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Ignition , combustion statement
Posted: Thu Dec 31, 2020 9:38 am
by Truckedup
This was said to me by a well known bike tuner. I said to him that lean mixtes are more prone to misfire. This is his reply ,is it accurate?
don't confuse ionization with combustion they are two different events. Air/Fuel mixture is an insulator with out any free radicals which makes it difficult to establish the spark. It is the spark created after the air ionizes that propagates combustion. Ionization is aided by heat and additional heat with combustion can actually be detrimental. An air/fuel mixture between the plug gap will ionize easier when the fuel/air mixture and the spark plug electrode is hotter. So it isn't a stretch to say an engine running on a lean mixture on the whole will be warmer than a rich mixture, thus it will be easier to establish ionization. This is contrary to combustion which is as you described, where it is harder to propagate combustion with a lean mixture.
Re: Ignition , combustion statement
Posted: Thu Dec 31, 2020 10:03 am
by RevTheory
It sounds like he didn't want to miss an opportunity to try and make himself look really smart to end up agreeing with your initial statement. Ask a guy for a light and you get the history of fire, lol.
Re: Ignition , combustion statement
Posted: Thu Dec 31, 2020 10:17 am
by panic
From "Nashville" (Robert Altman film): "You asked a lawyer what time it is, and he told you how to make a watch".
Re: Ignition , combustion statement
Posted: Thu Dec 31, 2020 10:32 am
by Momus
Is combustion exhaust a gas or a hot, slippery plasma as it passes the valve?
Re: Ignition , combustion statement
Posted: Thu Dec 31, 2020 10:49 am
by Truckedup
My simple mind thinks he is saying that first you need ionization that occurs easier with a lean mixture between the plug electrodes.....Then ,the actual burn is hard to start with a lean combustion chamber mixture. So he claims it's two events, yes?
Re: Ignition , combustion statement
Posted: Thu Dec 31, 2020 12:20 pm
by rebelrouser
I do not have an engineering background, just messed with engines and cars all my life, and do a little reading. One device that taught me a little about ignition systems was a old Champion spark plug tester. I worked in a shop that had one, and before fuel injection, the first big cold snap every year, we would always get cars towed in that would not start. The majority were due to gas fouled spark plugs because the vacuum pulloff on the carb went bad during the summer, and now the engines were being over choked. You could screw a fouled plug into that machine, and you had a window to watch as you sparked the plug and applied air pressure, until the spark winked out, a number on a dial gave you a relative state of the plug. Black hydrocarbons on the plug are much easier for the energy to follow than even trying to jump a gap. A fouled plug simply had yellow fingers of energy moving over the base, nothing would jump the gap. A clean worn plug would jump a good gap but the more rounded or worn the electrode the sooner the air pressure would blow out the arc. Just from that machine, the distance of the gap, the condition of the sharp edges on the electrode that the arc has to jump, and the amount of pressure or turbulence in the cylinder are the main factors in firing a plug. Everything I see and read about the design of racing spark plugs, gapping them etc. All relate very simply to that old spark tester.
And just to throw it out there, what brand new plug would fire the best on that machine? A Champion, at least in the late 70's when I was using it.
Re: Ignition , combustion statement
Posted: Thu Dec 31, 2020 1:57 pm
by hoffman900
Truckedup,
Same tuner you quoted here?
viewtopic.php?t=44265
Re: Ignition , combustion statement
Posted: Thu Dec 31, 2020 2:09 pm
by David Redszus
For a given spark plug gap, the required firing voltage is increased with increased cylinder pressure
and reduced with increased heat. And required voltage is increased nearly proportional with increased plug gap.
During the ignition delay phase, laminar flame properties control flame kernel development up to
approx 5% MFB. Thereafter, chamber turbulence resulting from squish velocity becomes the controlling factor.
The presence of combustion instability indicates that both ignition delay and turbulence are quite
variable and unstable. Watching combustion pressure traces on a scope resembles a Van deGraff generator.
The more uniform the mixture preparation, the more even the combustion becomes.
In the end, heat, mixture preparation, and turbulence are much more important than spark production.
Re: Ignition , combustion statement
Posted: Thu Dec 31, 2020 4:08 pm
by Truckedup
Re: Ignition , combustion statement
Posted: Thu Dec 31, 2020 5:03 pm
by gunt
ok
so i have see this before worded differently , look up the oldone.com , larry does post here , its in his archive i believe , but he refers it to stratospheres within the cylinder made up on the intake stroke .
now he did make use of it at the time on a high comp engine running what would be considered lean to produce mpg and still maintain power
but every gdi engine now a days fully utilities these processes within the cylinder , under full control up to 54: with out missfire
Re: Ignition , combustion statement
Posted: Thu Jan 07, 2021 1:41 pm
by Firedome8
I still use my champion spark plug cleaner tester. This is a scope display at full load 10 psi boost.
https://youtu.be/vOmPW-ze3Tc
Re: Ignition , combustion statement
Posted: Thu Jan 07, 2021 11:12 pm
by BLSTIC
gunt wrote: ↑Thu Dec 31, 2020 5:03 pm
ok
so i have see this before worded differently , look up the oldone.com , larry does post here , its in his archive i believe , but he refers it to stratospheres within the cylinder made up on the intake stroke .
now he did make use of it at the time on a high comp engine running what would be considered lean to produce mpg and still maintain power
but every gdi engine now a days fully utilities these processes within the cylinder , under full control up to 54: with out missfire
That would be because GDI, when combined with proper design and timing, can put different mixtures in different parts of the cylinder. Ideal is a rich pocket around the plug that ignites easy, with a lean mixture around it that is harder to light with a spark alone but burns easily when lit with a real flame.
FWIW the Gen1 Honda Insight got 24-28:1 with port injection, but only under specific conditions
Re: Ignition , combustion statement
Posted: Fri Jan 08, 2021 12:51 pm
by David Redszus
The best mixture ratio in the spark plug gap is stoich; neither rich nor lean.
But the best mixture in the chamber is slightly rich (depending on fuel type and conditions) which
is necessary to obtain stoich in the plug gap.
Re: Ignition , combustion statement
Posted: Sun Jan 17, 2021 10:47 am
by gunt
sorry to correct , but they don not need the chamber to be rich to have the plug at stoich , in fact they are able to have the total outter section in near full exhaust gasses with the used or egr , which includes running it through the vvt , so in the case of mitsubishi , where the total ratio is 54:1 and we know the area around the plug is rich the rest better be extremely lean to obtain the overall ratio of 54 .
Larry i believe ran an old vw at over 17: bk in the 80s on carbs ,
Re: Ignition , combustion statement
Posted: Tue Jan 19, 2021 12:53 am
by David Redszus
gunt wrote: ↑Sun Jan 17, 2021 10:47 am
sorry to correct , but they don not need the chamber to be rich to have the plug at stoich , in fact they are able to have the total outter section in near full exhaust gasses with the used or egr , which includes running it through the vvt , so in the case of mitsubishi , where the total ratio is 54:1 and we know the area around the plug is rich the rest better be extremely lean to obtain the overall ratio of 54 .
Larry i believe ran an old vw at over 17: bk in the 80s on carbs ,
I assume you are speaking about GDI and not normal fuel port injection or carburetors.
In that case normal nomenclature cannot be used since GDI is in effect a stratified charge engine.
Much like a diesel, normal a/f ratios do not apply since we are spraying a rich mixture at the spark plug
within a lean chamber. The mixture in the plug gap is still very near stoich or richer.
As you know, 54:1 is only a nominal value since it is far below the lean burn limit. The actual
burned mixture ratio is much closer to conventional engines. The main advantage is the GDI, like
a diesel, will burn all available fuel.