Hi,
Another "why don't they?" type question. What governs the shape of pistons, specifically the chamber and the surface of the piston? Just looking at engine sections that I've seen - the ones without the flat pistons, that is - they appear to generally favour a hemishperical shape for the piston which, I presume, is adopted to increase compression and improve the air/fuel mix. Wouldn't this work better though if the hemisphere was inverted, and the air/fuel fell into a sort of concave "bowl shape" piston rather than onto a convex surface? If I really need to, I'll make up a drawing to explain a bit better what I mean but, hopefully, you all get the drift. I haven't had much chance to read up on this yet, so hopefully someone can shortcut me and save me a few hours of fighting through googles opinion of how best to answer the question.
The Shape of Pistons
Started by
MarkWill
, Aug 22 2003 03:01
5 replies to this topic
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#2
desmo
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Posted 22 August 2003 - 03:15
Sounds like what I believe used to be called a 'Heron head'. Try googling that. 
#3
soubriquet
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Posted 23 August 2003 - 02:33
Ford did this with the Kent engine. Disadvantages of this arrangement include increased piston mass, and difficulty in arranging for angled valves, which slows gas flow.
#4
Bill Sherwood
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Posted 23 August 2003 - 09:02
I'm building a very fast 1.6 litre Toyota road engine, and I'm using the tapered squish areas like Ferrrari haev used for some time. Toyota have also started to use them in the 1ZZ-FE, 2ZZ-GE, and 2NZ-FE engines.
It supposedly allows for more spark advance and a touch more torque.
Here's the estimated power run, courtesy of Engine Analyser Pro 2.1D -
It's pretty ambitous, but the tricky bit is getting the piston tops & combustion chamber just right. The piston will likely end up a little dished in the middle, no tsure yet as I have to do some CC checks with plasticine, etc.
Fingers crossed ....
It supposedly allows for more spark advance and a touch more torque.
Here's the estimated power run, courtesy of Engine Analyser Pro 2.1D -
It's pretty ambitous, but the tricky bit is getting the piston tops & combustion chamber just right. The piston will likely end up a little dished in the middle, no tsure yet as I have to do some CC checks with plasticine, etc.
Fingers crossed ....
#5
MarkWill
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Posted 24 August 2003 - 03:31
Thanks for the lead, Desmo. I don't see why there's an increase in piston mass - concave or convex - you're carrying the extra mass over the surface no matter what the shape, or do you need a larger surface area for a concave piston vs. a convex? I'd thought about the valves, but I confess that to me, it seems to only mattter when you have lots of valves (which you don't always need).
Bill, what are you getting into? Looks like a very interesting performance hike from a 1.6 L road engine.
Bill, what are you getting into? Looks like a very interesting performance hike from a 1.6 L road engine.
#6
Bill Sherwood
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Posted 24 August 2003 - 06:31
(not trying to divert the thread here, sorry)
It's made up of bits of a few different Toyota engines.
7AFE block, overboard 3mm to take it to 1666cc. The 7A block is taller than the 4A, so I can run longer rods.
Steel rods, longer as mentioned. I was thinking about titanium rods but I'm nervous about how they'll last. The longer rods are for a better rod ratio.
Slipper pistons, I was looking at two rings ones but it's really a road car so for longevity it'll still have three.
Lightened Group A crank.
33mm inlet (vs 20.5mm) valves, 27.5mm (vs 25.5mm) exhaust valves in a 4AFE head.
There's at least five different types of 4AFE head, but only one is any good and it looks a far bit like the 2000 spec Cosworth F1 V-10 head I saw a year or so ago. They have a 22.5° included valve angle and so are again much like an F1 head. The cam buckets are 31mm diameter, the 4AGE ones are 28mm so I can run bigger cams with them, and I can get shimless buckets so they'll be very light as well.
It'll be dry-sumped, using a system that we've worked out that runs only two pipes, one from the scavenge tank and the other from it.
I'll be running it most likely with an M48 Motec, and it'll have two sets of fuel injectors; One set in the head and the other out in the middle of the inlet trumpets for higher speed running.
There's a lot of other little things here and there, but that's the main stuff.
It's made up of bits of a few different Toyota engines.
7AFE block, overboard 3mm to take it to 1666cc. The 7A block is taller than the 4A, so I can run longer rods.
Steel rods, longer as mentioned. I was thinking about titanium rods but I'm nervous about how they'll last. The longer rods are for a better rod ratio.
Slipper pistons, I was looking at two rings ones but it's really a road car so for longevity it'll still have three.
Lightened Group A crank.
33mm inlet (vs 20.5mm) valves, 27.5mm (vs 25.5mm) exhaust valves in a 4AFE head.
There's at least five different types of 4AFE head, but only one is any good and it looks a far bit like the 2000 spec Cosworth F1 V-10 head I saw a year or so ago. They have a 22.5° included valve angle and so are again much like an F1 head. The cam buckets are 31mm diameter, the 4AGE ones are 28mm so I can run bigger cams with them, and I can get shimless buckets so they'll be very light as well.
It'll be dry-sumped, using a system that we've worked out that runs only two pipes, one from the scavenge tank and the other from it.
I'll be running it most likely with an M48 Motec, and it'll have two sets of fuel injectors; One set in the head and the other out in the middle of the inlet trumpets for higher speed running.
There's a lot of other little things here and there, but that's the main stuff.
