fine wire from what is called the secondary winding.
Several hundred turns of heavier copper wire are wrapped around the secondary coil windings. Each end is connected to a primary circuit terminal on the coil. These windings are also insulated. The turns of heavier wire from the primary winding. study the winding connections in the cutaway.
The core, with both secondary and primary windings attached, is placed inside a laminated iron shell, the job of the shell is to help concentrate the magnetic lines of force that will be developed by the windings, This entire unit is then placed inside a steel, aluminum, or bakelite case. In some coil designs, the coil windings are encased in heavy plastic.
The coil is sealed to prevent the entrance of dirt or moisture. The primary and secondary coil terminals are carefully sealed to withstand vibration, heat, moisture, and the stresses of high induce voltages. the next figure shows some types of coils found on modern vehicles. Although their shapes are different , they all work in the same way.
Coil Operation
When the ignition switch is turned on, the current flows through the primary windings of the coil to ground. See figure. When a current flows through a wire, a magnetic field is built up around the conductor. Since there are several hundred turns of wire in the primary windings, a strong magnetic field is produced. This magnetic field surrounds the secondary as well as the primary windings. If there is a quick and clean interruption of current flow on its way to ground after passing through the coil, the magnetic field will collapase into laminated iron core.
As the field collapses through the primary winding, the voltage in the primary windings will be increase. This is called self-induction, since the primary windings produce its own voltage increase. The voltage induced in the primary windings is about 200 volts, since it consists of only several hundred turns of wire. Self-induction does not affect secondary winding operation, but can cause point arcing on contact point systems.
As the magnetic field collapses, it passes through the secondary winding, producing atiny current in each turn, The secondary windings prossess thousands of turns of wire. Since they are in series, the voltage of each turn of wire is multiplied by the number of turns. This can produce a voltage exceeding 100,000 volts. This is known as induction. the high voltage produced by the secondary windings exits the high tension coil terminal and is directed to the spark plugs.
Most coils have primary terminals marked with a (+) and (-), The plus sign indicates positive and the minus indicates negative. The coil must be installed in the primary circuit according to the way the battery is grounded. This alignment of the positive and negative terminals is called polarity. If the battery`s negative terminal is grounded, the negative terminal of the coil must be connected through the ignition module or distributor to ground as applicable. This is done to ensure the correct polarity at the spark plug.
Actual Coil output
Even though the voltage output of some coils can exceed 100,000 volts, the coil will only build up enough voltage to produce a spark. This may be as low as 2000 volts at idle on an older vehicle without emission control, or as high as 60,000 volts on a new vehicle with the leanest possible mixture and under a load .
To control the coil`s output, most engines have a distributor. The job of the distributor it to trigger the coil and to distribute the high voltage current to the right spark plug at the right time.
Methods Of Current Interruption.
To cause the coil`s magnetic field to collapse, the current flow through the primary windings must be interrupted instantly and cleanly with no flashover (Current jumps or arcs across space) at the point of disconnect. For about 75 years, the primary current flow was controlled by using a set of contact points to break current flow and collapse the coil primary field. Over the last 20 years, contact point systems have been replaced by electronic ignition systems, which uses transistor to operate the primary circuit.
Electronic ignition can produce the high voltage spark needed to fire the leaner mixtures used on modern vehicles. While the old contact point system could produce no more than 20,000 or 30,0000 volts, electronic ignition systems allow as much as 100,000 volts to be used. All modern vehicles use ignition systems with electronic primary circuit control. The basic difference between contact point ignition systems and electronic ignition systems is the method employed to interrupt the coil primary circuit.
Contact Points
The contact points used on older vehicles were a simple mechanical way of making and breaking the coil primary circuit. Note the contact point in the next figure. The stationary piece is grounded through the distributor contact point mounting plate. This section does not move other than for an initial point adjustment.
The second piece is the moveable contact point. It is pivoted on a steel post. A fiber bushing is used as a bearing on the pivot post. A thin steel spring presses the moveable contact arm against the stationary unit, causing the two contact points to touch each other. The movable arm is pushed outward by the distributer cam lobes, which are turned by the distributor shaft. The cam lobe on the distributor shaft opens and closes the points as it revolves. The number of lobes corresponds to the number of cylinders in the engine.
the cam moves the contact arm through a fiber rubbing block. The block is fastened to the contact arm and rubs against the cam. High temperature lubricant is used on the block to reduce wear. The moveable contact arm is insulated so that when the primary lead from the coil is attached to it, the primary circuit will not be grounded unless the contact points are touching.
Contact Point Dwell.
Contact Point Dwell.
The number of degrees the distributor cam rotates from the time the points close until they open against is called dwell, and is sometimes referred to as cam angle. Dwell is important as it affects the magnetic buildup of the primary windings. The longer the points are closed, the greater the magnetic buildup, However, too much dwell can result in point arcing and burning. If the dwell is too small, the points will open and collapse the field before it has built up enough voltage to produce a satisfactory spark.
When setting contact point gaps, as the gap is reduced, dwell is increased, When the gap is enlarged, dwell is decreased. The dwell cannot be adjusted on electronic ignition systems, but can be measured as an aid to diagnosis. Always check the manufacturer`s specification for dwell when setting points.
The next figure shows that the ignition points close at 1 and remain closed as cam rotates to 2. The number of degrees formed by this angle determines dwell.
Condenser
The condenser, sometimes called a capacitor, absorbs excess primary current when the contact points are opened, The condenser prevents point arcing and resulting overheating, pitting, and excessive wear. In addition to increasing contact point service life, the condenser allows the coil`s magnetic field to collapse quickly, producing a strong, instant spark.
Most condensers are constructed of two sheets of very thin foil separated by two or three layers of insulation. The foil and insulation are wound together into a cylindrical shape.
The cylinder is then placed in a small metal case and sealed to prevent the enterance of moisture. The next figure illustrates the construction of the typical condenser. The close placement of the foil strips creates capacitance, or the ability to attract electrons.
When the points are closed, the condenser is inactive as the coil`s magnitic field begains to collapse and voltage in the primary windings increase due to self induction. If a condenser was not used, the voltage in the primary circuit would arc across the points, consuming the coil`s energy before the magnetic field passes through the secondary windings.
However, the condenser attracts the excess primary voltage, prevent an arc across the points. By the time the condenser has become fully charged, the points have opened too far for current to arc. The magnetic field collapses through the secondary windings, producing a quick, strong spark.
Electronic Ignition.
Tje schematic in figure is a simple electronic ignition circuit. Note that there is no mechanical devices to make and break the circuit. The entire process is done electronically. Current flows from the ignition Switch, through the ignition module, to the coil. The ignition module contains the electronic components which cause the coil to produce a high voltage spark. Ignition modules process the inputs from other ignition components.
Ignition modules are sometimes installed on the engine firewall or inner fender to protect them from excessive engine heat. Other modules are located in the distributor, installed outside on the distributor body, or as part of the coil assembly.
Some typical electronic ignition modules are shown in next figure. Current from the ignition enters the module and passes through a power transistor before reaching the coil, as in figure C. The power transistor acts like a conductor, allowing full current to flow in the circuit. This begins the build up of the magnetic field in the coil.
When the power transistor is signaled by the triggering device (explained below) and other module circuitry, it becomes an insulator. Since current cannot flow through an insulator, this stops current flow through the coil primary circuit. When current flow stops, the magnetic field collapses, creating the high voltage current is complete, the process is repeated as current flow through the power transistor begins again.
