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High voltage spark and air compression

  1. Jul 25, 2009 #1
    Honestly I do not think automotive techs would know the answer to this question so being said from a physics stand point about the relationship between the high voltage spark on a spark plug and compression in a engine. I have had experiances where there was spark on the plugs but the engine would not start. Classic example is the 1990s hondas. I have seen examples that if the spark is weak, a strong engine with high compression will prevent the spark from jumping across the plug. Or am I wrong? What role does compression play in inhibiting the spark from a spark plug to jump the gap?

    I told the owner these HT coils have a weak dialectic insulation in its design. I told him because he never replaced the plugs, it loaded Coil with to much air gap resistance. The plugs had a excessive spark plug gap. The coil properly ohmed out on the primary and seconday and would fire out of the secondary of the coil using my hand to ground. The spark would not jump from the screwdriver to ground when inserted into the spark plug boot. Am I correct, that because the owner neglected to replace his plugs, The resistance leading from the top of the coil to the cathode on the plug was greater then the resistance in the coils secondary dialectric material that the spark blew a hole though it to ground?

    Am I correct that the coils KV also increased substantially from the excessive spark plug gap?

    Lastly, how is HT spark on a plug change with increase or decrease in compression?
  2. jcsd
  3. Jul 25, 2009 #2
    You are correct that the voltage required to produce a spark across a spark plug gap increases nearly linearly with the compression ratio. The spark plugs need to be gapped correctly, and the insulators kept clean. Starting an engine in very cold weather is especially difficult because the battery voltage is reduced by the high starter current and battery resistance. Resistance in the wire from the coil to the spark plug is important to limit the spark current AFTER the spark has occurred. Too high a spark current erodes the spark plug gap excessively. Also keep the outside of the coil clean.
  4. Jul 25, 2009 #3
    Thanks for the reply.

    I wonder if there is a chart that would show what the KV at the plug should be with the compression of the engine? This would also be different with a turbo or a supercharger? I imagine with very high compressions, the spark would jump the gap, but would be very weak if it is the wrong ignition setup.

    I suspect this would lead to high Hydrocarbons at the tail pipe.

    Being a automotive tech is not enough today to repair today's cars. You also need some understanding in physics and chemistry to resolve the issues. I have repaired countless cars that dealers or other independents could not resolve. Now I just wish some of my customers would seriously create a auto repair budget.

    BTW, I have built numerous electronic items from Doppler shift direction finders to Tesla coils. Anyone built a Tesla that responds to the strings of a electric guitar?
  5. Jul 29, 2009 #4
    Lortech, have a look on wikipedia for paschens law.
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