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Ignition Coil and Capacitor oscillations

by mrsummitville
Tags: capacitor, coil, ignition, oscillations
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mrsummitville
#1
Dec21-13, 02:26 PM
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BOB S had posted this in a now closed thread:

http://www.physicsforums.com/showthread.php?t=360072

When the points close, the capacitor is shorted and the coil primary charges up to about 2 amps (limited by coil internal resistance of ~6 ohms). When the points open, there is a very large V = L dI/dt voltage, ~300 volts, across the capacitor that resonates back and forth with current in the coil, for ~ 10 cycles. The coil secondary:primary turns ratio is ~100:1 so the coil secondary voltage is ~30,000 volts. See my LTSPICE simulation
http://www.physicsforums.com/attachm...6&d=1250393723
The points close at 0 and 4 milliseconds, and they open at 2 and 6 milliseconds.
Regarding this analysis ...
http://www.physicsforums.com/attachm...6&d=1250393723

My questions ...

Q1) Does this oscillation from the L-C circuit actually cause several (+/-/+/-) sparks on the spark plug each time the peak voltage on the secondary exceeds the break-over voltage?

Q2) How quickly does the loading from the spark plug via the secondary dampen out that oscillation in the real world?

Q3) So, how many "sparks" are there in one compression cycle?
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dlgoff
#2
Dec21-13, 03:45 PM
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The attachment was a simulation. The capacitor is there to reduce arcs across the points from the inductive back-voltage, thus increasing the point's lifetime. His simulation has the output on the capacitor which is not how auto ignition systems are designed.



Faraday's Law and Auto Ignition
sophiecentaur
#3
Dec21-13, 04:41 PM
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Quote Quote by mrsummitville View Post
My questions ...

Q1) Does this oscillation from the L-C circuit actually cause several (+/-/+/-) sparks on the spark plug each time the peak voltage on the secondary exceeds the break-over voltage?

Q2) How quickly does the loading from the spark plug via the secondary dampen out that oscillation in the real world?

Q3) So, how many "sparks" are there in one compression cycle?
Once the arc has struck, the LC resonance is very heavily damped (shorted right out) so you will get one main spike and a small 'ring' at DC. The spark itself will have a wide band of RF oscillation, during the current spike.


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