Re: What does a condensor do for a gas engine

Posted by MarkB_MI on June 03, 2007 at 05:30:55 from (216.234.125.196):

In Reply to: Re: What does a condensor do for a gas engine posted by RAB on June 02, 2007 at 02:23:23:

Hey RAB,

My explanation might be confusing, but that doesn't make it incorrect. I don't think I said anything that is not true. Yes, it would be nice if we could explain this behavior using only DC circuit fundamentals, but you can't explain transient conditions that way. We have to work in the time domain, and things get very messy there.

Current does indeed flow through the condensor, even though we all know that a condensor is an open circuit to steady state DC. The thing is that when those points open, the circuit is not at steady state at all. You have a big voltage spike generated by the coil primary winding, and that looks like high frequency ac to the condensor. At the instant the points open, the condensor is for all intents and purposes a dead short.

If you don't believe that current can pass through a condensor, try this simple example: Put your multimeter on the ohms scale and connect it to a condensor. You'll see the resistance initially go to a dead short, then climb to infinity. Reverse the leads and you see the resistance go negative, then climb through zero back up to infinity. Now your multimeter has to measure both voltage and current to determine resistance; it's just applying Ohm's Law. So there must be current flowing through the condensor. What happens when the condensor is in the vehicle is exactly the same thing, except is happening much faster because the current through the coil is lot more than what your multimeter is sourcing.

Trying to explain what's happening inside the condensor really just muddles things. Yes, the electrons at the ground side of the condensor must always stay at zero potential, but the coil side of the condensor can have a lot of potential once the points open. Current rushes into the condensor, causing it to charge. As the voltage rises, the current drops until it gets to zero. Then the current reverses back through the coil, increasing as the flux builds up in the coil, then dropping back to zero and reversing direction again. This cycle repeats, but the peak voltage at the condensor is a little less each cycle until it finally stabilizes at battery voltage.

Remember, current through a condensor LEADS voltage by ninety degrees. So the capacitor's voltage will be maximum when current through the capacitor is zero.

BTW, my education is as an electrical engineer, although I haven't worked in the field for many years. The example of the point-type ignition system is (or was) used as a classic example in sophmore ac circuit theory classes.

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