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Juanito84
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Joined: March 17, 2012
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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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If a water cooled engine cools its water with air, isn't it just an overcomplicated air cooled engine? |
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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
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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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If a water cooled engine cools its water with air, isn't it just an overcomplicated air cooled engine? |
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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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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 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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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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Jake Raby
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Joined: August 23, 2003
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Location: Aircooled Heaven USA
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| Posted: Thu Dec 05, 2013 7:35 pm Post subject: |
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Combustion chamber shape and chamber filling have everything to do with proper twin plug placement and flame propagation.
The larger the bore and the bigger the chamber, the more efficient an opposing twin plug arrangement will be.
There are benefits from chaning dwell times, splitting up the timing and etc, but I typically only found those gains beneficial when hunting for high MPG. Thats how I got 51 MPG from a 140HP engine back in 2005.
Two flame fronts colliding is whats supposed to happen, thats why twin plug engines optimize with much less overall timing, in my experience 22-24 BTDC is maximum timing for a twin plug engine. With twin plugs my CR starts at 11:1, else there's no benefit.
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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: Thu Dec 05, 2013 7:24 pm Post subject: |
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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.
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Fred Winterburn
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Joined: April 17, 2013
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Location: Ripley Ontario Canada
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| Posted: Thu Dec 05, 2013 6:43 pm Post subject: |
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| Juanito84 wrote: |
| Fred Winterburn wrote: |
| SGKent wrote: |
| it is done in aircraft for redundacy. Twin magnetos twin plugs. They can't just pull over when it comes to problems and fix a wire. |
Yes, And not only that, If you are running twin mags that are timed close together and you are running low octane gas, watch out. When the sparks occur nearly simultaneously, two flame fronts can collide and cause some serious engine damage over time. The cure is to time one mag slightly later than the other so that its spark is essentially wasted. Fred |
I'm afraid the flame front collission theory is a myth. First of all making them out of sync won't keep two flame fronts from "colliding". And what damage could two flames coming together really do? However there is one danger involved with twin spark. With two flames the cylinder pressure can rise over twice as fast. If it gets two high then detonation can set in. Usually you want the flames to end around 10 to 15º ATDC. Since two flames burn faster than one that's why dual spark engines should be timed retarded when compared to single spark.
You are right about combustion chamber design. A poor design leads to a slow flame. In ICE's the faster the flame the better short that of detonation. |
Well, I stand corrected and what you describe explains why there is detonation with cheap fuel in the Continental aircraft engines. Having one spark well retarded means the first and only flame front has taken more time to burn and hence no detonation. With higher octane fuel the flame fronts would expand more slowly and not result in detonation. You have solved the paradox that was threatening to short circuit something in my brain. However, I disagree with what you said about timing them differently not resulting in colliding flame fronts. Given how fast combustion actually occurs, retarding one spark far enough will effectively make it a wasted spark. That's how detonation was and is prevented with cheap fuel and twin mags in some aircraft engines. Thanks, 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 5:28 pm Post subject: |
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| Fred Winterburn wrote: |
| SGKent wrote: |
| it is done in aircraft for redundacy. Twin magnetos twin plugs. They can't just pull over when it comes to problems and fix a wire. |
Yes, And not only that, If you are running twin mags that are timed close together and you are running low octane gas, watch out. When the sparks occur nearly simultaneously, two flame fronts can collide and cause some serious engine damage over time. The cure is to time one mag slightly later than the other so that its spark is essentially wasted. Fred |
I'm afraid the flame front collission theory is a myth. First of all making them out of sync won't keep two flame fronts from "colliding". And what damage could two flames coming together really do? However there is one danger involved with twin spark. With two flames the cylinder pressure can rise over twice as fast. If it gets two high then detonation can set in. Usually you want the flames to end around 10 to 15º ATDC. Since two flames burn faster than one that's why dual spark engines should be timed retarded when compared to single spark.
You are right about combustion chamber design. A poor design leads to a slow flame. In ICE's the faster the flame the better short that of detonation.
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Fred Winterburn
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Location: Ripley Ontario Canada
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| Posted: Thu Dec 05, 2013 4:45 pm Post subject: |
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| Fred Winterburn wrote: |
| SGKent wrote: |
| it is done in aircraft for redundacy. Twin magnetos twin plugs. They can't just pull over when it comes to problems and fix a wire. |
Yes, And not only that, If you are running twin mags that are timed close together and you are running low octane gas, watch out. When the sparks occur nearly simultaneously, two flame fronts can collide and cause some serious engine damage over time. The cure is to time one mag slightly later than the other so that its spark is essentially wasted. Fred |
Of course, based on a little internet snooping, it depends on the engine. I was referring to a 65HP Continental engine as fitted to a J3 Cub, not a Porsche. This is a good link for explanation for why a twin plug set up would be a good idea. In this case it seems to be a band aid for a poor combustion chamber design. Fred http://rennsportsystems.com/letstalk-2/gasoline-detonation-twin-ignition/ |
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Fred Winterburn
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Location: Ripley Ontario Canada
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| Posted: Thu Dec 05, 2013 3:39 pm Post subject: |
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| SGKent wrote: |
| it is done in aircraft for redundacy. Twin magnetos twin plugs. They can't just pull over when it comes to problems and fix a wire. |
Yes, And not only that, If you are running twin mags that are timed close together and you are running low octane gas, watch out. When the sparks occur nearly simultaneously, two flame fronts can collide and cause some serious engine damage over time. The cure is to time one mag slightly later than the other so that its spark is essentially wasted. Fred |
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Steve Arndt
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Joined: August 01, 2005
Posts: 1797
Location: Boise, Idaho
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| Posted: Thu Dec 05, 2013 1:21 pm Post subject: |
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| Raby recommends 10mm plugs on the bottom side. |
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[email protected]
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Juanito84
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Location: Colorado Mountains
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| Posted: Thu Dec 05, 2013 10:02 am Post subject: |
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| Steve Arndt wrote: |
| Juanito84 wrote: |
| Is there anyone else for the job? |
RIMCO |
Muchos thank yous!
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Steve Arndt
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Location: Boise, Idaho
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| Posted: Thu Dec 05, 2013 9:53 am Post subject: |
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| Juanito84 wrote: |
| Is there anyone else for the job? |
RIMCO |
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Juanito84
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Posts: 2436
Location: Colorado Mountains
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| Posted: Wed Dec 04, 2013 9:38 pm Post subject: |
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So it seems this thread recomends either Great Planes, expensive pre tapped heads or do-it yourself. If I do contract Great Planes to do it will it mess with the quench area? I'd like to run a 0.040" quench but it looks like the plugs may protrude. Is there anyone else for the job?
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vwkirb
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Location: Athens, GA
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| Posted: Tue Sep 10, 2013 6:12 pm Post subject: |
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| Jake Raby wrote: |
.I made 250HP on pump gas last year with a 2.9 Twin Plug engine in my Wife's GL Vert and it still averaged 30 MPG.
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I think that is more better power to mpg than the 2013 Beetle. Hummm...... |
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gears
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| Posted: Mon Sep 09, 2013 12:46 pm Post subject: |
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When it comes to engines, there's a lot I don't understand, so I don't pretend to .. like how does an additional spark plug DOWNSTREAM of the exhaust valve make a top fuel car go faster?
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