Unit 4 Engine Ignition System and Starting System.doc

Engine Ignition System and Starting SystemPassage A Ignition System The purpose of the ignition system is to supply a spark across the spark plug electrodes in the combustion chamber at the correct time under all engine operating conditions. The three principal functions of the ignition system are:1. To covert low-voltage direct current obtained from the battery to high-voltage pulses ;2. To direct the high-voltage pulses to each spark plug to ignite the air-fuel mixure and begin the combustion process ;3. And to properly time the high-voltage puises for efficient operation.Two types of ignition systems are used:the conventional ignition system and the electronic ignition system. Conventional ignition system contains two basic eletrical circuits: the pirmary circuit and the secondary circuit . The primary circuit consists of the battery , ignition switch , resistance wire , coil primary winding , distributor contact point , and connecting wiring. The secondaty circuit consists of ignition coil secondary windings , distributor cap , distributor rotor, spark plug wires. 1. Ignition requires a source of electrical energy. This source is the battery. When starting the engine, the battery supplies the needed current. After the engine is running, current is supplied by the alternator or by the alternator and battery working together.2. The ignition switch allows the driver to start and stop ignition by opening and closing the circuit between the electrical energy source and the other ignition components. The switch usually has four positions: Off, Accessory, On, and Start.3. The voltage developed by the battery or alternator is not high enough to ignite the air-fuel mixture in the engines cylinders. The function of the ignition of coil is to step up , or transform, the low voltage available from the battery or alternator to the 35kv or more volts required for ignition . 4. The distributor has three jobs: It opens and closes a low voltage circuit between the source of electrical energy and the ignition coil. This supplies the ignition coil with intermittent surges of current. These surges are used to induce high voltage current inside the ignition coil. The distributor times the surges to engine requirements. This is accomplished with centrifugal and vacuum advance mechanisms. The distributor directs the high voltage developed by the ignition coil through high voltage spark piug wires or ignition cables to the spark plugs. 5. A spark plug is located in each of the engines combustion chambers. It gets the high voltage current developed by the ignition coil and directed by the distributor. The spark plug has an air gap that the current is forced to jump. As the current jumps across the spark plug air gap, a spark is developed. This spark ignitis the air-fuel mixture in the cylinder. The burning air-fuel mixture is used to develop power in the engine.6. The ignition system is made up of a primary and a secondary circuit. The wires and parts that direct the low voltage current from the battery are part of the primary circuit; those wires and parts that direct the high voltage current are part of the secondary circuit.In the early 1990s a new type of ignition system. Began to appear on many automobiles. It is called the distributorless ignition system, or direct ignition system. This system provides a high voltsge spark plug without many of the secondary system parts. The system can provide higher voltages and does so with fewer parts to wear out.The distributorless ignition system requires an engine control computer to control the primary current and develop the secondary current. A coil pack includes a separate ignition coil to provide high voltage to two separate engine cylinders. A crankshaft timing sensor is mounted over the crankshaft and sends a signal to the computer regarding engine speed and crankshaft position. Some systems also use a camshaft sensor to determine camshaft position relative to crankshaft position. This information is also sent to the engine control computer. In addition, this information, along with that from other sensors, allows the computer to control ignition timing. There is no need for the centrifugal and vacuum timing systems used on conventional ignition systems.In a conventional ignition system, the switching on and off of the primary voltage is done with a set of breaker points. The points are set inside the distributor and rode on a cam on the distributor shaft. This cam would have 4, 6 or 8 lobes, depending on how many cylinders the engine has. When the points are closed, current flows into the primary side of the coil creating the magnetic field. When the cam lobe opens the points, the current is turned off and the magnetic field collapses.In an electronic ignition system, the points are reliaced with a control module and the lobes on the cam are replaced with a trigger device. The trigger device uses a magnetic force field to induce a small “trigger” voltage in the control module to turn off the current to the coil. As it passes, the module turns the current back on. It is extremely accurate in when it does this. Another advantage of an electronic ignition system can produce higher voltages, up to twice the voltage a conventional system can produce. Passage B Starting System The starting system is the heart of the electrical system in your car. The starting system performs this function by changing electrical energy from the battery to mechanical energy in the starting gine s cankshaft. During canking, the flywheel rotates and the air-fuel mixture is drawn into the cylinders, compressed, and ignited to start the engine. Most engines require a cranking speed of about 200 r/min. The starting system includes the following components:1. Battery2. Cable and wires3. Ignition switch4. Starter solenoid or relay 5. Starter motor 6. Starter drive and flywheel ring gear Components in a simplified cranking system circuit are shown in Fig. 4-2 The starting system functions to crank the engine fast enough to allow the engine to start. The starting system and ignition system must work together for good engine starting. When the driver turns the key switch to START, current flows the battery through the key switch to the solenoid. The solenoid allows battery current to flow directly from the battery to the starter motor, which in turn cranks the engine. The automotive battery serves as an energy “bank” . It receives energy from the vehicle s charging sysytem and stores it until needed. Then it provides this energy to the many vehicle electrical systems on demand. The battery converts electrical energy into chemical energy during charging. It then converts it back into electricity during dischanging. The key switch ,or ignition switch ,controls the ignition circuit. It also controls the starting sysytem and the power for for the instruments and vehicle accessories. Most are steering wheel lock-type switches, which are mounted on the steering column. The starter motor , or starter ,is a compact but very powerful direct-current electric motor designed to crank the engine fast enough for it to start. It rotates a small gear called a pinion.A starter drive assembly connects the small pinion gear to the end of the starter motor , The pinion gear meshes with the ring gear on the engine s flywheel . When the driver turns the key switch ,the starter motor drives the pinion gear , which drives the flywheel and cranks the engine. The gear reduction ratio between the starter mtor and the engine must be high enough to provide enough torque to turn the engine over at cranking speed . There are approximately 15 to 20 teeth on the flywheel ring gear for every tooth on the drive pinion gear . This means the armature will rotate approximately 15 to 20 times for every engine revolution . Inorder to turn the engine over at 100 r/min, the armature must rotate at 1500 to 200 r/min . If the pinion gear remained meshed with the flywheel ring gear at engine speeds above 1000 r/ min ,the armature would be spun at very high speeds . These