General description
An ignition system is a system for igniting a air-fuel mixture. Ignition systems are well known in the field of internal combustion engines such as those used in petrol (gasoline) engines used to power the majority of motor vehicles. Ignition system is divided into two electrical circuits - the primary and secondary circuits. The primary circuit carries low voltage. This circuit operates only on battery current and is controlled by the breaker points and the ignition switch.
Principle of operation of the primary ignition circuit
The coil is the heart of the ignition system. Essentially, it is nothing more than a transformer which takes 12 volts from the battery and increases it to a point where it will fire the spark plug as much as 40,000 volts. The term "coil" is perhaps a misnomer since there are actually two coils of wire wound about an iron core. These coils are insulated from each other and the whole assembly is enclosed in an oil-filled case. The primary coil, which consists of relatively few turns of heavy wire, is connected to the two primary terminals located on top of the coil. The secondary coil consists of many turns of fine wire. It is connected to the high-tension connection on top of the coil (the tower into which the coil wire from the distributor is plugged).
Ignition systems can be divided into the following types:
Distributor Ignition
The distributor ignition system is the most common ignition system for early model year vehicles. Distributor ignition systems use one coil that fires one spark plug at a time on the compression stroke only. Viewing the primary ignition pattern requires that you have to monitor the voltage signal on the negative side of the coil’s primary circuit and to identify the trigger cylinder by using the RPM probe.
The classical or conventional ignition system consists of the following components: ignition coil, distributor, spark plugs, high-voltage wires and some means of controlling the primary ignition circuit. The primary circuit of the ignition coil can contain: points, points controlling a transistor, the transistor being controlled by some other means (breaker less) or electronic ignition. In point-type ignition systems the current in the primary circuit is controlled by a mechanical switch (or breaker). The mechanical points may control a switching transistor which opens and closes the primary circuit of the ignition coil. In breaker less transistor and electronic ignition a Hall effect, VRS (Variable Reluctance Sensor) or an optical sensor may be used to control the switching transistor.
Current flows from the positive terminal of the battery, through the ignition switch and/or relay, through a fuse and on to the positive terminal of the ignition coil. The current returns to the battery through the negative terminal of the ignition coil, on through the switching device (points or a transistor) through the vehicle chassis, and to the negative terminal of the battery. While current is flowing in the primary circuit a magnetic field builds up in the ignition coil. Due to the inductance of the ignition coil it takes some time (1-6 mS, depending on design) for the primary current to reach its nominal value. When the primary current flow is interrupted, the magnetic field collapses rapidly (in about 20µS) and a high voltage is induced in the primary winding (CEMF Counter electro motive Force). This voltage is transformed in to a very high voltage in the secondary winding. The amplitude of this voltage depends on the turns ratio (commonly 100:1). A 300V primary voltage, therefore, will be 30 000V in the secondary winding. The voltage will only build until the break down voltage of the spark gap is reached - the firing voltage of the spark plug.
Direct Ignition System (DI)
COP systems use one individual coil for each spark plug. Each coil is located directly on top of its spark plug and does not use any external spark plug wires. Each coil pack also has an independent primary circuit which must be tested individually.
The individual ignition coil by one running cycle of the engine generates one ignition spark. Therefore, in individual ignition systems is required synchronization of coils work with position of a camshaft.
At submission of the voltage to the primary coil, the current starts to flow by a primary coil and because of that in the core of the coil changes the value of the magnetic flux. Change of the magnetic flux value in the core of the coil leads to occurrence of the voltage of positive polarity on a secondary coil. Because the speed of increasing of the current in the primary coil is slow, the voltage arising on a secondary coil is small – according 1…2 kV. But in the certain conditions the voltage value can be sufficient for untimely occurrence of the spark between electrodes of a spark plug and as consequence, too early ignition of the air/fuel mixture. In order to prevent possible damages of the engine due to untimely occurrence of the spark, formation of the spark between electrodes of a spark plug at submission of a voltage to a primary coil should be excluded. In the individual ignition systems, occurrence of this spark is prevented by means of built-in diode EFU to the ignition coil switched consistently in a circuit of a secondary coil.
At the moment of closing of the output ignition cascade, current in the primary circuit sharply interrupts, and the magnetic flux promptly decreases. This fast change of the magnetic flux value is causes to occurrence of the high voltage on a secondary coil of the ignition coil (under certain conditions, the voltage on a secondary coil of the ignition coil can achieve 40…50 kV). When this voltage achieves the value providing formation of the spark between electrodes of a spark plug, the compressed in the cylinder air/fuel mixture is ignited from the spark between electrodes of a spark plug.
In some systems coils are not located directly on top of each spark plug and external spark plug HT leads are used. Each coil pack also has an independent primary circuit which must be tested individually.
DIS-Wasted Spark Ignition
DIS ignition systems use one coil for every two cylinders, also called “waste-spark” systems. A waste-spark system fires one coil for each pair of cylinders that are at top dead center (TDC) at the same time. These cylinder pairs are called “running mates.” One cylinder is at TDC on the compression stroke, while the other is at TDC on the exhaust stroke. The spark in the cylinder at TDC on the compression stroke ignites the air-fuel mixture to produce power. The spark in the cylinder at TDC on the exhaust stroke is “wasted,” hence the name “waste-spark.” Each waste-spark DIS coil is hooked in series with its two spark plugs. As the coil fires, secondary current creates a high voltage spark across the gaps of both plugs. One plug fires with the traditional forward polarity of an ignition system: negative (—) to positive (+) The other plug fires with opposite polarity: positive (+) to negative (—) Thus, one plug always fires with what has always been called “reversed polarity.” The voltage capacity of a DIS coil is high enough, however, to ensure that the available voltage is always high enough to fire the plug with reversed polarity when it’s on the compression stroke.
Fig.1 Primary ignition waveform
1. The ECU internal switch closes. Current rushes into the coil and begins to build, which is why voltage drops close to ground and essentially remains there until the firing spark.
2. The coil is now saturated with electricity, as indicated by the jump in voltage.
The coil is no longer charging up thanks to the ECU.
3. The ECU switch opens, unleashing all the built-up current. Amps drop like a rock and voltage skyrockets.
4. The spark line indicates the length of the spark event at the plug.
5. When not enough power is left for the spark, remaining power is rung out and the event begins all over again.
Procedure to verify functionality of the primary ignition circuit
— Ohmmeter and voltmeter measurements of the ignition coil primary winding —
1. Measuring the primary voltage
- Connect the active test lead to the ignition coil negative terminal (usually “1”) and the ground lead to the chassis ground.
Important note: To measure the primary voltage input voltage range of the oscilloscope should be set to ± 400V.
2. Measuring the primary current
- Connect an AC current clamp to the other oscilloscope channel. Range ±20A.
- Start the engine and left it idling.
- Compare result with the waveform in fig. 2.
Fig.2
Note: Primary voltage can rise up to 380V and primary current can vary from 8A to around 12A.
If the ignition module (ECU power switch) is combined into one unit with the coil primary winding, it is impossible to carry out diagnostics of the primary ignition voltage. In this case only the primary current can be observed with a current clamp.
1. Measuring the primary current
- Connect an AC current clamp to the other oscilloscope channel. Range ±20A.
- Start the engine and left it idling.
- Compare result with the waveform in fig. 3.
Note: Primary current can vary from 8A to around 12A.
Fig.3
— Possible reasons for failure of the primary ignition circuit —
» No supply voltage to the ignition coil.
• Make sure ignition is on.
• Check electrical connections to the ignition coil.
• Check for burned fuses and/or wires in the ignition coil circuit.
» Broken insulation between the coil’s primary and the secondary windings
» Bad ignition coil.