Ignition basics

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mikenixon
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Ignition basics

#1

Post by mikenixon »

Image

Above is an oscilloscope trace of an ignition event, taken as was once customary on its secondary (spark plug) side. The waveform's timeline is measured in milliseconds (thousandths of a second). At reference A the points have opened resulting in the ignition coil's primary winding powering down, making its magnetic field instantly collapse. This collapse mutually induces voltage in the coil's secondary winding, whose greater wire turns step up the voltage several hundred times, and the plug accumulates a head of bristling, aggressive voltage potential -- you can see this in the steep vertical line running from A to B. Raw voltage. In just a few thousandths of a second then, there's enough voltage to bridge the plug's gap, and spark happens, reference C.

This spark is really a wondrous thing to consider! The normally non-conducting air molecules inside the plug gap are super agitated by the electrical strain at the end of the plug. That 0.028" thick piece of of air literally becomes so bombarded, so overwhelmed, with Tesla coil like energy, it actually defies nature and becomes conductive. This is called "ionization", and it takes a lot of voltage to make this happen: around 5,000 volts under starting conditions. Now, let's focus on reference C again and consider four interesting things. First, notice that the spark's voltage is considerably lower than the peak voltage on the rise line. This shows it takes less voltage to feed the spark than it does to start it. Interesting. Second, also interesting is the spark's duration. Note that it lingers, on most ignitions several milliseconds. This is an important part of ignition, this duration, as it helps get combustion off to a good start. Third, did you catch that the spark's voltage decreases as the coil's energy is spent? That's electrical pressure dropping, like air rushing out of a punctured tire. And fourth, observe that the spark has a very sawtoothed look. It's jangly. This "electrical noise", as the engineers call it, actually produces radio waves that must be damped out by the resistor caps on the ends if the plug wires. That's why those caps are there.

Now look at reference D. Spark has stopped and look at what happens next. The voltage shoots upward again. Huh? Yes, with the plug no longer arcing, the ignition system suddenly doesn't have a load. So voltage surges. However, at point E we see that though the coil's voltage rises slightly, it isn't enough to ionize the plug gap again, so it immediately falls to zero. And even below zero, notice. But the coil isn't done yet. The secondary's voltage freefall cross-induces the primary, which reinduces the secondary, and the two windings go ping-pong, ping-pong for a considerable fraction of a second, until finally the coil is completely spent, noticeable at reference F. And even then the system isn't finished. One more thing is visible at point G, and that is that little blip on the zero line, a tiny backlash (negative, notice, because we're reading all of this on the secondary) when the points close in preparation for the next spark event. And just as with the coil, this blip also oscillates a little. After that, it's all over, that's the whole ignition event, completed in about a third of a second.

Ignition scope traces such as this one were used for diagnostics by mechanics for close to a hundred years. There's a lot there for those who know what they're looking at. For example, a scope might show all the engine's plugs in a row, and any that exhibit lower ionization lines than the others would indicate cylinders with lower compression. Or you might see a points closed blip whose first line is not the lowest, indicating dirty points. And there are many more of these tricks. Of course, scope traces are anachronistic now, what with engine scanners and other tools. But they nonetheless provide an interesting window into ignition function, and thus are useful for educational purposes.
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Re: Ignition basics

#2

Post by 05c50 »

Ahh, the days of "scopes". I was a tech (we were mechanics back then) at a new car dealership during the time of the electronic ignition introduction. Back then, tuneups were a very common item. I would always drag the SunScope (it was about the same size as the car) over to my bay and hook it up when I did a tuneup and the other guys would tell me that I didn't need it for a tuneup. Over the years I found that I could recognize a normal pattern look and easily diagnose a faulty plug, wire, points, etc. An intermittant miss caused by a fouled plug made line C more jagged and drop off at a steeper angle, a bad plug wire would make line D shoot up off the screen and if the point gap was off, G just didn't have the right wriggle. When electronic ignition came out, the patterns changed and weren't quite as meaningful.
Thanks for the memories.

........Paul
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Re: Ignition basics

#3

Post by mikenixon »

Yes, Paul. Same experience here. I bought my own scope and though I never used it to diagnose, it was fascinating to compare different ignitions, sketching out their secondary patterns in a little notebook. When Honda put CDI on their streetbikes (CB/CM 400) I found the waveform so much cleaner and abrupt it was unmistable but at the same time being almost just a horizontal line much less usable in a troubleshooting scenario. Honda's electronic ignitions on the other hand weren't so different from Kettering. Still pretty wiggly. A little tighter but still very readable. Later I taught using scope traces as material but it was pretty esoteric. And yes, I too remember those old monster roll-around "analyzers". Never used one professionally but had a Sun MEA available to use in teaching.
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Re: Ignition basics

#4

Post by Rednaxs60 »

Good article. I remember my Dad's equipment, never learned how to use it but it was interesting watching.

I now have a requirement for an oscilloscope to test TPS units for my '85 LTD. Found out the hard way that even though an aftermarket, new TPS modified to fit onto the engine of my '85 LTD should be good; however, this is not always the case. Had installed a new aftermarket TPS and had a hard misfire - ignition timing off at the lower RPMs - was not fun. Changed to another new aftermarket TPS and all was/is well. If a TPS is faulty or starting to fail, I have found it to be in the lower RPM range.

Found a web site that had oscilloscope pictures of the voltage curve of a good TPS unit and TPS units that were faulty. The good TPS had a smooth curve throughout the operating range. A faulty/failing TPS had a lot of "chatter" at the lower to mid voltage range. Definitely an eye opener. The other interesting fact is that a faulty TPS may be just enough in spec that no error codes will be generated by the ECU - my case as well. Makes you chase other issues that are similar but not related.

Looking for a used inexpensive oscilloscope, but have not found one yet.

Cheers
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Re: Ignition basics

#5

Post by mikenixon »

Rednaxs60 wrote:Good article. I remember my Dad's equipment, never learned how to use it but it was interesting watching.

I now have a requirement for an oscilloscope to test TPS units for my '85 LTD. Found out the hard way that even though an aftermarket, new TPS modified to fit onto the engine of my '85 LTD should be good; however, this is not always the case. Had installed a new aftermarket TPS and had a hard misfire - ignition timing off at the lower RPMs - was not fun. Changed to another new aftermarket TPS and all was/is well. If a TPS is faulty or starting to fail, I have found it to be in the lower RPM range.

Found a web site that had oscilloscope pictures of the voltage curve of a good TPS unit and TPS units that were faulty. The good TPS had a smooth curve throughout the operating range. A faulty/failing TPS had a lot of "chatter" at the lower to mid voltage range. Definitely an eye opener. The other interesting fact is that a faulty TPS may be just enough in spec that no error codes will be generated by the ECU - my case as well. Makes you chase other issues that are similar but not related.

Looking for a used inexpensive oscilloscope, but have not found one yet.

Cheers
Interesting. I discovered while with corporate Kawasaki that TPS is where OEMs scrimp. In fact, they are typically so poorly made that even the type of charging voltage regulators commonly used can prematurely wear them out. They're usually milar film you know, not wound wire, so they wear quickly. And some models of vehicles actually have TPS that are redundant, wired in parallel back to back, so when one fails the other is there to automatically take over. Kawasaki's 620 utility vehicles were that way. So, yeah, I know about crappy TPS.
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Re: Ignition basics

#6

Post by mikenixon »

mikenixon wrote:
Rednaxs60 wrote:Good article. I remember my Dad's equipment, never learned how to use it but it was interesting watching.

I now have a requirement for an oscilloscope to test TPS units for my '85 LTD. Found out the hard way that even though an aftermarket, new TPS modified to fit onto the engine of my '85 LTD should be good; however, this is not always the case. Had installed a new aftermarket TPS and had a hard misfire - ignition timing off at the lower RPMs - was not fun. Changed to another new aftermarket TPS and all was/is well. If a TPS is faulty or starting to fail, I have found it to be in the lower RPM range.

Found a web site that had oscilloscope pictures of the voltage curve of a good TPS unit and TPS units that were faulty. The good TPS had a smooth curve throughout the operating range. A faulty/failing TPS had a lot of "chatter" at the lower to mid voltage range. Definitely an eye opener. The other interesting fact is that a faulty TPS may be just enough in spec that no error codes will be generated by the ECU - my case as well. Makes you chase other issues that are similar but not related.

Looking for a used inexpensive oscilloscope, but have not found one yet.

Cheers
Interesting. I discovered while with corporate Kawasaki that TPS is where OEMs scrimp. In fact, they are typically so poorly made that even the type of charging voltage regulators commonly used can prematurely wear them out. They're usually mylar film you know, not wound wire, so they wear quickly. And some models of vehicles actually have TPS that are redundant, wired in parallel back to back, so when one fails the other is there to automatically take over. Kawasaki's 620 utility vehicles were that way. So, yeah, I know about crappy TPS.
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Re: Ignition basics

#7

Post by Rednaxs60 »

Thanks for the additional info. Have to agree that the quality of a TPS nowadays regardless of where it is from is hit and miss as to performance. This is the web site that I found that shows oscilloscope traces for good and faulty TPS units: http://autoditex.com/page/throttle-posi ... -22-1.html

Not having an oscilloscope, I have to use the tried and true method of repair by replacement and hopefully stumble onto a good one. The other issue I have is space is at a premium with the external alt mod installed. I have found that there is a lot of plastic on these units that can be cut away without affecting the internal rheostat to expose the connectors, and give more room.

Installed a TPS yesterday because I had a low speed stumble/hesitation that has been a small irritant, but not a show stopper. These small issues do become worse over time so I did the TPS change. I noticed the slow speed stumble/hesitation was gone, but did notice another RPM point, approximately 3000 RPM that did not feel quite right. It's more a feeling than an actual engine issue so I will continue my research and try to find a more appropriate TPS unit.

Just thought I'd share a few thoughts that are somewhat related, but not quite.

Cheers
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Re: Ignition basics

#8

Post by mikenixon »

Red... Don't forget that you can incrementally rotate that TPS to altercthe mixture curve.
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Re: Ignition basics

#9

Post by Rednaxs60 »

Mike - know about this. Spec is for the TPS voltage to be set with a 0.110 spacer (the one I use is 0.112" - close enough) under the throttle linkage to 0.475 to 0.479 VDC - not a lot of wiggle room. The lower the voltage the less the pulse width for injector opening when first started. I have heard of those who have gone below the 0.475 VDC level, not by much, but enough to get a lesser pulse width and fuel smell on start has been lessened. I set mine at the 0.475/0.476 VDC level.

Been perusing what is available, not a lot of choice for the space limitations.

Cheers
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Re: Ignition basics

#10

Post by mikenixon »

Gotcha.
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