Spark duration

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Model T Ford Forum: Forum 2016: Spark duration
Top of pagePrevious messageNext messageBottom of page Link to this message  By Bob Shirley on Monday, April 04, 2016 - 09:49 am:

Why dose the timer have such a long contact duration? Wouldn't the coil points last much longer if duration of the contact on the timer was reduced to a fraction of time spent in contact?


Top of pagePrevious messageNext messageBottom of page Link to this message  By John F. Regan on Monday, April 04, 2016 - 10:15 am:

The answer is yes it would reduce the wear and tear on the points and also reduce the current drain from the battery if running on battery alone. Ford did not have the benefit a scopes or other equipment that would allow them to actually see what was going on. I restored a Connecticut Electric timer on a 1908 Knox by making a new "donut" for the inside of the round roller type timer that was on that car. I made the contacts much smaller to reduce the dwell and it reduced the drain of the coil box but this car ran only on battery. The contact has to have more dwell for the instance when the lever is being moved to advance the timing simply because at some point it is possible that a few of the cylinders are firing on one of the magneto pulses while the other is firing on a different one. This is a very narrow window in real time but you need for the timer contact to span at least 2 full pulses from the magneto so that as you advance the timing you get what seems to be a smooth transition from one magneto timing pulse to the next one. Typically the dwell of the timer will encompass 3 or 4 timer pulses from the magneto. If running only battery then the dwell time can be very short compared to what Ford timers typically have. I have never actually experimented on a Ford magneto as to how narrow you can set it. On paper it is probably easy to figure out but one would need to have some extra dwell in there to compensate for small timing variables of timer/coils/linkage...etc.

As a side note I found some data in the archives were Ford tested some new timers for use on their cars in production and rejected those timers for NOT having enough dwell even though it was almost identical to what Ford had. It kind of proved to me that Ford was a bit fuzzy on how their own system actually worked. They were also probably very leery of changing anything at all due to the troubles they had with early coil boxes. Once they had a good system with the KW point design they didn't want to go back to the troubles they previously had so they very likely went into a mode of "DON'T CHANGE ANYTHING!"


Top of pagePrevious messageNext messageBottom of page Link to this message  By Mike Kossor - Kenilworth, NJ on Monday, April 04, 2016 - 10:22 am:

The timer contact duration must be long enough to ensure the coil fully charges operating at the fastest engine speed and minimum operating voltage.

A typical Model T coil requires about 0.004s to fully charge operating on 6VDC battery. Assuming a top engine speed of 2500 RPM that equates to 6x2500 = 15000 crank shaft degrees per second or 7500 CAM shaft degrees per second. Therefore, the CAM rotates 30 degrees in 0.004s at 7500 RPM (0.004 sec x 7500 degrees/sec). The timer segment must remain on contact for at least 30 degrees of rotation or performance may be limited due to insufficient coil/spark energy. Also note the this analysis assumes the timer segment grounding is perfect; no roller/brush/flapper bounce.

The coil will charge up faster as the operating voltage increases such as operating on 12VDC battery (minimum timer contact reduces to about half) or operating on properly functioning magneto.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Trent Boggess on Monday, April 04, 2016 - 10:47 am:

The duration of contact between the roller and the timer is 90 degrees per segment, 4 segments in all. When a Model T is running on its magneto with properly set up coils, the coil fires every 22.5 degrees of crankshaft rotation. So the coil to a particular cylinder send could a spark to that spark plug 4 times while the roller is in contact with a cylinder's timer contact segment. This is the way the system was designed to work.

It is not completely clear why the Ford engineers felt that the 4 possible firing points while the roller was in contact with a particular segment was necessary, as only the first of the four possible sparks will be at the correct point in crankshaft rotation and piston travel. The other three sparks will fire 22.5 degrees later than the one before it, meaning those sparks will occur at retarded points which are far from optimum. Moreover, if the first spark sets off the charge in the cylinder as intended, the following three sparks are completely redundant, and contribute nothing to the running of the motor.

There is another less obvious point: When the car is running on magneto, the coil will not necessarily fire the moment the roller comes in contact with a segment in the timer. The reason for this is that coils are set up to fire at the point where the current has reached a maximum as the 16 magnets pass over the 16 field coils. If the magneto is not at peak current when the roller touches a timer segment, a spark will not occur until the current reaches its peak in the next cycle. This could be as much as 22 degrees of crankshaft rotation later.

When a Model T is running on its magneto with properly adjusted coils, there are actually only two useable timing points in terms of crankshaft rotation. One of these is at about 4-1/2 degrees AFTER top dead center on a cylinder's compression stroke. This is the spark retarded position for starting the motor on the magneto.

The next advanced point of highest current is at 22-1/2 degrees of crankshaft rotation earlier, or about 18 degrees BEFORE top dead center on a cylinder's compression stroke. This position seems provides a good compromise for both low end performance and high end performance. Not the perfect point for high speed running, but useable.

The third opportunity for the coil to fire occurs another 22-1/2 degrees of crankshaft rotation earlier, or at 40-1/2 degrees BEFORE top dead center. Given the design of a Model Ts motor, this position occurs way to far in advance to provide good performance at high speed.

When all is said and done, there are only two (2) useable points for ignition when pulling down on the spark lever. The first occurs at, as stated, 4-1/2 degrees after top dead center on the compression stroke, and is a safe point to crank start the engine with little chance of a back fire. The second point occurs when the spark lever has been pulled down enough to allow the coil to fire at 18 degrees before top dead center on the compression stroke, and provides a good compromise between low speed and high speed performance. If you continue to pull down on the spark lever so that the coil fires at 40-1/2 degrees before top dead center on the compression stroke, this is too far advanced to be useful, and the motor will not run better. Often times this is felt as a hesitation or reduction of performance of the motor when the spark lever is pulled to far down. Any other movement of the spark lever is in dead space, and no change in engine performance occurs. The notion that small adjustments of the spark lever while the car is running on magneto is a myth and does not change the performance of the motor.

Ignition timing when the car is running on magneto, is one of the great compromises in Model T motor design. It was perfect in an era of first time car owners who did not understand the fine points of ignition. They could get good performance when the spark lever was pulled all the way up for starting, and acceptable performance when the lever was pulled down far enough to advance the ignition to 18 degrees. At that point there is no useful purpose in playing with the spark lever and all adjustments of the speed of the car are obtained by moving the throttle lever. It was simple and anybody could do it.

Caveats: This discussion assumes the car is in good running condition with a good magneto and properly adjusted coils. If the car is running on battery, or uses some other ignition system than the magneto and four coils, your results will will be different.

Respectfully submitted,

Trent Boggess


Top of pagePrevious messageNext messageBottom of page Link to this message  By Royce in Dallas TX on Monday, April 04, 2016 - 10:50 am:

Trent,

Well said.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Jerry VanOoteghem - SE Michigan on Monday, April 04, 2016 - 10:53 am:

Trent,

Do you mean to say 45 degrees per segment? If contact between roller & timer is 90 deg. per segment then there is no "off" time.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Trent Boggess on Monday, April 04, 2016 - 11:04 am:

Jerry,

If in terms of camshaft rotation, it would be 45 degrees per segment. However, in my post I was referring to crankshaft rotation, making it 90 degrees of crankshaft rotation.

April fool's day was three days ago. I KNOW you know this!

Respectfully submitted,

Trent Boggess


Top of pagePrevious messageNext messageBottom of page Link to this message  By Jerry VanOoteghem - SE Michigan on Monday, April 04, 2016 - 11:15 am:

Trent,

Thanks for the clarification. I kinda thought that's what you meant, as I know YOU knew this too. ;o)


Top of pagePrevious messageNext messageBottom of page Link to this message  By Ken Todd, ............Red Deer, Alberta on Monday, April 04, 2016 - 11:26 am:

"the CAM rotates 30 degrees in 0.004s at 7500 RPM"

I never thought a T engine that would run that fast.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Mike Kossor - Kenilworth, NJ on Monday, April 04, 2016 - 11:31 am:

Trent, the next magneto cycle begins 18 degrees BTDC but the coil does NOT fire at 18 degrees BTDC. The magneto voltage must build up sufficiently to permit sufficient coil current flow for the coil to pull open the points and generate spark. This all takes time so the actual piston position will be further advanced. A truly elegant feature of the Ford Model T low tension magneto ignition is its automatic timing advance; as engine speed increases, the magneto output voltage increases and the coil ramp time to fire decreases, keeping combustion timing relatively constant with piston position. A similar thing occurs with the position of the spark lever. Advancing the spark lever has no effect because it delays when the coil begins charging the coil but the magneto voltage also rises to a higher value by that time so the coil ramp time to fire is also faster with no net change in coil firing time.

(Message edited by mkossor on April 04, 2016)


Top of pagePrevious messageNext messageBottom of page Link to this message  By Mike Kossor - Kenilworth, NJ on Monday, April 04, 2016 - 11:34 am:

That should have been 2500 RPM (7500 CAM shaft degrees/second). Just wanted to see if anyone was actually following the analysis :-)

(Message edited by mkossor on April 04, 2016)


Top of pagePrevious messageNext messageBottom of page Link to this message  By Trent Boggess on Monday, April 04, 2016 - 12:58 pm:

Mike,

Mike,

You are correct. Model T coils, being electro-mechanical devices, do not fire instantaneously once the maximum current is reached. It takes a positive amount of time for the primary windings to generate enough magnetism in the iron core to saturate both the primary and secondary windings, and to pull the vibrator down far enough for the points to break, interrupting the primary windings, causing the magnetic field to collapse through the secondary windings and inducting the high tension spark to the plugs. The time it takes for this process to occur is somewhat variable (in milliseconds) and depends on the engine speed and current from the magneto. The time period to produce a spark gets shorter the greater the engine speed. This is what gives the coils something of a self advancing property.

For a magneto ignition system in good condition and properly adjusted coils, that process will begin at either 4-1/2 degrees after top dead center, or 18 degrees before top dead center of crankshaft rotation, depending on where the spark lever is at.

For purposes of explaining clearly what to most people a little known property of Model T magneto ignition systems, I chose to not bring the time delay between when the coil begins the process of firing and when it actually fires into the discussion. When writing on the Forum or teaching college students, it is best to start with a simple explanation before advancing to the more detailed explanation.

Respectfully submitted,

Trent Boggess


Top of pagePrevious messageNext messageBottom of page Link to this message  By Andre Valkenaers on Monday, April 04, 2016 - 01:15 pm:

Thanks Bob for your question.

A great thank you to John, Mike and Trent for the simple way of explaining the way it really works.

Wasn't there, a few years ago, an article in the Vintage about this ??

Are those sparks, after top dead center, not intended to burn the rest of unburned bad quality gasoline during the Model T period?

Thank you.

Andre
Belgium


Top of pagePrevious messageNext messageBottom of page Link to this message  By Royce in Dallas TX on Monday, April 04, 2016 - 02:10 pm:

There are three outstanding articles on the subject written by Dr. Boggess and Ron Patterson. Links to the three are here:

http://www.funprojects.com/pdf/The%20Model%20T%20Ignition%20Coil%20-%20Part%201. pdf

http://www.funprojects.com/pdf/The%20Model%20T%20Ignition%20Coil%20-%20Part%202. pdf

http://www.funprojects.com/pdf/The%20Model%20T%20Ignition%20Coil%20-%20Part%203. pdf


Top of pagePrevious messageNext messageBottom of page Link to this message  By Andre Valkenaers on Monday, April 04, 2016 - 04:29 pm:

Just found the article that I spoke about higher.
Sorry it wasn't in the Vintage but in the 2003 November- December Model T Times.
Title is :
The Model T Ford Ignition System & Spark Timing.
by Ron Patterson and Steve Coniff.

Tried to put a link or a copy here but I am not able to do so. Maybe someone is smarter as I am.

Andre
Belgium


Top of pagePrevious messageNext messageBottom of page Link to this message  By Royce in Dallas TX on Monday, April 04, 2016 - 05:10 pm:

Here is the article by Patterson and Coniff:

http://www.funprojects.com/pdf/Model%20T%20Ignition%20System-Final%20Artiticle.p df


Top of pagePrevious messageNext messageBottom of page Link to this message  By Mike Kossor - Kenilworth, NJ on Monday, April 04, 2016 - 06:06 pm:

Trent, I'm all for keeping it simple but thought it's misleading for folks to believe the coil fires at 18 degrees BTDC operating on magneto so thought further clarification was necessary. I tried to explain the timer contact duration in terms of DC battery operation with fixed coil dwell time for that reason.

I also concur coil operation is somewhat variable but it must be kept to microseconds NOT milliseconds if optimal engine performance is ever to be achieved. A single millisecond of coil to coil ignition timing variation translates to 15 crank shaft degrees at 2500 RPM! I doubt you could even get close to 2000 RPM with that kind of coil to coil spark variation alone.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Erich Bruckner, Vancouver, WA on Monday, April 04, 2016 - 07:37 pm:

I wonder how this equates to the dwell time of a typical anderson style flapper timer?


Top of pagePrevious messageNext messageBottom of page Link to this message  By Tom Carnegie Spokane, WA on Monday, April 04, 2016 - 10:23 pm:

I think that there are two things going on that made Ford choose 90 degrees of crank duration for the dwell of the timer. First, as Mike pointed out, the timer must have sufficient dwell to properly fire a coil on battery at high speed, such as when using the low pedal. If your battery were low, it could take 70 or more degrees to fire a coil at 2000 crank rpms. Therefore, the dwell should be over 70 degrees. The second thing is that the dwell should be such that it is a multiple of 22 1/2 degrees. This is because when running on mag, the timer can switch on and off when the mag is at its null point, which minimizes the initial current. In fact there would be no current when the timer switches on, and very little when it switches off. This would lessen wear from arcing. So, 90 degrees is the first possibility to satisfy greater than 70 and a multiple of 22 1/2 degrees.

Anderson timers have 90 degrees of dwell (more or less) just as roller and New Day and most other timers.

I don't think them guys back in the day were flying as blind as some folks think they were. Useful oscilloscopes were around in the late 1800's. Cathode ray oscilloscopes were first used in the late 1890's.


Here is photographic 'scope trace from 1890.

1


Top of pagePrevious messageNext messageBottom of page Link to this message  By Bob Shirley on Monday, April 04, 2016 - 10:56 pm:

Thanks to all for you knowledge and insight. A second question; the old gent that started me in the hobby in the early 60's always said not to run the automobile on battery any longer than necessary because the 6 volt battery would burn the points a lot faster than the mag. Is this truer? FYI, E Kinner was in his 70's had an early 13 he always called a 12. I spent every moment I could at his shop.We put together a 27 touring from pieces. We rolled Prince Albert when we took a brake and He told stories of when he was my age,13-14, going to the depot, down by the cattle loading pins, taking T's out of the box car. He said the bodies and frames were stood up in the box car and tied to the sides with rope. The fenders were wrapped in burlap and straw. They would put a piece of burlap under the fire wall and drag the body out then set it on the frame drop 4 bolts in put the fenders in the back of the touring, add oil water and gas and the real prize was getting to drive to the dealership.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Dan B on Tuesday, April 05, 2016 - 10:18 am:

Royce - Thanks for linking the white papers.

All the years of being on this forum and I must have missed them being posted before.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Dan B on Tuesday, April 05, 2016 - 10:21 am:

Love that story Bob. Any others?


Top of pagePrevious messageNext messageBottom of page Link to this message  By JohnH on Thursday, April 07, 2016 - 01:55 am:

Bob,
Re the question of coil points burning out faster on 6V, it is a myth as far as I can tell with my 13 year old points. In that time the car has done about 31,000km. It is true that point material will migrate from one point to the other (which point gives up material and which accumulates it is dependent on polarity). However, with coils on 6V, the process is so slow as to be insignificant. A slight file down every 6 years takes care of that.
From coils I have rebuilt for others that have been run only on 12V, the points life does seem to be reduced.
Of course, excessive current settings will also reduce point life.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Mike Kossor - Kenilworth, NJ on Thursday, April 07, 2016 - 11:14 am:

Tom, I agree with your reasoning as to the timer dwell angle with respect to magneto voltage pulses but the issue of timer contact arcing is not as simple as you described. Unfortunately, those interested to know why involves further complication which gets more difficult to explain and understand but will try to explain for the benefit of other interested readers.

Charging the Model T coil primary winding (coil of wire around a iron core) presents an inductive load to the magneto output voltage. The resulting coil current does NOT start flowing immediately, there is a time delay (phase shift) in which coil current starts flowing. During this time delay, the crank shaft (and timer brush) continues rotation until the magneto voltage (which tracks crank shaft position) becomes high enough to force enough current through the coil primary winding to cause the points to open and generate spark. The coil points remain open for a finite amount of time before closing (while the crank shaft continues rotation) and the coil charge/discharge process repeats.

Fortunately, we do have the modern convenience of using an oscilloscope to observe the relationship between magneto voltage and current as a function of time on a graphical display to see what exactly is going on. Here is a oscilloscope capture of magneto voltage and current for 90 degrees of timer contact (4 magneto pulses) operating at low engine 680 RPM. Time is measured on the horizontal axis at 5ms per division, voltage and current on the vertical axis at 20V/div and 2A/div respectively.

Timer Contact Low RPM

I have added annotations to the graphic to help explain what is going on. Note that at slower engine speeds there the coil fires a total of 4 times; on each of the magneto output pulses. The first and last firings are distinct, complete firings where the coil charges up to about 5A but of opposite polarity (first positive, the fourth negative). The second and third firings are weaker, incomplete firings, as the coil is still recovering (points still open as the second magneto pulse begins) from the previous firing. By the fourth firing, the coil has recovered sufficiently to behave similar to the first initial firing, but of the opposite polarity. Note that the fourth coil (current) firing occurs near the end of the magneto pulse (which follow crank shaft position, now approaching 90 degrees with respect to the first magneto voltage pulse). The coil current is still recovering from the fourth firing so there could be some timer contact arcing but much reduced as you correctly stated. The slower the engine runs and way the points are adjusted can increase the conditions for timer arcing to increase and will also depend upon the magneto output strength. There may be an engine speed that would cause a coil to fire at or near the fourth magneto voltage pulse as the timer contact moves off contact. I did not specifically look at that case so have no data to share for that case at this time.

The situation changes markedly at higher engine RPM as the next oscilloscope screen shot illustrates, time is not 1ms/div.

High RPM

There are only 2 coil firings, on the first and third positive magneto voltage pulse. There are no coil firings on the second or fourth negative magneto voltage pulses because the coil has not recovered from its previous firing (point contacts are still open). The coil current is zero as the fourth magneto pulse ends (which corresponds with 90 degrees of crank shaft rotation with respect to the first magneto pulse) so there is no risk of timer contact arcing in this case. This situation can be completely different with a mis-adjusted coil that requires much greater recovery time for the points so close and settle before the next magneto pulse.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Tom Carnegie Spokane, WA on Thursday, April 07, 2016 - 12:20 pm:

Yes Mike, I realize my answer was over simplified, but my statement "there would be no current when the timer switches on, and very little when it switches off", I believe describes the situation. However, at the risk of being pedantic, your statement "The resulting coil current does NOT start flowing immediately, there is a time delay (phase shift) in which coil current starts flowing.", is wrong. The current begins flowing immediately after the timer contacts and the voltage rises above 0 volts. The maximum current however, is phase shifted from the maximum voltage.


Top of pagePrevious messageNext messageBottom of page Link to this message  By arnie johansen on Thursday, April 07, 2016 - 12:22 pm:

Why is the second screen not 1 ms per division?


Top of pagePrevious messageNext messageBottom of page Link to this message  By Gene Carrothers Huntington Beach on Thursday, April 07, 2016 - 03:30 pm:

Tom, You and Mike are both certainly the great Teachers!

Pretty interesting stuff and most of us just think a little juice flows thru the wire to the spark plug when the key is turned on.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Royce in Dallas TX on Thursday, April 07, 2016 - 04:27 pm:

There is no need for any modern electronics to operate a Model T and enjoy the way it runs. Model T's are extraordinarily reliable, and the ignition is rarely a problem for the average guy who drives at Model T speeds, if you use original Ford design parts, adjusted and maintained as Ford specified.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Mike Kossor - Kenilworth, NJ on Thursday, April 07, 2016 - 06:37 pm:

Tom, good catch. I should have stated the resulting coil current does NOT peak immediately, there is a time delay (phase shift) in which coil current peaks with respect to the magneto voltage. Thanks for catching that.

Bottom line, you said: "running on mag, the timer can switch on and off when the mag is at its null point, which minimizes the initial current". You're wrong! The magneto current would be at its absolute maximum value as the mag reaches its null point due to the phase shift between voltage and current; that was my point. The only thing that prevents the coil current from being at its maximum value as the mag reaches its null (90 crank degrees rotation when timer contacts opens) is the points; they open just prior to interrupt the current flow. So I agree with your conclusion, coil current is low when the timer contacts open, but disagree with the rational of 90 degrees of timer contact (multiple of 22 1/2 degrees magneto pulses) is responsible for it because of the phase shift ignored in your over simplified explanation.

Thanks for keeping me honest and teaching me to be more concise :-)


Top of pagePrevious messageNext messageBottom of page Link to this message  By Mike Kossor - Kenilworth, NJ on Thursday, April 07, 2016 - 08:33 pm:

Arnie, that should be: "time is now 1ms/div


Top of pagePrevious messageNext messageBottom of page Link to this message  By Tom Carnegie Spokane, WA on Thursday, April 07, 2016 - 10:21 pm:

Mike, you about what I said ""running on mag, the timer can switch on and off when the mag is at its null point, which minimizes the initial current". You're wrong!"

I am not wrong. When the timer switches on (initial current) there is NO current if it switches on at the null point.

I agree that the end current would be at maximum at the trailing null point if the points hadn't just opened.

I will also try to be more concise.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Mike Kossor - Kenilworth, NJ on Thursday, April 07, 2016 - 11:28 pm:

Tom, I quoted the wrong statement. "In fact there would be no current when the timer switches on," (I agree) "and very little when it switches off. This would lessen wear from arcing." This is the statement that is wrong and motivated the discussion on magneto voltage and phase relationship.

Darn, I thought we were even. If I keep this up, I better switch to writing tax code. It doesn't matter if you screw that wording up, it still doesn't make any sense :-)


Top of pagePrevious messageNext messageBottom of page Link to this message  By Tom Carnegie Spokane, WA on Friday, April 08, 2016 - 01:33 am:

Mike, why is that statement wrong? Look at your traces. There is almost no current 90 degrees from initial contact in either one.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Michael Deichmann, Blistrup, Denmark on Friday, April 08, 2016 - 08:24 am:

Even though they may not fully understand it, I am impressed of how fancy the Model T magneto ignition system is, given it was designed in 1908.
And it is in full respect of these early inventors I run both my T's on stock ignition system.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Mike Kossor - Kenilworth, NJ on Friday, April 08, 2016 - 11:42 am:

Tom, the statement alone is accurate in describing the current 90 degrees from initial timer contact but not for the reason you stated, and you agreed.

"the (timer) dwell should be such that it is a multiple of 22 1/2 degrees. This is because when running on mag, the timer can switch on and off when the mag (voltage) is at its null point, which minimizes the initial current. In fact there would be no current when the timer switches on, and very little (current) when it switches off. This would lessen wear from arcing." The highlighted portion of the description is wrong.

Timer dwell of 90 crank shaft degrees is a multiple of 22 1/2 degrees coinciding with the magneto pulses (at proper spark lever position). The magneto output voltage is at nulls (0V) at 0, 45 and 90 crank shaft degrees of rotation from initial timer contact BUT the magneto current, driving a Model T coil (an inductive load), would be at its maximum peak value at those positions due to a phase shift (of 90 electrical degrees) between magneto voltage and current that was ignored in your analysis and lead to the wrong reason why magneto current is near a minimum value as the timer contacts break, 90 degrees from initial contact. The correct reason is the coil points open, interrupting the current flow from reaching maximum value leaving only residual current flow from the coil firing as the timer contacts open. The probability of timer contact arcing is reduced with properly adjusted coil points. NOT because the timer dwell (contact) is 90 crank shaft degrees in duration. The situation changes at higher engine speeds,due to the time required for the coil points to close after previous opening/firing but the magneto current remains far below peak values when the timer contacts open.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Tom Carnegie Spokane, WA on Friday, April 08, 2016 - 01:54 pm:

Mike, you have misunderstood what I said. I never said that the reason wasn't because the points had just opened and dumped the load. All I said was that at the trailing null point that there is usually little current. What I said was correct and you assumed that I said it for the wrong reason. I did not. I have studied countless wave forms and am well aware of what is going on.

Here is one of my 'scope traces that illustrates the point. (no pun intended)

1


Top of pagePrevious messageNext messageBottom of page Link to this message  By Tom Carnegie Spokane, WA on Friday, April 08, 2016 - 03:12 pm:

To be concise, in the example above the dwell of this timer was a bit short of 90 degrees. There was some current when the timer opened. There may have been less current if the timer had gone the full 90 degrees. The "no current at null point" caption is kinda dumb as of course there is going to be no current once the timer is open. The caption would better have said "little current when timer opens".


Top of pagePrevious messageNext messageBottom of page Link to this message  By Mike Kossor - Kenilworth, NJ on Friday, April 08, 2016 - 06:19 pm:

Tom, you are completely correct. Since the OP topic was: Why dose the timer have such a long contact duration? I did assumed your statement that the dwell should be such that it is a multiple of 22 1/2 degrees because running on mag, the timer can switch on and off when the mag is at its null point and there would be very little current when it switches off; was indeed interpreted completely wrong; thinking you were ignoring the phase shift of the magneto current because it would be, in fact, be at its maximum value at that point if it were not for the points opening; which although never mentioned, was implicitly responsible. My apology for the incorrect assumption, flawed interpretation, false accusations and resulting confusion that ensued.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Tom Carnegie Spokane, WA on Friday, April 08, 2016 - 09:45 pm:

Thanks Mike. What I originally wrote was confusing. I thought about saying that the trailing null point has nothing to do with the low-current situation, but that is not exactly true either. The null point is part of the entire waveform and the entire waveform "coerces" the coil to be in a low current state when it is.

Still confusing.

Again, thanks for sticking it out. I've been in these situations before on the forum and most folks "cut and run" rather than hash it out.


Top of pagePrevious messageNext messageBottom of page Link to this message  By arnie johansen on Saturday, April 09, 2016 - 09:18 am:

wow!!

Congratulations to both Mike and Tom for "keeping with it" and coming to an agreed conclusion.

Now it is up to the rest of us to try to understand what this all means!

Tom it appears that the red line is the timer opening and closing. What does the blue wiggly line represent, and bright blue line with the spikes represent etc. so we can try to understand your picture!

So much to learn, so little lifetime to do it in!


Top of pagePrevious messageNext messageBottom of page Link to this message  By Les Schubert on Saturday, April 09, 2016 - 09:46 am:

Mike and Tom
Very interesting postings. I have unrelated/related question; what is a practical maximum engine rpm for a "E-timer"?
Thx


Top of pagePrevious messageNext messageBottom of page Link to this message  By Dan McEachern on Saturday, April 09, 2016 - 10:24 am:

Great discussion! Us mortals now need to digest it all. Personally I think its a lot of fun to bring modern technology and analysis to something 100+ years old. But, with that said, I think I will stick to making gears! thanks guys- keep it up.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Dan B on Saturday, April 09, 2016 - 11:09 am:

This kind of discussion is great for the hobby.

While I agree with Royce that all of these modern electronics are not needed to operate a T, it certainly makes one appreciate the genius of the inventors of this stuff who did not have nearly the amount of tools that we have today to explain things. Really neat.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Tom Carnegie Spokane, WA on Saturday, April 09, 2016 - 01:10 pm:

Arnie, the blue all represents voltage. The line is wiggly because I stretched out the trace to show things a little better. The reason it is stair-stepped is because the digital interpolaters in the program can't interpolate at this resolution. What you are seeing is actually each sample.

The red represents current.

Here is a description of what is happening:

The blue at first heading downhill is from the magnets shoving on the electrons in the field coil. The electrons try to escape to ground. They push on electrons in the mag plug - they push on electrons in the mag wire - they push on electrons in the ignition switch - they push on electrons in the coil box wire - they push on electrons in the coil terminals - they push on electrons in the primary coil - they push on electrons in the points - they push on electrons in the timer wire - they push on electrons in the timer - Now, assuming you have all good connections and the key is on, the pressure from this pushing will reach all the way to the timer, but alas, the timer is not making contact, so (most of) the electrons just sit there under pressure. As the magnet passes the center on the pole piece, the pressure subsides. The magnet now approaches the next pole piece and the pressure begins to build again, this time in the other direction.

Now the timer is making contact, so the electrons can go through the timer and back to ground (the engine block). The red line indicates the quantity of electrons that are moving to ground. Electrons have inertia, especially when they are forced to spin, which they are as they are driven through the primary coil.

(A side note here: Mike's traces show the current going in the opposite direction from the voltage - this is just a function of the polarity of the amp probe.)

You will notice that current continues to rise even though the pressure (voltage) is dropping. This is from the inertia of the electrons. When the electrons flow through the coil, they produce magnetism in the iron core proportional to the current level. Somewhere near where the current is at its maximum, the magnetism pulls the points open. The field collapses, which results in a voltage spike (back emf). The coil action has been well described before, so I won't go into that. But, notice the nice squiggly current trace that shows the "ringing" of the tuned LC circuit.

After that, things are a mess. The points are trying to close, but can't really because every time they touch a little more magnetism is produced and they open back up and the field collapses and so on.

Sometime after the fourth pulse as the points are still mucking around trying to close, the timer breaks contact and the field completely collapses and there is no current and the coil is ready to spark again nicely.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Mike Kossor - Kenilworth, NJ on Saturday, April 09, 2016 - 11:54 pm:

Les, the E-Timer provides automatic timing advance up to 3500 RPM operating on 12V battery. I seem to recall you run your engines pretty fast but can't recall how high.

Dan B, Prior to Les's question, the discussion and waveforms in this thread pertain solely to the original, stock Ford ignition system, NOT modern electronics in the car. Modern electronic test equipment (oscilloscope) is used to display the magneto voltage and current to show their relationship and aid in understanding the operation of the stock ignition system; I agree, a true electro-mechanical marvel for its day.

Tom, What exactly do you mean by: "The null point is part of the entire waveform and the entire waveform "coerces" the coil to be in a low current state when it is."?


Top of pagePrevious messageNext messageBottom of page Link to this message  By Les Schubert on Saturday, April 09, 2016 - 11:58 pm:

Mike
Thank you for the information. I will try it on one of my "experiments ". I didn't mean to "hijack" the thread!!
Sorry and I truly mean it


Top of pagePrevious messageNext messageBottom of page Link to this message  By Tom Carnegie Spokane, WA on Sunday, April 10, 2016 - 02:04 am:

Mike, I meant that things happen (in reference to a point on the waveform) when they happen because the waveform drives the coil to behave as it is going to behave. For instance, the points open when they open largely because of the shape of the waveform. If you look at my trace above - there is virtually no place where the current is lower than at each subsequent (after the first) trailing null point. That is not because of the null point per se, but because the previous voltage peak "coerced" the coil to be in this state. The null point just happens to land on this moment of coercion. I was going to use the word "forces" rather than "coerces", but that seemed too strong as things can happen with electro-mechanical devices (marvels) that can cause them to behave differently than they should or you would like.


Here is the voltage/current relation of a T coil with its points shunted. You will notice that the current is not phase-shifted exactly 90 (waveform, not crank) degrees.

1


Top of pagePrevious messageNext messageBottom of page Link to this message  By Tom Carnegie Spokane, WA on Sunday, April 10, 2016 - 02:11 am:

In the "admitting when you are wrong" department, I would like to say that my statement:

"I agree that the end current would be at maximum at the trailing null point if the points hadn't just opened."

Isn't exactly true. It is essentially correct, but as shown in the graph above, not exactly correct.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Mike Kossor - Kenilworth, NJ on Sunday, April 10, 2016 - 08:55 pm:

Les, No apologies necessary. Dan B thought we were discussing modern electronics to make the Model T run which we were not, so just wanted to clarify that only your inquiry pertained to that. Otherwise, the discussion, waveforms, etc. pertained only to the original, stock, Ford ignition system with the aid of modern electronic test equipment to analyze and understand its operation.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Greg Adkisson on Sunday, April 10, 2016 - 09:44 pm:

Royce...Using modern methods to tune a Model T coil is not abusing the old methods of tuning, but rather improving upon the old method...if that were the case, we should not use digital calipers, machine tools, balance parts, modern bearings etc...just to name a few...I want my "15" to run smooth as it can for many reasons!
An ECCT tune is my choice. It's not a magic tune! Just a great tune...


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