What is the function of the capacitor (condenser) in the ignition system?
Also what would happen if there was no capacitor attached ?
- 1 decade agoFavourite answer
zephyr is wrong. What he is describing is the ignition coil which is an inductor. NOT a capacitor.
First, to understand what the capacitor (condensor) does in the ignition system, it is necessary to understand how an ignition system works. Current goes through the primary circuit in an ignition coil and then to some sort of switching system breaks the connection when a cylinder's spark plug is meant to fire. The magnetic field that's in the ignition coil collapses and induces a high voltage into the secondary circuit of the ignition system, causing an arc in the spark plug and then firing the cylinder.
The problem is that when the magnetic field collapses in the primary circuit, a short voltage spike is introduced to that circuit as well. The condensor/capacitor absorbs this voltage spike.
In modern cars, this capacitor is no longer necessary. A diode is used instead.
If no capacitor were attached to an older system which requires one, voltage (and thus current) can flow backwards into the battery causing an overcharge condition and/or short out the plates by arcing. In addition, the alternator diodes may fail.Source(s): Degree in Automotive Technology and student of electrical engineering
- TechnobuffLv 71 decade ago
Hey Joe, I think you're getting too far ahead with auto technology.
The system of battery/ coil/ condenser/ mechanical points is the Kettering system.
The condenser in the Kettering system charges when the points are closed, via the primary of the coil.
When the points open, the condenser DISCHARGES RAPIDLY through the coil.
The coil is energised, the spark occurs. But also, the collapsing magnetic field in the coil produces a charge in the condenser once again, while the points are open. When this field has collapsed, again the condenser discharges through the coil, but this time (and any successive time) the voltage produced at the secondary is less. The circuit "rings" or oscillates without necessarily producing a further spark.
When the points next close, the condenser again charges via the (low) resistance of the coil primary, ready for the next cycle.
The weakness of the Kettering system was that as the engine no. of cylinders/ revolutions increased, the capacitor (condenser) would no longer gain as much charge, as the points were no longer closed sufficient time to charge it. The spark energy decreased the higher the revs.
This is why schemes were introduced to extend the time the points were closed. The earliest of these was the "twin- points" found in, for example, the Ford V8 systems used up 'til about the '50's.One set of points opened producing the spark, very shortly after the other set closed. They were wired in parallel. The second set then opened before the primary set.
Another way was the "dwell extender" type systems, that electronically simulated lengthening of the "points closed" condition. Both systems allowed greater charge on the condensorat higher revolutions.
The condenser (capacitor) was never used as a transient suppressor.
Put simply, the Kettering system will not work without the condenser!
- 7 years ago
the current runs through the battery via some kind of resistance usually built into the coil into the low voltage side of the coil and to earth through the points.
The condenser/capacitor cannot be charged at the top is effectively earthed via the points.
When the points open the field built up in the low voltage side of the coil collapses the magnetic field of the coil instantly reverses producing back EMF. That is the collapsing field tries to keep the current flowing . The capacitor supplies the instantaneous current for this to happen thus preventing a huge arc across the points. Such an arc would wear away the points quite quickly and that kind of wear is a sign of a dud capacitor.
The low voltage side of the coil interacts with the high voltage side of the coil as a step up transformer the ratio between the low voltage side and the high voltage side is around 100 to 1 .
So a 12 volt surge would give a 1200 volt surge ..but back EMF in the low voltage side makes that many many times higher (the formula is E=-L delta I over delta t where L is inductance I is current t is time delta is change and E is voltage. ) So a huge spike from the low voltage side is multiplied by 100 on the high voltage side.
MY electronic theory does not support the idea of the capacitor being there to form an oscillator circuit. An ignition system will work perfectly well without one however the points will last a very short time indeed.
That is the field in the (primary) low voltage side of the coil builds up with the points closed. When the points open the Magnetic field in the coil instantly reverses and attempts to keep the current in the primary flowing. The capacitor supplies this instantaneous surge, Without the capacitor the collapsing field would create a huge arc across the points. The collapsing field greats a huge voltage spike that is amplified one hundred times by the secondary winding's ( high voltage side) .
An easy experiment to demonstrate this effect. remove the centre lead from the distributor and connect it to a spark plug resting on earth ( bare metal part of engine or body ) so that it can spark. Remove the capacitor from the distributor. turn the engine over by hand so the points are open. Turn ignition on ...with a well insulated screwdriver close the points and let them open....observe the large spark across the points and the healthy spark at the spark plug.
The capacitor coupled with the coil and the resistance dampen oscillations.
- 1 decade ago
The capacitor on the old ignition system has the job to create an oscillation circuit, producing a more powerful electric discharge to the cylinder to ignite fuel.
The ignition coil and the capacitor form what is called LC circuit that causes such oscillation and repetitive spikes (ringing) for some time.
Without the capacitor, the electric high voltage to the cylinder will be weak, sometimes not enough to cause the fuel ignition.
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- 4 years ago
NO, no and no, first of all you should make a difference between IDI and CDI systems which are totally different.
IDI is what I explain here because old cars use this system.
Capacitor does not hold any energy when points are closed because it is shorted.
When points open capacitor will keep the energy still flowing short while to the capacitor (because it was empty) maintaining the field in the coil primary, THIS prevents arcing on points !
Once the capacitor gets filled no energy flows through it and the primary field collapses and energy surges through the secondary to the spark plugs.
When capacitor is too small, it supresses less arcing on points.
When capacitor is too large it confuses the arc timing holding the energy in the coil too long.
When capacitor is missing, it does not influence the timing but points start arcing because high energy flows throug the points when disconnected. In this sense capacitor is essential for proper usage of the system BUT capacitor is not essential for creating sparks in IDI systems, but only to prevent arcing on points.
CDI-system works different and there the capacitor is essential for operation.
- zephyrLv 41 decade ago
The capacitor in the ignition system holds the electrical charge & stores it until it's ready to be used. You'll find this condenser under the ignition cap next to the contact breaker. The car's battery supplies a 12-volt DC charge which is not enough to ignite the combustion chambers thru the spark plugs, the capacitor (condenser) amplifies this charge to 10,000 volts DC (this strong direct current will give you a jolt if you aren't earthed to the car's body, but won't kill you, used to play this trick on the trainees) to give the plugs enough power to fire off.
- Anonymous5 years ago
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