5. Electrical and Electronic Systems.pptx

Electrical and Electronic Systems

1. ELECTRICAL AND ELECTRONIC SYSTEMS

2. Electronics in the motor vehicle

3. Electronics in the motor vehicle

4. Ignition systems The purpose of the ignition system is to supply a spark inside the cylinder, near the end of the compression stroke, to ignite the compressed charge of air/fuel mixture. For a spark to jump across an air gap of 1.0 mm under normal atmospheric conditions (1 bar) a voltage of 4–5 kV is required. For a spark to jump across a similar gap in an engine cylinder, having a compression ratio of 8:1, approximately 10 kV is required.

5. The ignition system must transform the normal battery voltage of 12 V to approximately 8–20 kV and, in addition, it must deliver this high voltage to the right cylinder, at the right time. If two coils (primary and secondary) are wound on to the same iron core, then any change in magnetism of one coil will induce a voltage in the other. This happens when a current is switched on and off to the primary coil. If the number of turns of wire on the secondary coil is more than on the primary a higher voltage can be produced. This is called transformer action and is the principle of the ignition coil.

6. Ignition Timing: For optimum efficiency, the ignition advance angle should be such as to cause the maximum combustion pressure to occur about 10° after TDC. The ideal ignition timing is dependent on two main factors, engine speed and engine load. An increase in engine speed requires the ignition timing to be advanced. The cylinder charge of fuel/air mixture requires a certain time to burn (normally about 2 ms ). At higher engine speeds the time taken for the piston to travel the same distance reduces. Advancing the time of the spark ensures that full burning is achieved.

7. Primary Circuit Operation When the ignition switch is on, current from the battery flows through the ignition switch and primary circuit resistor to the primary winding of the ignition coil. From there it passes through some type of switching device and back to ground. The current flow through the ignition coil’s primary winding creates a magnetic field. As the current continues to flow, the magnetic field gets stronger. When the triggering device signals to the switching unit that the piston is approaching TDC on the compression stroke, current flow is stopped. This causes the magnetic field around the primary winding to collapse across the secondary winding. The instantaneous change in the current induces a high voltage in the secondary winding.

8. A Crankshaft Position Sensor (CKP) is a magnetic type sensor that generates voltage using a sensor and a target wheel mounted on the crankshaft, that tells the ECU or the Ignition Control Module the exact position of the cylinder pistons as they come up or go down in the engine cycle. The sensor has 3 wires: power, ground and signal. They are powered with low voltage (4.75 to 24 volts) and have very little current draw (20 mA). There are two missing teeth which are used as a reference.

9. A Camshaft Position Sensor (CMP) is used to keep the engine's control module informed about the position of the camshaft relative to the crankshaft. By monitoring cam position (which allows the control module to determine when the intake and exhaust valves are opening and closing), the control module can use the cam position sensor's input along with that from the crankshaft position sensor to determine which cylinder in the engine's firing sequence is approaching TDC. This information is then used by ECU to synchronize the pulsing of sequential fuel injectors, so they match the firing order of the engine. Input from the camshaft position sensor is also required for ignition timing. The camshaft position sensor may be magnetic or Hall effect and mounted on the timing cover over the camshaft gear, on the end of the cylinder head. Operation and diagnosis is essentially the same as that for a crankshaft position sensor.

10. Secondary Circuit Operation The secondary circuit carries high voltage to the spark plugs. The exact manner in which the secondary circuit delivers these high-voltage surges depends on the system. Until 1984 all ignition systems used some type of distributor to accomplish this job. However, in an effort to reduce emissions, improve fuel economy, and boost component reliability, most auto manufacturers are now using distributorless or electronic ignition (EI) systems.

11. Mechanically-timed ignition The heart of the system is the distributor . The distributor contains a rotating cam driven by the engine's drive, a set of breaker points, a condenser, a rotor and a distributor cap. The system consists also from the ignition coil, the spark plugs and wires linking the distributor to the spark plugs and ignition coil. The system is powered by a battery and the engine operates contact breaker points, which interrupt the current to an induction coil. The ignition coil works as a step-up transformer which produces a high voltage from the secondary winding. The high-voltage end of the secondary winding is connected to the distributor's rotor.

12. As the engine crankshaft turns, it also turns the distributor shaft at half the speed (in a four-stroke engine). A cam is attached to the distributor shaft. During most of the cycle, circuit breaker is closed. As a piston reaches TDC, the circuit breaker opens which causes the current through the primary coil to stop. The magnetic field generated in the coil immediately collapses. This high rate of change of magnetic flux induces a high voltage in the coil's secondary windings that ultimately causes the spark plug's gap to arc and ignite the fuel. The capacitor prevents the points from arcing as they separate; if the points arc, then they will drain the magnetic energy that was intended for the spark plug. The capacitor temporarily keeps the primary current flowing so the voltage across the points is below the point's arcing voltage.

13. Spark Plugs: Spark plugs provide the crucial air gap across which the high voltage from the coil causes an arc or spark. The main parts of a spark plug are a steel shell; a ceramic core or insulator, which acts as a heat conductor; and a pair of electrodes, one insulated in the core and the other grounded on the shell. The shell holds the ceramic core and electrodes in a gas-tight assembly and has threads for plug installation in the engine. A terminal post on top of the center electrode is the connecting point for the spark plug cable. Current flows through the center of the plug and arcs from the tip of the center electrode to the ground electrode. The center electrode is surrounded by the ceramic insulator and is sealed to the insulator with copper and glass seals. These seals prevent combustion gases from leaking out of the cylinder. The resistor suppresses ignition noise generated during sparking which might interfere with other electronic components.

14. STARTING SYSTEM The starting system is designed to turn or crank the engine until it can operate under its own power. To do this, the starter motor is engaged to the engine’s flywheel. As it spins, it turns the engine’s crankshaft. The sole purpose of the starting system is to crank the engine fast enough to run. The engine’s ignition and fuel system provide the spark and fuel for engine operation, but they are not considered part of the starting system.

15. TEMPERATURE SENSORS The ECU changes the operation of many components and systems based on temperature. Nearly all temperature sensors are NTC thermistors. The engine coolant temperature (ECT) sensor is a thermistor. By measuring ECT, the ECU knows the average temperature of the engine. Temperature is used to regulate many engine functions, such as the fuel injection system, ignition timing, etc. The ECT sensor is normally located in an engine coolant passage just before the thermostat.

16. The intake air temperature (IAT) sensor is also called an air charge temperature sensor. Its resistance decreases as the incoming air temperature increases and vice versa. The ECU uses the air temperature information as a correction factor in the calculation of fuel, spark, and airflow. For example, the ECU uses this input to help calculate fuel delivery. Because cold intake air is denser, a richer air-fuel ratio is required. On engines equipped with a MAP sensor, the IAT is installed in an intake air passage.

17. PRESSURE SENSORS Most pressure sensors are piezoresistive sensors. A silicon chip in the sensor flexes with changes in pressures. The displacement is directly related to the voltage signal sent out from the sensor. A manifold absolute pressure (MAP) sensor senses air pressure or vacuum in the intake manifold. The sensor measures manifold air absolute pressure. The MAP sensor uses a perfect vacuum as a reference pressure. The MAP sensor measures changes in the intake manifold pressure that result from changes in engine load and speed. The ECU determines manifold pressure by monitoring the sensor output voltage. The ECU uses the MAP signals to calculate how much fuel to inject in the cylinders and when to ignite the cylinders.

18. High manifold pressure (low vacuum) resulting from full throttle operation requires more fuel. Low pressure or high vacuum requires less fuel. At closed throttle, the engine produces a low MAP value. Wide-open throttle produces a high value. The highest value results when manifold pressure is the same as the pressure outside the manifold and 100% of the outside air is being measured. The use of this sensor also allows ECU to automatically adjust for different altitudes. An ECU with a MAP sensor relies on an IAT sensor to calculate intake air density.

19. MASS AIR FLOW SENSORS The mass airflow (MAF) sensor measures the flow of air entering the engine. This measurement of intake air volume is used to calculate engine load (throttle opening and air volume). Engine load inputs are used to control the fuel injection and ignition systems, as well as shift timing in automatic transmissions. The airflow sensor is placed between the air cleaner and throttle plate assembly or inside the air cleaner assembly. There are different types of MAF sensors. The most commonly used design is the hot-wire MAF. In a hotwire-type MAF, a wire, called the hotwire, is positioned so the intake air flows over it. The sensor also has a thermistor, sometimes referred to as the cold wire, located beside the hot wire that measures intake air temperature. The sensor also contains a control module. Current from the ECU keeps the hot wire at a constant temperature above ambient temperature, normally 392°F (200°C).

20. The thermistor measures the temperature of the incoming air. The hot wire is maintained at a constant temperature. An increase in air flow will cause the hot wire to lose heat faster and the control circuitry will compensate by sending more current through the wire. The increased current flow keeps the hot wire at its desired temperature. The ECU simultaneously measures the current flow and puts out a voltage signal in proportion to current flow. The current required to maintain the hot wire’s temperature is proportional to mass airflow. Most MAF sensors have an integrated IAT sensor.

21. KNOCKING Knocking/detonation/pinging is a noise most noticeable during acceleration with the engine under load. Detonation occurs when part of the air-fuel mixture begins to ignite on its own apart from the normal flame front. This results in the collision of two flame fronts. One flame front is the normal front moving from the spark plug tip. The other front begins at another point in the combustion chamber. The air-fuel mixture at that point is ignited by heat, not by the spark. The colliding flame fronts cause high-frequency shock waves (heard as a knocking or pinging sound) that could cause physical damage to the pistons, valves, bearings, and spark plugs. Excessive detonation can be very harmful to the engine. Detonation is usually caused by excessively advanced ignition timing, engine overheating, excessively lean mixtures, or the use of gasoline with low octane rating.

22. KNOCKING

23. The knock sensor (KS) tells the ECU that detonation is occurring in the cylinders. As a result, the computer retards the timing. The KS is a piezoelectric device that works like a microphone and converts engine knock vibrations into a voltage signal. Piezoelectric devices generate a voltage when a vibration is applied to them. Engine knock typically is within a specific frequency range and a KS is set to detect vibrations within that range. The KS is located in the engine block, cylinder head, or intake manifold.