Interesting ignition reading

[CookMySock]

Mornin'

http://www.foxvalleykart.com/timing1.html

This is quite a good explanation of what seems to be similar to pocket bike ignitions. So if you ever wondered what was in the little box connected to your spark plug lead, then wonder no more.

Steve

[grantman]

Hi dangerous
Are you going to have a go and make one?


I think you could get a better power curve as I have herd pocket bikes dont really advance.

Is this correct?

Cheers
Grant

[CookMySock]

Are you going to have a go and make one? I think you could get a better power curve as I have herd pocket bikes dont really advance. Is this correct?

I wouldn't make that one. Yeah I heard the same, but I don't really know. I posted that link, coz it was a good description of what we have, ish.

If I did build something, I'd build this.
http://www.sportdevices.com/ignition/ignition.htm

Steve

[SS90]

I wouldn't make that one. Yeah I heard the same, but I don't really know. I posted that link, coz it was a good description of what we have, ish.

If I did build something, I'd build this.
http://www.sportdevices.com/ignition/ignition.htm

Steve

Two Stroke ignitions should never really advance (some will only advance say 2 degrees, up to a maximum RPM of say......3,000, (advancing from idle ignition point is advantage only in the lower RPM for a two stroke) and then either stay at that advance degree, " or maybe 3 degrees more than idle timing point) or, if it is more advanced ( infact pretty much all modern 2 stroke, if they have an expansion chamber or not, although the real advantages only really become apparent when a well designed expansion chamber is used ) they will actually RETARD back to LESS than the original ignition point.

It may indeed sound incorrect (a four stroke of course ADVANCES as revs increase, but a retarding ignition for a 2 stroke is quite a misunderstood (and often unknown concept)

A Two stroke (with-out getting too in depth) produces VERY weak power strokes (compared to a four stroke), the only reason they can make such high power, is the fact that they produce these "weak" power strokes TWICE as often as a four stroke.

(2 times not much = quite a bit!)

So, the way to make good power in a two stroke is to concentrate your efforts on making "more power" on the power strokes.

Getting into expansion chamber design and detail is REALLY involved, but, for this example (a retarding ignition), perhaps it would be best to look at only a small part of the expansion chambers operation.

An expansion chamber is a really busy piece of kit, and the correct expansion chamber for a particular engine set up will be different from others (one pipe does not work on all set ups....capacity is not a finite rule for expansion chamber design)

Only for this example, perhaps it is better just to concentrate on the "non resonance" part of the expansion chamber......for this example, lets JUST PRETEND that you have no expansion chamber, just a header pipe (this will save confusion)

When the spark plug fires (before TDC), it takes some micro seconds for the fuel to actually burn....that is, the fuel does not ignite EXACTLY when the plug fires....(If it did say 20 DEG BTDC, then the burning fuel will simply act against the piston when it is coming up (from the inertia of the flywheel etc), and simply overheat the piston and destroy it (detonation).

So, the only reason we fire a spark plug at 20 DEG (or so) before TDC on a two stroke, is because at LOW RPM the MAXIMUM PRESSURE POINT (the amount of degrees AFTER TDC...approx 15 DEG ATDC) arrives much more slowly than at 10,000 RPM.

Basically, the engine is running at a low speed, so the spark must fire earlier, so that when the piston is ON THE WAY DOWN, the burning air/fuel mixture PUSHES against the piston, giving maximum energy to the power stroke. Therefore, the higher the RPM....the LATER (less advance) the spark needs to be. (sometimes, in high performance engines, this can be UP TO - (yes, MINUS 5 deg)...that is to say, the ignition fires 5 degrees AFTER TDC

(see attached file, from an after market TZ250 ignition, compared to an original one)

But for "normal" road going stuff, from 7 deg retard to 12 degrees retard (from static, which is normally 25 deg static)

It depends on a lot of factors as to what amount of retard you have. (pipe design, compression ratio (primary compression as well), cylinder head design, available gear ratios (in the case of mini motos, CVT, so they would want quite a few degrees retard in my opinion)

The second (and very important) part of the advantages in a retarding two stroke ignition is that, when the ignition fires later, the "flame front" that is produced from the burn, instead of being localised on the crown of the piston (as it would if it happened earlier), the flame front is spread further out, spending less time on the piston crown, and simply going out the exhaust port earlier (than it would if the ignition fired earlier)

Think of it this way, if the ignition fires 15 deg BTDC, the burn would occur, say 2 deg ATDC (missing the "maximum pressure point on the piston by say, 13 DEG too early, which would rob a SMALL amount of power, but also lose OVERREV in the higher RPM's.....important for some aplications.)

Also, because the Air/fuel burns earlier, the exhaust port is still "closed" (or rather covered by the piston skirt) and, let's say the exhaust port opens....86 DEG ATDC...... there are 84 DEG of crank rotation that the burning air/fuel mixture stays on the piston crown, increasing engine temp.

Now, if we retard the ignition (in the higher RPM's) to get the maximum pressure point (approx 15 Deg ATDC) if the fuel now burns at this point, the exhaust port is opened sooner after the burn, so for example, the burning fuel only sits on the piston crown for say, 71 Deg of crank rotation, before being allowed to escape into the exhaust header.

This is sometimes reffered to as "passing the heat from the cylinder to the exhaust"

Now, this is why, (In this example) I pretended that this engine had no expansion chamber, just a header......... an expansion chamber will assist this action (heat transfer) even more, but is quite complicated to explain, and getting away from the ignition.

If you are not careful with your porting (going too wide, or too high) with your exhaust port...... with a retarding ignition (if like I say, the exhaust port is too wide, or high) you CAN cause localised heating on the exhaust side of the piston crown (because if you increase the exhaust port size too much.... the flame "roars" down the larger (too large) exhaust port........

(try opening the door on your log burner "just a little" and see how fast the flame burns...the effect is the same in a two stroke cylinder when it comes to the exhaust port width) which overheats the exhaust side of the piston crown, and, quiet often, will "eat away" this part of the crown.

This same damage (errosion of the exhaust side of the crown) CAN also be caused by ignition that is to advanced, so if you find such a problem in your engine, SOMETIMES too retarded can cause the same damage as too advanced timing........it depends on the size of the exhaust port.

Don't forget, that there are still some (commuter bikes,and old points ignition type ignitions) that have NO RETARD (or advance in the lower RPM), and simply have "static timing" (this is usually around 18 to 20 degrees).... in such situations (old school tuning) you can pick up a few ponies in the lower RPM by simply advancing this static timing point....however, as I have explained, such "tuning" is detrimental to the top end power of the engine, and usually results in holed pistons "randomly"....usualy when subjected to open throttle for too long.

I suspect a standard ignition on a pocket bike is set to something like 18 or 20 DEG (I don't know, maybe a little less, maybe 17 deg....that's very safe), because they have a centafugal clutch, and spend their whole day at pretty much max RPM negating the requirement for lower RPM advance......having a retarding ignition on such an engine would give you a few hundred RPM more at the end of the straight, as well as a cooler cylinder, allowing more cylinder compression to be run.

Hope that Helps!

[CookMySock]

Thanks heaps, bud. Thats great reading.

Steve