10-22-2006, 08:24 PM
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Stu,
When the combustion chamber is in the piston the head is flat at the top. It's not a "flat head" as it has overhead valves but you can see in these pictures that the entire combustion chamber is formed by the shape of the piston and cylinder. The only automotive engines I know of like this are pre 1965 larger Chevy V8s. I am sure there are others, but that's all I have seen. If this design is making a comback that would be fascinating.
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10-22-2006, 09:33 PM
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The Rover 2000 cars and Moto Morini bikes had a kind of reverse hemi. The combustion chambers were formed by a shallow "hemispheric" depression in the pistion itself. The head was just a flat surface with two valves side by side.
-TomP.
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10-22-2006, 09:43 PM
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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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10-23-2006, 02:52 AM
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 Quote:
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Originally Posted by Greg Gordon
Oh, and yes, there are many many pushrod Hemis. Overhead cams are definatly not a requirement.
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Here are some pics of a couple of engines I got laying around in my garage, the 16v is a Fiat/Lancia engine from an Alfa 155 Q4. Would you call this a Hemi? I don't. Most people call this a pent roof design, no idea where the term originated from but then again that doesn't really matter. Maybe because it is a "roof" with five surfaces?
Pros are: Squish, flat top piston makes around 9.5:1 static CR = small surface area to conduct heat away from the Combustion Chamber (CC). But it's not a hemi.
The Nord I would call a hemi head even though it has some reliefs in it around the valves (so it's really not perfecly hemisferical but who cares). No squish and as you can see on my geometry study the piston needs to me domed quite a bit to get a 9.0:1 CR. Not good. The TS has a slightly better design with the plugs to the sides, makes room for the valves to be less angled and therefore a lower roof is possible and less dome. The V6 is also better then the Nord but the plug is offset and the acess to it is quite tight.
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GTV 2000 -77 [URL=http://www.alfapower.nu/gallery/gtv]Pics[/URL] <= These are picture links you know...
75 Turbo -87 *sold* [URL=http://www.alfapower.nu/gallery/75T-Modificata]Pics[/URL] <=
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10-23-2006, 05:29 AM
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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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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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10-23-2006, 05:59 AM
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Quote:
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Originally Posted by BigSwede
The Nord I would call a hemi head even though it has some reliefs in it around the valves (so it's really not perfecly hemisferical but who cares). No squish and as you can see on my geometry study the piston needs to me domed quite a bit to get a 9.0:1 CR.
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Do you have a picture taken from along the crankshaft line?
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10-23-2006, 06:03 AM
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hahaha, it would be interesting to see a hemi combustion chamber with your definition. A true one that is. pretty funny looking pistons.
Cavity pistons with the diesels has do do with the fuel/air mixing in the chamber. Swirl/tumble inducing stuff + squish.
Edit:
Maldi, you posted when I wrote my reply... No I don't have a pic along the crank actually, is there anything in particular you are interested in? I have som shots of the cut head pieces (by themself) from the other direction, interested?
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GTV 2000 -77 [URL=http://www.alfapower.nu/gallery/gtv]Pics[/URL] <= These are picture links you know...
75 Turbo -87 *sold* [URL=http://www.alfapower.nu/gallery/75T-Modificata]Pics[/URL] <=
Last edited by BigSwede; 10-23-2006 at 06:06 AM.
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10-23-2006, 07:09 AM
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Yes, if it is not too much trouble!
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10-23-2006, 08:28 AM
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Quote:
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Originally Posted by Greg Gordon
Stu, that JE piston goes on an engine with a convetional combustion chamber. It's simply dished to lower the compression, almost certainly for some insane amount of boost.
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10-23-2006, 09:18 AM
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Diesels are a whole other animal. I would imagine they could have a hemi-but it would be very tight, the thing about diesels is the deck height compared to gas engines and the compression height of the piston-that is what allows extremely high compression, also the fact that the heads have a "precompression" chamber which is where the glow plug hides. Anything is possible! It's like when someone asks you, "how fast can you make my car?", the obvious answer is how much money do you have. If you got money-anything can be custom made-anything. I'm sure if any of us had a million dollars to invest, we could get Lunati, Scat or Eagle to custom make us forged steel crankshafts and H Beam rods, I would think we'd get at least a set of 1,000-10,000 of them for that price-but money is the only factor when doing high performance anything.
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