Combustion Flame Speed

[SteveS]

In the literature there seems to be considerable disagreement about combustion flame speed.............both about assumptions and the speed itself. The most cited speed is 32.8 ft/sec to 82.0 ft/sec...........with my question being what is a good number to use for average flame speed in contemporary four valve combustion chamber with 13:1 compression?

[beth]

Flame propagation speed changes with engine design but it also changes with RPM and load so I'm not sure there is an answer.

[hotrod]

Flame speed also varies with the AFR of the mixture and the specific gasoline blend being used. Each setup will also have a different effective flame speed due to mechanical configuration, depending on mixture turbulence, combustion chamber design etc.

Larry

[OldSStroker]

Flame propagation speed changes with engine design but it also changes with RPM and load so I'm not sure there is an answer.

Here's a quote frrom an old IC Engine text by Taylor and Taylor:

"Effect of Engine Speed on Flame Speed. The most important effect shown is the fact that the variation in crank angle occupied by flame travel as engine speed changes by a factor of 3:1 is very small. This means that flame speed must increase nearly in proportion to engine speed....

The increase of flame speed with increasing engine speed is due to the marked effect of turbulence..."

18-19000 rpm V10 F1 engines (4 valve 96-98 mm bore 13:1 SCR) reportedly had ignition leads approaching 45-50 deg. BTDC. That's 3 times the rpm of a 6000 rpm SBC or SBF engine that might run well with 30-35 degrees lead. That leads (pun intended) me to agree with the Taylors 1960s' text.

[beth]

"Effect of Engine Speed on Flame Speed. The most important effect shown is the fact that the variation in crank angle occupied by flame travel as engine speed changes by a factor of 3:1 is very small. This means that flame speed must increase nearly in proportion to engine speed.... "

Exactly, thats also why the total timing at 3000 still works well at 9000 in most cases. Air flow is important and usually equated with power but the largest gains left to be discovered in the IC engine will come from combustion speed and efficiency.

beth

[automotive breath]

Here's some interesting reading by Neels van Niekerk on squish, turbulence and its effects on burn rate.

http://home.earthlink.net/~scloughn/id21.html

One approach that can be taken to estimate average flame rate at a given RPM is to calculate it mathematically using ignition advance and an estimate of burn completion of 14 – 18 degrees ATDC and RPM.

After reviewing Neels van Niekerk’s writing; burn completion of 30 – 40 degrees ATDC mat be more accurate.

[moper]

After reading that article..Now I have a question. He refers to "squish area ratio" in it. Is this a volume measure of the squish area over total cylinder volume, or over cumbustion space volume? I Have read about the "lines" being ground into the head side over a quench area. It was thought this gave the flame front a path to any mixture in the quench area. Is it possible these lines simply add to the squish area volume, thus increasing the rate of burn, as opposed to getting a better burn and more heat from the small amounts trapped in the squish area? Or am I way off base?

[Darin Morgan]

After reading that article..Now I have a question. He refers to "squish area ratio" in it. Is this a volume measure of the squish area over total cylinder volume, or over cumbustion space volume? I Have read about the "lines" being ground into the head side over a quench area. It was thought this gave the flame front a path to any mixture in the quench area. Is it possible these lines simply add to the squish area volume, thus increasing the rate of burn, as opposed to getting a better burn and more heat from the small amounts trapped in the squish area? Or am I way off base?

The "squish area ratio" is

Area of squish quench pads/( Bore²*.7854)

This is the way I remember it and have used it through the years. A Pro Stocker has a ratio of about 45%. Thats extremely high! You will not be able to get half that in most head designs. You will be lucky to achieve 25% in most cases. The SBC 10° heads from CFE/Foltz are 46.2%. The lower the valve angle and cant, the higher you can get the squish quench area ratio. We don't use all of it or keep it perfectly flat. We utilize all of it to our advantage by modifications to the squish pad or piston in order to manipulate the air fuel mixture in such a way as to accelerate the deflagration process.

Thats a fancy way of saying we point the air fuel mixture to the center of the chamber over the spark plug so we burn it faster, cleaner and make more power.. The idea is to get the majority of the air fuel mixture over the plug and have it very agitated. We accomplish this by manipulation of the chamber, squish quench pads and piston dome design.

Poor a mold (rubber only) of the chamber down through the spark plug hole with the piston at TDC.
Take the mold out and slice it laterally. You will see where the chamber is separated into two sub chambers because the center is choked off.

You will see why cutting the center out from between the two valve relief's on any engine can have positive effects on power. The chamber is " choked" in that area. Its splits the charge at or near TDC.

[moper]

Thanks Darin.
Kind of makes the quench domed BB Mopar KBs look like a crazy bandaid for a huge poor chamber..lol.
The quench pad then is the only area considered, and the head's grooves would not be part of that formula. Got it.

[beth]

Darin wrote:


Poor a mold (rubber only) of the chamber down through the spark plug hole with the piston at TDC.
Take the mold out and slice it laterally. You will see where the chamber is separated into two sub chambers because the center is choked off.

You will see why cutting the center out from between the two valve relief's on any engine can have positive effects on power. The chamber is " choked" in that area. Its splits the charge at or near TDC.

I have had great success going a step further with 4 valve pistons. The center of the dome is brought down resulting in two seperate domes on each end of the piston. This also helps intake breathing at a crucial point in 4 valve intake demand. This combined with a 360 degree squish band burns very well.

[Unkl Ian]

Darin: is it possible to have too much quench percentage ?

Thanks.

Ian

[MadBill]

[quote="OldSStroker...18-19000 rpm V10 F1 engines (4 valve 96-98 mm bore 13:1 SCR) reportedly had ignition leads approaching 45-50 deg. BTDC. That's 3 times the rpm of a 6000 rpm SBC or SBF engine that might run well with 30-35 degrees lead. That leads (pun intended) me to agree with the Taylors 1960s' text.[/quote]

There was a Honda tech paper a few years ago which covered their work re determining just how high a 4 stroke could rev before the combustion process could no longer keep up. I don't know the bore diameter, but they ran it as high as 33,000 RPM.

[SteveS]

The way I framed the question and the lack of mentioning RPM.....made the question impossible to answer and frankly dumb. I recently downloaded a couple of SAE papers and through a variety of methods engineers are measuring and or modeling flame speed with the usual equation being laminar flame speed.......about 1 M/sec.....times some expression of turbulence intensity which varies with RPM. It looks like the practical limit is around 100 M/sec for F1 designs............but it is another case where the SIIC engine "perpetuates" itself. In this instance, the faster it spins the faster it can go.....at least on paper.

[Rod Rocks]

… A Pro Stocker has a ratio of about 45%. That’s extremely high!... … We don't use all of it or keep it perfectly flat. We utilize all of it to our advantage by modifications to the squish pad or piston in order to manipulate the air fuel mixture in such a way as to accelerate the deflagration process.

That’s a fancy way of saying we point the air fuel mixture to the center of the chamber over the spark plug so we burn it faster, cleaner and make more power. The idea is to get the majority of the air fuel mixture over the plug and have it very agitated. We accomplish this by manipulation of the chamber, squish quench pads and piston dome design.
…

Darin,
If I understand what you are saying your piston and head modifications help to direct homogenized squish flow into the area of the plug where it will best promote combustion. This is a turn from what I’m accustomed to that being two flat surfaces coming together forcing the mixture laterally into the chamber cavity.

Can you give specifics on modifications required to get the majority of the squished mixture into the central portion of the chamber cavity near the plug?

[Darin Morgan]

… A Pro Stocker has a ratio of about 45%. That’s extremely high!... … We don't use all of it or keep it perfectly flat. We utilize all of it to our advantage by modifications to the squish pad or piston in order to manipulate the air fuel mixture in such a way as to accelerate the deflagration process.

That’s a fancy way of saying we point the air fuel mixture to the center of the chamber over the spark plug so we burn it faster, cleaner and make more power. The idea is to get the majority of the air fuel mixture over the plug and have it very agitated. We accomplish this by manipulation of the chamber, squish quench pads and piston dome design.
…

Darin,
If I understand what you are saying your piston and head modifications help to direct homogenized squish flow into the area of the plug where it will best promote combustion. This is a turn from what I’m accustomed to that being two flat surfaces coming together forcing the mixture laterally into the chamber cavity.

Can you give specifics on modifications required to get the majority of the squished mixture into the central portion of the chamber cavity near the plug?

Remember that a Pro stock head is designed from scratch so we can put the valves in the center making the quench area on both sides almost equal! We also use chamber mods which are areas that are laid down to direct the mixture one way or the other or to eliminate vortex generation on the quench pad itself. The spark plug is as close to center as we can make it so having equal quench squish on both sides so the mixture is being directed laterally as you stated. Its hard to explain with words but our quench squish is almost 260° all the way around the chamber. The mixture has no choice but to be moved to center. I have .160 thousands of flat on the intake side of the chamber. It does not seem like a lot but if you take it out, its disastrous to say the least. The engine will drop 8ft/lbs and it will kill the fuel flow 10lbs/hr. I will see if I cant get some pictures so I can explain it better.