Spark Burn Voltage
Spark Burn Voltage is the voltage required to maintain the arc between spark plug electrodes. It can be seen right after the Spark Firing Voltage Peak, flowing for the duration of Spark Burn Time until the end of the ark. Spark Burn Voltage can be found under different names like: "Spark Line", Spark KV" all representing the same thing. Ideally this voltage line should be flat straight line ( when viewed with an oscilloscope ). Flat line would represent clean fuel burning. Additional resistance in the secondary coil circuit ( wanted or unwanted ) will result in a sloped line. Most spark plugs today have interference-suppression resistors inside them, which can burn out or not make good contact with the central electrode. This leads to increased resistance which sometimes ONLY occurs under load. Testing spark plugs with ordinary multimeter in this case may not indicate a problem. Use of oscilloscope however will.
1. 2. 3. 4.
Figure 1. represents a good Spark Burn Voltage Line. The actual voltage value can vary from system to system, from 400V to a couple of KiloVolts. 1KV=1000 Volt.
Figure 2. represents a higher secondary resistance. Brake in HT insulation, interference-suppression resistors, spark plugs etc. If the line is not steady and jumps up and down, the spark plug in question is heavily sooted or oil-fouled. If all cylinders lines jump, then the high resistance is in the "central" secondary lines ( from coil tower through center HT Lead, central distributor cap tower and brush, to the rotor.) ( or suspect all spark plugs and HT leads)
Figure 3. represents very lean air/fuel mixture. As the fuel is available in the mixture diminishes quicker, the spark needs more "energy" to sustain itself. Hence the increase in voltage as it progresses to the end.
Figure 4. represents very reach air/fuel mixture. The fuel ignites multiple times in the chamber at different burning spots.
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