High Voltage Photography Using Ford Model T Coil
(NOTE: this commodity was published in Vintage Ford, Vol. 36, No. 3, Pg. 22)
The data given here is a combination of info obtained from Steve Coniff in the course of a number of conversations, Gail Rodda'southward "Model T Ford Parts Identification Guide", Book ii, input by Ron Patterson, and my own observations, measurements, and conclusions.
My current 1925 T does non have a magneto due to a tramp nut left in the transmission past an earlier owner of the car. In the past I have been involved in making Tachometers for Model T Fords; this has necessitated my collecting and evaluating data on Model T magnetos and so that I could pattern tachs for magneto powered ignitions and for bombardment powered Ford coil cars.
Development OF FORD MAGNETOS
I continue to be amazed that the Ford people got magnetos and Ford coils going at all considering the primative diagnostic gear they had and marginal theory. They seemed to find that the small magnets and small-scale pole pieces of the very early magnetos gave a wave form that was a series of positive and negative peaks with close to naught output in between - hence the reputed "notchy spark accelerate" of early Ts. In that location was no risk of a spark in that expressionless zone regardless of what one did with the spark advance lever!
Every bit fourth dimension went by Ford rapidly increased the magnet size and so played with magnet pole pieces, single and double stacked coils, and the coil mounting plates until virtually 1916. They also went from the double stacked coils, which favored voltage ouput (and mayhap a hotter spark), to the single stacked coils with heavier copper windings, which had higher current output capability. This was done in the mid teens when they needed the extra current to ability horns and headlights. The weakest magneto appears to have been the 1912, when they had gone to a smaller coil pole and had done zippo to beef up the magnetic coupling. The wider magnet pole pieces, used in 1913 and 1914, and the larger coil pole shape, used from 1915 on, helped to spread the waveform out from positive and negative spikes to the nowadays waveform. This later magneto (about the aforementioned from and then to 1927 with only variations in the coil mounting plate) had more energy in the waveform and could produce a better, hotter spark. These changes, derived in the main from Gail Rodda "Model T Ford Parts Identification Guide", Volume ii, Pages x,11,12, 22, and 23, are laid out in the timelines shown in the post-obit figure.
MAGNETO WAVEFORMS
I have digitized the a scope trace of a 1916 T magneto waveform with the engine in neutral and without any electrical load on the magneto (the auto has a distributor). The waveform has an overall triangular shape, brought well-nigh by the wider coil poles.
Here are the digitized recordings of the waveform at 3 different RPMs. There are easily recognized irregularities in the waveform, particularly in the 500 RPM view. These are due to variations in the magnet strengths and positions on the flywheel.
Note that if this were an early (particularly 1912) magneto, the irregularities would transform into longer periods of near goose egg voltage.
Here are the digitized recordings of the waveform at 3 different RPMs. There are easily recognized irregularities in the waveform, particularly in the 500 RPM view. These are due to variations in the magnet strengths and positions on the flywheel.
Note that if this were an early (particularly 1912) magneto, the irregularities would transform into longer periods of near goose egg voltage.
The variation in magneto voltage at 1500 RPM indicates that a magnet to coil space change of nigh 16 percent was taking place while the engine was running (inversely proportional to the square of the magnet pole to scroll pole spacing - ordinarily nearly 0.025 to 0.040 inches). This was most likely due to forrad and backward shifting of the crankshaft. It is interesting to note that the Ford Service Transmission, in Section 992, puts an upper limit of 0.015 inches of crankshaft endplay; this would let fifty-fifty larger variations in the magneto output voltage.
Unfortunately, this automobile had a distributor, so magneto-timer-curlicue waveforms and spark advance timing effects could not be explored.
The magneto coils from 1909 to part way through 1911 were on a stamped coil sail with the poles about the size of a 25 cent coin; so the poles were smaller until the oval poles came into use in 1915. This would indicate that the magneto waveform was more of a fasten during 1912; that is until the broader magnet pole piece was used in 1913 and 1914.
MAGNETO OUTPUT VOLTAGE
The output voltage of this 1916 magneto, data from A. 50. Dyke, "Dyke's Autombile Encyclopedia" 1925, Page 1082, data taken from two 1926 T'southward, and data from an electrical motor driven magneto test rig are shown hither. All information, other than Dyke's, were taken with a diversity of analog multimeters. Some of the data were obtained via phone conversations. The 40 volt at 2000 RPM data point is questionable - information technology is probably a chip also high.
The two 1926 magnetos were in well tuned Model Ts that are ofttimes driven at high speeds. These two sets of data were obtained while the engines were running with timer-coil ignition loading the magneto.
The 1916 was in an older stock engine, without a magneto load, and it output a slightly lower voltage than is shown in the other data.
These data correspond what 1 should expect from a properly setup and operating Model T Ford magneto. The Ford Service Transmission, in Section 995, chosen for a magneto output of 7 volts at 400 RPM.
I have seen a 1914 that did not yield more than ten Five RMS at whatever engine speed, withal the car was fully capable of running on the magneto - this is probably a marginal case.
MAGNETO WAVEFORM TIMING
Ron Patterson has pointed out that Ford cartoon T-701C was used to define the flywheels and magnet positions in all cars 1919 to 1927. This cartoon sets the main magnet mounting bolt position as avant-garde by 7 degrees from centerline of the two dowels that prepare the crankshaft - flywheel alignment. The contumely magnet pole and pole piece retaining bolts near the rim of the flywheel are offset 11.25 degrees to each side of the main bolts.
This seven degree beginning from the dowel centerline has been confirmed by measurement of Gail Roddas flywheel pictures 4a, 5a, and 6a on Page 11 of his second book (after geometry corrects were made to correct for parallax due to the photographic camera angle). These pictures show the 1913/1914, 1915/1916, and 1917/1918 flywheels.
Measurements of 1911, a 1913/1914, and a machined 1927 flywheel, washed by Steve Coniff, produced similar results.
The 2 crank dowels are vertical when number 1 and four crank throws are at Top Dead Eye (TDC). This is clear in the adjacent figure taken from the rear of a crankshaft; the hump is the number four creepo throw in the background.
Magneto coils are spaced 22.five degrees apart and are xi.25 degrees each side of 12 oclock, or TDC. A magnetic pole passing laterally across a curlicue, which is wound around an atomic number 26 core pole, causes a voltage peak equally it approaches the coil and a contrary polarity peak after information technology passes and and so recedes. Hence, with the coils of a magneto wound in alternate directions, the unloaded voltage peak occurs with the flywheel magnet pole midway between coil poles.
This data would indicate that the output peak voltage waveform timing should be xi.25 + 7. = 18.25 degrees BTDC and 22.5 degree increments to each side of that value.
The values and then available via the spark advance control should therefore be 40.75 BTDC, xviii.25 BTDC, 4.25 ATDC, and 26.75 ATDC. A good coil appears to burn once near the peak of a continuous sine moving ridge or the peak of a magneto triangular wave. It may fire at a different time if it is continued via the timer at some signal earlier in the waveform.
Observations are that a properly tuned coil fires one time per waveform elevation, while a poorly tuned curlicue seems to burn down more randomly and with less vigor. This would say that tuning using a examination machine is probably more valid considering it shows the uniform single sparking, which 1 cannot see if using the simple 6 VAC current depict examination, unless one uses an oscilloscope. My observations likewise signal that either DC or Ac tuning results in about the same current depict and that testing at 6 volts or 12 volts DC yields similar electric current draws. On 12 VDC, the points closed on time draws more current merely the off time tends to be longer - hence about the aforementioned average current draw. The DC point vibration frequency seems to exist near 500 HZ.
I selected a coil for further testing, from a few erstwhile ones on hand, on the basis that it showed a more than vigorous spark than the residuum. This is an apparently unrestored KW whorl that is probably very representative of many in routine service in Model Ts today. The capacitor in this curlicue was measured at 0.12 microfarads and tests good with no leakage. The electric current depict was measured at several different DC and Air-conditioning supply voltages while the curl was continuosly firing every bit shown in the next figure. Annotation that the electric current draw assumptions stated higher up are well supported. The amperage tends to go down as the supply voltage increases - this is due to the fact that the coil points are open a larger percentage of the fourth dimension as the ringlet cycles harder with increasing voltage.
TIMER - Coil FIRING
A series of tests were performed with the above KW coil. These were done with the coil in a test fixture that was built up equally a part of my prior tachometer evolution and test piece of work. The coil was held in a 1926-1927 curlicue box that was fired by a New Day timer which was in turn driven by a variable speed motor. In this case the timer connections for all four roll positions were connected together so that the single whorl in the box was fired at each cylinder position of the timer. The coil high voltage spark output was connected to a standard spark plug firing in air. The plug ground return was via a slice of 16 estimate wire that had about 120 turns of 26 guess wire wrapped over most 1 ane/four inches of its length. This was a current transformer that could detect the firing electric current as the plug sparked. The betoken rise time of a spark breakdown is among the fastest of all electronic signals and was well beyond the 60 MHZ response of the oscilloscope in utilize here. The signal was also quite small; so the monitoring signal cable could non be properly terminated with its characteristic impedance. Equally a effect, the spark current firing indicate tended to echo upwards and downward the signal cable. A positive current bespeak could only as often show a die-away every bit though information technology were a negative betoken. A sample output signal from the tests to be performed is shown in the next figure. The visible coil current betoken is in reality simply the die-away RC time abiding tail of the real signal, merely it serves as a good indicator that the spark plug did fire and at what time. A vital signal to be seen in this figure is the fact that the spark plug simply fires right at the inital leading border of the coil points opening - there is no "burn time" as some presume.
This pic, as well as all of the similar data pictures used in the post-obit discussion were taken using a xv ampere threescore HZ supply transformer with a variable output. This transformer was selected and so as to have a very low output impedance to be similar to that of a Model T Ford magneto. The pictures were taken with a tripod mounted digital camera aimed at the iv trace scope screen. The camera was operated in a continuous manner at about 2 frames per second at a 1/xv second exposure. The motor driven timer was run at a speed so that the timer contact times precessed across the 60 HZ driving signal voltage waveform. The oscilloscope was synched to the 60 HZ line. Some of the resulting pictures were partially or totally blank when the oscilloscope was in the retrace fashion. A total of about 1600 pictures were taken in the process of shaking downward the method, and finally recording the meaningful data. The final data consited of virtually 500 pictures that formed a stable set in terms of scope display of waveforms, of which well-nigh 269 had useable indications of timer contact closure and the commencement resulting spark generation and were used in the data reduction. It should be noted that the data from only 4 pictures were eliminated from this last gear up as being of questionable nature - such as a shot at the 4.four VRMS coil firing threshold, where the curl did non fire until ii voltage peaks after the timer contact closure.
All information pictures showed a clean single curlicue firing at the 4.4 volt RMS threshold driving voltage. Many of the pictures taken at 6 volts RMS displayed a closely timed double firing of the coil. The pictures at eight and 10 volts RMS showed multiple coil firings - please bear in mind that this test KW spark coil was a run of the mill coil that had non benefitted from a professional rebuild and tuneup - it is simply the best of a small litter.
The final data pictures were readout using a digitized figurer display in which the data points of interest could be read to an accuracy of about two pixels, where 93 pixels = 22.five degrees. Other factors probably introduced another few pixels of possible variation. All data were recorded in a database, where the final information reduction and plotting was done. It was assumed that a positive Air conditioning driving waveform peak represented the 18.25 degree BTDC wave and a negative elevation was that next one following TDC. All data reduction therefore assumed that the effective TDC was located following the positive waveform and before the ensuing negative waveform. The position of the timer contact closing, the resulting firing of the roll, and the related Ac driving waveform peaks were all recorded in the database. The precessing of the timer contact closure across the Ac driving waveform enabled a set of data to be recorded that represented all possible timer advances. These sets of data were recorded at the four.four volt coil firing threshold, and at 6, 8, and ten volts RMS.
The reduced data are plotted in the following figure. Note the scatter in the 4.iv volt data; this is indicative of the threshold nature of this voltage. The higher voltage data all tend to follow a smooth curve. The eight and 10 volt data points are very close to each other, and it was decided not to extend the data to even higher voltages, which in any case would have been beyond the output ability of the Air-conditioning transformer in utilise. A coordinated data gear up of this blazon would not be possible without the utilize of a very stable driving power source. A Model T engine driven magneto could not be held at a sufficiently stable speed for the time required to shoot multiple sets of hundreds of pictures.
A clear upshot to be seen in the effigy is the sharp advance in the first spark timing as the timer contact closure advances and causes the firing to movement from the voltage elevation after TDC to the next one before TDC. This change appears to move to a point of less timer advance every bit the driving voltage increases.
All of these data show a mutual feature. When the timer contact closure was almost to or a bit forward of the height of the driving waveform, the starting time spark occured on the behind of the voltage peak. As the contact closure advanced forward from the peak, the commencement firing likewise advanced up to the fourth dimension of and then ahead of the peak voltage. Further advance of the timer contact closure beyond the time of the driving waveform nada voltage crossover acquired the timing of the commencement spark to receed back down the behind of the voltage summit - this additional timer contact advance actually causes a slight retarding of the first spark. This is evidenced in the figure by the dropoff in the first spark advance with increased timer accelerate at the right end of each curve. The advance effect can exist seen in the following sequence of photos taken at 6 VRMS.
Note that here the supply voltage is sufficient that, for this particular coil, a 2nd firing begins to occur as the timer contact advances ahead of the voltage peak.
MAGNETO - Ringlet FIRING
The above tests were repeated using a 1926/1927 type magneto, driven by a variable speed electric motor, that was fix up as a examination demote by Don Dechant. Both Tony Bowker and Don Dechant helped run this examination. The timer and recording system were the same as used in the sine moving ridge tests above. Runs were made at 500 and chiliad RPM on the magneto.
The electrical surround was noisier, the scope trigger point tended to vary a chip, and the magneto could not be held at exactly the aforementioned speed at all times. This combined caused the data reduction to exist done in a slightly different manner. As a issue, the data tend to have a bit more than besprinkle = of the social club of a few degrees as seen in the next figure.
Information technology is significant to note that the scroll firing point was ever somewhere on the back side of the magneto waveform; it never fired at or before the elevation voltage. This meant that the timing of the first spark never reached as much as xviii degrees before TDC. This is probably due to 2 factors; the magneto appeared to have a higher impedance than the transformer used in the sine moving ridge tests, and the more or less linear ramp of the magneto waveform could not provide equally fast a voltage rising equally a comparable frequency sine moving ridge.
CONCLUSIONS
Both sets of tests show a mutual characteristic in that each grouping of first sparks is nearly 22.5 degrees broad. The groupings in each set are also about 22.v degrees offset from each other and are of similar shape. This indicates that each group is a consummate set of information points for firing on a particular positive or negative bicycle of the magneto waveform. If the spark accelerate lever should chance to be positioned and then that information technology was before or later the extreams in the figures hither, the kickoff spark would occur at points that are shifted 22.5 dgrees both vertically and horizontally from the points shown in the figures, and they would be and then far advanced or retarded that the engine probably would not fire properly.
Another important point is that the information from the tests using the magneto have a flatter distribution of timing of the outset spark. That ways that a Model T Ford driver is basically only selecting what timing group (advanced or retarded) the engine is firing on, and has piddling command, beyond a few dgrees of spark timing, within that group.
Whatsoever resistance in the magneto to spark curlicue excursion volition have a detrimental upshot and crusade the timing of the kickoff spark to lag. It is important to keep all wiring in the ignition excursion in height shape. Utilize skillful approximate wiring, be sure to have clean, solid connections, and be sure that the ignition switch contacts are in skillful status. I take seen a number of old switches that made very poor contact. An ohm meter examination is not adequate - one must bank check the voltage drib across the switch when information technology is nether load. The peak currents can be several amps, and an ohm of resistance can result in volts existence lost.
The two 1926 magnetos were in well tuned Model Ts that are ofttimes driven at high speeds. These two sets of data were obtained while the engines were running with timer-coil ignition loading the magneto.
The 1916 was in an older stock engine, without a magneto load, and it output a slightly lower voltage than is shown in the other data.
These data correspond what 1 should expect from a properly setup and operating Model T Ford magneto. The Ford Service Transmission, in Section 995, chosen for a magneto output of 7 volts at 400 RPM.
I have seen a 1914 that did not yield more than ten Five RMS at whatever engine speed, withal the car was fully capable of running on the magneto - this is probably a marginal case.
MAGNETO WAVEFORM TIMING
This seven degree beginning from the dowel centerline has been confirmed by measurement of Gail Roddas flywheel pictures 4a, 5a, and 6a on Page 11 of his second book (after geometry corrects were made to correct for parallax due to the photographic camera angle). These pictures show the 1913/1914, 1915/1916, and 1917/1918 flywheels.I selected a coil for further testing, from a few erstwhile ones on hand, on the basis that it showed a more than vigorous spark than the residuum. This is an apparently unrestored KW whorl that is probably very representative of many in routine service in Model Ts today. The capacitor in this curlicue was measured at 0.12 microfarads and tests good with no leakage. The electric current depict was measured at several different DC and Air-conditioning supply voltages while the curl was continuosly firing every bit shown in the next figure. Annotation that the electric current draw assumptions stated higher up are well supported. The amperage tends to go down as the supply voltage increases - this is due to the fact that the coil points are open a larger percentage of the fourth dimension as the ringlet cycles harder with increasing voltage.TIMER - Coil FIRING
A series of tests were performed with the above KW coil. These were done with the coil in a test fixture that was built up equally a part of my prior tachometer evolution and test piece of work. The coil was held in a 1926-1927 curlicue box that was fired by a New Day timer which was in turn driven by a variable speed motor. In this case the timer connections for all four roll positions were connected together so that the single whorl in the box was fired at each cylinder position of the timer. The coil high voltage spark output was connected to a standard spark plug firing in air. The plug ground return was via a slice of 16 estimate wire that had about 120 turns of 26 guess wire wrapped over most 1 ane/four inches of its length. This was a current transformer that could detect the firing electric current as the plug sparked. The betoken rise time of a spark breakdown is among the fastest of all electronic signals and was well beyond the 60 MHZ response of the oscilloscope in utilize here. The signal was also quite small; so the monitoring signal cable could non be properly terminated with its characteristic impedance. Equally a effect, the spark current firing indicate tended to echo upwards and downward the signal cable. A positive current bespeak could only as often show a die-away every bit though information technology were a negative betoken. A sample output signal from the tests to be performed is shown in the next figure. The visible coil current betoken is in reality simply the die-away RC time abiding tail of the real signal, merely it serves as a good indicator that the spark plug did fire and at what time. A vital signal to be seen in this figure is the fact that the spark plug simply fires right at the inital leading border of the coil points opening - there is no "burn time" as some presume. MAGNETO - Ringlet FIRING
The above tests were repeated using a 1926/1927 type magneto, driven by a variable speed electric motor, that was fix up as a examination demote by Don Dechant. Both Tony Bowker and Don Dechant helped run this examination. The timer and recording system were the same as used in the sine moving ridge tests above. Runs were made at 500 and chiliad RPM on the magneto.
The electrical surround was noisier, the scope trigger point tended to vary a chip, and the magneto could not be held at exactly the aforementioned speed at all times. This combined caused the data reduction to exist done in a slightly different manner. As a issue, the data tend to have a bit more than besprinkle = of the social club of a few degrees as seen in the next figure.
Information technology is significant to note that the scroll firing point was ever somewhere on the back side of the magneto waveform; it never fired at or before the elevation voltage. This meant that the timing of the first spark never reached as much as xviii degrees before TDC. This is probably due to 2 factors; the magneto appeared to have a higher impedance than the transformer used in the sine moving ridge tests, and the more or less linear ramp of the magneto waveform could not provide equally fast a voltage rising equally a comparable frequency sine moving ridge.
CONCLUSIONS
Both sets of tests show a mutual characteristic in that each grouping of first sparks is nearly 22.5 degrees broad. The groupings in each set are also about 22.v degrees offset from each other and are of similar shape. This indicates that each group is a consummate set of information points for firing on a particular positive or negative bicycle of the magneto waveform. If the spark accelerate lever should chance to be positioned and then that information technology was before or later the extreams in the figures hither, the kickoff spark would occur at points that are shifted 22.5 dgrees both vertically and horizontally from the points shown in the figures, and they would be and then far advanced or retarded that the engine probably would not fire properly.
Another important point is that the information from the tests using the magneto have a flatter distribution of timing of the outset spark. That ways that a Model T Ford driver is basically only selecting what timing group (advanced or retarded) the engine is firing on, and has piddling command, beyond a few dgrees of spark timing, within that group.
Whatsoever resistance in the magneto to spark curlicue excursion volition have a detrimental upshot and crusade the timing of the kickoff spark to lag. It is important to keep all wiring in the ignition excursion in height shape. Utilize skillful approximate wiring, be sure to have clean, solid connections, and be sure that the ignition switch contacts are in skillful status. I take seen a number of old switches that made very poor contact. An ohm meter examination is not adequate - one must bank check the voltage drib across the switch when information technology is nether load. The peak currents can be several amps, and an ohm of resistance can result in volts existence lost.
0 Response to "High Voltage Photography Using Ford Model T Coil"
Post a Comment