Engine Tech

And in accordance with the engine’s particular firing order, we’re now back to the combustion process. But for purposes of a clearer understanding of how this combustion spark was developed, let’s return to the engine’s battery and see what takes place from wet cell to “fire.”
In spark ignition engines, there are two basic methods by which ignition voltage is produced. One requires battery voltage input, the other uses permanent magnets and the disruption of magnetic fields to produce secondary voltage output. This second form of ignition voltage is called a magneto (so named because of its use of magnets for the production of ignition voltages). However, since battery ignition systems are the more common, let’s first examine how they develop ignition voltage.

We mentioned that battery voltage flows into the primary side of the ignition coil. Interruption of this voltage flow is required to produce secondary (plug) voltage. This is accomplished as follows: Voltage passes from the battery through a set of contact points inside the ignition distributor (see schematic illustration). While the points are closed, this flow of voltage continues through the primary side of the ignition coil until the points are caused to open. A “distributor cam” (see illustration) opens and closes the ignition points which act as a switch in the delivery of ignition voltage to the spark plugs. When the points are opened, the primary/ secondary magnetic field collapses, resulting in an “induced” voltage to the spark plugs. Timing of this voltage is governed by a voltage-conducting rotor mounted on the same shaft as the distributor cam. A metal tip is located on the outer end of the rotor so that ignition voltage is passed from the rotor to each of the engine’s spark plug wires connected to the distributor cap (see illustration).

On the chance that all this good theory is not coming into focus, let’s simplify it a little. First, compare the movement of electrical current to water. What we’re trying to do is get electrical energy into an engine’s combustion chamber at the proper time and with the required amount of force. As cylinder pressure increases (a result of high compression ratios, lean air/fuel mixtures, or early spark ignition timing), more “water pressure” or voltage strength is going to be required. This means that more ignition voltage will be needed to initiate combustion. And if you haven’t already figured out where all this is leading, an obvious solution is the use of an HEI (high-energy ignition) system.

Such systems provide the necessary voltage to initiate combustion during conditions of lean air/fuel mixtures or otherwise high precombustion cylinder pressure. As a natural result of such conditions, multiple-spark ignition systems have evolved for both passenger car and race engines. The object here is the reduction of misfire (lost power) during lean air/fuel mixture conditions, contaminated air/ fuel mixtures, or high-rpm engine operation. Such systems provide additional ignition spark as air and fuel (mixed to some degree of combustibility) move past an engine’s spark plug. Elements (and their functions) of a conventional battery ignition system include the ignition coil with a primary-to-secondary coil ratio typically of 1:100. This means, for example, that for every single coil of primary wire there will be 100 secondary coils. More exactly, an ignition coil of 200 turns of primary wire will have 20,000 comparable turns of secondary wire, usually wound in layers and insulated from adjacent layers by some form of coated paper. In order to increase the intensity of the magnetic field developed as a result of voltage through the primary windings, a soft iron core is frequently used. And to further aid the dissipation of heat within the coil unit and help prevent insulation failures among all the coils of wire, oil is often used to fill the inside of the coil assembly case.

E. Spark plug heat range is related to ignition timing and net engine output. What you can boil it all down to is the length of the heat path from electrode to cylinder head (or cooling) area and the actual surface area of plug insulator (usually porcelain) exposed to combustion heat. As a rule of thumb, the shorter the cooling path, the colder the plug temperature. The longer the path, the hotter the plug will operate. For engines of high-combustion temperatures, colder plugs are required to prevent preignition and lost power. Lower combustion heat engines require hotter spark plugs to prevent the buildup of deposits that prevent proper plug operation. F. Here you can see the relationship among components of a conventional battery-spark ignition system. Functionally, voltage passes through the primary side of the ignition coil and ignition points (closed). When the points open, the magnetic field generated by the primary voltage collapses, inducing a much higher secondary voltage into the windings of this side of the coil. Properly timed with the distributor, this voltage passes from distributor rotor to one of the engine’s spark plugs, and the process repeats according to the firing order.

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