A short study on ignition timing and combustion

acotrel · Sep 12, 2014

Magnetos are very dangerous things. Back in the 50s, long before I owned my Triumph, a friend was racing it. Halfway down the main straight on Phillip Island in the rain, a plug lead came off and went down his boot. The fellas were all wondering why this idiot was jumping around on his bike. My uncle had a good trick to do with a magneto. He'd say 'look -this magneto doesn't work' and he would hold the live end of the lead with one hand with the magneto in the other, however the bastard would turn it backwards until he enticed you to hold it. Then he would give it a flick in the forwards direction. He was the guy who used to blow smoke into my face when I was a baby after promising to tell me a story, - during the war years.
He was responsible for my addiction to racing motorcycles. Had the 1953 650cc Thunderbird, and took me to see his mate race a 7R AJS at the Geoff Duke/Gilera meeting at Fishermans' Bend in 1954.

texasSlick · Sep 12, 2014

acotrel said:
One thing that I found with my old triumph - whenever I lifted the head, because of the high crown of the pistons, the side near the plug was always an acceptable colour, the other side was coked up.

This is in keeping with Jim's summary....the high crown "shades" the piston side opposite the plug, and inhibits flame propagation. This is a good example of why one should stay with flat top pistons if possible.

The best possible configuration would be to have dual plugs, one on either side of the piston crown, fired by independently controlled coils or magnetos.

BTW....trying to fire two plugs off the same coil or magneto will not work....one will fire, the other will just sit there.

Slick

acotrel · Sep 12, 2014

The only really good thing about a magneto is you don't have to bugger about with batteries. I'd like to have the money I've wasted getting to race meetings only to have the magneto start to misfire. A rotating magnet magneto is marginally better, however with the lucas SR type you still need to take out the bolts that hold the internal coil and replace them with longer lock-wired ones.
The advantage of moving to a programmable system is that you can tailor it to get the optimum advance right across the rev range and then jet to it without doing damage through detonation. If you are trying to get max performance you are usually in the danger zone of the balance between jetting, timing and comp. ratio.
One thing that puzzles me is that high comp. ratios seem to give more power. I think that is because the jetting is richer to give the same combustion conditions as lower comp.

comnoz · Sep 12, 2014

texasSlick said:
BTW....trying to fire two plugs off the same coil or magneto will not work....one will fire, the other will just sit there.

Slick

??Any wasted spark ignition fires two plugs from one dual tower coil. Jim

Hobot · Sep 12, 2014

Ugh both correct, if, both plugs center terminals fed from same coil polarity terminal so sparks-electrons can only jump gap in same direction then gap with the least resistance will pass all the spark energy current bypassing the higher resistance plug, but if the plugs are wired so electrons jump off one center post to hook then travel though head to the ground hook of the other plug to jump from its hook to center post then both plugs will always fire together but the opposite polarity spark can be up to 15% weaker, which does not translate to 15% weaker combustion pressure, so undetectable power difference.

Opening gaps tends to retard spark d/t extra time to build voltage up enough to jump across higher resistance.

comnoz · Sep 12, 2014

hobot said:
Opening gaps tends to retard spark d/t extra time to build voltage up enough to jump across higher resistance.

That is true only if the ignition system is very borderline in it's capability to fire the larger gap.

If using a capable ignition system then opening the gap will create a larger kernel and effectively advance the timing. Jim

Hobot · Sep 12, 2014

Ah so JIm, an exception to old school rule of thumb.

texasSlick · Sep 12, 2014

@comnoz

Are dual tower coils internally wired with dual parallel windings? If so, they are essentially two independent coils in one case.

Slick

comnoz · Sep 12, 2014

texasSlick said:
@comnoz

Are dual tower coils internally wired with dual parallel windings? If so, they are essentially two independent coils in one case.

Slick

A dual tower coil only has one primary and one secondary winding. The two towers are connected with one on one end of the secondary winding and the other on the opposite end of the secondary winding. It will fire a twin plug head with no problem if it is charged with enough energy to fire two gaps under compression.

Of course a single tower coil will not fire two plugs in parallel. Jim

motorson · Sep 12, 2014

Jim's explanation explains why a dual coil is trying to fire to itself instead of ground. So, like hobot said, the spark goes through one plug down, across the head and back through the other plug up. The Lucas Rita fires both coils at the same time but they are both trying to fire to ground. (Thus, wasted spark, but not like a single coil wasted spark.)

acotrel · Sep 12, 2014

With wasted spark there can be an issue with 'dwell time' at high revs i.e. the time available for the coil to do it's thing. A commando doesn't usually rev past 7000 RPM, and is not often ultra high comp. so shouldn't have a problem using a double ended coil, even with points.

comnoz · Sep 12, 2014

motorson said:
Jim's explanation explains why a dual coil is trying to fire to itself instead of ground. So, like hobot said, the spark goes through one plug down, across the head and back through the other plug up. The Lucas Rita fires both coils at the same time but they are both trying to fire to ground. (Thus, wasted spark, but not like a single coil wasted spark.)

True although it really makes no difference. With two coils the charging voltage is divided between two coils. With a twin tower coil the output voltage is divided between two plugs. The plug in the cylinder that is not under compression wastes very little voltage to jump the gap.

Hunt mags are wasted spark with a twin tower coil. Jim

comnoz · Sep 12, 2014

acotrel said:
With wasted spark there can be an issue with 'dwell time' at high revs i.e. the time available for the coil to do it's thing. A commando doesn't usually rev past 7000 RPM, and is not often ultra high comp. so shouldn't have a problem using a double ended coil, even with points.

A twin cylinder 360 degree engine with wasted spark would not run into dwell issues until at least 18,000 rpm. More with a low impedance coil. Jim

acotrel · Sep 12, 2014

hobot said:
Ugh both correct, if, both plugs center terminals fed from same coil polarity terminal so sparks-electrons can only jump gap in same direction then gap with the least resistance will pass all the spark energy current bypassing the higher resistance plug, but if the plugs are wired so electrons jump off one center post to hook then travel though head to the ground hook of the other plug to jump from its hook to center post then both plugs will always fire together but the opposite polarity spark can be up to 15% weaker, which does not translate to 15% weaker combustion pressure, so undetectable power difference.

Opening gaps tends to retard spark d/t extra time to build voltage up enough to jump across higher resistance.

Steve, if the electrons flow through the secondary circuit with the same energy at both ends across the gap, why does the different polarity make one spark weaker than the other ? Are you suggesting it is because one electrode is smaller if the spark is jumping in different directions in the two plugs ? How would the ignition system designers prove that ?

'Opening gaps tends to retard spark d/t extra time to build voltage up enough to jump across higher resistance.'
I would like to see you prove that.

Snorton74 · Sep 12, 2014

I think Kenny would call this one of those 'strong signal threads'. Good stuff Jim, thanx.

acotrel · Sep 12, 2014

comnoz said:
acotrel said:

With wasted spark there can be an issue with 'dwell time' at high revs i.e. the time available for the coil to do it's thing. A commando doesn't usually rev past 7000 RPM, and is not often ultra high comp. so shouldn't have a problem using a double ended coil, even with points.

Click to expand...

A twin cylinder 360 degree engine with wasted spark would not run into dwell issues until at least 18,000 rpm. More with a low impedance coil. Jim

Would you run into dwell issues if the electronics in the black box did not respond quickly enough to give the coil time to recuperate after firing ?

comnoz · Sep 12, 2014

acotrel said:
comnoz said:

acotrel said:

With wasted spark there can be an issue with 'dwell time' at high revs i.e. the time available for the coil to do it's thing. A commando doesn't usually rev past 7000 RPM, and is not often ultra high comp. so shouldn't have a problem using a double ended coil, even with points.

Click to expand...

A twin cylinder 360 degree engine with wasted spark would not run into dwell issues until at least 18,000 rpm. More with a low impedance coil. Jim

Click to expand...

Would you run into dwell issues if the electronics in the black box did not respond quickly enough to give the coil time to recuperate after firing ?

You would need to provide about 2 or 3ms burn time before you could charge the coil again to get full energy out of the coil. The discharge could be stopped at any time by re-applying voltage to the primary winding.

A low impedance coil can be charged fully in .5 to .7 ms

An old high impedance coil can be fully charged using points in about 5ms.

pete.v · Sep 12, 2014

When does octane adjustment come into the discussion. Doesn't this effect both timing and combustion and concidered a key adjustment tool?

Hobot · Sep 12, 2014

Yes Alan, from what I've read on sale pitches to actual testers the reversed polarity slightly weaker spark side is d/t the blunter emitter ground hook. The only down side of our common type wasted spark systems occurs on non 360' twins like Harley's so wasted spark may fire some remaining mixture out of time of need for slightly rougher running. Nortons are on full exhaust stroke when wasted spark side hits so a non issue.

comnoz · Sep 13, 2014

pete.v said:
When does octane adjustment come into the discussion. Doesn't this effect both timing and combustion and concidered a key adjustment tool?

In a perfect world timing will be set to burn the mixture to a point where maximum cylinder pressure is achieved at a particular number of degrees after top dead center. Octane would be high enough to provide the needed knock resistance at the optimum compression ratio.

In the real world on pump gas in a high compression engine the pressure peak usually comes a little too late because the timing must be retarded to prevent knock.

Note that retarding the timing to prevent knock and making the pressure peak come late decreases the performance more than reducing the compression ratio enough to prevent knock.
What we really need is a variable compression ratio engine so the compression ratio could be set for the available fuel instead of varying the timing. There have been a few attempts to use this technology.

The fuel burn rate is determined by the temperature, the cylinder head design and swirl/tumble characteristics. It does not vary with the octane rating. [contrary to popular belief] Therefore varying the timing to prevent knock is not the optimum way because that moves the pressure peak also. However It is the only practical way to do it on a Norton.

There are two common ways to determine when the pressure peak is at the right crank angle.
One is with a cylinder pressure sensor and monitoring equipment -very expensive. That is how a car maker would do it.
The second way is with a static speed dyno where you hold the rpm at a set speed with a brake and vary the timing looking for peak torque. That is how I do it. Jim