MTFCA Forum

Noticed where a new way of calculating speed (and possibly engine speed - rpm) by measuring magneto out put. The meter has been presented on the forum.
A meter that allows for measuring the output of the magneto.
From 1921, a page showing the relationship of magneto output and speed.

The only thing I am curious about, if you are at stopped idle, - say 200 rpm, - does it still feel the car is motoring along a 5 MPH?

George

Good chart, for info too.

As for the new mag. speedo/tach I can say if car isn't moving you kinda know, there isn't any wind in your face.

But at idle the T is shaking, so you think you are, unless you notice you're not passing any trees, houses, or signs!


Meter display about 500 RPM's at reasonable idle, of course not moving.



Happily running around my neighborhood, a bit under our posted limit of 25mph. Meter is fun to see the speed.

Nobody out today either.

We're all quarantined, but at least can move on the roads some, are to really remain at home. At least we can move.

My neighbor's daughter lives on the island of Tortula in the BVI's. Lockdown, only can go into your own yard, got out for 3 days to get stuff, after shut in for 7 days, now have to stay inside for another 17 days. No one can come or leave the island.

I designed something similar to this a while ago (at least I assume it's similar - if there is any detail it'd be interesting to see?).

Mine was effectively a rev-counter also calibrated in MPH, driven either by magneto, timer, or magnet/hall effect device on a wheel.

For the magneto drive I used a MEMS gyro/accel on the gear pedal to determine which gear was in use, or if the vehicle was in neutral. This utilised the angle of the lever arm. The code (I used an embedded micro to translate the input to various meter output types) would 'calibrate' the meter in MPH or KPH according to the requisite gear/frequency ratio, and if in neutral would return the meter to zero. The latter was ok if you weren't coasting!

It's an effective way of returning speed from one of the several available inputs, but in the end I decided I quite liked the bareness of the 'instrument panel' on the T A personal choice, I can see the attraction and may return to a variant of it at some stage, it's good to see others doing this as well.

That chart is incorrect. Don't rely on it.

The data chart presented was used and posted in many places but for all my research there was NO PUBLISHED set of specs on the magneto unit as a whole. NO minimum nor Maximum appeared anywhere. The data was taken and appeared in the service bulletins and was copied by other publications (non ford) but it was always the exact same data. The only TYPO mistake for sure was in the copy published above here which appears to show 0.5 Volts in the "Volts" column in the row labeled as 200 RPM while the Service Bulletin "Essentials" show that first entry as 5 volts. A more glaring error appears in ALL copies of this data and it kinda authenticates where the data source comes from. Dykes Motor Encyclopedia has the exact same data published as Ford magneto output and it was taken from the Ford Service Bulletin. The most glaring mistake is on the row that is labeled "1000 RPM" and at the far right column shows the cycles are measured to be 146.4 but there is no way that the magnets could have moved and the correct cycles is calculated to be 133.33 and that is simply too big of a goof to ignore. If you calculate the cycles for each of the speeds presented as exact whole numbers they "SHOULD" have been as follows:

200 RPM = 26.6667...(actual Ford measurement = 26.4)
400 RPM = 53.3333...(actual Ford measurement = 52.8)
600 RPM = 80.0 .......(actual Ford measurement = 80.0)
800 RPM = 106.6667..(actual Ford measurement = 106.4)
1000 RPM = 133.333..(actual Ford measurement = 146.4) (big error in either measurement or RPM used by Ford)
1200 RPM = 160.0.....(actual Ford measurement = 160.0) Kinda looks TOO exact at this point like maybe 1200 wasn't really done.

The error in the cycles of the 1000 RPM reading appears everywhere this data is posted and no other set of data is shown anywhere that I could find. It appears that everyone who published any data on the "output" of the Ford T magneto used the same source and you can look up page 80 in the DAN R. POST edited copyright edition of the Model T Ford Service Bulletin Essentials. All output data that appears in all books I have seen have these same data and the error appears again and again so Ford alone published the only data we have. There are many devices that have been built to measure magneto output but the first set of data was flawed for sure and while the volts column came close to doubling in voltage when RPM doubled it was slightly off and no way to know for sure how much voltage was indeed enough. The "AMPS column could only have measured the huge numbers shown by measuring AMPS TO GROUND methinks but that makes no useful measurement which makes be think the data was take by workers as instructed with no real understanding of what was in fact being measured. Just my opinion and no challenge is being presented to those who may see it differently. I wasn't born when these readings were taken.

As an FYI the chart was posted earlier on the forum in 2012.
http://www.mtfca.com/discus/messages/25 ... 1338086883

I can see me driving down the road with a killawatts meter plugged into an outlet on the dash! People asking what is that for, why it's my speedometer I reply.
I use the Killawatts meter to set RPM on my AC Kohler, 59-61Hz = approx 1800 RPM 115-120 volts AC.

Matt, great minds think a like. I started on an "E-Dash" project a while back: a magnetic mount, 2 line LCD display powered by 2 wire connection to the Magneto post that would simultaneously display:

Speed (MPH) Magneto Voltage
Engine RPM Engine Temperature

Micro controller based, of course, so MPH easily calibrated regardless of gear ratio or tire size combo, and programmable engine coolant alarm in the event of rapid temperature rise associated with oil loss or cooling system fault. Speed warning possible too but generally not an issue in a typical Model T

I too decided to abandon development as it would be an unwelcome modernization clearly visible on the dash even though it was mag mount and easily removed. Then again, GPS displays seem common in many Model Ts.... Perhaps something to consider again if sufficient interest in such a device warrants. Deriving noise free DC power to operate a fragile uC from the Magneto while firing coils generating voltage spikes up to 300V was a real challange.

The new Speedometer/Tach is informative and looks period correct. Very nice design John.

For some more information on the magneto vs speed chart this was found dated 1916.
The Ford Supplement found in the Dykes 6th edition from 1916, provides the following information of the magneto output and speed with factory gearing.
At an engine speed of 500 RPM in low gear a speed of 5.589 MPH and in high gear 12.295 MPH is achieved.
At an engine speed of 1,500 RPM in low gear a speed of 16.767 MPH and in high gear 36.885 MPH is achieved.
The magneto at 500 RPM is achieving a voltage (AC) of 14 volts. At an engine speed of 1,500 RPM the voltage output is 30 Volts AC.
The magneto is producing 5 amperes, it was not indicated if this is with a load of coils and light or without a load.

It needs to be mentioned that between 1916 to 1918 the coil sizes in the magneto were changed.

Nor does it take into consideration of the strength of the magnets over 100 years later.

If the chart is wrong, what should it be?
I don’t know how to compute the voltage as offhand I don’t know the strength of the magnets or the number of turns in each of the sixteen coils. However I think the frequency should be easy to compute.
Each magnet passes all sixteen coils with a North Pole and then a South pole on each revolution. So the output will change sixteen times for each engine revolution. So at 1000 rpm will convert to 266Hz (1000*16/60).
So yes there is an error, 200rpm will be about 53Hz not 26Hz and so on.

Tony, I Feel that it is eight cycles per engine revolution but as my sister said to me one day, " What would know, you are just an old truck driver"....N.

Tony, here's the math:

1000 RPM / 60 sec = 16.667 RPS (Revolutions Per Second)

Each engine revolution produces 16 magneto pulses: 8 positive and 8 negative pulses = 8 complete cycles per engine revolution so Magneto output frequency equals engine speed (in RPS) x 8 so that is: 16.6667 x 8 = 133.333Hz

Conversion formula is:
Mag frequency = (Engine RPM/60) x 8

Mag frequency = Engine RPM x 0.133333

Magneto Trivia question: What are the physical positions of the flywheel magnets with respect to the field coils for maximum (peak) output voltage and minimum (zero) output Voltage?

Mike...I feel that equal distance between any coil pole piece.. Is she right???...N.
Mike.. Still waiting for your answer re Luke's F.A.C.T. thread, ttf on magneto..N.

Mike, Another feeling.. I think your zero should have been maximum(--)minus voltage. (AC wave form)
Zero voltage would be when the magnets are on top of the coil pole piece...N.

Final answer? Is he right?

Each magnet has a north and south and they are arranged so there are a total of eight norths and eight souths in a complete revolution. My original post ignored the placement of the magnets are arranged to make eight poles not sixteen.
Therefore I am wrong. At 1000rpm frequency will be 133 Hz (1000*8/60)

So the chart will look like this.
Rpm. Freq
0200. 26.6
0400. 53.2
0600. 79.8
0800. 106.4
1000. 133.3
1200. 159.9

This seems to agree with some measurements quoted above. By the way the waveform is far from a sinusoidal, it contains many harmonics and is almost a triangle.

I have only seen the output once on an oscilloscope while helping Buzz Pound when he was preparing his paper on how when using the magneto, the timing of the spark is largely controlled by the amplitude of the voltage resulting no the timing being either 4 degrees ATDC or 11 ATDC. It was all written up in The Vintage Ford about thirty years ago.

There is an earlier posting from Tom Carnegie that contains several images of the magneto waveform FYI:

http://www.mtfca.com/cgi-bin/discus/sho ... ost=914602

Yes, but what are the fly wheel magnet positions with respect to the magneto coils corresponding to the magneto output voltage waveform?

A. Maximum (peak +V) positive voltage output
B. Maximum (peak -V) negative voltage output
C. Zero (0) magneto voltage output

Just trivia for anyone remembering electromagnetics from HS physics class. My initial assumption was dead wrong, oops; sorry Mr. Winburn

Poppie, not sure I understand your answers as they pertain to magneto waveform points A,B and C above. Please clarify.

I am not an electrician and I hated physics. I guess that is why I became a pharmacist - drugs are my life.

I would presume that the max voltage output is when the spool is directly over the coil, and the poles of the magnets are equally spaced over the coils. That should produce maximum flux. Then the minimum would be when the spool is equidistant between 2 coils.

That is my answer and I am sticking to it until Mike or someone else corrects me.

...- drugs are my life

David, that describes a coworker from days past...you know they have clinics for that now...

David , and Mike.
I feel that your "PRESUME" is correct regards to max flux... but WRONG for the rest.
Voltage is produced when the magnets pass over the coils from one coil to the next coil.
I feel that max voltage either + and/or - would be when the magnets bobbin is crossing at the mid point between each coil winding and the next coil winding. The answer for Mkosser A and B tirivia Question.
Zero voltage would be when the magnet spool is directly over a coil pole on the bobbin. THE answer for Mkosser C trivia question...N.

Neil has it right (Ding, Ding, Ding). On first thought, mistakenly thought the maximum (peak) output voltage would occur in position C because that is the position for in car re-charge when you zap the magneto coils and thought the output would be reciprocal - NOT! The maximum magneto voltage output occurs when the maximum number of magnetic field lines cut the maximum number of coil windings. That, of course, occurs exactly in between two adjacent coil windings (Position A or B) The polarity of the peak will depend how the magneto coils are wound and connected (determinable by the right hand rule). The next magneto output voltage peak occurs when the crankshaft is rotated 22.5 degrees but will then be of the opposite polarity. Zero magneto output voltage occurs when the magnet pole piece (bobbin) is directly over the coil center core. All of the magnetic lines of flux are directly concentrated into the iron core leaving zero lines of flux cutting the coil windings.

Mike, Thanks for the detailed simple explanation!