4 Valve Head: Single Central Spark Plug vs Dual Spark Plugs

[nicholastanguma]

I'd imagine it's about acceptable misfire limits, with throttled driving and EGR there's a lot of incombustible mixture in the chamber and only one little spark to ignite the combustible portion. Still if things get hot enough from compression you can still have combustion after the timed ignition point.

Good points, actually, thanks for posting.

[MetricMuscle]

There are plenty of modern moto engines--modern I suppose meaning designs from the last 20 years or so--that use twin plugs, but it seems to me there are more with single plug. Admittedly, I've never taken an official survey, so I could be wrong about this.

In former times, when two valve head design was the norm, some form of hemispherical combustion chamber shape was the norm, too. Ergo twin plugs provided some real performance benefits, however small or large per application.

Modern combustion chamber design favors compactness and large squish area, and coupled with modern flat top piston design, I'm kind of having trouble believing that twin plugs provide better performance than a single centrally located plug.

That being said, I obviously don't know for certain, or else I wouldn't have started this thread. If I'm wrong please do correct me.

There does seem to be more to this magical trickery than we are aware of. Honda even uses dual plugs on the VLX 600 Shadow, a small bore non-performance minded, air cooled, 3-valve engine.



It's also interesting how Feuling went about designing his heads. Still uses lots of valve angle which creates higher volume CC which compromises piston design by needing some pop up. In the YouTube video he also lists one of the advantages of his 3-valve design as being able to run more timing which I thought is not the goal.

[elle]

The new H-D Milwaukee 8's four valve design works very well in stock form, but when high performance tomfoolery gets thrown into the mix, such as high comp pistons, high lift cams, all that sort of good stuff, the bridge between the engine's two exhaust ports gets overheated beyond factory design parameters.

Overheating the wall between the two exhaust valves indicates slow burn of the mixture. The out of center spark plug on the KTM RC8 TwinSpark engine gets shut off above 6500rpm when the piston induced charge motion / turbulence is high enough for fast burn.

[dorset]

In former times, when two valve head design was the norm, some form of hemispherical combustion chamber shape was the norm, too. Ergo twin plugs provided some real performance benefits, however small or large per application.

i race today in the former times, a 650cc triumph vertical twin with two valves and two plugs per cylinder, symmetric. the 1930s combustion chamber is a true hemisphere, negligible squish. stock timing was 38btdc for 9:1. at 11.75:1, immense domes separate the chamber into two burn regimes. i run retarded at 30btdc.

it works very well. with two plugs, i had detonation opposite the plug:



with four, i do not, and its faster:

Modern combustion chamber design favors compactness and large squish area, and coupled with modern flat top piston design, I'm kind of having trouble believing that twin plugs provide better performance than a single centrally located plug.

no room for me to experiment with that, unfortunately. the pistons are prehistoric:

[nicholastanguma]

In the YouTube video he also lists one of the advantages of his 3-valve design as being able to run more timing which I thought is not the goal.

I, too, was a bit perplexed when I heard him say that. The Fueling head's large squish band and very compact combustion chamber mean a higher comp can be used, and the whole point of the single centrally located plug is supposedly to be better than the twin plugs, so...?

At this moment I still don't quite get it.

[nicholastanguma]

i race today in the former times

You and I could probably have a political discussion and walk away without hating one another.

[Tom68]

In the YouTube video he also lists one of the advantages of his 3-valve design as being able to run more timing which I thought is not the goal.

It may be that the chamber is cooler allowing the extra timing. More exposed water jacketed aluminium to absorb heat.

[dorset]

You and I could probably have a political discussion and walk away without hating one another.

more than liklely.

its a complicated world, and the people who look for simplicity seldom find it.

[elle]

In the YouTube video he also lists one of the advantages of his 3-valve design as being able to run more timing which I thought is not the goal.

It may be that the chamber is cooler allowing the extra timing. More exposed water jacketed aluminium to absorb heat.

You would only want more timing (ignition advance) for more power, if the heat release rate (dQ/dt) is too low for your specific application. Generally speaking: the higher the heat release rate is, the higher thermal efficiancy will be. Which is exactly what everyone wants, more mechanical work out of chemical heat.

Aluminum as a material is not used to absorb heat. It is used because of its specific weight and thermal conductivity.

[ptuomov]

In the YouTube video he also lists one of the advantages of his 3-valve design as being able to run more timing which I thought is not the goal.

I, too, was a bit perplexed when I heard him say that. The Fueling head's large squish band and very compact combustion chamber mean a higher comp can be used, and the whole point of the single centrally located plug is supposedly to be better than the twin plugs, so...?

At this moment I still don't quite get it.

The way I would interpret that is that is as follows: there’s less need to retard the ignition timing from the best torque ignition timing for lower octane fuel.

[nicholastanguma]

The way I would interpret that is that is as follows: there’s less need to retard the ignition timing from the best torque ignition timing for lower octane fuel.

Thank you.

[hoodeng]

"It's a complicated world, and the people who look for simplicity seldom find it". As quoted by Dorset.

Never truer words have been said!

When Frank Whittle was showing Rolls Royce around his developing W1 centrifugal flow jet engine he espoused it's simplicity, the representative from Rolls Royce [Hooker or Hives] replied ,
" Don't you worry about that, we will soon design the simplicity out of this thing".

Another i think was attributed to L Iacocca of Chrysler fame when questioned about designing components/processes to be simple, said something to the effect of,
" Some things end up complicated in their design just to work properly"

Cheers.

[David Vizard]

Quote:- when the two flame fronts inside the combustion chamber collide with such force they produce shockwaves which can damage the engine. No that is not what happens!

See my Youtube video Episode 67 power sparks and how to use them

DV

[nicholastanguma]

Quote:- when the two flame fronts inside the combustion chamber collide with such force they produce shockwaves which can damage the engine. No that is not what happens!

See my Youtube video Episode 67 power sparks and how to use them

DV

Holy moly, David Vizard himself responded to one of my threads.

[Tom68]

See my Youtube video Episode 67 power sparks and how to use them

DV

I see you mention surface discharge, I turned the threads off these used Nascar plugs and ran them projected in an old school 327 on LP Gas.

I was firing them with a MSD 6AL switched by points, when the 6AL failed I just went back to kettering ignition, never missed a beat, even though LP is supposed to be hard to fire.

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