Just ran across an interesting Wikipedia article on wasted spark.
</font><blockquote><font class="small">Quoting Wikipedia:</font><hr />
The wasted spark system is an ignition system used with some four-stroke cycle internal combustion engines. In a wasted spark system, the spark plugs fire in pairs even though one is on its compression stroke and one is on its exhaust stroke. The extra spark on the exhaust stroke has no effect and is thus "wasted".
This design halves the number of components in a typical ignition system, while the extra spark, against much reduced dielectric resistance, barely impacts on the lifespan of modern ignition components. <b>In a typical engine, it requires only about 2 to 3kV to fire the cylinder on the exhaust strokes. The remaining coil energy is available to fire the spark plug under compression (typically about 8 to 12kV).</b>
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I wasn't aware it's so much easier to fire a plug during exhaust.
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Here's an interesting article dealing with spark plug construction and heat behavior.
</font><blockquote><font class="small">Quoting Wikipedia:</font><hr />
The central electrode is usually the one designed to eject the electrons (the cathode) because it is the hottest (normally) part of the plug; it is easier to emit electrons from a hot surface, because of the same physical laws that increase emissions of vapor from hot surfaces (see thermionic emission). In addition, electrons are emitted where the electrical field strength is greatest; this is from wherever the radius of curvature of the surface is smallest, from a sharp point or edge rather than a flat surface (see corona discharge). It would be easiest to pull electrons from a pointed electrode but a pointed electrode would erode after only a few seconds. Instead, the electrons emit from the sharp edges of the end of the electrode; as these edges erode, the spark becomes weaker and less reliable.
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The development of noble metal high temperature electrodes (using metals such as yttrium, iridium, tungsten, or palladium, as well as the relatively high value platinum, silver or gold) allows the use of a smaller center wire, which has sharper edges but will not melt or corrode away. These materials are used because of their high melting points and durability, not because of their electrical conductivity (which is irrelevant in series with the plug resistor or wires). The smaller electrode also absorbs less heat from the spark and initial flame energy. At one point, Firestone marketed plugs with polonium in the tip, under the (questionable) theory that the radioactivity would ionize the air in the gap, easing spark formation.
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Some interesting items from an autoclub.atilim.edu.tr publication.
</font><blockquote><font class="small">Quoting p241:</font><hr />
The ignition coil supplies much more secondary voltage than the average engine requires. The peak voltage that a coil produces is called its available voltage.
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Generally, a primary circuit voltage loss of one volt may decrease available voltage by 10,000 volts.
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When there is a spark plug in the secondary circuit, the coil voltage creates an arc across the plug air gap to complete the circuit. Figure 11-31 compares typical no-load oscillation to a typical secondary firing voltage oscillation. At about 15,000 volts, the spark plug air gap ionizes and becomes conductive. This is the ionization voltage level, also called the firing voltage, or required voltage. As soon as a spark has formed, less voltage is required to maintain the arc across the air gap. This reduces the energy demands of the spark causing the secondary voltage to drop to the much lower spark voltage level. This is the inductive portion of the spark. Spark voltage is usually about one-fourth of the firing voltage level.
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The spark duration or burn time of the trace indicates the amount of resistance and efficiency of the spark voltage. Burn time on most ignition systems is between 1.6 and 1.8 milliseconds.
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</font><blockquote><font class="small">Quoting p243:</font><hr />
The firing voltage on the spark plug of 0.035 inch is about 6 to 8 kV; 0.045-inch air gap is 8 to 10 kV; and 0.060 air gap is 10 to 12kV.
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</font><blockquote><font class="small">Quoting p244:</font><hr />
Voltage reserve is the amount of coil voltage available in excess of the voltage required. Under certain poor circuit conditions, there may be no voltage reserve. At these times, some spark plugs do not fire and the engine runs poorly or not at all. Ignition systems must be properly maintained to ensure that there is always some voltage reserve. Typically, an ignition system should have a voltage reserve of about 60 percent of available voltage under most operating conditions.
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</font><blockquote><font class="small">Quoting p245:</font><hr />
In any system, the connections to the primary winding must be made correctly. If spark plug polarity is reversed, greater voltage is required to fire the plug. Plug polarity is established by the ignition coil connections.
One end of the coil secondary winding is connected to the primary winding, Figure 11-41, so the secondary circuit is grounded through the ignition primary circuit. When the coil terminals are properly connected to the battery, the grounded end of the secondary circuit is electrically positive. The other end of the secondary circuit, which is the center electrode of the spark plug, is electrically negative. Whether the secondary winding is grounded to the primary + or [-] terminal depends on whether the windings are wound clockwise or counterclockwise.
The secondary circuit must have negative polarity, or positive ground, for two main reasons. First, electrons flow more easily from negative to positive than they do in the opposite direction. Second, high temperatures of the spark plug center electrode increase the rate of electron movement, or current. The center electrode is much hotter than the side electrode because it cannot transfer heat to the cylinder head as easily. The electrons move quickly and easily to the side electrode when the air-fuel mixture is ignited. Although the secondary operates with negative polarity, it is a positive ground circuit.
If the coil connections are reversed, Figure 11-42, spark plug polarity is reversed. The grounded end of the secondary circuit is electrically negative. The plug center electrode is electrically positive, and the side electrode is negative. <b>When plug polarity is reversed, 20 to 40 percent more secondary voltage is required to fire the spark plug.</b>
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</font><blockquote><font class="small">Quoting p251:</font><hr />
The arc of current across a spark plug air gap providestwo types of discharge:
• Capacitive
• Inductive
When a high-voltage surge is first delivered to the spark plug center electrode, the air-fuel mixture in the air gap cannot conduct an arc. The spark plug acts as a capacitor, with the center electrode storing a negative charge and the grounded side electrode storing a positive charge. The air gap between the electrodes acts as a dielectric insulator. This is the opposite of the normal negative ground polarity, and results from the polarity of the coil secondary winding.
Secondary voltage increases, and the charges in the spark plug strengthen until the difference in potential between the electrodes is great enough to ionize the spark plug air gap. That is, the air-fuel mixture in the gap is changed from a nonconductor to a conductor by the positive and negative charges of the two electrodes. The dielectric resistance of the air gap breaks down and current travels between the electrodes. The voltage level at this instant is called ionization voltage. The current across the spark plug air gap at the instant of ionization is the capacitive portion of the spark. It flows from negative to positive and uses the energy stored in the plug itself when the plug was acting as a capacitor, before ionization. This is the portion of the spark that starts the combustion process within the engine.
The ionization voltage level is usually less than the total voltage produced in the coil secondary winding. The remainder of the secondary voltage (voltage not needed to force ionization) is dissipated as current across the spark plug air gap. This is the inductive portion of the spark discharge, which causes the visible flash or arc at the plug. It contributes nothing to the combustion of the air-fuel mixture, but is the cause of electrical interference and severe electrode erosion. High-resistance cables and spark plugs suppress this inductive portion of the spark discharge and reduce wear.
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</font><blockquote><font class="small">Quoting p252:</font><hr />
The heat range of a spark plug determines its ability to dissipate heat from the firing end. The length of the lower insulator and conductivity of the center electrode are design features that primarily control the rate of heat transfer, Figure 11-54. A “cold” spark plug has a short insulator tip that provides a short path for heat to travel, and permits the heat to rapidly dissipate to maintain a lower firing tip temperature.
A“hot” spark plug has a long insulator tip that creates a longer path for heat to travel. This slower heat transfer maintains a higher firing tip temperature. Engine manufacturers choose a spark plug with the appropriate heat range required for the normal or expected service for which the engine was designed. Proper heat range is an extremely important factor because the firing end of the spark plug must run hot enough to burn away fouling deposits at idle, but must also remain cool enough at highway speeds to avoid pre-ignition. It is also an important factor in the amount of emissions an engine will produce.
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