10-23-2006, 04:29 AM
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Registered User
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Join Date: Jan 2004
Location: SALE Victoria Australia
Posts: 1,238
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 Quote:
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Originally Posted by Gordon Raymond
Seems to me that diesel pistons have the hemispherical cut in them as well.
I've only worked on one, but I've seen a bunch. Let's get the engineer back in here on that. Can a diesel have a true hemispherical combustion chamber, or must it have some sort of flattened wedge, with the hemisphere cut in the pistons? Why?
Gordon Raymond
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Gordon, the problem with making the piston form the comb chamber shape would be that you increase piston mass to make the piston strong enough. It's better to leave that mass (strength) in the head. I can't see why a diesel couldn't have a hemispherical head design, but if it's an older style pre-chamber design, then the chamber is not really hemispherical because the prechamber forms part of the comb chamber (whew !). From what I recall, the piston tops and the head surface in my old 6.2 GM disel are dead flat.
Modern direct-injection diesel engines will have a myriad of piston shapes I'm sure.
Now, to enter into the original argument, I say that a hemi head (lower-case H) simply means that the cavity cut into the head begins as a TRUE hemispherical shape. I'm talking basic geometry here.
It has nothing to do with the ignition source, nor the positioning of such. It also has nothing to do with the number or positioning of the valves. All valves will change the comb chamber shape from a true hemispherical shape. lastly, there is no definition as to how much of a sphere is required to form the cavity in the head. It could be 1% of the volume of a sphere, or it could be 50%.
I do have some trouble with piston shapes however. One part of my brain says that a true hemispherical combustion chamber is only formed using a flat piston, but another part of the brain says it's irrelevant. So a hemispherical HEAD is one animal, a hemispherical COMBUSTION CHAMBER may be quite different.
Now in reality, there are only some combinations of combustion chamber volume, valve position etc that work. However, this is heavily dependent on the fuel used. We have arrived at similar designs in 4-stroke car engines because they mostly use the same fuel. Petroleum.
Throw 2-stroke petrol, 2-stroke diesel (quite different), and 4-stroke diesels into the mix and we see an unlimited variation on a theme to arrive at the same place.
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