The solution for this is to have a duration or life to the spark. In most ignition systems, once the spark jumps, the electrodes will continue to burn for at least 1, microseconds and maybe more. The idea is to keep the gap lit up long enough to ensure that some volatile fuel and air will be ignited.
The required spark plug firing voltage is also affected by the pressures inside the cylinder. The pressure results from compression and also from heavy loading of the engine. The largest firing voltages happen typically in snap throttle acceleration conditions.
The change in engine speed, the change in fuel ratio, and the load on the engine contribute to the higher required voltage. Current technology coils are of several basic types. Pencil coils are typically an inch in diameter and up to four inches long. The normal practice for pencil coils is to include the switching electronics needed to control the coil, in a small pocket near the outer end of the coil package. Pencil coils have the advantage that they are made to fit within the small area available between the cams.
While they package very well on to the engine, pencil coils have the disadvantage of their location. That part of the engine typically operates at very high temperature.
The metal surrounding the coil can interfere with the magnetic action of the coil, reducing its output. This is a major consideration in direct injection engines. Pencil coils are also more expensive to manufacture, and possibly less reliable in actual use, than conventional coils. Coil Near Plug CNP applications use a short, plastic insulated stick or a rubber spark plug boot to reach down into the engine and the spark plug. This keeps the transformer portion of the coil up and away from the metal of the cylinder head.
While CNPs do not package as neatly as pencil coils, their operating temperature is lower and thus their reliability is greater. CNPs are perhaps one-third less expensive to manufacture than comparable coils. Some CNPs include integrated electronic packages while others do not. Still in use on some engines are the coil pack type ignition coils. Inside the coil packs are double ended coils that can be connected to simultaneously fire the plug for one cylinder under compression and also a second cylinder on its exhaust stroke.
Still, this arrangement allows a single coil pack to serve a pair of cylinders. On the next rotation of the engine, what was a wasted spark will become a useful spark when that cylinder is under compression. The disadvantage of coil pack type systems is that they still use spark plug wires and boots to conduct the spark from the coil to the spark plugs. Spark plug wires and boots have long had warranty-related problems and are being design-eliminated from most current technology engine designs.
Coil packs are also comparatively big and heavy as compared to CNPs. There are still millions of road vehicles and agricultural vehicles that still use a distributor, distributor cap, rotor and spark plug wires.
With this type of system, cylindrical, oil-filled coils may be used. In this type of system, the coil is designed to supply sparks to as many as eight cylinders. The cap and rotor act as a high voltage switch that distributes the output of the coil to the required spark plug. The cap, rotor and spark plug wires are all subject to wear and contamination issues.
The net result is that these systems have lower output voltages and lower reliability than the newer systems that replaced them. The basic construction of an ignition coil begins with two coils of wire.
The primary consists of approximately turns of what is called magnet wire. For the typical ignition coil, the primary is a single strand of copper wire of approximately 22 gauge. The wire is insulated with a thin layer of polyester insulation. This insulation allows the primary to be wound into a coil shape to maximize inductance without the individual turns shorting out to their neighbors.
Winding the coil in a circular, tube like shape helps to concentrate the inductance. The secondary of the coil is typically wound on to a plastic bobbin that has been divided into sections or bays.
The wire used in the secondary is much finer than the primary wire. This too is magnet wire. Typically, this type of wire is a single strand, 42 to 44 gauge wire that is coated with a dozen or more very thin layers of insulation. For comparison, this wire is smaller in diameter than the average human hair. It depends on the type of coil being made, but as many as 10, turns of the fine wire is wound onto the secondary bobbin.
The secondary coil will have a resistance of between 6, and 10, ohms. The primary coil mounts inside the secondary coil. Stretched out, there might be as much as half a mile of wire wound onto the secondary bobbin.
Inside both the primary and the secondary coils is the lamination stack. The lamstack can be 20 to 30 layers of thin electrical grade steel that is laid out in a rectangular path. A gap of roughly 1mm 0. This gap is important to the storage of magnetic energy in the primary of the coil. Some coils use a very special powdered metal whose particles are insulated with plastic to form the magnetic core of the coil.
Today's coil-on-plug assemblies come in a variety of physical and wiring configurations. The typical configurations are two wires, three wires and four wires. Some ignition coils contain a solid state driver module which is part of the ignition coil and is controlled by the Powertrain Control Module PCM. Others have the primary winding wired to and directly controlled by the Powertrain Control Module.
One of the circuits that should be common to all ignition coils is the battery voltage power supply. It is always recommended to refer to service information for the specific vehicle you are working on. When a magnetic field has been created by applying an electric current to a coil of wire, any change in the electric current increase or decrease in current flow creates the same change in the magnetic field. If the electric current is switched off, the magnetic field will collapse.
The collapsing magnetic field will then induce an electric current into the coil Figure 4. Figure 4: If an electric current used to create a magnetic field is switched off, the magnetic field collapses, which induces another electric current into the coil.
In the same way that increasing the speed of movement of a magnetic field across a coil of wire will increase the voltage induced into the coil, if a collapsing magnetic field can be made to collapse more rapidly, this will induce a higher voltage. Additionally, a higher voltage can also be induced into the coil if the number of windings in the coil is increased. If two coils of wire are placed next to or around each other and an electric current is used to create a magnetic field around one coil which we call the primary winding , the magnetic field will also surround the second coil or secondary winding.
When the electric current is switched off and the magnetic field then collapses, it will induce a voltage into both the primary and the secondary windings. Figure 5: The magnetic field in the primary winding also surrounds the secondary winding. Collapsing the field induces electric currents in both windings. If no voltage is indicated in either direction, it can be assumed that there is an interruption in the secondary circuit. If a voltage is indicated in both directions, the high-voltage diode is faulty.
During all testing work on the ignition system, please note that faults established during tests with the oscilloscope are not necessarily faults caused by the electronic system; they can also be caused by a mechanical problem in the engine. This may be the case, for example, if compression is too low in one cylinder, which means the oscilloscope shows the ignition voltage for this cylinder to be lower than that of the other cylinders.
Although "diagnosable engine management systems" are installed in today's vehicles, a multimeter or oscilloscope must be used when checking ignition systems.
In order to interpret the displayed measuring results and figures correctly, additional employee training is usually required. One important pre-requisite for successful diagnostics is a careful visual inspection at the beginning of the troubleshooting process. We would like to demonstrate the diagnostics procedure for a dual-spark ignition coil using the following example, "misfiring".
Each cylinder has a main plug and a secondary plug. The ignition coils are triggered by the ignition output stages integrated in the engine control unit. In this example, the repair procedure is shown using a Mega Macs diagnostic unit. The schematic illustrations, figures, and descriptions are intended solely as explanations of the document text, and cannot be used as the basis for carrying out installation and repair work. Condition for diagnostics work: Engine mechanics, battery, starting system, and fuel system OK.
Connect the diagnostic unit to the pin OBD connector. Depending on the vehicle manufacturer and date of registration, a different diagnostic socket and additional adapter may be required. Sufficient battery voltage and the correct connector are required in order to establish communications with the electronic control unit.
Insufficient supply voltage to the electronic control unit could be an indication of a wiring defect or a defect in the vehicle battery. Note: If several fault codes are displayed, clear the fault first. Once this is done, carry out a test drive with the diagnostic unit connected. Monitor the parameters and read out the fault memory. Before the actual diagnostics process begins, the engine wiring harness and plug connectors must be checked for damage as far as possible. Kinks, lack of strain relief, and "marten bites" in the wiring harness are all possible causes of this.
Switch on the ignition. A voltage of more than Measured value: Measurement OK. In order to check the voltage supply under load, we recommend repeating the measurement while actuating the starter. In order to prevent unnecessary fuel injection, you must remove all the injection valve connectors first.
A signal must be clearly identifiable on the oscilloscope. In this example, the measurement is successful. In order to avoid damaging the spark plug connector, it is essential to avoid rotating the ignition coil. Check the plug slot for soiling caused by oil and water penetration. Remove and check the spark plugs. Use the multimeter to check the removed ignition coil.
In order to measure the secondary coil, measure the probes directly at the high-voltage outputs of the ignition coil. Here, care must be taken that the spark plug connector and the high-voltage cable for the second plug fit properly. Attach the ignition coil using the fixing screws. Once this is done, insert all the plug connections for the ignition coil and the injection valve connectors. During the diagnostics work, additional faults were detected by the electronic control unit.
These must be cleared before the test drive. Carry out a test drive with the diagnostic unit connected. Once this is complete, read out the fault memory again. Always take the vehicle manufacturer's specifications into account during all testing and diagnostic work. Depending on the manufacturer, additional vehicle-specific testing methods may have to be taken into consideration. During work on electronic ignition systems, contact with live components can result in potentially fatal injuries.
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