Helllo, This is a post from ww2birds, his question didnt get answered,.....its something that has been doing me ead in as well, ........so anyone ?











Guys,

I have a 3W106, but only have about two gallons through it .. runs like a top, and have never had this kickback problem with it (and from the discussion, I sure hope I don't!).

But a few posts ago on this thread, a question came up that I have often wondered about. On the C+H ignition, when you install it, you use the supplied timing wheel set the sensor for a spark at 28 degrees BTDC. I presume that the C+H ignition is more like 2-3 degrees BTDC at idle and only moves to 28 degrees at full throttle .. so how is it that we can flip it and get it to start and idle when the microprocessor does not yet "know" the RPM, and can't set the timing to the retarded position that is appropriate for idle?

Clearly, somehow it DOES know, since I have never ever had it kick back, and never had a moment's trouble with any C+H ignition .. and then, by analogy, how does the 3W ignition do this, and could it be making a mistake "sometimes" as suggested.

Like I said, I have often wondered about this...

Any of the ignition gurus know???

Dave

Hi,
The timing defaults to the retarded position and advances following a curve as sensor pulse frequency increases with RPM.

Dave

Dave when you say it defaults to retarded do you mean at low revs it does this?, I understand thats what its supposed to do, but that doesnt explain why you set the static timing at 28 degrees btdc, in your scenario you would set it 0-3 degrees btdc, I f i'm understanding you

If I remember correctly it goes back to full advance when under 400 rpm or something like that. I am not sure of the exact # now. A speed way lower than the engine is capable of idling at. But still a rpm speed lower than you are flipping it over when starting it. So under 400 rpm, full advance. Over 400(starting to idle and up) it is retarded untill it reaches it's pre programmed advance curve. It's determined by the speed at which the magnet passes by. Something like that as I remember it anyway.

C&H Ignitions "SS" or Synchro Spark ignitions electronically retard the timing from the fully-advanced position verses advancing from the fully retarded position. C&H uses auto-retard logic and senses within the first flip of the prop what is required. C&H Ignitions are timed at full advance and retard from this setting. Others are timed at say 4 degrees and advance either electronically or mechanically from this setting. Others are fixed, this is a compromise between starting and maximum power, with max power suffering.

Our engines can accelerate from idle to full power in maybe 250 milliseconds. At an average rpm of, say, 3000, the engine fires 50 times per second, so in 250 ms. you'll see something like 10 to 15 power strokes. So if the timing must increase linearly (it doesn't which makes it more complicated) you need to advance the timing something like 2 to 3 degrees every rotation. This is quite a bit of change.

If set up for electronic auto-advance, the electronics module gets a signal from the crank position sensor/pick-up coil long after the time when the spark should have been generated. Therefore on that particular rotation it is too late and the only thing it can do is fire the plug immediately, so you get at least one cycle when the timing is off. The ignition module has to remember that the engine is accelerating, remember the rpm, and try to anticipate at what point the spark should be fired for the next rotation. As you can see things can change pretty rapidly, a full rotation could see the engine no longer accelerating.

The whole Auto-advance operation is some guesswork and a lot of computation to get it right. In an environment with rapidly changing RPM values some error may occur. The calculations are much simpler and error is much less with an auto-retard system. Auto-advance works, but it's not quite as precise in all conditions.

With auto-retard the module can calculate the firing angle over just a few degrees of crank angle in the current firing cycle. Auto-advance systems inherently must calculate over the last full 360 degrees of crank rotation and then average that with several previous cycles. This is because when auto advance senses in the current firing cycle that it should have fired, it is already 20 odd degrees too late to do so.

Auto-retard provides extreme accuracy and anticipates rapid increases in RPM and adjusts the timing for better engine response. This is achieved because the calculation time required is so minuscule as compared to the ignition firing rate.

Auto-retard requires an extremely accurate calculation be done extremely fast because it occurs within the current firing cycle. Auto-advance requires only remembering past cycle(s) and guessing what the next cycle should be. Both methods can be accomplished with discrete components. Newer auto-advance systems use micro-processors to reduce part count and increase accuracy.

As I recall, the CH Syncro Spark measures the pulse width generated by the first flip and calculates the amount of retard necessary. If you flip through very slowly then you can get a bad kick back. Always flip it like you mean it.

My apology for such a simple explanation.

Ken

The timing at start-up is controlled via an element of the modules program. The program "element" engages as the magnet passes the sensor (first flip). The Hall Effect sensor switching on/off each time the magnet passes bye allows the module to calculate RPM which in turn is used to calculate timing needs. When the engine is running the timing is calculated relative to RPM and/or rate of RPM increase

FWIW: C&H Ignitions with SS are user programmable to the limits of full advance at either 4000 or 6000rpm and 20 or 26 degrees full advance. Default is 26 degrees at 4000rpm.

Thanks Michael, so the first spark on electronic units like my moki is always at full advance eh?, that is what I thought all along, many people have said otherwise but your explanation is the first that makes sense to me

Crusty, if your Moki has a CH ignition and you flip it through fast, then the first spark is well retarded.

TGK (Terry) is with CH Ignitions and knows the theory well. Please see this thread:

http://www.rcuniverse.com/forum/m_23...tm.htm#2311892

Ken

Ken No I have a moki ignition, first spark is not retarded yet static timing is set at 28 degrees btdc, I presume it retards at second firing.
thanks for the link, ill read it
p.s. I use a electric start and the engine is only 22cc, so I guess the retard is not really neccesary, come to think of it idle is quite rough, so its even possible that my unit is faulty and not ever retarding, no-one else seems to use these units and I can find no info on the net, ill have to strobe it sometime

I suspect the pick-up coil(s) (if it has one vs. a Hall Effect Sensor) when excited by the magnet, cause a charge value to be introduced into the ignition module. This charge is then used to electronically reduce the time between firings. This reduction in time artificially causes the plug(s) to fire progressively before TDC. The amount of time reduction would be based upon the amount of charge (which would increase as RPM increased). This type system would always start at retard but it is anyone's guess as to how much advance one would get - especially as components age and values change.