Hi all. Today I was asked what angle of dwell was on a Model T timer. My reply was "I have no idea and don't really need to know" this guy is using model T coils and a specially manufactured roller timer on a stationary engine. Looking through my T ignition literature I can't find any reference to dwell angle. Is it relevant to T's?
Not in the sense that it applies to a distributor which opens and closes the points mechanically. Now, as far as how long or how many degrees of crankshaft revolution the timer makes contact, it would be close to 90, I think, but the first spark fires the mixture. The rest are wasted.
Dwell angle is a term that does not apply to the Model T ignition system. You must suspend any concepts associated with coil, distributor, and point ignition systems.
Dwell, (also referred to as cam angle in the book I just read), is basically the time the points stay closed and is determined by the point gap. No relation to the T/coil system at all.
I have to disagree. Dwell is a time period where the points are closed and allows current to flow in the coil. This time allows the coil to saturate. On the Model T system, the cushion spring serves a similar function. When the timer contact closes, current will flow thru the coil. As the magnetic field grows the contact is pulled down. Yet the cushion spring allows the contacts to remain closed for a short period of time. Again allowing the the coil to saturate.
Dwell is an angle, but can also be considered a time period for closed contacts. The cushion spring does the same thing. It allows the contacts to stay closed for a period of time. My 2¢, Mike
Mike and Royce are both correct.
The Model T Ford ignition system and the more modern single point distributor system are fundamentally the same. For every function in one there is an equivalent function in the other.
Ron the Coilman
Well, Not exactly. A properly adjusted Model T coil does not reach magnetic core saturation. That would be very bad!
The coil point contacts remain closed long enough for the magnetic field to become strong enough to pull open the coil point contacts to produce spark. The current flowing in the primary winding of the coil typically reaches a peak value of approximately 6A when this occurs; producing a spark energy of approximately 60mJ.
The rise in coil current occurs nearly linearly with time until the points open; in about 1.5 to 2ms depending upon the operating voltage applied to the coil. Magnetic core saturation would result in an abrupt increase in coil current limited only by the DC resistance of the coil and operating voltage without any significant increase in magnetic field.
Mike, while I would imagine that modern, computer controlled, coil-on-plug systems achieve optimum coil energy over the full range of engine rpm, was that true for older, single coil, fixed dwell angle systems? I would think that at low rpms, the fixed dwell angle system would apply more energy to the coil than needed, and that at some high rpm point there wouldn't be enough time for the coil to reach whatever percentage of full saturation was deemed optimum.
Mark, modern computer controlled ignition systems can advance when coil charging begins depending upon engine RPM so that the primary current reaches the same specified value by the time spark must be fired. Spark energy remains constant because spark energy depends upon the primary coil current at the time spark fires. That is exactly how the E-Timer functions to provide spark advance.
Not that familiar with the single coil, fixed dwell systems. A fixed dwell time system would be the limiting factor for performance as engine RPM increases due to the same finite time needed to charge the coil unless the operating voltage increased with engine RPM as in the case of the Model T operating on magneto. The higher operating voltage will function to advance the spark to some degree.
Thank you all for the replies. An interesting debate and a little different.
Cheers. Alan in Western Australia
What I meant by no relation to the T coil system is that the points on a dist. system are mechanically operated. You therefore have an exact ammount of measureable dwell or time the points are closed and it's directly related to the point gap. The T accomplishes the same thing but with a different operating system. Wouldn't constantly changing mag voltage alter "dwell" on T coils because of the way their opened and closed?
Charlie, there is an exact amount of measurable dwell ANGLE the points are closed. However, the time the points are closed on a mechanically operated distributor system depends upon the speed of the engine. The fixed amount of time needed to charge the coil will still function to retard the ignition timing if no mechanism is included to advance coil charging with respect to when spark is needed.
The Model T magneto output is directly proportional to engine speed. The higher the engine RPM the higher the magneto output voltage driving the coil which does function to shorten the dwell time the coil points are closed and help maintain spark energy at higher RPM.
I should really not get into this discussion at all, because it is WAY over my head, however, I think I have made one "observance" due to something you just said Mike:
I do understand the old single coil distributor type ignitions systems completely, and this whole thread seems to me to be confusing two terms,....."dwell angle" and what you just said Mike,....."dwell time".
In the old system, "dwell angle" is the angle of distributor shaft/cam rotation while the points are closed. "Dwell time" is the length on time (depending on rpm) the points are closed while the distributor shaft/cam goes through that angle.
And as has been pointed out by several early in this thread, "dwell angle" ONLY relates to the old distributor/single coil ignition systems.
Harold, I concur with your understanding. To further distinguish the Model T ignition system, the Model T timer contact for each coil has a specific dwell angle (with respect to the CAM shaft rotation) in which the coil is activated. Once the coil is activated, however, each individual coil points determines the dwell time they are closed prior to delivering spark; not the timer. That is why it is of critical importance the coil points are adjusted for the same, correct dwell TIME. Accurately measuring thousands of a second was not easily done back in the day so it became common practice to monitor the average RMS coil current with a periodic stimulus; a magneto turned by hand, as in indicator of the coil dwell time; the HCCT. Modern electronics make it trivial to measure Millionths of a second of timing variation so precise adjustment of coil dwell time is possible using modern techniques; the ECCT.
I think he just snuck in an advertizement. LOL.
Forgot to mention you can easily translate the coil point dwell time into equivalent dwell angle (with respect to the crank shaft) by multiplying the coil point dwell time (td) by the angular velocity of the crank shaft (vc) which is simply the engine RPM x 6 so the equivalent dwell angle(degrees) is: td x RPM x 6. For example, at 1000RPM the equivalent dwell angle of the typical coil point dwell time of 2/1000 sec is: 2/1000 x 1000 x 6 = 12 degrees. Double the RPM (2000) and you double the dwell angle (24 degrees) for a fixed coil dwell time of 2/1000 sec. That is a huge amount of ignition retarded which has to be compensated for by advancing the spark. This example describes 12V battery operation where the coil point dwell time remains fixed at 2/1000 sec.
Ooops, now I'm guilty of subliminal education.
Mike, you bring up an interesting point. The dwell time on a modern engine changes with speed, yet the battery voltage that drives the ignition is relatively constant. So as engine speed increase the energy in the spark will decrease.
The Model T magneto output increases as engine speed increases. The cushion spring delay will be less as engine speed increases, but the driving energy from the magneto is greater.
So if you could or would measure the joule output of a modern spark and compare that to the Model T magneto spark, would you find that the modern spark joule energy reduces with speed? And would you find that the Model T spark energy stay constant or maybe increases?
I find it amazing that just a few parts, a coil, a capacitor and a couple of contacts can generate should interest. Mike
Mike, not necessarily, modern electronics can detect camshaft position well in advance of the firing time and modern electronics can then delay the spark appropriately based on engine RPM so that the coil dwell time (coil charging current) remains the same without impacting ignition timing regardless of engine RPM. Spark energy can be approximated by the current flowing in the coil primary winding at the time of firing using the equation : spark energy (j) = 1/2 x L i^2. Where L is the primary coil inductance and i is the current flowing in the primary coil winding when the points open to generate spark.
I'm sorry, Mike, when I said modern, I meant 1950's/60's ignitions, just a set of points and coil. The really new stuff is amazing in what it does. So for those not so modern ignitions, the ones I played with as a kid, they could or would run out of gas (joules) as the engine RPM max'ed out. Thanks Mike
Thanks for the clarification Mike, I agree, that was a limitation of that era of ignition systems you described.