I have worked with model t's for several years and until a couple of years ago had assumed the mag output waveform to be a sine wave. According to scope patterns and written comments on the forum by persons that I have developed great respect for the output is more a triangular waveform. This question has bugged me for sometime, why is the mag output not pure sine? I hope I am not asking a dumb question. Lenney
The nature of the Model T magneto waveform, when viewed on an oscilloscope, depends upon the RPM the engine is running and the physical shape of magnets and shape of the individual field winding's and their relationship to each other. The shape of these parts changed several times over the years of Model T production.
At lower running RPM it clearly has a saw tooth shape, but at higher RPM it is a continuous sine wave waveform.
On a somewhat related question, when a Model T is running on a properly set up mag, and you move the advance lever through its range from full retard to full advance, does the timing switch between 3 or 4 fixed settings? Or is it continuous like a Model A?
Like Lenny, I have read a lot of what appears here and elsewhere and I'm not clear about this. Some have described moving the lever and at certain points the engine starts to run much much better in an almost switch-like fashion. And no, I am not talking about switching from battery to mag.
Ron, thank you very much. In my elementary way of thinking I had come to the conclusion that it is the relationship of magnet pole size versus stator pole size. I am pleased to see that I wasn't totally wrong. You are an asset to this hobby. Lenney
Gary. Ron Patterson posted an excellent article dealing with this question somewhere on the 2016 forum. It is very thorough and easy to understand. Lenney
To answer your question briefly: on magneto the movement of the spark lever will provide three, or possibly four, distinct timing "areas". Think of them as climbing a staircase, with different levels of timing and not a continuous change as with a Model A.
But, if you are running on battery and coils & timer the timing advance is gradual, kinda like a Model A.
Ron's very good article:
And another of his:
Hope this helps. Bill
When running on battery, the timing is advanced with every notch the rod is moved from full retard to full advance. When running on magneto the timing is advanced according to the position of the magnets in relation to the magneto coils. So it advances as you move the rod but only when the timer passes the point where the current comes from the next pole on the magneto. However it also advances to a certain degree with engine speed even when the rod is not moved.
I have never seen a T magneto waveform that was shaped anything like a saw tooth. I have also never seen the waveform change significantly with speed.
Here is a view of a waveform @ 500 rpm
Here is a view of a waveform @ 2000 rpm
Here are the two superimposed. I have changed the time scale so that the two waveforms are the same basic size.
As you can see, there is no significant difference in the shape of the two waveforms.
These were cropped out of the following two traces.
So, Lenney, to answer your really good question:
The reason why the T puts out the odd shaped waveform that it does is because of harmonic distortion. The magnets interact with the field coil in a manner that produces a series of harmonics at different amplitudes. The strongest of these harmonics is an in-phase third-order harmonic that is attenuated approximately 15 db from the fundamental.
If you wanted to see this in action, you could use a program such as Audacity. Combine a sine wave of say 60 hz with a sine wave of 180 hz (down 15 db) and the resultant wave would look much like a T mag waveform.
I wonder whether Henry and his engineers really understood all this.
As Ron said: “…the physical shape of magnets and shape of the individual field winding's and their relationship to each other…”. And as Tom said “…The magnets interact with the field coil …”. Is what produces the shape of the waveforms. And the waveforms, as Fourier showed can be described as a series of sine waves. These sine waves are commonly called a Fourier series today, but with all due respect that is not harmonic distortion.
Gilbert, I believe that they did. They weren't flying as blind as we may think nowadays. Plus, they were really smart. I don't know if Ford had access to them, but oscilloscopes of various forms were available in the late 1800's. Even cathode-ray tubes were around before the turn of the century. Even earlier than that they had electro-mechanical graphing devices. If anyone is interested I can explain how they worked.
Sam, it is harmonic distortion. It is mainly third order harmonic distortion as a result of saturation of the field coil core. Nearly all electricity generating devices with an iron core experience this. Even transformers can experience it. If you don't believe me, try Googling it.
Sam, you can read all about it here.
When people ask simple questions about extremely complex issues and their technical interactions I find it better to offer simple answers that a layman can understand. This is why we co-authored the articles cited above.
Others will try to divert your attention with esoteric technical jargon few fully understand and buttress the arguments with fancy diagrams even fewer understand; trying to convince you they are correct. Understanding these complex issues is not about what you a told, but rather about what you are NOT told! They are entertaining, but will never get you to fully understand all the interactions of the complex technical matter.
This tactic has been used very effectively used on this Forum to frame other technical issues.
In the past these tactics were called a con, but today it’s called fake news: to get you to believe what they think you should believe.
Ron, I can agree to that. Thanks.
Ron, just because some may not get it, others may. I certainly don't see how giving them wrong information as you did helps them to understand. It wasn't simplified, it was wrong.
I think there is room for all levels of discussion on this forum.
Things you can't unsee.
Tom, you said:
"I have never seen a T magneto waveform that was shaped anything like a saw tooth."
Did you mean
"....anything BUT a saw tooth"?
Your excellent waveforms (and thank you for them) look like sawtooth to me!
I guess that the advantage of the sharp peak over a sine wave is that the actual timing - the point at which the coil triggers - is much more precise.
If you have a good magneto and timer with a properly set up coil my thinking is that the coil should fully magnetize and open it's contact before the half cycle peaks. This would give an automatic advance within the half cycle as motor speed changes. I am impressed with the simplicity yet complexities of the lowly model T ford. Lenney
Ron I read and made copies of your two articles, great for my own understanding and I will pass them on when others ask, how it works. Excellent
As a practical matter here is everything any Model T Ford owner needs to know about the magneto.
The magneto is a sub-system of the ignition system.
The magneto is an alternating current generator. It produces current pulses every 22.5 degrees of crankshaft rotation regardless of engine speed. The frequency and voltage of the current pulses are controlled by engine speed. The current pulses produced have sufficient power to operate the ignition trembler coil. The magneto output is not a pure sinusoidal waveform; its shape is controlled by the physical design and positioning of the magnets, field winding and gap. Fundamentally this design remained unchanged over the period of Model T production, with minor changes in component shape and size affecting the exact nature of the magneto waveform shape and its power capability.
Anything further would only interest design engineers.
For more information, including a description of all the other ignition system sub-systems, and their relationship to the magneto and a discussion about exactly how spark timing is controlled see our article entitled “The Model T Ford Ignition System and Spark Timing” cited above.
I'm scared to post. All I try to do is help people by saying what I think is the truth - with nice pictures and links to back up what I say and I get jumped on. Oh well, I have big shoulders. I will soldier on.
No, Chris, I said what I meant. A sawtooth wave is asymmetrical. It rises gradually, then falls sharply. Google "sawtooth wave" to see what I mean. The T wave is largely symmetrical, therefore nothing like a sawtooth in my opinion.
Lenney, I have never seen a model T coil that was set up correctly or incorrectly fire before the voltage peak and I have looked at hundreds and hundreds of waveforms. In my observation, there is no appreciable advance in timing as the speed of the T engine changes when running on mag. One might think that the coil would fire sooner as the voltage rises faster at higher speeds, but this is largely offset by the current phase-shift which increases as the frequency of the mag wave increases. Less phase-shift at low r.p.m. and more at high r.p.m. I could show you hundreds of waveforms that demonstrate this, but I won't, as 'scope traces seem to upset some folks delicate sensibilities for some reason.
I, and others I know are not design engineers and find this stuff interesting. Some folks are happy just to know that it works, others like to know more. There is room for all of us here. If you don't like technical posts, don't read 'em. If you do like them, but don't understand something, ask questions. I find it strange that people try to shout down or otherwise dissuade others that are having fun, pursuing knowledge and not hurting anyone.
Thank you Tom
I can honestly say I am learning from all who have contributed to this post even if there is disagreement on some details or aspects of the answer to the question. To be sure the points of disagreement are a bit beyond my understanding of electrical theory but that just means I've got more to learn.
I especially appreciate your mention of the availability of oscilloscopes in the early days of the Ford Motor Co as I was also wondering just how well understood a lot of this was at the time. Saved me from having to do the google search for that question.
I'm essentially a lay person when it comes to the T ignition. In that sense, it is adequate to know that the mag generates AC to power the coils. Fine. At the same time, I find generally interesting and amusing some of the details like the shape of the wave form where I, in my basic electricity understanding, would have expected a classic sine wave. Does it really matter in my world? No, but it is interesting just the same. For others who choose want more detail, it may be enlightening to see the scope traces. I can't see any harm in hearing more than I need.
I think the articles referenced in the links above were very well done and provide an excellent explanation of how the system works and kudos to Ron and others who contributed to them. An article in the Vintage Ford needs to provide a concise explanation that by virtue of space and general interest can't delve into all of the details of wave forms etc. One of the nice things about the forum is that there is an opportunity to dive below the surface for those those who have an interest. I appreciate the efforts of Tom and others in providing some of that. OK, let's all have a nice day and enjoy our T's.
Now I understand better how the Etimer run off a battery, not the magneto is capable of timing control which relates to more pulling power etc...