Posted by John T on December 12, 2010 at 13:50:44 from (66.244.97.31):
ATTENTION SPARKIES AND WANNABEES:
This friendly discussion got lost below between Jim and myself and I would like to get more opinions regarding this question. It concerns the voltage necessary to arc jump current across the spark plug gap in an internal combustion engine.
I always thought as the compression increased inside the combustion chamber it took a higher voltage to fire the plug. I thought if the plug was outside the tractor at atmosphere the plug would fire at a lower voltage then if it were inside where the pressure may be say 100 PSI, and as the compression increased, the voltage necessary to fire the plug likewise increased. Thats the how n why I always thought why in extreme high compression racing engines they had to use the high voltage capable coils which rose maybe to 30,000 to 40,000 volts (versus maybe 10,000 in a stock engine, depends on compression and fuel) to insure the plugs fired.
NOTE the voltage of a coil is NOT instantly say 10,000 volts, it ramps up only high enough to the point where current arc jumps the plug gap and no higher. i.e. even if you use a so called high voltage 40,000 volt coil in a stock set up, it will still fire the plug at the same voltage the old stock coil did, its just that the HV coil has the CAPACITY to achieve higher voltage if necessary as in high compression engines (under my theory)
So I did a very quick search AND ALTHOUGH THIS IS NOT PERFECT OR CONCLUSIVE what I did find seems to me to confirm my belief that voltage has to increase (to fire the plug) as compression rises.
Please take a look and provide opinions/reasons/theories why or why not my thinking was correct or wrong as rain??????
http://www.stealth316.com/2-sparkplugtech.htm#j4c
Factors Affecting Required Voltage
The following operational factors affect the required voltage. • Spark gap - the voltage needed to ionize the electrode gap is directly proportional to the gap width and the required voltage increases with a larger gap. • NOTE Compression pressure - required voltage increases in direct proportion to cylinder pressure or as the air density increases. • Engine load and speed - required voltage changes in direct proportion with engine torque and throttle opening because these change the cylinder air density. • Air-fuel ratio - required voltage is at a minimum with a stoichiometric (14.7) air-fuel mixture because the air is just damp enough to improve electrical conductivity; rich mixtures increase required voltage because grounding across the insulator increases slightly, and lean mixtures increase required voltage because the air is drier. • Electrode temperature - between 200º and 600ºC (~400º and ~1100ºF) required voltage decreases rapidly as temperature rises; above 600ºC required voltage falls at a much-slower rate. • Electrode diameter - required voltage increases as the electrode diameter increases and as the roundness of the electrode edges increases. • Electrode polarity - using the model that electrons flow from the negative electrode to the positive electrode and knowing that electrons are released more freely when metal is very hot (even though resistivity of metals generally increases with temperature), because the center electrode is hotter than the ground electrode, the required voltage can be lower when the center electrode has the negative polarity (true for half of our spark plugs).
An important electrical phenomenon appears when a Xenon Timing Light is hooked up to a Spark Plug Lead on a Petrol Engine.
The Timing Light will flash when the Compression Pressure inside the Cylinders is high, and not flash when the Compression Pressure inside the Cylinders is low. Essentially the Spark Plug can short out the Spark Plug Lead when the Compression Pressure is low and doesn't leave enough Voltage to trigger the Timing Lamp.
NOTE:::::::::::This is because it takes more Voltage to fire the Plugs when the Compression Pressure is high and minimal Voltage when the Compression Pressure is low.
When the Engine is decelerating the lamp will stop flashing because the Compression Pressure inside the Cylinders is very low. It's low at this time because the Engine is compressing the high vacuum that was created in the inlet manifold with the throttle closed and the Engine decelerating. This weak spark requirement is the reason why Engines polluted heavily on the overrun prior to VK Commodores.
The Lamp will flash brightly under acceleration and when there is enough Compression to allow the Engine to idle.
If you use a very expensive and sensitive Timing Light you may not see the symptom.
The observation explains why Engines with weak Ignition stall on take off. Take off is the time the Ignition needs are greatest and Idle the time when the Ignition requirements are least. Clearly the overrun case needs even less spark, but no spark at all is needed for the overrun case.
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