Flat Tops vs Dished Pistons

I honestly don"t think one is better than the other in a traditional engine. What dictates the use of one over the other is a desired compression ratio.Now some designers that use an extremely shallow combustion chamber(typically 4 valve engines)use a dished piston as there upside down combustion chamber.There is less valve shrouding to deal with and as a result better flow.Flatops work well,with no dome in the chamber ,good flame travel properties are realized,(as are dished pistons).. AS stated the most negative piston design is a high dome. Poor flame travel and possibly poor cylinder purge on overlap.The possibility of poor fuel/air mix in the combustion chamber.(Swirl?). Unfortunately,in some cases, Chamber design can and does dictate piston dome design..... Craig

For a diesel, I think that the pistons almost universally have bowls. Feel free to correct me.

I ask because on another forum there was a question about which head to use on a 300cid Inline Six buildup. Some said the 87-96 300cid or the 240cid head because they both had the heart-shaped chambers, and another said that flattops were worth more than the CC design. I had read somewhere that dished pistons were a good thing, but with the overwhelming preference to flat tops......


-=Whittey=-

Iveymike,I can"t say if automotive type diesels use a dish or a flatop piston. I do know some of the Catepillar and Detroit diesels use flattops.At least in the past they did . I dealt with Detroit diesels in the military and on occasion viewed the internals. Whittey, I would again say use the piston design that suits you"re need. Compression ratio and intended purpose will play the lead roles in you"re selection.Combustion chamber volume will also come in to play in you"re finalized compression ratio. Make you"re pick from there.. Craig

With automotive diesels it depends on the type of injection. Usually if the engine is of the IDI or indirect injection type, they use a flat top type of piston as the precombustion chamber,(also known as the antechamber, auxillary chamber or turbulance chamber take your pick) is around 30% of the total clearance volume. Some like International diesels have a slight groove(for lack of a better term) in the piston running from the outlet of the prechamber to the middle of the piston. So I guess you could say it was a flat top with a narrow dish, or perhaps a channel would be a better description, on one side.

Direct injection diesels use a 'chamber' built into the top of the piston that looks like the letter W . Known as a Mexican hat design, when the piston starts toward TDC, the air starts to swirl in the hat and helps to create better turbulance when the fuel is injected.


phasers

Phasers,Interesting. It makes one wonder why piston crowns in other ("gas&quot engines are not configured to induce swirl and beneficial turbulence. This seems to be left up to combustion chamber design. Craig

Aha! Here is the quote i'm talking about:
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Some have expressed a view that the small chamber heads are a better design, the fact that they have to be used with a dished piston appears ignored in this assumption. My personal opinion is that a small chamber with a dished piston is no better and possible not as good as a large chamber with flat top piston.
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300cid Inline Six (gasoline).


-=Whittey=-

Cycle, I'm surprised you don't recall the weird pre production efforts of Suzuki way back when. Apparantly the didn't work.

Whittey, My two cents worth---In my experience, chamber in piston designs like the Ford Kent 1600 make power on a par with earlier 116E designs with a combustion chamber in the head. This brings up the question of valve shrouding and final combustion chamber shape (and ,of course, the bottom line).
In my work with hemi, semi-hemi, pentroof chambers, the closer to a flatop piston, the better the results. Valve shrouding here would not seem to be a problem, but turbulance (swirl?) and squish area would.
Just for the heck of it, I'd like to see what an analysis of the Chevy 235 Blue Flame 6's combustion chamber and valve placement design, based on modern theory.


Rod

Rod,No I sure don"t recall this. But ,I will have to check that out , just for my own education. Whittey , Barring piston design , it is possible for one cylinder head to have a better designed combustion chamber.(obvious) As Rod stated ,swirl,turbulence,valve shrouding,etc. would all play a part in my cylinder head choice. Even spark plug location and the direction it faces. Another consideration is open or close chamber design. I believe from a performance stand piont cylinder head parameters must be taken into consideration long before piston choice. Although I do understand they go hand in hand......... Craig

One more item to add:
The piston serves more than just a slug to move up and down! It is an integral part of the combustion chamber. Its surface shape should represent a desired control of the combustion sequence, directing the flow of combustion gasses inside the chamber to enhance the process. It is easy to place a flat piston top in a dished chamber, and it would work relatively well. It is also easy to place a dished piston in the same head to reduce compression without changing head castings, the same for a domed piston. In the old days, I remember (sorry Rod!) the concept of “make a big combustion chamber, then stuff it full of piston dome”!

Feuling ( has done a lot of research on small combustion chambers and valve sizes and shaken up some of the engineering world (myself included) with non conventional designs that seem to work exceedingly well. The same for “The Old One”.

Some of the diesel piston shapes have to do with the combustion control processes, and to eliminate noise. A simple reshaping of the dish can reduce particulates and improve power, without changing the compression ratio.

Ideas?
Franz