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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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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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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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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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