Rather advanced questions on electrical theory!

dumb_ricer · 10-19-2005

Okay, so we all know how a ignition coil works. You have a primary winding and a secondary winding. The primary winding gets energized with a relatively high amperage and approximately 14volts, and then voltage is cut off, and the magnetic field caused by the current collapses causing current to jump over to the secondary coil, causing HUGE voltage and very low amperage. When this happens, it allows a spark to happen across the spark plug gap. But then you have to figure that the spark plug stays sparked for a given amount of time, and isnt just a tiny flash, otherwise you wouldn't get proper flame propogation acrouss the combustion chamber right? Still with me? good.

Alright, now here is my series of 2 questions.

1. We all know that the primary and secondary windings on the coil are seperate, and that source voltage cannot change. We also know that you cannot create energy, you can only change its form. So for the fact of keeping it simple, say you have 10volts and 10amps in a circuit, and volts times amps equals watts. So you have 100 watts of current flowing through the primary winding. Now you cut off the power, the magnetic field collapses, and in theory lets say you get .1amp at 1000 volts. Keep in mind these are not actual numbers. So you still have the 100amps of power, it has just changed state. Now today in class, I was told that there is NO drop in wattage and that 100 percent of the magnetic field collapsing is transferred over to the secondary winding. I cant see there being a 0 percent drop, and it being an EXACT 1:1 ratio between the primary and the secondary. Are my teachers full of shit and trying to avoid further confusion by saying it is an exact 1:1 ratio, or is there a "parasitic" loss in amperage due to it having to form and collapse a magnetic field, and transfer it all to another wire. It just doesnt make sense that it could possible be a exact 1:1 with no drop in wattage even though they are completely different circuits. What do you guys know/think?

2. Of course we know that the amount of gap and amount of resistence in the area between the electrode of the plug and the ground is going to determine the amount of voltage to complete the curcuit, but what then decides how long it is going to stay sparked? If the magnetic field on the primary collapses instantly, and electricity travels at the speed of light, and the magnetic field collapses instantly, transferring energy, then how can the spark plug possibly spark for any given amount of time, saying 2-3ms?

So what the hell is up and what am I not understanding? Or what are my teachers not understanding?

Matt D. · 10-19-2005

To answer your first question in simple terms, no system is 100% efficient. We don't live in a perfect world, and it's absurd for your teachers to think that way.

For the second question, I think it is probably best answered if you'd ever have the chance to look at an oscilloscope of the timing components and their triggers. Certain events have to occur for the ignition to fire, and they last for a certain amount of time, possibly 2-3ms like you mentioned. From what I always understood about coils, when one side has current running through it the other side is live, so if the ignition event were 2-3ms then that's how long the spark would exist.

**1ViciousGSX** · 10-19-2005

You are making it too complicated.

First off, the primary voltage does not change. It stays at the same voltage it was at prior to firing. The part you are not picking up on is that the only change in your circuit is the ground is being removed. You have a voltage (let's say 14 volts) at the primary + side of the coil and a ground on the - side of the coil. Your circuit at this point is completed. Electrons are flowing from the + side of the electrical system/battery to the - side of the electrical system/battery. Electrons will always take the path of least resistance and that's what they are doing now. This is where the term "short circuit" comes from.

As you open the ground, you've got positive electrons looking for a way to continue over to the - side of the system/battery. They find a path through the secondary windings of the coil. Since there is also a magnetic field present, they build up in voltage until enough energy is created to jump the gap across the spark plug electrodes. The energy created in the magnetic field and secondary windings could also be looked at as a capacitor. Once the spark plug fires, the rush of electrons under high voltage jumps across the gap. But since the secondary windings of the coil have a high resistance to current flow, once the built up energy is released, there is not enough current flow available through the secondary winding to continue to fire the spark, so it dies out.

When looking at CDI (capacitive discharge ignition) systems there are basically two designs out there, which are ussually combined into one unit. One of which uses capacitors to build up and store additional engergy to help push additional current flow through the ignition coil. If I remember correctly, my AEM C2DI can raise the primary voltage as high as 540 volts prior to firing the coil. This additional voltage "push" helps maintain enough current flow through the secondary windings to help jump the spark across the plug gap and maintain it for longer. The second design type uses mulitple spark technology to allow the ignition coil time to "saturate" the secondary windings with electrons and fire again as needed, ussually several times for the same combustion cycle. As rpm goes up, the number of mulitple sparks goes down. My AEM C2DI does this also.

For every combustion cycle this get repeated.

So it really is quite simple.

LightningGSX · 10-25-2005

It seems your idea of wattage is an issue.Wattage is just a measurement of work done or power consumed.Voltage and current will fluctuate according to Ohms law, but the total amount of work being done(watts) will stay the same.Anyway, it is practically impossible to transfer all the energy, there are losses in both the coils' core and the copper winding, as well as back EMF in the primary.As far as time goes, it is related to the amount of resistance in the secondary and plug/wire.The time is L/R where L is the inductance of the secondary circuit.

BTW Vicious, electrons can only be negative and "flow" from negative to positive.

**JET** · 10-26-2005

Like Vicious said, the coils are basically acting like a capacitor. When the energy stored in them is used up, the spark will stop.

**1ViciousGSX** · 10-26-2005

Uh yeah, I said it backwards.

**1ViciousGSX** · 10-27-2005

Um, maybe I did say it correctly the first time. I understand what you are saying, but in reality, if we remove the ground to the coil, and the coil fires across a sparkplug to the ground, then it does seem as if the electrons are jumping from the positive side of the coil to the ground. Which would mean the electrons are flowing from + to -.

Discuss.

LightningGSX · 10-27-2005

Electricity flows from bumping valence electrons from atom to atom.Its kinda hard to visualize, but electrons have a negative charge.So essentially they are transfering lower potential from - to +, and electricity flows from higher to lower potential, so electrons move from - to +, but the charge flows from + to -.A while back, I suspect car makers had the same confusion, since at one time vehicles had a "positive grounding" scheme.

On a side note, waste spark systems like ours, the secondary technically isn't grounded.It fires from coil to sparkplug to head to sparkplug and back to the coil.Thats why grounding the head isn't critical as it is in some vehicles.

LightningGSX · 10-27-2005

BTW , the back EMF is the reason old point ignition systems needed a capacitor(condenser) to prevent a spark jumping the points.