Piston Dish Purpose

[Mike Rappold]

What is theory/purpose behind a piston top that is dished slightly? Like the MAC 67 piston.

Thanks in advance.

Mike

 

[lohring]

A perfect combustion chamber would be a sphere with the ignition point in the center. Also, the squished mixture should be directed at the plug. This is done in direct injection engines to provide a rich mixture at the plug even if the mixture in the rest of the chamber is too lean to ignite. A plug with the element pulled out in a chamber with a dished piston gives an ignition point at the center of an approximate sphere. The squish should aim the mixture at this point with a lot of local turbulence. However, in a two stroke scavenging is also very important. Since most small gasoline two strokes use domed pistons, I assume dished pistons don't help enough. In compression ignition engines the combustion chamber shape is very important. Dished and more complex piston shapes are common. Here, turbulence is necessary for complete combustion. Glow ignition systems are probably somewhere in between these two types of combustion chambers.

Lohring Miller

 

[Mike Larson]

Lohring, thanks for the information.... creating a few questions as we think......

Dished, and creating the looked for sphere, and ignition in the center, would have thought of creating serious heat to the piston center, with the possible burned the hole in the center scenario?.... or, the better you get to spherical burning allieviates the heat in the center?....

Limited knowledge... (mine) has some action of critical cooling oil/mixture cooling the cylinder wall sides.... so...

Is this where angled squish band mechanics come into play?... experimenting with angle and dish in theory then are we actually looking for the spherical charge more than anything else?.... In thought, sensibly, a centered charge on the piston center, would be the most efficient working force?... all else equal, what forces the critical oil/cooling mixture to the wall sides, the most efficiently....

Question of, why, Flat to flat,(common), flat to angled, dished to angled and so on..... quite a few guys really engineering buttons, with effects differently, what has shown to be best?..... Search the sphere?.... ( rounded squish edges vs square common thought)

I have a mentor... pulls coils religiously..... on my engines, at least, plug life is seriously reduced if any at all..... pull them, and i think the pipe scavenges it to an early death...... pulling the plug changes the ignition timing ever so little, maybe to gain, maybe not?......

In theory, domed may force the charge better to the sides,( cooling the wall?, surely a seizure, if it wasnt?).. as it looks impossible to get the sought sphere, domed?. Equalling, maybe serious squish angle to get it there?....

Critically you hear the gas guys easily burning holes alot.... domed to flat?... oil, and lean?......

Nitro only, i search the glow plug to tell me whats going on..... a simple button/volume/angle may change things, possibly.

Engine and lathe guys abound, whats shown best and why?....... way more to it, easily..... more to it than throwing a button on...

thanks Mike

 

[lohring]

I only know about the basic principles of combustion chambers. There was a great discussion here at http://www.intlwaters.com/index.php?showtopic=41685&hl=%20head%20%20button&st=0 It will probably create more questions than answers. The only way I know to get good answers is to try different ideas. The basics are head volume and squish velocity. See Marty Davis' and similar spreadsheets for calculations. After that the sky's the limit.

Lohring Miller

 

[dwilfong]

I have posted some very interesting info in the food for thought thread, Heck it even has pictures of the flame kernel as it is forming in the chamber. :huh:

The spherical chamber is less efficient at high RPM. pancake is the way to go.has to do with heat transfer. the pancake chamber holds more heat in making more power.

HEAT= POWER

Heck a Italian writing his doctoral thesis on glow engs. GO FIGURE

Spend some time on the read it is very enlightening, No guessing or conjecture with this info.

I am still digesting all the info

David