Piston Failure???

[Walt]

Has anyone witnessed/experienced or heard of a piston failure due to a faulty ignition on our applications. More specifically detonation due to a fault in the timing circut..Thanks Walt

[main_gears_bent]

Whose application?

[main_gears_bent]

Are you meaning the RC application in general or a particular brand?

[3ddd]

well i would think that the pinging would be so loud that you would stop it then, and yes i have heard of this,it is sometimes caused by ignition timing but mostly by heat but still takes awhile to eat up a piston

[Kiwi]

Guys,

I've been piston broke a few times but not from advanced ignition. I think retatrded was closer to the point.


Seriously on a two stroke its really hard hard to hear the pinging or pre ignition you hear guys talking about. I'm pretty deaf so its absolutely impossible for me to hear it even if it is there.

Given that most new ignitions advance as much as 30 degrees with rpm and the very limited if any adjustment is available its really tough to get them too far advanced.

As a rule of thumb I use the method whereby the piston should be at TDC and the magnet should be just entering the reed switch (pickup ) That will give you around about 5 degrees of retard and the engine will start without ripping your hand off. Then its a gentle adjustment millimeter by millimeter until your engine fires perfectly and does not bounce back when flipping it.

If it starts easily and never backfires it is highly unlikely your ignition will be too far advanced.

Kiwi

[Jaybird]

So if it pops backwards sometimes while starting it's advanced too far?

[Kiwi]

In my estimation yes,

WHY???

Because the spark when you start an engine is supposedly retarded by a few degrees. If it fires and kicks the engine back then to my mind that means the spark MUST be occuring before TDC. If it is after TDC how can the engine possibly kick back.

I have always used the engine kick back at start as a good indicator of static timing. If it kicks back on me move the ignition a mm in the direction the engine turns. That retards it a degree or so and see what happens. You will soon find a point where your engine flicks and fires, runs smoothly and responds well.

Then go do your needle settings and you wont burn or break pistons due to an over advanced ignition system.

ONE THING THATS IMPORTANT HERE HOWEVER. Always flip your engine like your trying to throw it into the next ball park. That way you have some inertia in your swing so if the engine does kick a little bit your going to carry it through. I have seen far too many accidents happen when people do a pansy throw on the prop. If they kick back and your all limp wristed they will bite you badly.

Kiwi

[KrisW]

	Quote: Originally Posted by Jaybird
	So if it pops backwards sometimes while starting it's advanced too far?

NOT necessarily!

Walt. . what kind of ignition???

A set advance curve, and proper operating ignition, but poor quality gas, CAN lead to detonation and piston damage. Whether the ignition is not working properly will depend on the ignition involved. Some are dirt simple, and almost impossible to over-advance. Others can have a timing chip fail and be locked at full advance when they should not be. It all depend son which ignition is involved.

About the popping back thingie. . .

With CH type ignitions, that use a timing RETARD for starting, if the magnet is too far away from the pickup sensor (Hall Effect) the ignition will stay at full advance. . and yes, you will get bit.

Then there is a posibility that a "Synchro-spark" delay chip has gone bad and your ignition is locked at full advance of 28 degrees.

I have never seen a DA/3W style ignition, using a pickup coil, not Hall Effect trigger, exhibit problems with being too advanced. Engnes with a variable timing ring, like the BME's, can be set too far advanced, though.

A few things about the advance mechanisms in current Model ignitions:

DA and 3W both use an "advance" type ignition, that starts off at a specific static setting (usually 10-14 degrees BTDC) and then "Advances" the faster the engine turns. ON the older style 3W, and still current DA ignitions, the magnet passes an actual "Coil", not a hall effect trigger, and the magnetic field passing the coil makes an electronic "spike" that trigger the ignition. Cool thing is that the faster the magnet is travelling, the faster the spike is made, so as the engine speed increases, the spike occurs quicker (in degrees before TDC).

The effective limit on this type of advance mechanism is about 12-14 degrees, meaning that at about 4500-4800 rpm this type of ignition timing is fully "advanced" at about 26-28 degrees.

Falkon uses an electronically timed "advance", that uses a timing chip and Hall Effect trigger. It starts off at 4 degrees BTDC, then at a predetermined rpm (about 1500-1800 rpm) switches to a timed "delay" that actually holds the ignition from firing for about 350 degrees of duration, so that effectively as the magnet passes the Hall effect trigger you are starting the process of firing the NEXT ignition pulse. Since the current ignition cycle has already ended, it's okay to do it this way, and you never lose a firing cycle. This "advance" mechanism is programmable, and winds up with 28 degrees total "advance" at about 4500-5000 rpm, but is locked in solidly at 4 degrees below 1500 rpm for most engines.

CH uses a "synchro spark" module to delay the timing a set amount for starting, then slowly reduces the delay to effectively "advance" the timing above 1500 rpm. You time the engine at about 28 degrees BTDC, then when you flip the prop to start a built in delay of "X" microseconds is in place. As soon as the engine fires and begins to run, another set delay is established (above about 500 rpm) so that at 1200 rpm you have about 4-6 degrees of timing. Then as the speed increases, the delay progressively decreases, and at 4500-5000 rpm there is zero delay, and effective timing of 28 degrees BTDC. This advance curve is also programmable.

Funny thing with the CH, though. . at 1200 rpm the engine has a set delay of X amount. Well, at 800 rpm the delay is still X amount, but the engine is turning slower, so the timing is effectively ADVANCING because the delay is the same, but the engines speed is decreasing. This can lead to some strange idle characteristics below 1000 rpm. You will have 4 degrees at 1200 rpm, and as much as 8-10 degrees at 900 rpm. It's very difficult to get an engine to idle lower than that, so below 900 rpm it becomes kind of a moot consideration

Those are the three basic types of timing mechanisms used today. 3W now uses ignitions that have Hall effect triggers instead of Coil pickups, and apparently use a "delay" advance very similar to the Falkon unit.

And popping back??? It really depends on the type of ignition as to WHY it is happening.

So. . what kind of ignition is it??

[RTK]

Just a quick comment on the pinging/detonation sound.
Kiwi is right, in our model engines the sound is not the same as our car engines either. The sound I hear is more like a bag of big rocks being shaken. It is not very loud and can be hard to hear, but when it is happening you will know it and better do something about it.

[Walt]

My mention of our application refer's to model aircraft engines. I'm not asking for anything pertaining to kicking back, indications or what to look for. I would rather not mention a brand name. This is a twin that has blown 3 pistons in 3 flights. I can assure all the ground has been covered with anything that would or could cause this situation. I am hoping to find a similar situation that someone has had. I'm pretty sure this particular ignition has cause this problem from the day the engine was first opened from the box. Walt

[KrisW]

	Quote: Originally Posted by Walt
	My mention of our application refer's to model aircraft engines. I'm not asking for anything pertaining to kicking back, indications or what to look for. I would rather not mention a brand name. This is a twin that has blown 3 pistons in 3 flights. I can assure all the ground has been covered with anything that would or could cause this situation. I am hoping to find a similar situation that someone has had. I'm pretty sure this particular ignition has cause this problem from the day the engine was first opened from the box. Walt

For detonation. . .look for a sparkly etched surface of the piston, melted or cracked ring lands next to the exhaust port, specks of aluminum on the sparkplug electrodes and insulator, and possibly broken rings.

If you'd care to PM me with the engine make and model, I can probably give you a few pointers on setup, timing, and other things that will help with the problem, Walt.

[Kiwi]

Walt,

Is it always the same cylinder failing or is it / has it been both?

Kiwi