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Fred Winterburn
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Joined: April 17, 2013
Posts: 423
Location: Ripley Ontario Canada
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 Posted: Thu Dec 05, 2013 7:59 pm Post subject: |
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| Juanito84 wrote: |
Yep, you got it!
As far as retarding one over the other not causing flame "collision" depends on several factors. The flame lasts for several degrees, picking up speed as it burns. Some flames can take over 50 degrees of crank rotation from start to finish even in ideal conditions. Lets say one is timed at 20 BTDC and the other at 0 TDC. The goal is that both end between 10 to 15 ATDC but may last longer in poor conditions. The flames really don't get rolling until after TDC meaning the second plug has a very good chance of starting a flame. With the increasing pressure and heat caused by the burning fuel between 0TDC and 15ATDC both flames will take off and collide. Of course the first will be bigger but I'd imagine it would take a spark at at least 5 ATDC to not cause a second flame. |
Flame front propogation and total fuel burn time are two different things. A second spark several crankshaft degrees retarded from the first spark will do nothing. It is wasted. Fred |
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Juanito84
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Joined: March 17, 2012
Posts: 2436
Location: Colorado Mountains
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| Posted: Thu Dec 05, 2013 8:26 pm Post subject: |
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| Fred Winterburn wrote: |
| Juanito84 wrote: |
Yep, you got it!
As far as retarding one over the other not causing flame "collision" depends on several factors. The flame lasts for several degrees, picking up speed as it burns. Some flames can take over 50 degrees of crank rotation from start to finish even in ideal conditions. Lets say one is timed at 20 BTDC and the other at 0 TDC. The goal is that both end between 10 to 15 ATDC but may last longer in poor conditions. The flames really don't get rolling until after TDC meaning the second plug has a very good chance of starting a flame. With the increasing pressure and heat caused by the burning fuel between 0TDC and 15ATDC both flames will take off and collide. Of course the first will be bigger but I'd imagine it would take a spark at at least 5 ATDC to not cause a second flame. |
Flame front propogation and total fuel burn time are two different things. A second spark several crankshaft degrees retarded from the first spark will do nothing. It is wasted. Fred |
Well I may be wrong, and propagation does has everything to do with it. But as far as I understand the flame doesn't propagate through half the cumbustion chamber until at least 3/4 into the flame duration, usually just after TDC. That, in my mind, still leaves a big chance for the second plug to create a flame even several degrees after the first. Now even if that new flame only grows a miniscule amount before the first flame engulfs it you still have two flames technically "colliding". True, if the first flame propogates in direction of the second plug and covers it rather quickly then you won't have a second flame. But then again if you already have flame propagation at the location of the second plug only a few degrees after the first plug fired, what would be the advantage of starting a propagating flame there just a few degees before?
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If a water cooled engine cools its water with air, isn't it just an overcomplicated air cooled engine? |
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Fred Winterburn
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Joined: April 17, 2013
Posts: 423
Location: Ripley Ontario Canada
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| Posted: Thu Dec 05, 2013 8:54 pm Post subject: |
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[quote="Juanito84"][quote="Fred Winterburn"]
| Juanito84 wrote: |
Yep, you got it!
As far as retarding one over the other not causing flame "collision" depends on several factors. The flame lasts for several degrees, picking up speed as it burns. Some flames can take over 50 degrees of crank rotation from start to finish even in ideal conditions. Lets say one is timed at 20 BTDC and the other at 0 TDC. The goal is that both end between 10 to 15 ATDC but may last longer in poor conditions. The flames really don't get rolling until after TDC meaning the second plug has a very good chance of starting a flame. With the increasing pressure and heat caused by the burning fuel between 0TDC and 15ATDC both flames will take off and collide. Of course the first will be bigger but I'd imagine it would take a spark at at least 5 ATDC to not cause a second flame. |
There would be no advantage to the second flame front if it only gained a miniscule amount of time. I think the point I was making was that for a second spark plug to make a difference, it had to compensate for a poor head/piston design in most cases. The flame does progress pretty quickly but the burn time is a separate matter. I keep reading conflicting reports which I think is because the authors are mixing up the two. One thing is absolutely for sure, a multi-spark ignition with only one spark plug does nothing to improve ignition if the first spark was a quality spark and not timed incorrectly (ie, too far advanced)./ I will aquiesce and admit (now that I have been corrected) that under some circumstances a twin plug set up will increase power and economy by increasing the burn rate. By the same token, with an aircraft engine that was timed on high octane gas and has to use low octane gas on occasion, it's a good idea to time one mag retarded so that it will still provide a redundant spark for reliability, but usually wastes the spark to prevent detonation. The detonation is not caused by colliding flame fronts as I was taught , but rather fast burn times with low octane and the timing too far advanced. The mechanism for this is something I'll have to ruminate on. Fred |
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Juanito84
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Joined: March 17, 2012
Posts: 2436
Location: Colorado Mountains
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| Posted: Thu Dec 05, 2013 9:37 pm Post subject: |
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Ok, I think I see our missunderstandings. I never ment that retarding one spark won't cancles its advantages. That's absolutely true. What I ment in my first post on this subject was that
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| The cure is to time one mag slightly later than the other so that its spark is essentially wasted |
doesn't guarentee that two flames, although perhaps very different in size, won't exist and "collide". Like we both have said, a later second spark forfiets its benefits, but I don't see how "slightly later" guarentees the impossibility of a second flame. Just like you said it is "essentially wasted". But was there a moment when gas was pinned between two oncoming flames somewhere in the cylinder. I believe so.
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Juanito84
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Joined: March 17, 2012
Posts: 2436
Location: Colorado Mountains
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| Posted: Thu Dec 05, 2013 9:53 pm Post subject: |
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I was wondering, Mr Raby, you mentioned using dual spark only over an 11:1CR. I take that to be static CR, correct? What kind of dynamic CR do you think dual spark can help and handle, if you don't mind me asking? And if it's more about super high profile cams that have a long valve duration that also help allow an >11:1 CR does that mean there's no real benefit of dual spark with a not so long duration cam or even a low end torque cam? Or does dual spark make 11:1 CR possible with just about any cam?
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Jake Raby
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Joined: August 23, 2003
Posts: 7433
Location: Aircooled Heaven USA
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| Posted: Fri Dec 06, 2013 5:49 am Post subject: |
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Twin plugs love cylinder pressure. I was not dropping dynamic cr by using a long cam, doing that would have been counterproductive.
I started playing with this a long time ago with ACVW engines and had our first twin plug feature way back in 2000 in the July issue of VW Trends.
Twin plugs open doors to radically positive results, if the engine is designed properly and the head work is done optimally.
I've ran an 11.4:1 engine making 240 HP on 87 octane fuel.
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Jake Raby
Raby Engine Development
www.rabyenginedevelopment.com
"I've never given anyone Hell, I just told them the truth and they thought it was Hell" |
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Juanito84
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Joined: March 17, 2012
Posts: 2436
Location: Colorado Mountains
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| Posted: Fri Dec 06, 2013 10:22 am Post subject: |
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Thank you Mr. Raby!
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