Chapter 4 The Contact-Point(or Conventional) Ignition System1. Purpose of ignition systemThe ignition system supplies high-voltage surges as high as 15.000 volts (47.000 volts in some electronic system)-to spark plugs in the engine cylinders. These surges produce electric sparks across the spark plug gaps. The heat from the sparks ignites, or sets fire to the compressed air-fuel mixture in the combustion chambers. These sparks occur at exactly the right time according the firing order of cylinders.2. Classification of ignition systemTwo kinds of ignition systems are used on vehicles today.Contact-point ignition systemElectronic ignition systemIn the older system, movable contact points are used to make and break an electric circuit. This system is still used in some cars today. But most cars now have electronic ignition. In this system, the make-and-break of the electric circuit is done by an electronic device. The electronic system is much faster in action, and more accurate.3. Requirements to ignition systemThere are three requirements to ignition system(1)High breakdown voltage that can puncture the spark plug gaps.(2)Sufficient energy of sparks that can ignite the mixture of air and fuel.(3)Correct ignition time that can adapt to the changes of engine operation.3.1 Breakdown voltageInfluencing factors upon breakdown voltage:(1)Spark plug gapsGreater the gaps, higher the breakdown voltage.(2)Pressure and temperature of the mixture in cylindersHigher the pressure and lower the temperature, higher the breakdown voltage.(3)Temperature of spark plug electrodesHigher the temperature of electrodes, lower the breakdown voltage.(4)Engine operating statusThis includes engine speed, load, compression ratio, ingredients of mixture ,etc. For example, high speed needs low voltage and when starting and accelerating, it needs high voltage.3.2 Sufficient energy of sparksUsually to ensure the ignition, the ignition energy should be 50~80mJ. When starting, it needs up to 100mJ.3.3 Correct ignition time3.3.1 Firing orderThe ignition system gives sparks to the cylinders according to firing order. For example:4-cylinder in-line engine: 1―3―4―2 or 1―2―4―36- cylinder in-line engine: 1―5―3―6―2―4 or 1―4―2―6―3―53.3.2 Ignition advanceThe influencing factors of ignition advance include engine speed, load, starting and idle, octane value of gasoline, compression ratio , etc .speed↗, advance↗load↗, advance↘for starting and idle, advance↘octane value↗, advance ↗compression ratio↗, advance↘4. Composition and operating principle of ignition system4.1 Composition of ignition systemIt includes battery, ignition switch, ignition coil, distributor, condenser(or capacitor), spark plugs, and high tension cables , etc.As the distributor cam rotates, it causes contact points to open and close. When the contact points are closed, they connect the primary winding(W1)of the ignition coil with the battery. Current flows through the coil, causing a magnetic field to build up around it. Then, when the points are opened, this disconnects the primary winding from the battery. The magnetic field collapses, creating a short pulse of high voltage in the secondary winding(W2)of coil. The high voltage surge flows through the distributor cap and rotor to the spark plug in the cylinder that is ready to fire. This is the spark plug in the cylinder in which the piston is nearing the end of the compression stroke.5. Influencing factors of secondary(winding)voltageFactors that influence upon the secondary voltage include:(1)Engine speed and number of cylindersWith high engine speed and great number of cylinders, the closed time of contact points is short. This reduces the current of primary winding, and then decreases secondary voltage. (2)Carbon deposits of spark plugsCarbon deposits are often caused by excessive rich air-fuel mixture ,weak ignition and use of spark plugs of incorrect heat range. It seems a conductor with some resistance. This will cause electric leakage to decrease secondary voltage.(3)CondenserTheoretically smaller condenser helps to increase secondary voltage, since bigger condenser delays the charging and discharging time of circuit. But too small condenser will cause electric sparks at contact points. This loses some energy and decrease secondary voltage.Usually it is about 0.15~0.35μFFor distributed capacitance of the circuit, it is small about 47~70 pF.(4)Contact-point clearanceIf the contact-point clearance is too wide, the closed time of contact points is short .This will decreases secondary voltage.If the contact-point clearance is too small, contact-point sparks are easy to be produced.This will also decrease secondary voltage.(5)Temperature of ignition coilHigh temperature of ignition coil will increase the resistance of primary winding. This will reduce primary winding current and decrease secondary voltage.6. Main components in the ignition system6.1 Ignition coilThere are two types of ignition coils according to their different constructions of magnetic circuit:Open magnetic circuit—often used for contact-point ignition system.The ignition coil has two windings. One is the secondary winding, which consists of tens of thousands of turns of a very fine wire. The other is the primary winding, which consists of a few hundred turns of a relatively heavy wire. The primary winding is in the primary circuit. The secondary winging is in the secondary circuit.About ballast resistance (or calibrated resistance):●Function:(1)Prevent secondary circuit from fire missing and low voltage when engine speed is high.(2)Prevent primary circuit from overheating when engine speed is low.●Operation:The ballast resistance’s value can change(↗or↘) with its temperature ,and then controls primary current automatically.(1)At high speed, closed time of contact points↘,primary current↘,then ,ballast resistancebecomes cold, resistance↘,current↗.(2)At low speed, closed time of contact points↗,primary current↗,then ,ballast resistancebecomes hot, resistance↗,current↘.When starting engine, bypass or short the ballast resistance circuit through a cable from the solenoid switch of starting motor. This is to provide a high voltage to produce strong spark.6.2 DistributorThere are two separate circuits through the distributor. One is the primary circuit. The other is the secondary circuit. It also includes ignition advancer.(1)The primary circuit includes the battery, ignition switch, primary resistance, primarywinding, contact-points(or breaker), condenser, and primary wiring.(2)The secondary circuit includes secondary winding, central high tension cable, rotor,distributor cap, spark-plug high tension cables, spark plugs.(3)Ignition advancer includes centrifugal-advance and vacuum-advance mechanism.Note: The distributor shaft and cam rotate at the same speed as the engine camshaft. This is half the speed of the crankshaft.6.2.1 BreakerThe breaker consists of a pair of contact-points(stationary contact-point and moveable contact-point) and a cam.The contact-points open and close with the distributor cam rotating. The number of cam lobes are the same as there are cylinders in the engine.The distributor cap and rotor form a rotatory switch.When the coil secondary winding produces a high-voltage surge, it passes through central high tension cable to the center terminal of the cap. Then the high-voltage surge goes through the rotor blade and jumps to the outer terminal toward which the rotor blade is pointed. From there, it passes through the outer terminal and through the spark-plug high tension cable to the spark plug. The plug produces a spark which ignite the compressed mixture in the cylinder.6.2.3 Condenser(or capacitor)The condenser is in parallel connection with the contact points. It has two functions. One is to help the circuit to build up high breakdown voltage, the other is to protect the contact points from burning.6.2.4 Centrifugal advanceAs engine speed increases, the spark must occur in the combustion chamber earlier in the cycle. This will give enough time to ignite the mixture and initiate combustion.To produce the advance based on engine speed, the distributor has a mechanical centrifugal-advance mechanism. The action of the centrifugal-advance mechanism causes the cam assembly to move ahead and the contact points to open and close earlier.Main parts of centrifugal advance include advance weights, weight springs, cam assembly and main-shaft assembly.6.2.5 Vacuum advanceWhen the engine is operating at part throttle, less air-fuel mixture enters the engine cylinders on the intake strokes. There is less mixture to compress. With lower compression, the mixture doesn’t burn as fast when ignited.To produce an advance based on part-throttle operation, the distributor has a vacuum-advance mechanism driven by intake-manifold vacuum. The action of the vacuum-advance mechanism causes the moveable breaker plate to rotate so that the contact points open and close earlier.Main parts of vacuum advance include diaphragm, return spring, and diaphragm lever(or linkage),etc.6.3 Spark plug6.3.1 Construction:It consists of center electrode, side electrode ,and insulator, etc.The space between center electrode and side electrode forms spark-plug gap.6.3.2 Spark plug heat rangeThe heat range depends on the shape of the insulator in the spark plug. It also depends on how far the heat must travel from the center electrode to the much cooler cylinder head(or length of the insulator skirt).There are two types according the heat range:(1)Hot plug: Heat path is long. This is for small power, low compression ratio, low speedengine.(2)Cold plug: Heat path is short. This is for high power, high compression ratio, highspeed engine.7. Adjustment of ignition system7.1 Contact-point clearance adjustmentThe contact-point opening decreases gradually due to wear on the contact-arm rubbing block. The opening should be checked periodically and adjusted if necessary. There are two ways to do this, with a thickness gauge(or feeler gauge) or with a dwell(angle) meter. Dwell is the number of degrees of cam rotation that the points are closed. As the point opening is increased, the dwell angle decreases.Contact-point clearance can be adjusted by loosening a hold down screw and turning an eccentric screw. The standard clearance is about 0.35~0.45mm.7.2 Spark-plug gap adjustmentThe gap can be adjusted by bending the side electrode. (Note: Many mechanics recommend replacing spark plugs at periodic intervals, instead of cleaning and regapping them.) Usually the standard gap is about 0.7~0.8mm for contact-point ignition. For electronic ignition, the gap can be increased up to 1.0~1.2mm.7.3 Ignition timing adjustmentThe ignition system must be timed so that the sparks jump across the spark-plug gaps at exactly the right time. The ignition timing is normally set at idle, or at a speed specified by the engine manufacturers. Adjustment is made by turning the distributor in its mounting.To make the adjustment, loosen the clamp screw that holds the distributor in its mounting. Turning the distributor in the direction opposite to cam rotation advances the spark(or counterclockwise). Turning the distributor in the direction of cam rotation retard the spark(or clockwise).To time the ignition, a timing light can be used for modern cars.。
