How do Atom Computer Ignition modules work?

[jnjvan]

The chances are excellent. You turn the mag plate to get the peak (and the points opening) at the correct crankshaft timing angle.

JV

[legendre]

quote jnjvan:

The chances are excellent. You turn the mag plate to get the peak (and the points opening) at the correct crankshaft timing angle.

JV

Only one of those events can be precisely set. You either time for optimal spark energy, or you time for the most efficient crank angle – you can’t have both.

Fortunately, the former has a fair amount of angular ‘slop’ – the latter, not so much.

[debe]

Optimum spark energy the way I see it is factory set & is not adjustable as the cam is keyed to the flywheel & its magnets. Presumably this has been worked out as the optimum spark energy. The only adustment left is the firing point before TDC. The only exception I see is using an electronic module in place of the points, the optimum spark energy is fixed by the trigger point of the generated pulse. In practice ive found the electronic module advances the timing by about 2 degrees, & I haven’t found this a problem in any of the engines ive used. Looking at a CRO picture of the wave form using a module, its triggered at about 2.5 to 3Volts

[vintin]

My understanding is that the resonate circuit formed by the inductance of the coil and the capacitance of the condenser results in longer spark duration. Another important role of the condenser and a good reason for the cap to have the correct value. The scope photo clearly shows the LC circuit ringing.

It seems obvious to me that the opening of the points hence the firing of the plugs in relation to piston TDC is more important than the plugs firing at peak coil output. I may have missed it but I don’t remember anyone speculating on the width in degrees of peak coil output.

[legendre]

@debe & VinTin

Then all three of us are in agreement. In this type of magneto, the flywheel magnets & pole shoes are arranged to provide the maximum spark energy at or near the angle where the ignition event should be set to occur. When the ignition timing is correctly set (in terms of angle relative to TDC), the ignition event (points break) will roughly coincide with the angle of maximum spark energy.

They don’t always perfectly coincide, as the manufacturing just isn’t that precise – nor does it need to be. If we’re off the max peak by a couple of degrees either direction, we’ve still got ~90% or so of max energy, and plenty to generate sufficiently powerful spark. Properly timing the ignition event relative to TDC is the more critical tuning parameter.

The situation is different with an accessory magneto, like a Lucas, or other engine-driven accessory mag. On those systems, you can set the point gap to fire the mag precisely at the magnetic peak – and then the entire mag can be timed to the engine, as it’s driven by a gearwheel that sits on an un-keyed taper. You can set your perfectly-timed mag to fire at any crankshaft angle by changing the position of the drive gear- best of both worlds.

[jnjvan]

quote legendre:

quote jnjvan:

The chances are excellent. You turn the mag plate to get the peak (and the points opening) at the correct crankshaft timing angle.

JV

Only one of those events can be precisely set. You either time for optimal spark energy, or you time for the most efficient crank angle – you can’t have both.

Fortunately, the former has a fair amount of angular ‘slop’ – the latter, not so much.

But you can have both: You time for optimal spark energy with the points, you time for crank angle by rotating the magneto plate (which moves the coil and the points relative to the crankcase). The points gap will have a small effect on timing vs the crank, but not much. By far the dominant factor for crank angle timing is the mag control lever.

Your statement is true for lawnmower engines, and others with fixed ignition timing, but not for 99.9% of outboards. The spark control on outboards is very similar to the accessory mag you mention in another post.

I do agree with you that the former has some slop. That’s why you still have spark if you set the gap a few thou either side of the recommended .020 setting.

John Van

[legendre]

quote jnjvan:

But you can have both: You time for optimal spark energy with the points, you time for crank angle by rotating the magneto plate (which moves the coil and the points relative to the crankcase).

This thing about being able to "move the mag plate" is a HUGE red herring. Think about any other engine with dynamically variable timing – be it an automotive system, a motorcycle, or any other system with variable timing.

Just because the operator is able to control the timing (via the engine speed lever) doesn’t account for the upper & lower bounds of the timing range. Again, ignore the fact that the timing may be changed and just think of it as a fixed timing system, with the engine speed lever moved to "FAST" and as such, the throttle wide open. Can we agree that the ignition timing must be as specified in terms of degrees relative to TDC at the full-speed settings? I can guarantee you, it IS.

So then, what timing angle is set by setting the engine speed lever to FAST – if we’ve timed the magneto for maximum energy? Why should it end up at the required angle, for full-speed operation?

If what you’re saying were correct, then there would be no need to time +any+ ignition system with variable advance – either manual, or automatic – for anything other than max spark energy, as the timing would some how be magically correct whenever the engine ran up to full advance.

This concept just isn’t sinking in, is it? If not, let me phrase it this way:

When an OMC engine is set to full speed, and hence full spark advance – what is the more critical factor: That the plug(s) fire at the correct crankshaft angle, relative to TDC – or that the plug(s) fire at the moment of greatest spark energy. Very recently, we all seemed to agree that the former was correct.. Yes?

[The Boat House]

• This reply was modified 6 years, 1 month ago by The Boat House.
• This reply was modified 3 years, 9 months ago by The Boat House.

[jnjvan]

Tubs said it well.

Because the points move with the coil, you get the max spark condition at any point of spark advance. This is what I have been trying to explain. The critical timing for max spark, (which is set by the geometry of the coil laminations, the magnets, and the timing of the points opening) is not affected by the position of the spark advance. That is the genius of the rotatable mag plate. The plate insures that the correct geometry for "max" spark is maintained at any point of spark timing relative to TDC.

Of course the spark advance is critical. But you get your cake and eat it too: Ideal spark advance and ideal spark energy.

JV

[debe]

Well said JV, that’s what I tried to explain earler. Just obviously didn’t come across correctly.

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