Electronic ignition, variable advance?

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keith5700

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Looking ahead to when I need an ignition system for the 1/4 scale V8, I recall reading about an electronic ignition system which had a variable advance curve. I've tried to find it again now but can't.

I can see some systems advertise variable advance, but it seems all they do is retard the timing, say, 20 degrees, for starting purposes, and once running, switch to full advance.

Does anyone know of an ignition system for small motors which has an actual timing advance curve built in?
Even better would be if the curve was adjustable. Or even, even, better if the timing was programmable, against rpm and load.
I know a full sized car stand alone ignition system would be ideal, but is far too big and expensive for my purposes.
Or, is this sort of ignition control not needed with small model engines?

Thanks in advance...
 

Swede

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In theory, an electronic retard/advance is do-able, and probably a PIC or similar chip would be a good way to go, but it'd take a guy with the ability to both program, AND execute a circuit that'd be immune to the spikes and other weirdness in an ignition system.

Variable ignition that is manually controlled is generally pretty easy to do, and can be set up both with regular points or the more popular hall effect chip + magnet. All you have to do is alter the switch location around the arc over maybe 40 degrees.

I set up such a system on one engine, and the effect of ignition timing is very obvious when it is varied. For example, at idle, I found that retarding the spark a bit vastly improved running. I thought that a greatly retarded spark would help starting, but surprisingly the engine starts easily just about anywhere from 30 to 0 degrees BTDC. But while running, at any particular throttle setting, there's a spot on the ignition timing curve that is best, and it's something you can hear with your ears. I'm sold on variable timing for anything more than a simple hit & miss or similar.

Does your system have to be electronic? If you went mechanical, it'd be easy to tie the ignition timing to the throttle, so it'd advance the spark as the engine throttles up.
 

Lakc

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Here is one place that has done it, sorry its in French. I have a lot of links for this but unfortunately they are on a different computer. Of several different styles they all typically trigger ~ 35-40 degrees BTDC and involve a calculation of a preprogrammed delay depending on rpm, this gives an effective advance. I intend to do a lot of ignition system work coming up, although I cant say how soon it will be.

http://www.transmic.net/
 

dieselpilot

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Most engines will run without any advance retard at all as long as the flywheel or prop is heavy enough.

The RCEXL ignition for RC retards timing about 24° for starting and advances linearly from something like 1000RPM to no retard at 4000RPM. This is adequate for most any small engine. These ignitions are developed most for airplane applications, and the retard makes it easier/safer to hand prop. The retard at low RPM also makes idle slightly better, but not necessary. All of these engines run very well when the carb is properly selected and tuned.

Cars have complex ignition timing maps, partly because the engines are knock limited, and partly due to efficiency, and emissions standards.

While I agree in general with Swede, fuel mixture goes hand in hand. Simple carbs on our engines will likely supply a variable A/F ratio through the operating range and subsequently varying optimum ignition timing. If the engine has no load, unstable operating temperature, etc. the ignition curve is not going to do much in my opinion.
 

keith5700

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Thanks for replies,
Swede, linking the advance, mechanically, to the throttle is a good idea. I'll use that as my back-up plan.

Lakc, that French site looks like it would do the trick, but there's too much to take in whilst I'm researching all the other required areas for a first IC engine. Besides, everytime I make some electronic stuff it blows up as soon as I connect the battery!

dieselpilot, I totally understand what you say. I have no idea whether variable advance will be of any benefit on a small engine. In a past life I have programmed aftermarket ECU's for full sized engines, and assumed it would be desireable on a model. In the future I would like to fuel inject the engine I'm making, but that's a project in itself, and too complex for now.
I'll have a look at the RCEXL, and see if that would be useable.

Cheers.

 

Lakc

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keith5700 said:
Lakc, that French site looks like it would do the trick, but there's too much to take in whilst I'm researching all the other required areas for a first IC engine.
I hear you there. Electronics is another fun hobby, but an entirely different gear so to speak. I am still trying to get my first IC engine running, on glow plugs to start, before I get fancy.
Once I do have an ignition system to share with everyone, things will get easier for all the following engines.
 
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