halloweenz28

alright here it goes, ive debated asking this or not so that i wouldnt get half @$$ answers and incorect info thrown back and forth so im really looking for the top dogs to answer this thread, im studying to be a mechanical engineer and im a freshman, however unfortunately i havent learned the answer to this yet and ive been looking everywere, but anyway:

backround info for my question

compression ratio is affected by
1)piston shape(dome, flat, dished...)
2)the stroke of the crank
3)the combustion chamber size on the cylinder head used(58,64,72....)

the bore would have an effect as well if all things were kept constant but for this question i want to keep the variables as:

the combustion chamber size and the piston shape

the constants are everything else: same stroke, same bore, same compression ratio

the question: so if we have decided on a compression ratio of lets say 10:1 (arbitrarily chosen for the question) would it be better to have a domed piston with a large combustion chamber , or a dished piston with a small combustion chamber?? (these were extreames but i used them to help clarify the question in the answer you dont have to be specific)

now i understand that it depends on engine displacement and i also understand the effects of metalurgy and the transfer of heat so could anyone tell me if its better for the combustion to be located more in the cylinder bore, or more in the combustion chamber

anything that i have written incorectly please feel free to correct me and any other information that pertains to this topic would be greatly apreciated as well as a book that would adress this topic.

thanks alot to anyone who could give me some insight i would really apreciate it :hail:

you can personal message me and i could give you my email if you want to send me something that could help

formularpm

A flat-top/small chamber combonation is usually ideal, it can be difficult to get adequate compression with a dished piston without a very small chamber. To put it simply, with a flat top (or dish) there is no dome blocking the spark plug, and the expanding gasses are free to travel unrestricted through the chamber, while with a domed piston the gasses have to travel up and over the dome before completely filling the chamber. Also, a flat top piston has better 'quench' characteristics to more successfully mix the air and fuel at the top of the compression stroke. Id stay away from a domed piston unless youre running enough (12:1 +) compression to necessitate it. If you are looking at dished pistons, a d-cup dish is usually the best choice, part of the piston is still flat, so it has better quench characteristics compared to a full-dish piston.

ede

i'd say dished piston with small chamber, but i'm just a dumbass so ignore me and what i have to say

Daz

iTrader: (1)

if you can take the shape/flow characteristic of the chamber shape then it would be the dish/small chamber combo.BUT open chamber designs(like:BBC open oval port vs. he bath tub shap eclosed chamber)tend to unshroud the valves better.So there could be a scenario where the combustion characteristics is great on the piston side but not that great on the head side.Piston side is only half the equation.So IMO the most correct answer would be that the best piston are only the ones that matches the heads. Or you could ask ede the only expert here

Daz

jwscab

Theres alot of factors involved with chamber design. Ideally, when the chamber is at the minimum volume, and the air/fuel charge is ignited, you want the flame kernal(more or less, tip of spark plug) as centrally located as possible. You'd want a homogenous air/fuel mixture, and a shape that has the most volume centered at the kernal, like a sphere. Now, real world, alot of other factors come into play. You have chamber shapes to help promote homogenous air fuel mixture; small crevises to deal with, such as gaps between piston and bore, piston and cylinder head from deck gap, and physical location of spark plug. There is also the fact that sharp edges promote detonation. Another thing to deal with is the chamber shape necessary to house the valves, while not shrouding them while open.

After all that, I'd say you would want as compact a chamber as possible, clearances and crevices at a minimum, a flat top piston, or if necessary, a small dish that matches the head chamber to create that above scenario.

Thats why 4 valve type cylinder heads are very efficient; centrally located plug, the chamber is round to match the bore, not much shrouding, not many areas that are 'sharp' that promote detonation. They require alot of valvtrain though, and other designs can be close in efficiency.

I don't design this crap, but thats my take on it.....(shrug)

F-BIRD'88

The most efficient combustion chamber shape is a sphere. Smallest surface to volume ratio.
Its the most thermo efficient. The best place for the spark plug is right in the middle of the sphere.
least flame travel distance.
its hard to get a high enough compression ratio with a sphere shaped combustion chamber.
The next most efficient chamber is a pent roof design
with twin quench areas . Usually 4 valve/ cylinder.
the most power efficient 2 valve combustion chamber is a Hemisphere or "Hemi" combustion chamber.
Also tough to get a high compression ratio without a
high dome piston. But allows large unshrouded valves per cubic inch for high airflow. good for a supercharger.

A shallow figure eight combustion chamber with the plug in the middle between the valves combined with a flat top or matching 8 shaped dished piston is the most fuel efficient 2 valve combustion chamber. Low surface to volume ratio
unshrouded valves central located spark plug. Very high swirl and tumble, makes a homogineous air fuel mix allowing leaner airfuel ratios and low emmisions.
fast combustion. (Fueling cylinder head.) Can be 3 valve head too. Allows lean burn , higher compression ratios/ per octane, high fuel mileage and low emmisions.
A motor with a high(er) compression ratio is more mechanically efficient.

next A wedge chamber with a small chamber and either a flat top or matching D dish is best for power and thermal efficiently (surface to volume ratio.) The closer to the center of the chamber the plug tip is the better.
A tight quench clearance between the piston flat and cylinder head creates piston cooling,
high swirl , fast combustion , low(er) octane tolerance.
Can be hard to get a high compression ratio in small ci motors and hard to get large enough valves in small bore engines. For highest power output a large bore/ short stroke oversquare design is best.

Wedge chamber with a large dome piston creates a convoluted combustion chamber with more surface to volume ratio per compression ratio and a long convoluted flame burn path. This chamber is more likely to detonate, requires more ignition advance, requires more octane, makes less power than others. its the least efficient. More chance of poor emmisions. (unburned hydrocarbons)
The least desireable configuration.

halloweenz28

??? i get the feeling that you were being sarcastic becasue of my comment
i diddnt mean to be a jerk or anything if thats the way people take it, and i really didddnt want to discourage anyone from giving their imput, but i always see great thread topics go down the drain with false information and people bickering back and forth

thank you everyone who gave their imput

thats what i was thinking too,

would it be correct to say
that the larger cc cylindar heads would be used on engines that would be larger and otherwise have incredibly high compression ratios even with a dished piston?

and that the domed pistons would be used to further raise the compression ratio (to something much higher) if all of the other ways to increase compression were already done (small chamber, thin head gasket, big stroke)

so it seems that the best possible piston type would be a flat top because it would alow the best cylindar filling (expelling of exhaust) of the air fuel mix, have less surface area and less edges, this in combination with a cylindar head that would give the desired compression ratio would be optimum then?

how big of a negative impact (if any) would a dished piston have on performance compared to a flat top piston because from what i understand, the dished piston is not as good as the flat top becasue it moves the flame front further away from the piston (because of the depression in the piston) and doesnt use the maximum amount of cylindar pressure because it allows the burning gasses to expand more

these are my QUESTIONS and information that i have collected in my research and if anything is wrong at all please correct me so i dont do the very thing that i diddnt want (give false info)

Daz

iTrader: (1)

you summarized everything the opposite of what was said..well good luck

Daz

halloweenz28

hahahaha crap
did i really? i thouht i had it right
are you sure i was totaly wrong? i know that the dish would have a better volume to surface area ratio (as was said) and i understand that, but i thought that it was counteracted by the fact that the dish allows the combustion to decrease in pressure more before it moves the piston causing less power

anyway
thanks about the good luck

halloweenz28

maybee i just worded things wrong but just to add to what i wrote above than the flatop would (becasue of the ability to use the power of the flamefront and the pressure of combustion more efficiently

Guest

I think you'll find most engines have little if any sort of dish or -cc value overall on the piston side, or small percentage wise when compared to the size of the chamber in the head. Moreso than the actual volumes, the design of the chamber and the piston top are alot more important.