Automobile Ignition

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Bob Scott
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Automobile Ignition

Post by Bob Scott » Tue Oct 06, 2009 8:45 am

Since I owned my first used car, which I bought for $200 when I was young and poor, I've done my own engine repair due to lack of finances to hire a wrench-head to do it for me. I noticed that the ignition timing was modulated by two systems, "vacuum advance" and "mechanical advance". I got by for years not figuring out that the vacuum advance makes the ignition more advanced during idle, not high RPM. ?? If it's the other way around, it should be called "vacuum retard".

Also, why is that capacitor (condenser) on the points contacts? I got by thinking that it absorbed spark energy across the points. If you leave out the capacitor, the point arc like crazy and the engine won't run well. Now I'm thinking that the capacitor/coil form a resonant tuned circuit at a low enough frequency to give the points a chance to separate without arcing. The cap slightly delays the inductive discharge. The same system is used in horizontal output circuits in television where the H output is tuned to a frequency that is 7 to 10 times the H frequency.

So if the car capacitor/coil resonance is tuned to a specific frequency, there is a specific time delay for voltage to build up after the points separate, voltage high enough to make the ignition spark. A spark delay is not good for engines at high RPM. That's what I think the variable centrifugal mechanical advance is for. I think that it cancels out the tuned circuit delay. Is anyone familiar with this concept?

I will go see Wiki.
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reloadron
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Re: Automobile Ignition

Post by reloadron » Tue Oct 06, 2009 11:31 am

Been awhile since I replaced the points, capacitor and plugs in a small block GM but the memories are still there. :smile:

I believe the vacuum advance does just that, it advances the timing and can do it much quicker on demand than the centrifical mechanical advance. Eventually the centrifical will catch up to where the vacuum advance is in timing. This is a pretty good read on the subject.

As to the capacitor. I believe the capacitor served two purposes. It did surpress the arc flash of the points but I think it also mainly allowed current to continue to flow once the points actually broke open to allow the flux lines ( magnetic field) of the coil to collapse quicker making for a better higher voltage spark. For some reason I tend to recall something like that and I may be way off base.

Ron

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MrAl
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Re: Automobile Ignition

Post by MrAl » Tue Oct 06, 2009 11:59 am

Hi,

Ron, that's something to think about, but capacitors dont actually absorb energy they store it, so what would happen
is the cap would take the energy and then give it back to the coil and then back to the cap and back to coil etc.,
until something like the resistance of the coil and esr of the cap dissipated all the energy. This produces a set of
humps in the voltage characteristic so maybe the second or third hump isnt as large as the first and that helps
break the arc? I would think a lot of energy is dissipated in the arc that forms in the spark plug.
I've never looked at this in detail either though. We would have to look at where all the energy goes once the
contacts start to open. I might be able to simulate this on the computer.

A LITTLE LATER:

Interesting, it looks as though the capacitor may actually be used to suppress a second high voltage pulse
that can occur when the points open. This sounds strange though, because this assumes that the high
voltage pulse that gets to the spark plug actually occurs when the points CLOSE. This would contradict
using the coil in the boost mode, which i thought was the case in automobiles, so this probably isnt right.

I'll do a couple more experiments, but the real problem is that i dont know the value of the capacitor
used nor it's series resistance. I also dont know the typical coil inductance nor it's series resistance either.
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Bob Scott
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Re: Automobile Ignition

Post by Bob Scott » Tue Oct 06, 2009 12:33 pm

reloadron wrote:This is a pretty good read on the subject.
NAB! Thanks Ron. That tells me two things. The vacuum advance advances the timing at idle, not at WOT, and now I know that when I changed out the worn-out cam in my 79 Impala wagon with a mild racing cam, I should have changed the vacuum advance to match the lower available manifold vacuum at idle. NOW he tells me! Too late now by 2 decades. :smile:

Those were the days! You could change out the cam in a Chev on a Saturday afternoon, standing in the engine compartment (after you take out the radiator) and be finished by Dinner. Nowadays the new Chevs don't have enough room for a whole hand to fit under the hood, the cam goes in sideways, and you have to remove the motor mounts to change a fan belt.
As to the capacitor. I believe the capacitor served two purposes. It did surpress the arc flash of the points but I think it also mainly allowed current to continue to flow once the points actually broke open....
I agree.
......to allow the flux lines ( magnetic field) of the coil to collapse quicker making for a better higher voltage spark. For some reason I tend to recall something like that and I may be way off base.
I believe the current is kept flowing in the capacitor, delaying the magnetic field collapse, in order to give the points a chance to separate before the collapse generates the HV.
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reloadron
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Re: Automobile Ignition

Post by reloadron » Tue Oct 06, 2009 2:51 pm

MrAl in part:
Interesting, it looks as though the capacitor may actually be used to suppress a second high voltage pulse
that can occur when the points open. This sounds strange though, because this assumes that the high
voltage pulse that gets to the spark plug actually occurs when the points CLOSE. This would contradict
using the coil in the boost mode, which i thought was the case in automobiles, so this probably isnt right.

I'll do a couple more experiments, but the real problem is that i dont know the value of the capacitor
used nor it's series resistance. I also dont know the typical coil inductance nor it's series resistance either.
Before I forget the cap values I think (no shortage of I Think) were in the .2 to .33 uF and they were rated at maybe 650 VDC? I don't know why that pops into my head.

I was a kid who grew up in the 50s to early 60s. My earliest experience with high voltage and a spark plug was when I grabbed the spark plug on an old Reo engine lawn mower. Curiosity on such things damn near did kill me. I just had this natural curiosity for engines and ignition systems. My dad was an EE and a ham radio operator who encouraged my interest. I began to mess with automotive ignition coils and made a few discoveries.

I collected old automotive batteries from gas stations that had a bad cell or two but were suitable for my experiments. I discovered that when I connected a coil and with ground connected touched the positive wire to the battery nothing really happened, maybe a tiny spark. However, as I moved the positive wire off the battery terminal it drew an arc. That was when the plug fired. I later understood with my dad's help that the field was collapsing and that was what generated the arc I saw. That was also what induced a high voltage in my coil secondary. My spark plug would fire. The plug only fired when the current path was broken.

Years ago I did a cool experiment based on something I read in PE (The old Popular Electronics) that I grew up with. I found an old automotive ignition coil. Old as in prior to the High Energy Discharge or whatever they call it today. PE had a circuit I knew looked familiar and it was little more than a everyday Lowes light dimmer. I drove the coil using a light dimmer. I found a clear glass very large 120 VAC light bulb and connected the coil common to the base (both leads) and using aluminum foil made a cap for the bulb. The high voltage went to the cap. Instant lightning storm inside the bulb. That was pretty cool.

However, and totally off topic. My best memories were of the early car radios. Early 50s car batteries were 6 Volts for 6 Volt systems. Then came the 12 Volt systems. The radios were all tube type radios. So where di they get the B+ (remember B+)? THey used a vibrator to produce a square wave a very ugly square wave at that to drive a transformer which kicked up the voltage to 250 volts. Then we sent that to a tube, a rectifier tube which gave us the DC plate voltage for the tubes in our radio. The old 5Y3 dual diode could deliver 350 plus mA per plate. As a kid I found that stuff incredible. :shock:

I loved rebuilding engines especially the Chevy 327 (originally 265 then 283) blocks and the Pontiac 326 and 389 blocks. Today I "farm it out". :smile:

Ron

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kheston
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Re: Automobile Ignition

Post by kheston » Tue Oct 06, 2009 4:12 pm

Bob,

When I was a kid, I figured out what the capacitor was for the hard way...by assuming it wasn't required and leaving it out. I believe it is there to preserve the contact points, that is, the size of the spark between the contact points when they open is greatly decreased with a condenser (capacitor) in-line. Smaller sparks fry the points more slowly. Had I known this, I wouldn't have stranded myself half-way around the block on that particular test drive.

As for timing advance, well, I think we have smog regulations to blame for the confusion. Nowadays, timing is advanced at idle and at high rpm in most cars (electronically on late-model stuff). You see, idling vehicles burn fuel very inefficiently and release lots of hydrocarbons. The timing advance at idle is aimed at reducing the unburned fuel that leaves your tailpipe. This is different from my uncle's '68 Ford 1/2-ton pickemuptruck, the vacuum line is connected to the carburetor at a point up near the venturi, where vacuum is high when more air flows (at higher rpm). In that old Ford, the timing advances when you'd think it should: at times when the engine is buzzing faster and the combustion needs to start sooner to complete the burn by the time the piston is traveling back downward.

By the way, there were springs in those old distributors that used centrifugal force to advance the timing a few degrees before the vacuum advance kicked in as well.

HTH
Kurt - SF Bay

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