The ignition system is designed to convert battery voltage into the high voltage required to ignite the air/fuel charge in the cylinders. There are two basic types of ignition systems as defined by SAE: Electronic Ignition (EI) and Distributor Ignition (DI). Although both types are electronic, SAE assigned the term ‘Electronic Ignition’ exclusively to distributorless designs. In contrast, Distributor Ignition (DI) is the term applied to systems that use a cap and rotor to deliver secondary energy to the spark plugs.
IGNITION SYSTEM BASICS
The ignition system consists of a primary and secondary circuit. The primary circuit is the low voltage portion of the system and includes the battery, ignition switch, primary coil winding, triggering mechanism and switching device. The triggering mechanism detects crankshaft position and relays that information directly to the ECM or to an ignition control module. Depending on the system, this is accomplished using a magnetic sensor and reluctor, a Hall effect sensor and shutter wheel, or a slotted disc and photo optical sensor. The switching device controls the ground side of the primary coil winding based on the crankshaft signal as well as other sensor inputs (e.g. ECT, IAT, etc). The secondary circuit is the high voltage part of the ignition system and includes the secondary coil winding and spark plugs. The additional secondary components needed to distribute high voltage energy vary according to system type. For example, in a conventional distributor ignition system, a cap, rotor and high tension wires are used to transfer coil energy to the plugs. However, in a distributorless design , such as Coil-Over-Plug (COP) , the cap, rotor, and wires are eliminated. When current flows through the primary winding of the ignition coil, it creates a magnetic field around the winding. Once the current is interrupted by the switching device (i.e. ignition control module and/or ECM), the magnetic field collapses and induces a voltage into the winding. This ‘primary voltage’ reaches several hundred volts. At the same time, the collapsing magnetic field induces a voltage into the secondary winding. Since the secondary winding is made up of many turns of fine wire, compared to the few turns of heavy wire used in the primary, ‘secondary voltage’ is measured in kilovolts (thousands of volts). Generating adequate secondary voltage to fire the plugs ultimately depends on the condition of the primary circuit. This means that there must be a low resistance path from the battery through the dosed ignition switch to the coil and switching device. Excessive voltage drop at any point along this path can cause problems ranging from a misfire to a no-start condition.
Many EI and DI systems use a Hall-effect sensor to detect crankshaft position. This device has three connecting wires including external power, signal and ground. The sensor is triggered by a thin metal ring known as a shutter wheel. On DI systems, the shutter wheel is splined to the distributor shaft. On vehicles with EI, it is an integral part of the crankshaft vibration damper. The shutter wheel consists of a series of vanes that pass through a narrow area on the sensor. The vanes can be thought of as doors, while the space between the vanes can be thought of as windows. When a window is positioned inside the Hall-effect sensor, the sensor pulls the signal circuit low (close to zero volts). When a door is positioned inside the sensor, the sensor turns off, and the signal circuit rises close to source voltage. The rotation of the shutter wheel creates a digital (ON/OFF) signal that the ECM uses to regulate coil dwell and spark timing.
PERMANENT MAGNET (PM) SENSOR
A PM sensor consists of a soft iron core surrounded by a coil of fine wire. Unlike a Hall-effect device, the PM sensor produces its own voltage based on its proximity to a rotating wheel (reluctor). Depending on the engine, the reluctor is either mounted to the end of the crankshaft or is an integral part of it. As the reluctor rotates past the sensor, it changes the density of the magnetic field radiating from the sensor’s tip. This results in the production of an AC (Alternating Current) voltage that varies in proportion to engine speed. Since the ECM is a digital computer, the AC signal must be conditioned before it can be used for rpm calculations. This is accomplished by an analog-to-digital converter located inside the ECM.
While less popular than the Hall-Effect or permanent magnet sensors, photo optical triggering is another technique used in some ignition systems. With this method, a slotted disc rotates between a pair of Light-Emitting Diodes (LEOs) and phototransistors. Depending on the application, the outer diameter of the disc either contains 360 slots, each of which corresponds to one degree of crankshaft rotation, or 350 1-degree slots and a single 10-degree synchronization slot. These slots provide the ECM with a high-resolution signal for precise fuel and spark timing control. The inner section of the disc contains one slot for each engine cylinder, providing a low-resolution (piston position) signal. On systems that use a sync slot on the outer diameter of the disc, the inner slots are of equal size. However, on systems where there are 360 1-degree slots near the edge of the disc, the inner slots are asymmetrical in order to provide cylinder identification. The optical sensor is typically powered by battery voltage, while the phototransistors control two 5-volt signal circuits from the ECM. As the slots pass between the LEOs and the phototransistors, the light beams from the LEOs are alternately interrupted. When the light beam from the LED strikes the phototransistor, the transistor turns on. This causes the 5-volt signal to be pulled low. When the light beam is blocked by the rotating disc, the transistor turns off, which causes the signal voltage to go high (5 volts).
WASTE SPARK IGNITION
Waste spark ignition is a distributorless system that uses a separate coil to fire one pair of spark plugs. The coils and plugs are grouped according to ‘companion cylinders,’ which is the term applied to cylinders whose pistons are at top dead center at the same time. For example, in a typical V6, cylinders 1/4, 2/5 and 3/6 are companions. When one piston is at TDC on the compression stroke, the companion cylinder’s piston is at TDC on the exhaust stroke. The cylinder on the compression stroke is known as the ‘event’ cylinder, while the cylinder on the exhaust stroke is referred to as the ‘waste’ cylinder. Each coil, along with its attached wires and spark plugs forms a series circuit. When a coil discharges, current flows through one spark plug in the normal manner (center electrode to ground electrode). To complete the circuit, current flows through the opposite plug in reverse (ground electrode to the center electrode). Although the plugs are fired simultaneously, most of the available energy is applied to the ‘event’ cylinder. This is because there is little resistance in the cylinder on the exhaust stroke. When the cylinders reverse roles, the current follows the same path through the spark plugs. However, the majority of secondary voltage is applied to the opposite cylinder, since it is now on the compression stroke.