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.
Direct ignition is a general term used to describe systems that use individual coils for each cylinder. On some engines, the coils are mounted on the rocker cover and connect to the spark plugs using short cables. In Coil-Over-Plug systems, the coils are mounted directly on top of the spark plugs. This design minimizes secondary resistance by eliminating the plug wires.
IGNITION WAVEFORM ANALYSIS
As previously mentioned, the production of adequate secondary voltage depends upon the proper operation of the primary circuit. Since it is impossible to obtain a primary waveform from a distributorless ignition system, and because any problem in the primary will show up in the secondary trace, an analysis of secondary waveforms is sufficient for diagnostic purposes.
An ignition waveform can be bro ken down into three specific areas including the firing, intermediate and dwell sections. The firing section begins with a high vertical spike called the ‘firing line.’ The firing line occurs at the instant primary current is interrupted (coil’s magnetic field collapses). The height of the firing line (firing kV) depends upon the level of secondary resistance in the circuit. For instance, if 12kV is needed to overcome all of the resistance in the number one cylinder, then the coil must develop 12,000 volts for a spark to occur. However, if secondary resistance exceeds the coil’s maximum output, the cylinder will misfire. Simply put, firing kV is the amount of voltage required to initiate a combustion event. Generally speaking, the difference in firing kV between the lowest and highest cylinder should not exceed 20%. Any firing voltage beyond this range indicates a problem in that particular cylinder. If the firing kV for all cylinders is too high or too low, then look for a problem that would affect all cylinders equally, such as an open coil wire (DI systems), or an excessive imbalance in the air/fuel ratio (DI and EI systems). Be aware that when analyzing the firing lines in a waste spark system, you will notice more than a 20% variation in firing kV between half of the cylinders. This is because the plugs that fire in reverse require greater secondary energy than the positive firing plugs. Consequently, the firing kV for the negative cylinders will normally be more than 20% higher than the positive cylinders. Once the spark has been initiated, a horizontal line will be displayed approximately three quarters of the way down on the firing line. This point is called the spark line. The height on the voltage scale that the spark line corresponds to is referred to as the ‘burn voltage.’ Burn voltage indicates the amount of energy required to sustain the flow of current across the plug electrodes. The duration of the spark, known as the ‘burn time,’ is dependent upon the amount of secondary resistance and coil reserve. Under normal conditions, a properly functioning coil can maintain current flow for at least two milliseconds, which is vital for good combustion. Be aware that firing kV and burn time are inversely proportional. This means that any increase in kV demand will result in decreased burn time and vice versa. For example, if one injector is leaking, the affected cylinder will run too rich. Since fuel is more conductive than air, the added fuel will reduce secondary resistance. Consequently, firing kV will be lower and burn time will be longer. If the injector is restricted, the opposite will occur. The lean mixture will cause increased resistance, resulting in a higher kV demand with shorter burn time. High internal resistance, such as a lean mixture or excessive EGR, will cause the spark line to slope upward. This is be cause these conditions increase the demand for burn voltage. As you can see, observing the spark lines can pinpoint problems affecting individual cylinders. In the event all of the spark lines seem irregular, look for a problem that could affect all cylinders equally. For instance, if all of the spark lines slope upward, the increased resistance may be the result of low fuel pressure or excessive EGR.
When viewing the intermediate section of the waveform, you should see a series of diminishing oscillations that represent residual coil energy. Although the primary circuit is still open during this time, there is not enough energy left in the coil to sustain the spark. This residual energy gradually dissipates between the switching device and coil. Under normal conditions, there will be three to four coil oscillations following the burn event. Be aware however, the more voltage required to initiate and sustain the spark, the less energy will remain afterward. Consequently, there will be fewer oscillations in the intermediate section if firing kV or burn voltage is excessive. If the intermediate section shows less than normal coil oscillations and the firing and spark lines appear normal, the coil is most likely shorted. The fewer oscillations indicate that there is less residual energy due to a weakened magnetic field. The most likely symptoms of a shorted coil will be a misfire under load, or a hard start cold, since secondary resistance is highest under these conditions.
The dwell section begins once coil energy has been completely exhausted. A sharp downward spike indicates that the switching transistor has turned on to initiate the flow of primary current. The several oscillations seen at the beginning of the dwell period are the result of a phenomenon known as ‘inductive reactance.’ In short, this is a normal condition in which the coil resists sudden changes in current flow. Consequently, there is a slight delay before primary current reaches its maximum level. Once the ignition module or ECM determines that coil saturation is sufficient, it begins limiting primary current. This is indicated by the slight hump seen near the end of the dwell period.