A short study on ignition timing and combustion

john robert bould · Sep 29, 2014

Fast Eddie said:
john robert bould said:

The responce to how much power to fire two plugs, 40 years ago i drilled /tapped a second plug hole in my BSA single, simply split the HT lead , the differance to tick over was large! pull one plug and it slowed,hooked back on [rubber gloves] and it speeded up..so even a 6 volt battery had enough poke to fire both plugs...carnt say i conducted precise tests I guess it was like setting fire to a barn...blow lamp or two matchs...the barn burns quicker when a match is applied same time at Both ends!

Click to expand...

Hmmm...

A mate of mine was a serious Velo racer in BHR. He set up one of his Velo race bikes for twin plugs. He found exactly the same as you did John.

We both thought 'well there you go, that's PROOF that it is better'... Simples innit?!

But when he put it on the dyno it produced LESS power with twin plugs!

Carnt see why it made less power, unless he didnt retard to suit?

I'm not sure if he ever got to the bottom of it. I simply concluded that the whole twin plug thing, in fact the whole sequence of events that happens inside the combustion chamber, is a tad more complex than I had realised!

lcrken · Sep 29, 2014

comnoz said:
Believe me, I have looked hard at the possibility vs the difficulty of installing a second plug. It could be done and I have enough alternator to power it. It would not be easy and some head finning would have to be sacrificed. Since the ignition I have has done a pretty good job of delaying pressure before TDC I am not sure how much could be gained.

I actually have a plasma ignition unit here that I want to play with first. It will do much higher frequency than my modified MSD board. It will require going to individual coils instead of wasted spark as I have now, so some ECM changes and a cam sensor will be required. That will also let me run synchronized injection. Jim

I also doubt that there is much gain to be had from twin plugs in a Commando head. It's already a pretty good design for a 2-valve head, as indicated by the fact that it needs so much less advance than less efficient designs like the same period Triumphs and Harleys.

Were you looking at doing something like this old MEZ conversion on a 750 head? Looks like it would be a real PITA to do.

A short study on ignition timing and combustion

Ken

WZ507 · Sep 29, 2014

Fast Eddie said:
Hmmm...

A mate of mine was a serious Velo racer in BHR. He set up one of his Velo race bikes for twin plugs. He found exactly the same as you did John.

We both thought 'well there you go, that's PROOF that it is better'... Simples innit?!

But when he put it on the dyno it produced LESS power with twin plugs!

I'm not sure if he ever got to the bottom of it. I simply concluded that the whole twin plug thing, in fact the whole sequence of events that happens inside the combustion chamber, is a tad more complex than I had realised!

This is too obvious a question, but.... on the dyno did they just do a couple rips and render a verdict or did they sweep ignition timing to find the best power spark advance?

jseng1 · Sep 29, 2014

I used to fly small planes. Before each take off I would run the motor at full power and switch off each magneto one at a time (dual magnetos). The power would drop several hundred RPM with only one magneto. Pretty obvious.

worntorn · Sep 29, 2014

I have twin plug heads on my 1360cc Vincent project bike. Andy at Pazon made up a special Smatfire ignition for this, it uses two drivers, one dedicated to each two plug high efficiency coil. He normally uses one driver, even for two plug Smartfire igntion, but recommends the dual driver setup if you are running high compression. This is a high draw EI system and requires a good charging system, though nothing too outrageous.
The two plug ignition in itself might only add a few horsepower, however it allows you to roll the timing back a considerable amount, in this case, with quench heads, 10.5 to one CR and twin plugs, 24 degrees total advance vs 38 degrees for stock non quench, single plug, low compression heads. Note that all three of those listed differences play a part in the timing number reduction. Two plug ignition alone is good for five to seven degrees of timing retardation. He also/made up a custom igniton curve to my specs, which were determined after much consultation with various experts.
So it is not just the power advantage of the two flame fronts that helps, but also the ability to run very high compression without fear of detonation. Then there is the added positive effect of being able to have the spark occur later to reduce power loss due to high cylinder pressure before TDC of compression stroke, as discussed earlier in this thread.
The two flame fronts are also thought to be more effective as bore size goes up.
I mainly wanted two hot sparks happening to give every advantage to ease of kick starting, which was bound to be difficult with the displacement, head type and CR. I figure one hot spark is better than one mediocre or weak spark and two hot sparks are better yet. It seems to work, as starting is excellent provided the engine rolls thru.
As far as power is concerned, I suppose I could remove one plug lead and ground it, but really the timing should be advanced for single plug operation, then pinging becomes a potential issue.
I can attest that the throttle response is brutal, takes some getting used to!

So the dual plugging might be more effective for all out high performance modded Commandos, especially big bore versions . For a standard 8 or 9 to one bike , kickstarting is easy and there is generally no problem with pre igntion, so much of the dual plug advantage is lost.

Glen

Triton Thrasher · Sep 29, 2014

jseng1 said:
I used to fly small planes. Before each take off I would run the motor at full power and switch off each magneto one at a time (dual magnetos). The power would drop several hundred RPM with only one magneto. Pretty obvious.

It's not so simple. Twin plug setups run with less advance. If only one of the two plugs sparks, the timing is too retarded.

acotrel · Sep 29, 2014

Triton Thrasher said:
jseng1 said:

I used to fly small planes. Before each take off I would run the motor at full power and switch off each magneto one at a time (dual magnetos). The power would drop several hundred RPM with only one magneto. Pretty obvious.

Click to expand...

It's not so simple. Twin plug setups run with less advance. If only one of the two plugs sparks, the timing is too retarded.

That has always been the problem with twin plug heads on bikes. I can remember one guy in particular with a Triumph twin, who fitted the extra plugs, by skew machining into the dead space on the other side if the combustion chamber. It worked very well, however in the situation where something causes a ding in a valve, you might feel the drop in performance if you were practising on the bike, I don't believe you would ever feel it during a race. It once happened to me, and I kept racing the bike for several meetings before I found the cause of it going slightly off-song. I don't believe you would ever know if one of the two plugs in one cylinder had fouled, the difference would be like feeling a small continuous gust of wind while you are riding. You would be relying on memory too much to make you aware of the performance drop - impossible.

Fast Eddie · Sep 29, 2014

jseng1 said:
I used to fly small planes. Before each take off I would run the motor at full power and switch off each magneto one at a time (dual magnetos). The power would drop several hundred RPM with only one magneto. Pretty obvious.

All that's obvious there Jim is that the revs dropped.

On the Velo engine, we saw the same, if set to tickover nicely on one plug, when the second was added, the revs were higher. If set to tickover nicely on two plugs, when one was pulled it would stall, but this seemingly obvious occurrence, did not equate into a measured power gain (it was slightly less)

I tried twin plugs on a Triumph twin, it was a bit of a casual experiment using the centre plugs and standard outer plugs together, this had no effect, positive or negative at all!

I suggest that if it really were obvious that twin plugs were better, then they'd be standard on all engines. As it is, they are a rarity. So I'd say they do work, but only in very specific circumstances.

john robert bould · Sep 29, 2014

What ever means of firing the mixture, fact is the energy cannot exceed the fuel /air mix...it may be slower with one plug,quicker with two...the BTUs are same. The power differance with two plugs is just differant..thats all.

worntorn · Sep 29, 2014

There is added cost and complexity involved in using a dual plug ignition system, this alone is enough to prevent some manufacturers from using it.
Ducati did nothing more than add a second plug per cylinder to their MultiStrada in 2012, they reported a 5% increase in torque and 10 % increase in fuel efficiency.
Some BMWs use two plug heads.
The newish Chrysler Hemis use two plug heads.
When a new Porsche gets the Factory horspower upgrade treatment involving a change to an even higher compression ratio than stock , all engines get switched to dual ignition two plug heads in order to make the higher CR work. Aside from preventing detonation, Porsche claims the two plugging on it,s own adds about 3% to Max HP and 5 percent to midrange power.
So there are quite a number of examples of two plug heads in automotive or motorcycle use, but all high CR high output engines, not really very similar to our Commando engines.

Glen

WZ507 · Sep 29, 2014

john robert bould said:
What ever means of firing the mixture, fact is the energy cannot exceed the fuel /air mix...it may be slower with one plug,quicker with two...the BTUs are same. The power differance with two plugs is just differant..thats all.

I agree that the energy can not exceed the BTU "potential" of the fuel air mixture present, but the actual BTU output does not necessarily get anywhere near entitlement, as the actual output is a function of Otto cycle efficiency, which in turn is a function of compression ratio (CR), combustion chamber design, ignition system/timing, etc. If 2 engines are essentially the same, and differ only in compression ratio, where one engine has 5:1 and the other engine has 15:1 CR, the Otto cycle efficiency of the high CR engine is nearly 50% higher than the low compression analog!

Hobot · Sep 29, 2014

Do realize that for dual plugs both to fire - require two separate ignition systems or all the spark will cross lowest resistance gap - until it fouls up resistance enough the spark jumps across the other instead. I think if the wasted spark dual plugs systems like our ole boyah's were installed in same chamber they'd both fire though would need second system for another chamber. To extract most the combustion power requires getting as much can of it burnt after TDC but before 20' ATDC. To get a handful % of more power out of already highly tweaked engines is significant. The all out single Goldstars racers run dual plugs. There was example of dual plug Commando head done by same shop that once offered bathtub heads.

acotrel · Sep 30, 2014

john robert bould said:
What ever means of firing the mixture, fact is the energy cannot exceed the fuel /air mix...it may be slower with one plug,quicker with two...the BTUs are same. The power differance with two plugs is just differant..thats all.

If you look at a 12 to 1 comp. piston out of a Triumph, after it has been raced for a long time, the side of the crown away from the plug is usually pretty coked up, while the side near the plug can almost be bare. If you simply centre plug the head, that seems better. I did it by welding a pad over the centre and using a 10mm plug set well back and firing through a slot as per 50s Gilera practice, and I added about 4 degrees to the ignition advance. The problem was that the welding softened the head, and a crack appeared behind one of the inlet valve guides. You can fit a 10mm plug without welding the head, however I think the threads become part of the valve seats. I don't think twin plugs or even central plugs would improve a Norton engine , you don't usually use high crown pistons to raise the comp. ratio. Which is probably also better because the piston weight is lower.

chasbmw · Oct 1, 2014

i have a feeling that in the 1950s Jaguar twin plugged their racing engines.....if BMW had twin plugged their airheads when unleaded fuel was introduced into the US, they would have not had to lower the compression ratio on their bikes.... but then they had to wait a further 30 years until Bosch came out with twin plug ignitions that were fitted to BMWs and Ducatis. works well with the wasted spark ignitions of BMw twins.

tricatcent · Oct 1, 2014

On one of my Triumphs I installed twin plugs. I was able to install the extra plugs in vertical holes exactly on the opposite side to the original plugs. No welding was required. I used 10mm plugs. The extra plugs enabled me to run 34 degrees advance rather than 38 degrees. The engine had 10.8 compression ratio, but would run perfectly on 92 octane gas. This also improved the stability of the idle. That had been a problem because this bike had 2 1/2 lbs taken off the flywheel. I think really is was a mistake using flywheel that light, although it does help the bike to accelerate really well in the lower gears. This is a road bike and it has run well that way for about 15,000 miles.

I think on a Triumph the twin plugs are more of an advantage than they would be on a Norton.

Nigel

jseng1 · Oct 1, 2014

jseng1 said:
I used to fly small planes. Before each take off I would run the motor at full power and switch off each magneto one at a time (dual magnetos). The power would drop several hundred RPM with only one magneto. Pretty obvious.

It was obvious with small planes. The prop is always there for load and any increase in RPM also means more power - for planes.

Hobot · Oct 1, 2014

A bit hard to picture where the 2nd plug sticks out in head and how they did it

texasSlick · Oct 1, 2014

jseng1 said:
jseng1 said:

I used to fly small planes. Before each take off I would run the motor at full power and switch off each magneto one at a time (dual magnetos). The power would drop several hundred RPM with only one magneto. Pretty obvious.

Click to expand...

It was obvious with small planes. The prop is always there for load and any increase in RPM also means more power - for planes.

Agree....

First Law of Thermodynamics: You cannot get something for nothing. If you get higher RPM on two plugs, (and it so occurs in small aircraft engines), then the engine has higher kinetic energy. If the engine is working against a load (prop, dyno, or just moving the vehicle down the road), higher RPM equates to the engine doing more work. This work represents "something", and there must be something else to pay the price...that something else can only be more fuel converted into heat by burning. Conclusion: two plugs burn more fuel in the cylinder rather than allow some to escape unburnt or to finish burning in the exhaust pipes. More fuel burned in the cylinder = more power or more fuel economy depending on engine operation.

Slick

acotrel · Oct 2, 2014

hobot said:
Do realize that for dual plugs both to fire - require two separate ignition systems or all the spark will cross lowest resistance gap - until it fouls up resistance enough the spark jumps across the other instead. I think if the wasted spark dual plugs systems like our ole boyah's were installed in same chamber they'd both fire though would need second system for another chamber. To extract most the combustion power requires getting as much can of it burnt after TDC but before 20' ATDC. To get a handful % of more power out of already highly tweaked engines is significant. The all out single Goldstars racers run dual plugs. There was example of dual plug Commando head done by same shop that once offered bathtub heads.

The better way to fire twin plugs might be with a double ended coil for each cylinder and no wasted sparks. That way if one plug fouls the motor would cut out on one cylinder and you would be aware that one plug has stopped working and can do the necessary. I think that in a combustion chamber which has a high crown piston poking up into it, twin plugs or a centre plug are necessary if you want to get the most. A uniform almost too-lean mixture in the combustion chamber would give the best results. What we are talking about is a chemical reaction - the oxygen needs to be close to the fuel molecules for max power. If part of the charge is unburned as happens in the normal Triumph combustion chamber the answer is obvious as far as specific output is concerned (power delivered per fuel consumed).In a Triumph head a single centre plug is probably sufficient to get an evenly distributed flame front. When I was using the Lucas SR4 magneto, I had the option of firing the twin plugs alternately. (I used to pair the leads to both plugs). I don't know what that would have achieved, and I discussed that with Phil Irving, who also did not say much except the swirl might have been a lot different with the flame front originating at different positions, and the plug readings might change when setting the main jets.

Hobot · Oct 2, 2014

I'm more a chemist than electrical engineer, so technically ignition is to somehow cause the reactants to reach or exceed their "activation" energies, which is basically adding enough mechanical thermal motion to cause outer electron shells to start absorbing and exchanging photons and electrons with exothermic excess to spread the reactions, short of occurring all at once everywhere. You can get a bit more power out of same gun powder charge with a bigger primer ignition centered in shell, same goes for engines. The thinner flatter more open the combustion space the faster more thoroughly the reactants can combine in the given flash off time. Don't know what dual plugs worth in a Norton but Jim's super duper sparker gives a sense it could be detectable. For Peel will try the plasma blast experiment using the powerarc as trigger and spark path to conduct the big back surge though. if we could project a tuned micro wave that resonated with the reactant molecules we could set most the charge off at once so could get nearly full combustion just after TDC and not so much pressure to work against so run cooler more efficient. Of course ideally we'd like it to become a diesel at hi rpm no need of a spark. The charge at TDC in hi CR engine is more a liquid than a gas btw.

A short study on ignition timing and combustion

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