In cylinder turbulence/ burn rate

[automotivebreath]

I'll start this thread with a quote form Neels van Niekerk, find a link to the writing below.

"Without turbulence in the combustion chamber we would burn the mixture at the laminar
burning rate which is ten to twenty times slower than the turbulent rate. This would make
practical engines that rev higher than about 1500rpm an impossibility."

When we look at the high RPMs of formula 1 engines it is obvious that burn durations are
very short. I'm certain many aspects of the engine design allow combustion of air/fuel
mixture in the very short time available at these extreme RPMs.

I don't have interest in turning these extreme RPMs, I only use this as an example.
My goal is to generate high levels of mixture motion at much lower RPMs to reduce burn
rates substantially. My questions are:

Is piston movement/squish action largely responsible for the fast burn rate at high RPM?

Is this more important than intake flow induced mixture motion or do they play equal roles?

Is there a point where in cylinder turbulence becomes excessive, if so what are the results?

Squish action

[DavidVizard-GFN]

This is a great set of questions. It is a subject that is rarely coverd in detail and really warrents such. Suffice to say at this point that if you ar building a typical SB Chevy or Ford the quench and swirl can make a big difference to the output. But the overall picture is not quite as simple as it at first may seem. What I would like to do here is start planning an article for the not to distant future - say 6 weeks or so. How about reminding me about that time?

DV

[automotivebreath]

Thanks for the response, a detailed article would be great.

You are right about the subject not being given the coverage it deserves.
I have read everything I can find on the subject several times. My reference
was to something on two strokes only because that is one of the clearest
writings on turbulence and combustion.

With my last engine I utilized something from days gone by when top level
racers ran big dome pistons to fill tall chambers. They had found that
directing squish action into the roof of the chamber eliminated combustion
problems associated with the dome obstruction.

What I did was use a 0.160" tall dome shaped to mirror the combustion
chamber near the plug side squish zone. This small dome acts as a ramp
to redirect the squish action into the spark plug area. The results were
outstanding, ignition advance requirements at low RPM reduced by 10 degrees.
Here's a crude drawing of what was done.

[automotivebreath]

With bore sizes maxed out and RPM as high as the antiquated push rod design
will allow; it’s often found that the flame can no longer keep up with the piston.
Those with unlimited resources have found that by moving valve centerlines
they can create a small squish zone around the parameter of the intake valve
to provide catalyst for combustion in the otherwise dead area.

Unfortunately the chambers on my SBC heads are already bigger than the bore.

Also note shallow chamber depth and central spark plug location.

[automotivebreath]

Here’s a quote from T.O.O. from the late 90s. Even after 10 years this
sort of talk seems eccentric to most, including myself. David, I’d love to
hear your thoughts?

Originally Posted by Larry Widmer

The ability to induce the proper swirl frequency and depth of rotation is
paramount to maintaining a layered homogeneous mixture which will provide
a lengthened primary burn followed by a rapid secondary burn, all of which
will yield considerable resistance to detonation, greater over all combustion
efficiency and fewer bad guys coming out the exhaust. The over all time of
burn is so short and complete that the spark advance may be reduced to
the extent that you're not doing as much "negative work", and the exhaust
gas temperatures generally are in the 800 degree area, which means that
the heat of the burn provided considerable better thermal efficiency, and
something we and our competition noticed early on was the sound of the
exhaust....it was almost a "whisper" rather than what you'd normally hear
from an un muffled race engine.

I cast some small block Chevy heads back in the mid 80's, and although I
did rotate the deck to lessen the 23 degree valve angle, and reduce chamber
volume. The plug position was optimized, and of course the ports were
adequate, and the inlet ports were properly biased to promote swirl. The
pistons were "unique" in shape...all I'll say is they had no dish, except two
.120" valve reliefs. They were certainly of the domed variety. Those small
blocks were 358 cid. engines with 1.75-1 rod length to stroke ratio, very
short cam timing...235 degrees @ .050", and the intake manifolds were
some of my Edelbrock "specials" with Murray Jenson prepared Holley 830 cfm
carbs. Those engines had "over" 16-1 static CR, and dynamic compression
was so high we had to use custom starters run off 24 volts.

They were installed in some Camaros and two pick-up trucks. They all ran 91
octane unleaded pump gas. They never detonated; the mileage was 37
(combined) for the cars and 25 for the trucks. The Camaro's had Turbo 400
automatic transmissions, and from off idle you'd swear that there was at
least a 454 under the hood...the throttle response was almost too quick.
Those "loaded" cars all ran 12's with ease. The trucks had pulling ability that
no body imagined, and were a dream to drive, especially compared to their
street counterparts.

So, yes. If something as crude as a small block Chevy can be that efficient,
some of the more "modern" chambered heads can certainly do the same and
considerably better.

Everything Larry talks about in the quote is common knowledge today so I look
for clues, how can he accomplish what most others cannot? I go back to when
he talks about the dome piston; that must be the secret. Now perhaps he was
doing something like the drawing I first posted. Or rather he could have utilized
the idea used in the piston he designed for the air cooled crowd using a dome
to evacuate the area under the intake valve. Here’s an example done for a
late model LT1.

[automotivebreath]

Originally Posted by DavidVizard-GFN

... What I would like to do here is start planning an article for the not to
distant future - say 6 weeks or so. How about reminding me about that time? DV

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.

[automotivebreath]

Then there is something called surface turbulence, with this example small
grooves are cut in the back side and face of the valve. It appears these are
designed to induce mixture motion during the intake stroke and again during
compression. I suppose these would also have an impact on combustion as
the flame travels across the rough surface.

[Dusty]

that must transfer alot of heat into the valve and seat

[automotivebreath]

Adding that much surface area is a down side that the claimed benefits
must overcome. If the flame is faster as a result of the modification
the heat will have less time to transfer to the engine parts resulting in a
cooler running engine.

[automotivebreath]

He calls himself the "metric mechanic", he likes his idea a lot. Heres some of his
work on a combustion chamber. Again adding surface area.