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Hi!

That's the first thing I thought: "loss of compresion", so i did not understand the trick...

Regards!
 

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Technically they work to assist the atomized mixture flow and disburse better, then supposedly help create a more even pressure on the top of the piston during combustion, but it only works on a side valve 4 stroke lawnmower engine and then only if you don't mind changing burnt head gaskets every week.......
 

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What the grooves do--

First of all, the grooves DO WORK, when properly applied.

AND, it ain't just on 'lawn mower motors'. Lots of work done with 'em on diesels with cylinders you could likely stand up in !!

First thing you have to have for them to work properly is a decent amount of squish/quench area in your chamber. You DO have that incorporated into your piston chamber design already, DON'T you ???

Next, the squish/quench clearance has to be pretty tight for the grooves to do much of anything. As in about .020---.025" on an engine of typical Alfa proportions. If you're not capable of building an engine with this tight a squish clearance that will live, don't bother.

What properly designed grooves do is DIRECT the super to trans sonic flow that comes out of the squish area in a manner so that consistent vortices are created in the roomier part of the chamber--rather than the random turbulence that one gets without such grooves.

Heard the old saw that 'too tight a squish will give you a 'roughness' ' ??
Well, the real translation of that is that random, highly active turbulence in the chamber will give you wildly varying burn rates, and is tantamount to changing the timing on each power stroke !! The consistent vortices that the grooves can give cure this problem.

With the consistent, faster burn rate that tight squish and the grooves yield, first off, the engine will need significantly less ignition advance. Don't take this into consideration, and, yep, head gaskets and other such will have a real short half life.

Benefits--

1. faster burn gives greater detonation resistance on a given fuel.
2. faster burn gives an inherently more efficient cycle, thus lower bsfc (better fuel economy) AND more torque/power.

Greg
 

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This applies to a hemispherical head how?

All I see it reffered for use in is pent roof and wedge heads, (both good for compression, but a bit on the pathetic side AFA burn pattern), and I'll give the benifit of the doubt on diesel (even though they fire as a result of compression and a pocketed piston crown rather than spark......)

I stand by what I said as do the pile of old time racers, engineers, and engineers who race around these parts.


Well, the real translation of that is that random, highly active turbulence in the chamber will give you wildly varying burn rates, and is tantamount to changing the timing on each power stroke !! The consistent vortices that the grooves can give cure this problem.
So will a properly atomized A/F mixture coming through proper port work and induction tuning.

Much cheaper than a screwing up a head with grooves if even only from a rebuildable core aspect. :shrug:
 

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Well--

Glad to see we've sorted out those who might want to learn something from those who like losing to Duckworth and ricer designs !!

First of all, re diesels--a combustion chamber (or the burning air/fuel mix inside it) doesn't give two hoots whether it's located in the head or the piston crown !! In fact, good fuel atomization has another VERY good benefit, but, at least for now, the answer to what that is will be a pop quiz !!

Second---good fuel atomization is a wonderful thing, as are port induced swirl and tumble. BUT--they won't even APPROACH speeding up the burn the way properly directed , squish induced turbulence will.

Third--if you think squish induced swirl is only for pent-roof and wedge type chambers, try taking a peek at the chamber shape in the most advanced two valve true hemi chambers that presently exist--as built by folks with names like Black, Donovan, and Indy (nee Mopar). Clue: they get their shape (and their squish) via educated and effective design of the piston crown.

Recommended reading:

Glassman 'Combustion'

Obert 'The Internal Combustion Engine'

NACA --A WIDE variety of papers written by guys who tested (true hemi headed piston) engines to DESTRUCTION --because many guys lives depended depended on that last bit of performance !!!

Suggest you not refer vaguely to un-named 'engineers' when trying to debate a REAL one !!

Greg
 

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Suggest you not refer vaguely to un-named 'engineers' when trying to debate a REAL one !!
You wouldn't know thier names any more than they'd know yours anyway, so what would be the point?

Though I can assure you they're nowhere near as anal as the one who claims to be.

I leave you to the thread, you may call it a 'victory' if it makes you feel better.
 

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Ahhhh--

What's the point--lemme see--perhaps that in a technical/scientific discussion one references one's sources so that they may be verified ??

Of course, then again, if one has filled one's mouth with not so tasty foot, one might make a petulant excuse for not disclosing one's sources--either because they are imaginary, non-existent, or incompetent ??

And, of course, to distract attention from the above, one might consider making an implied slur of the opponent--'You MEANIE !!! You just wanted to win , and thus FEEL better !'

(Typical debate tactic of the incompetent, uneducated, immoral, and/or traitorous -- attack the messenger when you have a faulty message !!!)

BTW--since you clearly demonstrated that you didn't know the answer to the quiz, I will share it with the others here---

Well atomized (NOT evaporated yet !!!) fuel mixed with air once inside a closed cylinder acts as an interal coolant. It's latent heat of vaporization absorbs some of the heat generated during the compression stroke--thus bringing the compression process toward iso-thermal and away from adiabatic. This lowers the peak pressure in the cylinder, lowers peak temps, reduces bearing and side thrust loadings, and increases detonation resistance. Net effect is positive, even though the lower peak temp reduces cycle efficiency to a small degree.

NOTE: spraying injected fuel on the back of closed intake valves does NOT provide the above benefits, as the fuel is already EVAPORATED before it gets inside the cylinder. You need good carbys--such as DCOE's--to get the above benefits--or injectors that use compressed air atomization and only squirt atomized fuel into the port air stream when it is MOVING, or directly into the chambers. Hence--the fuel economy and power gains from direct injection. Not to mention the bsfc superiority of DCOE's over conventional port injection !!

Bear
 

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Bearbvd, Can you name one NACA report that talks about these grooves? If so I would like to read it. This thing sort of reminds me of the turbonator, I am not saying it won't work, but I can't see how it would do anything good on an Alfa Nord or and Alfa V6.

Greg Gordon
 

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Sorry, Greg, I can't name (or number) any specific NACA report addressing grooves. There are a LOT of those reports , mostly '30's--'40's vintage !!!! LOTS of fascinating info in them, such as most all one could ever want to know about water injection.

Too bad most of the seriously talented guys involved in that work went off into work on turbines--but, then again, turbines were the 'hot new thing' during that period, so, of course, they skimmed off the best talent !!

Such grooves are pretty common in big diesels--usually in the pistons, NOT the head. And, likewise, I think it would be wiser to put them in the piston on any spark engine, including an Alfa, if one wanted them on an Alfa.

Basically because it's less 'invasive'.

I suspect that the ones pictured were done in the head just so the builder could use off the shelf flat-top pistons without bothering (too lazy ??) to come up with a set-up/tooling to do them on his pistons.

The combustion chamber doesn't care in the slightest whether the grooves are on the piston crown or in the head. The point is to direct the squish turbulence in a consistent manner.

There are some modern (mostly Asian design) spark ignition engines that use various types of groove/texturing on the piston crown, there IS something to it. I've never tried such stuff on an Alfa engine, but may well do so soon.

I AM a FIRM believer in using 'squish' design pistons in Alfas, been doing THAT for about 35 years. The benefits of having squish in an Alfa Nord are striking.

Clearly, if no squish area, no point in grooves !!

Bear (also a Greg)
 

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Clearly, if no squish area, no point in grooves !! Bear (also a Greg)
I've been reading some reports around some websites and all of them refered to heads with squish area design. That's the point, I think.

Anyway I's a wonderfull technical discussion.
Thanks a lot for the posts!

Best regards,
ALvaro
 

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The best info on hemispherical heads I found was for race bikes.
As the squish area goes all the way around in a hemi it is not clear how you would do it.
I guess you could add some even spaced all the way around.
but I would see if any of the bikes have done it. I think if the squish is working on the hemi that is as good as it gets. but I think the squish is not effictive many times do to too much clearance.
for it to work right it needs to be very tight almost touching at redline.
who here has even meshered the squish ring? let alone changed it so it is tight all the way around.
 

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Do you know of any published SAE papers on the subject? I did a search and came up empty handed.

I would think that a gentle slope over the roof of the chamber leading to a final concentrated expulsion of the gases would be more effective.

The company I work for has a in piston design using a gas chamber inside the body of the piston connected to the combustion chamber by a small port that has great result on fuel consumption and emissions. Some of these small changes have great effects on combustion.
 

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Hi guys , I saw somewere on the Net that it is possible to gain squish area for the Hemispherical heads with special pistons, they are supposed to be domed and too close to the wall of the chamber(the domed crown) within the specific clearance(s) thus creating a '3D' squish area....that 's what they said anyway.I 'll check for the link if u want.
 

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Bearbvd, thanks. I am pretty familiar with NACA reports. That's why I would be excited to read one that had information on this. However they are tricky for the average person (obviously not you) to read because they use aviation, not automotive terminology. There seems to be more misinformation from misunderstood NACA reports out there than good information. This is especially true with water injection information.

Greg Gordon,
www.hiperformancestore.com
 
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