In cylinder turbulence/ burn rate

[DavidVizard-GFN]

Originally Posted by automotivebreath

Hi David, As a reminder to start planning an article on mixture motion, I'll post
in this thread from time to time. I hope you can find the time to fit it in, it's a
interesting subject. I personally believe we are only beginning to touch on
the benefits of in-cylinder turbulence.

Anyone interested? Jump right in.


The designer of this port has taken the concept of swirl to extremes, to me it
looks like he created a high RPM fuel/air centrifuge.

Funny this port should crop up at this moment in time as we have a set of those 7 liter (litre for those back home in the UK) head going through the shop at this moment in time.

The port dam does not appear to promote swirl in the normal manner but acts more as a vortex generater. This should be good for faster combustion and for breaking up any fuel stream there may be in the system. However there should not be too much fuel wash as this is an injected engine and there is some 45 psi to break up the fuel just a short ways up from the intake valve.

Although the dam may be good for vortex generation it certainly is not that good for flow. At present the bench is showing the more that comes out of the dam the better the flow and the swirl.

Some Cup car heads were done this way about 10 -15 years ago but refinements in carburation and the ports themselves lead to the dam becoming smaller and smaller until it eventually disappeared.

The real issue here is that it is a great talking point as the number of people looking at this combustion thread has shown. Still got to get to that article I promised. Right now the tach is pretty much redlined here getting the front cover done and the two lead tech stories out for the PRI edition of Stock Car Racing. but I will get to that combustion feature just as soon as I can.

[automotivebreath]

Originally Posted by tommurphy73

I am still not 100% on why exactly an increase in turbulence prevents detonation. Tom

Tom to get a better vision of the role turbulence plays in reducing detonation
you must consider pre flame reactions, formation of “hot spots” and the role
of free radicals. As the fuel/air mixture is heated and compressed in front of
the flame front “hot spots” begin to form, very small centers of auto ignition.
These are considered areas of locally high reactivity due to imperfect mixing of
residual exhaust gas with the fresh charge. This is very complex may be
better explained with this quote.

In a running engine, air is drawn in at some ambient temperature, and this
air then begins to pick up heat from the hot internal engine surfaces it
contacts. Finally it enters the actual cylinder, where is it heated by even
hotter surfaces. Trapped there, it is heated again by the process of
compression.

In this heating process, some little discussed chemical reactions begin to
occur in the fuel. Called preflame reactions, these take the form of slow,
partial breakdown of the least durable types of fuel molecule. Fuel
hydrocarbons have three basic forms; straight carbon chains, branched
chains, and ring structures. Temperature is a measure of average
molecular activity, but there are always some gas molecules moving
significantly faster than the others. These faster moving molecules hit and
break some of the less durable fuel molecules, dislodging some of their
more weakly bonded hydrogen atoms. This released hydrogen is very
reactive (normally hydrogenous travel in bonded pairs, but his is atomic
hydrogen) and instantly pairs with an oxygen from the air to form what is
called a radical, a chemical fragment that is highly reactive because if
contains and unpaired electron. Its attraction for the missing electron is so
great that it can snap one out of other chemical species it happens to
collide with, thereby breaking it down as well.

At some point in the compression stroke, the spark ignites the mixture and
combustion begins. The burned gases, being very hot, expand against the
still unburned charge, compressing it outward into the squish band. This
compression rapidly heats the unburned charge even more, accelerating
the preflame reactions in it. As a rule of thumb, the rate of chemical
reaction doubles every seventeen degrees F. All this while, the population
of reactive molecular fragments radicals is increasing in the unburned
endgas. If this process of heating takes long enough, and reaches a
temperature high enough, this population of radicals becomes great
enough that its own reaction rate one radical creating more and more
through further reactions accelerates into outright combustion. This is
autoignition.

Now why does this heated, chemically changed endgas detonate instead of
simply burning? The fuel in the endgas is no longer ordinary gasoline. The
preflame reaction that have taken place in it have changed it into a
violent explosive much like a mixture of hydrogen and air, or acetylene
and oxygen. It is in a hair-trigger state, being filled with reactive
fragments from preflame reactions. When it autoignites spontaneously,
combustion accelerates almost instantly because the material is so easily
ignited now. The combustion front accelerates to the local speed of sound,
igniting the material it passes through simply by suddenly raising its
temperature, through the shock wave it has now become.

[automotivebreath]

Rookie you and I agree, as the engine RPM goes up so does the speed of the
engine parts and intake flow. This equates to increased mixture motion resulting
in faster flame travel. As to why an increase in ignition advance is sometimes
required at higher RPM, the generation of in cylinder turbulence does not keep
up with the pace with the reduction in burn time allowed. Twice the RPM equals
half the time for combustion but does not create enough turbulence for a flame
that is twice as fast, so we tune to compensate.

[automotivebreath]

David thanks for the interaction and the information on the vortex generator.
That is exactly the type of information I’m hoping for, clearing up
misconceptions and separation of yesterdays thinking from today’s technology.
I fully understand deadlines and have no problem at all waiting. From your
sneak preview, it sounds like we will be hearing about improved air/fuel mixing
at the carburetor as well as proper port shaping for maximum mixture motion
with out the penalty of flow restricting swirl dams.

[automotivebreath]

Does anybody have knowledge or experience with this style piston? It has a
concave dish leaving a small squish band around the circumference. I have
heard they are sometimes used in cup engines. What is the theory behind them?
What does the cylinder head combustion chamber used with these look like?

[rookie]

Originally Posted by automotivebreath

Does anybody have knowledge or experience with this style piston? It has a
concave dish leaving a small squish band around the circumference. I have
heard they are sometimes used in cup engines. What is the theory behind them?
What does the cylinder head combustion chamber used with these look like?

On the other hand, Judson hoped to eek out a few extra ponies by running concave dish pistons instead of the more common reverse domes. "A concave dish sacrifices some quench compared to a reverse dome, but the Nextel Cup guys have been using them for years and say they're good for an extra 3-4 hp."



A quote made by Judson Massingill that is in the paper version of the mag. Article bit does not appear on line is.
“Some people swear that a concave dish piston is worth some power over a reverse dome, but when five guys that finished ahead of you have reverse domes, it makes you think twice.”

Small Block Ford - Popular Hot Rodding Magazine

[automotivebreath]

Thanks Rookie; I just don't get it, there taking a cylinder head with a generous
squish to bore ratio and eliminating most of it by opening the chamber floor. I
have read that these pistons are worth 4-8 HP over a reverse dome design, I'd
love to understand the reasoning, it goes aganst my normal thinking.

[Dusty]

just my two cents but those pistons would allow the swirl to continue where as a dome will hurt that on compression. So in effect I guessing they let the mixture move about more freely. Any thoughts??

[automotivebreath]

You make a good point Dusty, the mixture movement initiated during intake
would move freely in the dish with no obstruction throughout the compression
stroke all the way until squish action at TDC. That same swirl would decay
much faster if the dish mirrored the shape of the combustion chamber.

The same would apply for unobstructed flame travel to the far reaches of the
dish during reverse squish early in the power stroke.

That would explain the even tan tint in the dish posted by rookie!

[rookie]

Yea like Dusty said,

I was kinda guessing i'ts like the Hemi chamber approach, the flame is free to move about the chamber and push more evenly on the piston this is my speculation only.

Makes you wonder how much quench do you realy need? Does a Hemi have a quench pad?