Electronic Ignition System Operation


Electronic Ignition System Operation

The primary circuit of an electronic ignition system is controlled electronically by one of the sensors just described and an electronic control unit (module) that contains some type of switching device.

Primary Circuit Control

The system consists of a distributor with a magnetic pulse pick-up unit and reluctor, an electronic control module, and a ballast resistor. As described earlier, when the tooth of the reluctor passes the pick-up, an electrical impulse is sent to the electronic module, which contains the switching transistor. the pulse signals the transistor to open the primary circuit, firing the plug. Once the plug stops firing, the transistor cloeses the primary coil circuit. The length of time the transistor allows current flow in the primary ignition circuit is determined by the electronic circuitry in the control module. Some systems used a dual ballast resistor. The ceramic ballast resistor assembly is mounted on the fire wall and has a ballast resistor for primary current flow and an auxiliary resistor for the control module. The ballast resistor has a 0.5-ohm resistance that maintains a constant primary current. The auxiliary ballast resistor uses a 5-ohm resistance to limit voltage to the electronic control unit.

There are some electronic ignition systems that do not require a ballast resistor. For instance, some control units directly regulate the current flow through the primary of the coil. Hall-effect systems do not require ballast resistors either. The signal voltage is not changed by the speed of the distributor as it is in an inductive magnetic signal generating system.

Some systems can be enhanced with additional sensors that increase the capabilities of the control module. The module can be equipped with either a barometric pressure switch or vacuum switch. The barometric pressure switch enables the module to retard the ignition timing 3 to 6 degrees when the vehicle is operating at low elevations. The vacuum switch does the same, when the engine is under hard acceleration or heavy load. Other modules have the ability to retard ignition timing during start-up or when engine knock is detected.

Timing Advance

As stated, early electronic ignition systems changed the timing mechanically just like breaker point systems.

CENTRIFUGAL ADVANCE At idle, the firing of the spark plug usually occurs just before the piston reaches top dead center. At higher engine rpm, however, the spark must be delivered to the cylinder much earlier in the cycle to achieve maximum power from air/fuel mixture since the engine is moving through the cycle more quickly. To change the timing of the spark in relation to rpm, the centrifugal advance mechanism is used.

This mechanism consists of a set of weights and springs connected to the distributor shaft and a distributor armature assembly. During idle speeds, the springs keep the weights in place and the armature and distributor shaft rotate as one assembly. When speed increases, centrifugal force causes the weights to slowly move out against the tension of the springs. This allows the armature assembly to move ahead in relation to the distributor shaft rotation. The ignition's triggering device is mounted to the armature assembly. Therefore, as the assembly moves ahead, ignition timing becomes more advanced.

VACUUM ADVANCE During part-throttle engine operation, high vacuum is present in the intake manifold. To get the most power and the best fuel economy from the engine, the plugs must fire even earlier during the compression stroke than is provided by a centrifugal advance mechanism.

The heart of the vacuum advance mechanism is the spring-loaded diaphragm, which fits inside a metal housing and connects to a movable plate on which the pick-up coil is mounted. Vacuum is applied to one side of the diaphragm in the housing chamber while the other side of the diaphragm is open to the atmosphere. Any increase in vacuum allows atmospheric pressure to push the diaphragm. In turn, this causes the movable plate to rotate. the more vacuum present on one side of the diaphragm, the more atmospheric pressure is able to cause a change in timing. The rotation of the movable plate moves the pick-up coil so the armature develops a signal earlier. These units are also equipped with a spring that retards the timing as vacuum decreases.

Spark Distribution

The distributor cap and rotor receive the high voltage from the secondary winding via a high-tension wire. The voltage enters the distributor cap through the coil tower, or center terminal. The rotor then sends the voltage from the coil tower to the spark plug electrodes inside the distributor cap. The rotor mounts on the upper portion of the distributor shaft and rotates with it.

The distributor cap is made from silicone plastic or similar material that offers protection from chemical attack. It is attached to the distributor housing with screws or spring-loaded clips. The coil tower contains a carbon insert that carries the voltage from the high-tension coil lead to the raised portion of the electrode on the rotor. Spaced evenly around the coil tower are the spark plug electrodes and towers for each spark plug.

An air gap of a few thousandths of an inch exists between the tip of the rotor electrode and the spark plug electrode inside the cap. This cap is necessary to prevent the two electrodes from making contact. If they did make contact, both would wear out rapidly. This cap cannot be measured when the distributor is assembled; therefore, the gap is usually described in terms of the voltage needed to create an arc between the electrodes.

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