Combustion Flame Speed

[automotive breath]

This timeline was developed to better illustrate what Rob explained about the combustion process. Burn rate is not steady, it starts slowly increasing rate as combustion temperatures rise, gaining speed along the way. At the end of the delay period, cylinder pressure begins to rise above cranking pressure.

The illustration shows reduction in burn time as a result of increased turbulence created with piston velocity as RPM rises.

Math is not my strength, make corrections as required.




I retract this statement corrected by both Beth & Rob. I will rethink and respond at a later date.

In practice I have found combustion chamber modifications that generate turbulence in the vicinity of the spark plug during the delay period have a pronounced effect on the entire burn. The objective is to reduce delay period and begin transition to turbulent flame sooner, the result is increased flame intensity and reduced burn duration.

[ford-swap]

Hey guys (and gals),

I figured I would check in now, as there are some really good things going on in this thread. I am by no means an expert in this field (though I am literally learning every day). Here are some thoughs.

Turbulence intensity in he vicinity of the spark plug, during the spark, is generally not regarded as a good thing. As the spark is generated, any significant turbulence in this region can push the developing flame kernal into the center electrode, or the ground electrode. These relatively cool surfaces can quench the flame, and cause either poor ignition, or a misfire if severe enough. Only after the flame kernal has developed enough to be of similar size to the turbulene in the chamber, can the turbulence actually 'shred' the flame front' and help to advance it.

As Beth stated, 2 valve vs 4 valve stuff can be significantly different. Our research group just recently got it's hands on a very new AMG Mercedes cylinder head. The chamber has zero traditional squish zones, though I can't comment on how the pistons interact with the chamber as, well I don't have those.

-Rob

[Tycho]

..Or could the tighter squish act to suppress detonation, allowing the engine to tolerate the timing it needed all along?

Agreed,but in that case it's just not as detonation-prone with tight squish. If it needs more ignition,it's not burning faster...it's just knocking less.

Indeed, this is a more appropriate assesment of the situation. Worth noting, the quench pads were similarly clean in the tighter quench engine (with the carbon buildup biased towards the exhaust).

I suppose the fact that both of these engines were street driven has a profound impact on the carbon buildup patterns as well seeing that most of their lives were spent at partial throttle.

Regardless, I see clean quench areas as an indication that the mixture is being effectively displaced into the combustion space dictated by the chamber and piston.

*Sorry, I've become a thread detractor rather than contributor. Good dicussion, I continue to read each post twice.*

[automotive breath]

...Turbulence intensity in he vicinity of the spark plug, during the spark, is generally not regarded as a good thing. As the spark is generated, any significant turbulence in this region can push the developing flame kernal into the center electrode, or the ground electrode. These relatively cool surfaces can quench the flame, and cause either poor ignition, or a misfire if severe enough. Only after the flame kernal has developed enough to be of similar size to the turbulene in the chamber, can the turbulence actually 'shred' the flame front' and help to advance it. ...-Rob

Rob,

Thanks for checking in. I’m trying to understand what you are saying and how it relates to the results I am seeing. Could it be that using squish action to direct turbulence into the flame kernel during development provides benefit because the squish velocity is very low due to the piston distance from the head? Or perhaps the turbulence is generated late in the delay phase?

Here is an example from xwave in the UK

Hi all,

Has been a while since my last post.
I wanted to give plenty of time to reasonably evaluate some results & driving a few thousand miles before I posted.

Well I can say I am very pleased with the results.
I have upped the boost a little more, and still:
Fuel economy has been at a steady 28-30 UK MPG, consisting of mainly longer runs on A roads /Motorway runs, including some town driving, and being stuck in slow moving traffic on the M6 for over 1 hour - which is to be expected if you live in the UK.
Engine has been smooth and Quiet
The Turbo spooling sound is noticable, unlike pre groove, where it wasnt noticable at all.
Plenty of low end torque (takes off from 2400 RPM)
Engine oil still clean.
More Power (from increasing boost a notch)
I cant / dont need to put my foot right down, as now the extra torque is causing some slipping of my clutch!
(my 16T will need to wait a little beore going in!)
The engine seems to be running cooler. (I say seems as I think my temp guage has temp compensation board, so its not as straight forward to read from the gauge accurately, but has never been past (or even up to) dead centre, even on the occasions we have had exceptionally hot days for the UK. Pre groove the temp gauge was pretty much dead centre, but difficult to give an accurate indication on this.

Today I decided to index the plugs (a little better by moving them to other chambers) , and at the same time take a look at them visually.

Visually, from the plugs, the engine appears to be running well.

I was a little surprised that the plugs were pretty spotless round the ceramic surround centre electode side facing towards the main groove (or centre of the chamber) with light carbon deposits on the opposite side (near the chamber wall).

Regarding the indexing of the plugs, as you can see from my head groove photo (below), the spark plug on the right was probably in the WORST position regarding indexing terms.
Interestingly, the white surround on the centre electrode was still spotlessly clean on the edge near the main groove, and the coating on this spark plug ground electrode was this light blue colour, talked about.



Furthermore, the exhaust tailpipe exit, has a nice coating of light grey carbon, suggesting a good running engine and mixture.
My older exhaust pre mod, was much more sooty.

Also, (probably not due to the grooves in the most part) Oil consumption is very much reduced to zero. This is probably because beore I was running Mobil 1 (5w 30) or (10W30) cos 10W 40 doesnt seem available in the UK. I think this was too thin. Im now running 10w40 Amsoil. I am also not getting any in my Oil catchcan like beore. Like I say, this is prob not due to the grooving.

Overall all very +ve, and Im very happy. Recommended.

[beth]

Turbulence intensity in he vicinity of the spark plug, during the spark, is generally not regarded as a good thing. As the spark is generated, any significant turbulence in this region can push the developing flame kernal into the center electrode, or the ground electrode. These relatively cool surfaces can quench the flame, and cause either poor ignition, or a misfire if severe enough. Only after the flame kernal has developed enough to be of similar size to the turbulene in the chamber, can the turbulence actually 'shred' the flame front' and help to advance it.

I agree completely. My opinion is squish turbulence does not normally reach the flame kernel during ignition delay. It depends on the engine physical properties, squish width, finish, and area but I would suspect it's about 10 degrees BTC before any significant changes occur. Uncontrolled squish action is an easy way to increase turbulence thus the reason it has been used for so many years, almost any squish is better than none as long as it is not too close to the spark plug location.

My experience has been any increase in burning quality and speed results in a measurable decrease in ignition lead and fuel use while increasing torque.

[KennyM]

Beth, It has been my experience also on the Ford N351 heads burning alky.
We ran 30 ° total timing and detonated gaskets out of the motor.
Had to reduce the total to 25° to get out of the problem even with correct air/fuel ratios.
I take it these heads are fast burn in compairison to many others that use well over 35° lead?

[beth]

I have only seen pictures of the N351. It has a well shaped chamber and balanced squish areas. If you are making excellent power at 25 degrees then they are burning well. I bet you are seeing good fuel mileage as well.

[ford-swap]

Beth, It has been my experience also on the Ford N351 heads burning alky.
We ran 30 ° total timing and detonated gaskets out of the motor.
Had to reduce the total to 25° to get out of the problem even with correct air/fuel ratios.
I take it these heads are fast burn in compairison to many others that use well over 35° lead?

Kenny,

A good friend of mine has run these heads in a blown alky sbf. The chambers are indeed very nice in them. We never played with timing all that much, mainly because we too kept blowing head gaskets. In reading the literature he got with the cylinder heads, we found that they stated the nominal thickness of the decks was something like 0.25"

This was from a fairly old set of the N351 heads, and it looks like there have been several revisions. The newer heads might be better, might not.

-Rob

[dbusch]

is squish next to the intake valve desired on all heads or just canted 10* heads? i am working on a set of Yates (no cant)heads that have about .120" of squish on the intake side of the chamber, and contemplating removing it. I would say i shouldnt, based on the info here.

i also asked this before...what about on an inline valve head? would it be desirable to leave any squish next to the intake valve here too? most inline heads dont have this anyway, but a few heads do when put on a large bore. A Glidden SBF head has .250" of squish here...

in the past, i have always ground the chamber out to the bore line next to the intake valve (excluding the Glidden SBF head)...now i am thinking this is a mistake.

[Ron E]

is squish next to the intake valve desired the on all heads or just canted 10* heads? i am working on a set of Yates (no cant)heads that have about .120" of squish on the intake side of the chamber, and contemplating removing it. I would say i shouldnt, based on the info here.

Very good question. I'm anxious for the experiences/opinions on this. I normally would remove that .120" also. It's one of those cases that begs for a A-B-A test. It's also unlikely to happen outside of a funded R&D setting.

[beth]

I have seen intake ports with a small squish band help to put some of the wet flow back into suspension that normally would be left on the bore surface. I believe that that adds to combustion efficiency. I also believe a small squish band in that location will help to homoginize the mixtures located there at TDC. I believe the narrower a squish area is, the more it produces small eddys somewhat like a reverse waterfall. Small squish areas tend to have a more local effect where wide squish areas have local and cross chamber effects IMO.

[dbusch]

In addition to my other questions, I wonder if the radial clearance of the intake valve has something to do with it too. Maybe there is a point where you can have too much, and not just because of reduction of squish.


Darin???