The nomenclature is only confusing when you try to use terms across different disciplines.
For example, "dwell angle" would not be understood by electronics engineers, and automotive mechanics would probably not understand the term "duty cycle".
Well dwell angle and duty cycle really do mean different things. Duty cycle means the portion of time that a signal in On out of one complete cycle which simple compared to computing dwell time.
Dwell time (an angle) is the time that the points are closed in a mechanical ignition system. it's the time in which the ignition coil charges up so that when the points open and the magnetic field collapses the di/dt generates a high voltage pulse that is feed into the ignition coil(s). But with engines that on time happens one, two, four, six, either or more times per revolution of the engine. And it also depends on if the engine is a two stroke or four stoke (or something else!) engine. In two stroke engines there is one ignition pulse per cylinder per revolution but there is one one ignition pulse for each cylinder and for every other stoke on a four cycle engine. And to confuse things further, distributors are
usually driven at 1/2 of the engine speed in a four stroke engine.
But before we go too buried in the details, the Dwell time happens a number of times during each engine revolution,
i.e. each cycle. In a four cylinder four stroke engine it happens four times per engine revolution. IIRC 30 degrees is a pretty normal dwell time, that means that the points are closed for 30 degrees of the engine revolution and then opened and that repeats four times per engine revolution (per cycle).
So duty cycle means the total On time in a cycle and is expressed as a percentage; but Dwell is measured in degrees (or your unit of choice) and is a single event and it can happen multiple times per cycle. In a single cylinder four stroke engine it would only happen once every other cycle!
Confused yet? If you're not then consider that engines can run at various speeds so the
time each Dwell takes will also vary in time (but not in angle).
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Yeah, I think that I have too much time on my hands this morning.