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Overview of Ignition Technology
The Stages of a Spark Discharge
The ignition process is key to the operation of internal combustion engines. Four-cycle engines require external combustion, and this is accomplished by discharging electrical energy through a spark gap. Heat transfer and gas ionization (splitting apart of gas molecules) caused by the spark discharge initiate a flame kernel. The flame kernel then grows into a flame front that spreads through the combustion chamber. The stages of a spark discharge include breakdown, arc, and glow discharges.
Break Down
Most engines require nearly 12,000 volts to fire the spark gap. When this voltage level is reached, a very high current flows for several hundred nanoseconds, an almost inconceivably short period of time, as energy stored in the capacitance of the coil, secondary wiring, and spark plug is rapidly discharged. Because the break down phase is so short, it accounts for only a small percentage of the total ignition energy. To better put this into perspective, at 6,000 RPM the crankshaft rotates less than 1/100 of a degree in 200 nanoseconds!
Arc Discharge
The break down phase is followed by an arc discharge. An arc discharge requires a considerable current flowing in the gap, usually on the order of several hundred milliamps. During the arc phase, the voltage across the gap drops to about one hundred volts. An arc discharge is very visible and high levels of energy are transferred to the flame kernel near the center of the spark gap. The duration of the arc discharge for a conventional ignition is usually no more than a few microseconds. Again, to put this into perspective, at 6,000 RPM the crankshaft rotates through less than 1/10 of a degree in 2 microseconds!
Glow Discharge
A glow discharge is less intense and the sustaining voltage across the gap is higher. A neon light is a good example of a glow discharge. It looks nice, but it doesn't generate a lot of heat. Much of the energy in the glow discharge is lost heating the electrodes or ionized gas near the electrodes. Conventional ignition systems have a short arc phase (some microseconds) after the initial breakdown followed by a glow discharge that lasts 1-2 milliseconds. At 6000 RPM, the crankshaft rotates through 36 degrees in one millisecond.
Capacitive Discharge
Capacitive discharge (CD) ignition systems are widely used in automotive racing applications with high RPM requirements. A special high voltage power supply charges a capacitor to 450 volts or more. Energy is stored in the electrostatic field within the capacitor and is proportional to capacitance times the square of the applied voltage. An electronic switch discharges this stored energy into the coil primary. A high voltage pulse then appears across the secondary windings and fires the spark gap.
Spark Plug Wires
Three types of spark plug wires are commonly available: original equipment style carbon core suppression, low resistance spiral core, and solid core. Carbon core suppression wires cause some energy loss due to their high resistance (about 5,000 ohms/foot). Replacing carbon core suppression wires with low resistance spiral core wires only increases spark energy by about 10%. Contrary to any claims, you will not see a performance improvement by changing spark plug wires. On the other hand, carbon core wires can deteriorate over time and any wires more than a few years old are candidates for replacement. If you are going to install new spark plug wires, buy a set of low resistance spiral core wires. Most are less than 500 ohms/foot. At that point, almost no energy is lost in the wire. There are some new versions with very low resistance (50 ohms/foot), but the advantage is insignificant for the short lengths encountered in motorcycle applications.