In cylinder turbulence/ burn rate

[rookie]

Dusty I hate to keep asking but could you smack DaveI mean ask David his thoughts if you see him again.

[automotivebreath]

Sorta like a Himi only much better. Here's a piston out of one of the local super
stock cars, yes the dome is a perfect round sphere but the valve reliefs become
huge obstructions. These pistons are run with 0.050 piston to head clearance,
the dome becomes the squish area and the valve reliefs become the
combustion chamber at TDC.

[automotivebreath]

In an engine that's octane limited, that tiny squish band might be just whats
needed to stir up the OH radicals that are popping up in the local hot spots.

[Dusty]

Heres some food for thought for you guys go look up "Particle Image Velocimetry" and some of its siblings there are alot of interesting papers of it being applied to engines

[automotivebreath]

Although not PIV perhaps a sibling. One of my favorites, a CFD showing flow
vectors and equivalence ratios to highlight in cylinder mixing of the fresh charge
with residual gas. It's a great time to study combustion and auto-ignition with
all of the money going into HCCI.

"...this means that generally in the cylinder there will be fuel richer regions with
lower temperature and leaner regions with higher temperature. This could be very
significant, since high temperature leads to advanced ignition timing and faster
burn rates, while a lower equivalence ratio has the opposite effect..."

"...the effect of mixing in the cylinder is very important since it controls the local
temperature and composition in the cylinder. There is a correlation between
local equivalence ratio and residual gas fraction with temperature. The assumption
of homogeneous composition could be quite inaccurate even under premixed
conditions in the intake port..."

Gas exchange

[automotivebreath]

"velocity magnitude contours on a cross-sectional plane for one complete engine cycle"

action

[tommurphy73]

From the animation there is very little velocity on the comression stroke for the last half of the upward movement. It would have been nice if they had added squish at the top also to see its effect.

Tom

[automotivebreath]

Tom, I haven't had any luck finding animated squish, it would be great to get a
good visual image of the action as the piston and head come together and then
apart again.

Its a last opportunity to archive homogeneous mixture and near
100% mass fraction burn, most just hope for something in the 90% range and
fire the remainder in the cat. Even in a great running engine we have no way
of knowing how much of the combustion develops useful work.

[DavidVizard-GFN]

Automotivebreath

The more I see you guy’s digging up on this subject the more I realize that it’s a full time job and then some. (and I am working on that story – among 500 other things).

Here are a couple of points to consider. First a high dome in a hemi creates the worst surface area to volume ratio of any combustion chamber known to man – and it’s not good. A flat top or dish works well that’s partly why those big WW2 aero engines with 20 lbs of boost made such big numbers,

Secondly (if practice follows basic theory) as the piston quench clearance tends toward zero so the ejection speed tends toward the speed of light!

Unfortunately the quench is a last opportunity to not only break up fuel droplets but also to clump them together (we will get to that later).

At this point I would like to say that you guys are looking at a very complex subject and trying to understand it is a challenge for us all. I think what we need to do here is come up with a bunch of priorities so we end up fixing or avoiding problems we can - and understanding there are some more minor things that will not be easy to fix and so time should not be unduly spent on said fixes.

Rookie - the pressure on a piston is pretty even as any pressure change travels across the piston at the local speed of sound.
Dv

[MaxRaceSoftware]

Originally Posted by DavidVizard-GFN

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.

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.

i discovered the same thing or had the same results
on the Dyno and the DragStrip.

Worked with 3 Teams, 1 was ProStock Truck
another B/ED , another was A/ND , and then a LS-7 Engine .

With that Vortex-fin the Flow CFM gained between 15 to as much as
25 CFM -VS- without that Fin.
But in all the above Test Cases...the extra Flow CFM gains showed
zero MPH and ET gains down the DragStrip.

On the B/ED they did a A-B-A Dyno + DragStrip tests
results were no real differences in ET or MPH,
it was as if you did absolutely nothing to the Cylinder Head,
even though the Vortex-generator fin was increasing FlowBench CFM
by 15 to 25 CFM !

When you ground out that Fin..you lost FlowBench Dry Flow Numbers,
but the RaceCars ran the same.
When you put back the Fin..you gained FlowBench Dry Flow Numbers,
but the RaceCars ran the same.

Basically a great waste of time that only resulted in
FlowBench Bragging Rights ..., but no DragStrip ET/MPH differences