Anatomy of a Model T Coil Spark

[MKossor]

Recent data I took on a Model T coil with properly maintained and adjusted points when studying ignition spark generation. Though this data might be of interest to the more technically minded Model T enthusiasts.

Basic view

Detailed description of events numbered in order they occur in time.

There are a lot of things going on in a very short time scale. Even with the simple, low combustion, Model T engine, microseconds DO matter for optimal engine performance.

For Context, here is another oscilloscope screen shot illustrating the relationship between coil primary current (Yellow trace this time) and voltage measured across the coil points (Blue trace this time). The Red box illustrates the area of focus in the previous screen shots.

[It's Bill]

I can testify from personal experience after having been zapped by one of these things, microseconds do indeed matter! Bill

[TXGOAT2]

I have read that the Model T coil emits about 15,000 volts at the secondary. Evidently that is an average of the voltage over the duration of the discharge/spark oscillation, probably at some nominal primary voltage.

[JohnH]

Very interesting; thanks for posting, Mike. I like the timeline of the firing process. The primary current waveform is good illustration for the benefits of equal dwell time for a set of coils.

[MKossor]

Pat, I'm not sure if the coil secondary voltage actually ever reaches 39.6kV, the primary voltage spike only lasts for a few nanoseconds as spark initiates. I believe the longer duration secondary voltage could reach a peak voltage of 200V x 80 = 16kV that lasts for 10s of microseconds during the beginning of the spark "Burn" interval.

John, Thanks for the feedback. Spark ignition waveforms are very interesting and instructive in diagnosing SI engine problems. SUN used to make a large automotive ignition analyzer for service shops that is now obsolete thanks to modern onboard diagnostics. Gone are the days where the technician must be knowledgeable in the proper instrumentation and interpretation of diagnostic displays such as I posted. Simply replace the component the ODBII Diagnostic Trouble Code (DTC) tells you to replace. Rumor has it there are additional symbols being added to the "Check Engine" light new for 2022 that makes it very easy for the non-technical minded driver to fully comprehend at a glance. Here is an example:

New DTC Symbols:
Check Engine - $ (Technical Description Example: Loose Gas Cap)
Check Engine - $$ (Technical Description Example: Replace O2 sensor)
Check Engine - $$$ (Technical Description Example: Ignition timing error, check/replace timing chain, gears, tensioners)
Check Engine - $$$$ (Technical Description Example: Too many DTCs to list, Don't even think of buying this car!


Trying to keep interest, knowledge and discussion of the inner workings of Spark Ignition systems going thanks to the MTFCA Forum.