Spark plugs

[SteveD CB500K0]

Basic electronics Delta.

The spark plug acts like a capacitor. Put a resistor in series and you get a low pass filter (low frequencies get through).

This has the effect of rounding off the sharp “attack” edge of the voltage pulse to the plug, softening and lengthening it.

The high frequency interference is caused by the sharp attack edge of the pulse, so soften it and you get less interference.

The shaping of the spark pulse helps the fuel burn (as long as the combustion chambers are designed to use it).

But I’m an electronics engineer not a mechanical so my knowledge stops there!


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[K2-K6]

It's a much more intricate area than is readily given importance to I feel, and dominated by the bigger and fatter the spark,  then the better it is stance. That seems more marketing huff and puff than based on factual engineering. 

Do you mean with reference to the R&D specifically related to resisted ignition,  or how resistance affects spark and ignition effects?

It may,  by way of logical explanation be more relevant in the thread I was writing about detonation than isolated as it's really an extension of that field from an informative point of view.

[SteveD CB500K0]

Love how the “D” in R&D gets turned into a smiley


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[K2-K6]

I crossed over with you there Steve so may seem slightly out of sync.

My D is not showing smiley on Android tablet 

Interesting to get electronic engineer view of it. 

As you say the lengthening of spark assists mechanical layout of combustion chamber.  Combustion is often mistakenly viewed as single time event of explosion by people selling "high power" ignition systems.  Reality is its a controlled burn with progressing stages.  These engines have a fairly conservative design (evidenced by the total ignition advance being around 35 degrees) and rely on the squish area pushing unburnt mixture toward the spark location on approach to peak combustion pressure. Any lengthening of spark duration would be a welcome characteristic to facilitate this design.

It follows that if they required suppression for their developing markets, USA etc,  then the engine design would incorporate that element as a primary lead during development, making it logical that they should always be run with resistance to fit that specification.

If you flip the analysis,  and now remove resistance,  the resulting shortening of spark duration should have a detrimental effect on completion of combustion burn.  It can also lead to detonation.

[deltarider]

In all the automotive literature (at least that I have read) there isn't a single hint to this. But... I am always eager to learn. Tell me where I can find it (internet fora excluded) and I will have a look at it. From what I know a 10kΩ dissipates more energy into heat than for instance a 5kΩ resistor. In spite of having asked around multiple times, nobody so far has delivered any proof for this 'lengthening' of the spark duration.

[Nurse Julie]

Blimey, this is all too tech for me. It's s spark plug and it sparks. D7ES's haven't been around for a long while as far as I can remember so the 'new' or 'alternative' or 'superseded' is the EA type which work perfectly well on all our bikes.

[K2-K6]

I've linked to this before,  be interesting what you feel about it in this context.

https://www.crypton.co.za/Tto%20know/Ignition/burn%20time.html

The statement directly following the graph references spark duration. 

[K2-K6]

In all the automotive literature (at least that I have read) there isn't a single hint to this. But... I am always eager to learn. Tell me where I can find it (internet fora excluded) and I will have a look at it. From what I know a 10kΩ dissipates more energy into heat than for instance a 5kΩ resistor. In spite of having asked around multiple times, nobody so far has delivered any proof for this 'lengthening' of the spark duration.

I've been thinking about this statement,  and tentatively agree that the fixed resistance in HT circuits doesn't,  in itself, makes the spark duration change.

Change of spark duration appears as a result of resistance across the spark plug gap seems to be accepted as true.  The variations in pressure, fuel mix, atomisation etc, give a variety of changing conditions which alter the ionisation phase (spark propagates across gap) different start and termination qualities then exist.

How true it is I'm not sure,  but it seems that originally resistance in HT was initiated in aero engines to better control the harmful degradation of plug electrodes and so increase service life along with reliability.  Interference of other systems seems to have been a byproduct.

The fixed resistance appears to give a "collar" to bring a minimum threshold at which the secondary coil is charged before release,  a bit like a wier on a river, making available the quantity of volts to reliably cross the gap consistently.
That's interesting on these systems with two plugs on one coil. Firing the non loaded plug may be easier for the coil to exit through (non power stroke and combustion pressure needs less volts) making it vulnerable to reduced sparking on the power cylinder (the important one).  Making sure the two routes out have the same system resistance looks to be intrinsic to power cylinder performance. 

Checking that the caps are matched pairs would seem to be useful as some measured on her have shown discrepancy in the past.

[Johnwebley]

  interesting discussion ,

 I run Boyer ignition,boyer coils and 5K ohn caps.

 I tend to also have 32thou plug gaps,

I am more than happy with starting and power ,
when fitting new plugs ,if set to 27thou the bike feels a little sluggish,


  any thoughts ? comments ?

 

[Bryanj]

I seen to remember Boyer saying to increase gap John

[K2-K6]

They've a setup that's not fully disclosed,  Boyer that is, making it difficult to comment on their settings.

We've asked them from the forum before but they wish to keep confidential their methods for commercial reasons,  which you can understand.

They obviously know what they're doing,  make very good systems and it's rare to find criticism of them,  so you have to go with their advice or definite individual experience. 

From a general discussion point of view,  the plug gap offers the ultimate resistance in the circuit. So wider gap (with same coil resource)  would imply shorter duration spark,  but if the gap is wider and causes more volts to build in HT circuit before it jumps, then that may in itself provide more resources to prolong arc time.  It seems a very movable set of parameters as each one influences the other.

If it runs best at 32thou,  then I can't see any problem with that. 

They have previously had info on their site (can't find it now) that indicates multi firing of plugs under certain conditions,  it's not clear if that is driven by circuit load sensitive measurements or just run out of their chip controlling strategy to set parameters programmed in.

They also seem to fire all of the plugs all of the time,  which I don't understand the reason for.

[florence]

Thanks to everyone for the advice.  It has paid off, new coils, new plug caps and correct plugs and all runs very sweetly indeed.

[mike the bike]

Reading this thread had got me wondering if my HT caps are within spec.   I need to have a measuring session.

[deltarider]

Reading this thread had got me wondering if my HT caps are within spec.   I need to have a measuring session.

Caps do detoriate over time. I remember back then there were aftermarket 10.000km (6000 miles) service kits* that did not only include sparkplugs, filters and oil, but also plugcaps. Now in my experience they live longer than 10k but they do detoriate and resistance will increase.
*I believe the brand was Dino.

[florence]

I would second that, the old caps had been on for 24 years and I think the plugs were the same age, so it was about time really.