Combustion Flame Speed

[automotive breath]

Timdog, on this subject I have more questions than answers, hopefully others will share their thoughts. Numerous factors influence burn patterns some have been discussed earlier, like squish velocity and wet flow.

One factor that has a huge impact on the burn pattern is mixture quality. In an ideal engine the air and fuel would be uniformly mixed in a perfect concentration. In reality this is far from true, some areas of the chamber are wet with fuel, other areas are very lean and in some places it contains concentrations of remaining burnt gasses for the previous cycle. This is partially why we run rich air/fuel mixtures.

Daren’s example of the wet flow vortex on the squish pad is a perfect example of rich and lean areas in the chamber. This combined with fluid flow during combustion and state of tune will influence the burn and create the patterns. Mean squish velocity and the piston position when the flame reaches the squish region are other important factors. In addition some areas are exposed to higher temperatures than others.

Flame intensity although often understood is of utmost importance. One brilliant example of altering flame intensity is the small squish region around the intake valve discussed earlier. The area is cooled by the intake charge, the turbulence generated by the squish accelerates combustion to promote complete burn in the area

[beth]

I would not assume that the extent of burn patterns on squish areas of pistons or heads correlate directly to the burn speed or efficiency, especially on this type of head (offset chamber/offset plug). If we take the photographed head and mill down the squish pad .100 it will color beautifully. Will it make more power? Open chambers color very well and are great for burning a larger amount of the fuel mixture. Does this mean more power? No, power comes from burning the most mixture at the correct time, too soon and we make negative torque, too late and it's just heat out the exhaust. Retarded timing will color a tight squish pad more. A slow burn will color the squish pad more. A large squish pad with too much clearance has the effect of saving a portion of the mixture to be burned later in the cycle when it does little or no good. Is it more important to color the squish pad or could it be that moving it's contents into an area that will be consumed during peak cylinder pressure is better?

Note the coloring in the exhaust port of the illustrated head. By coloring it would seem the most efficient burning was going out the exhaust.

great discussion

beth

[timdog]

This is the only picture i have with me now and it shows the piston, not the combustion chamber.
So if i'm understanding correctly, the white areas could actually be where it is running very lean?

[thundair]

Could I drop a few questions in to keep up.

Is the piston above using tumble for the intake?

And is there any wetting of the cylinder wall that would cause the rings to scrap off wet flow and send into eddy's as the piston heads for the squish.

Thanks in advance for putting up with a Retardo Montibon




I don't know anything but the people that do

[SStrokerAce]

I hate to add in something that seems obvious but are these changes to the chamber (especially the grooves) requiring a lower amount of total timing for best power? That what we are after here, less negative TQ.

The only two things other than looking at wet flow like Darrin has posted AND looking at chambers post running that really mean anything in the end is TQ on the dyno AND timing needed to effectively burn the air/fuel. What are the improvements seen in these terms.

I realize that if you can prevent detonation then there is a higher potential to make more power with more igintion lead OR more compression. Then again are these grooves being added to the combustion space backing the compression ratio off enough to stop the detonation?

Bret

[Unkl Ian]

That's where you need an A-B-A test to prove the value of one change.



Actually,it wouldn't be too hard to calculate the volume of the groove,
and figure out the difference in CR.

Then again are these grooves being added to the combustion space backing the compression ratio off enough to stop the detonation?
Bret

[SStrokerAce]

That's where you need an A-B-A test to prove the value of one change.



Actually,it wouldn't be too hard to calculate the volume of the groove,
and figure out the difference in CR.

Then again are these grooves being added to the combustion space backing the compression ratio off enough to stop the detonation?
Bret

Why don't you just mill the heads down to reduce the chamber volume as much as you increase it with the groove.

I still want to know about the ignition lead.

Bret

[automotive breath]

...I still want to know about the ignition lead.

Bret

I wish I could offer you something more specific, I can't honestly do so. I ran this modified engine with a very tight squish clearance earlier this year and was impressed with the results.

I like to run my engines on the edge of detonation; I get the dynamic ratio as high as I dare for the fuel in use and control cylinder pressure with the tune-up. I decide to raise the compression of a modified engine depending on the initial compression; in this instance I raised the compression slightly. For what it's worth, this engine responded to the modification with a reduction in detonation tendencies. After the modification it accepted more ignition advance but ran quicker with less.

The grooves on a SBC head measure about one cc.

[automotive breath]

Clean areas on the piston and head can be very misleading. When analyzing them I like to consider three possibilities.

1. Wash, this is normally found in the coolest area of the chamber, the fuel doesn’t burn and the area is normally wet and clean. This can be caused by mixtures that are too rich (too cool to evaporate and burn) or too lean (not enough fuel to support combustion)

2. Mechanical contact, the piston is hitting the head at RPM wearing a clean area.

3. Minimal boundary layer, this is an area that burns so lean that it leaves little signs of combustion.

Knowing wet flow patterns and squish clearances helps to determine what’s what, what areas are squish? I’m not familiar with 4V engines but that piston appears to have mechanical contact and wash at the bore, I think thundair has that part nailed

...And is there any wetting of the cylinder wall that would cause the rings to scrap off wet flow and send into eddy's as the piston heads for the squish....

[automotive breath]

I would not assume that the extent of burn patterns on squish areas of pistons or heads correlate directly to the burn speed or efficiency.... Is it more important to color the squish pad or could it be that moving it's contents into an area that will be consumed during peak cylinder pressure is better?
beth

This could be a good example of what Beth is referring to. This engine was running great with a close piston to head clearance and complete arc of unburned squish area.

[shawn]

I would think that someone could model this in CFD fairly easily to give an idea of what the heck is going on. My guess would be that there is more than meets the eye. It seems that about the time we think we have a clue as to what is happening, it turns out your really taking two steps forward and one step back. If you think about the large column of air, fuel, and who knows what else moving up the cylinder it's amazing that you can make a change that is even remotely controlable. Add to that a dome,valve reliefs, and other pockets here and there and it really starts to complicate the matter. I had a really smart (famous) engine guy tell me once that there isn't a change you make to an engine that doesn't make a difference, it's just up to you to figure out what that difference was.
shawn

[SStrokerAce]

I had a really smart (famous) engine guy tell me once that there isn't a change you make to an engine that doesn't make a difference, it's just up to you to figure out what that difference was.
shawn

Shawn, WHO said that?

Bret

[automotive breath]

...there isn't a change you make to an engine that doesn't make a difference, it's just up to you to figure out what that difference was. shawn

Hi Shawn,

It’s been almost a year now, Sun Aug 28, 2005 11:11 pm, I evacuated for Hurricane Katrina the night I found a discussion about the modifing heads on speed talk. I commented that night because I was impressed with the results I had achieved with the modification. During the last year I have been studying combustion because I need to know what’s going on. I believe I have a fairly good understanding what's happening inside of my grooved engines. The results are repeatable and noticeable by most that get involved around the world.

I'm doing about three to four sets of heads a month now, the only reason I'm not doing more is because I spend 40 hours a week in an office and race every weekend. I’m constantly receiving messages from people around the world that are achieving like results. I can assure you, even those that don't understand it like it. The group of people here are the brightest that can be found on a racing forum anywhere, when they realize what can be accomplished, they will understand.

[beth]

I would not assume that the extent of burn patterns on squish areas of pistons or heads correlate directly to the burn speed or efficiency.... Is it more important to color the squish pad or could it be that moving it's contents into an area that will be consumed during peak cylinder pressure is better?
beth

This could be a good example of what Beth is referring to. This engine was running great with a close piston to head clearance and complete arc of unburned squish area.

I didn't mean to imply the squish area shouldn't be colored, just wanted to inject other possible reasons for clean or colored areas. This chamber looks like it was run excessively rich and that could be the reason the squish is clean.

[timdog]

Is the piston above using tumble for the intake?

And is there any wetting of the cylinder wall that would cause the rings to scrap off wet flow and send into eddy's as the piston heads for the squish.

Don't know about the tumble as i can't measure it with my bench.

Again, don't know on the wetting but the bore and piston do suffer bad wear on the exhaust side.


Great thread. So many clever thinkers on here.