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Piston area ? 1
- Thread starter Rat5
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TugboatEng
Marine/Ocean
Pressure acts perpendicular to the surface. While the dome has more area, the resultant forces are not inline with the cylinder and contribute nothing to the torque generated. As only forces inline with the cylinder count towards torque, the cross section area of the cylinder is the only number you need.
BrianPetersen
Mechanical
When you look at secondary effects, the one with the "spherical" (dome?) shape is probably going to suffer from horrible combustion chamber shape (slower combustion), higher heat loss (more surface area), higher unburned-hydrocarbon emissions (more surface/quench area, more crevice volume), etc., and the piston will probably weigh more. Pistons of that shape are not found on engines of modern design because of all that.
LittleInch
Petroleum
And if it's a true sphere it has only a teeny little bit of contact with the cylinder wall so expect more leakage past the piston seals...
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
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1
- #6
Combustion chamber design is a niche unto itself in the engine design world as torque correlates back to combustion efficiency, which correlate back to charge mixing and the motion of the charge in terms of swirl, squish, and tumble. The massive increase in power density over the last 20 years is largely due to a focus on combustion development and the necessary simulation tools. BTDT, don't let anybody tell you its easy nor done well without spending many millions of dollars.
enginesrus
Mechanical
The piston with the larger surface area eats up more heat, and represents a loss of power.
I honestly can't think of any situation in which flat top pistons aren't the best solution insofar as they don't drive excess compromise in other areas. They have the minimum surface area, lose less heat, have the largest possible squish height, and suffer less quenching. This article from JE Pistons elaborates...
Lou Scannon
Automotive
Well, there is a case for managing charge motion before and during combustion, be it in conventional 4-stroke or 2-stroke otto, and all diesel engines, add to that interacting with fuel spray characteristics in diesel and direct-injected otto engines.
This is the first time I came across an aftermarket piston supplier being cited as an authority on combustion recipe fundamentals. Of course your italics take care of all of the above if you desire.
"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz
This is the first time I came across an aftermarket piston supplier being cited as an authority on combustion recipe fundamentals. Of course your italics take care of all of the above if you desire.
"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz
Hemi said:
The article simply explains what is obvious to anyone who has done the engineering analysis as I have. Is there some aspect of reduced heat loss, better flame propagation, and less quenching you’d like to refute or are you just going to stick with ad hominem (“a piston manufacturer can’t possibly know what they’re talking about”)? Of course your name does suggest a certain bias.
gruntguru said:
Hemispheres, on the other hand, are domed. I have been unable to find a photo of heavily domed pistons when googling "formula one piston" or "NASCAR piston." What I keep finding are flat pistons with shallow cutouts for the massive valves those engines use.
This is a post about rods, but it has a lot of pictures of pistons- a lot of which are relatively heavily domed. Of course for an engine with a very short stroke and a high compression ratio target in design, it's almost impossible to get there with a pure flat-top piston.
BrianPetersen
Mechanical
Maybe a sphere might be "ideal" if it were not for charge turbulence and the inability to get the spark plug dead center in the sphere.
Two-stroke engines are unconstrained by having to accommodate intake and exhaust valves. The cylinder head of the last generation of Yamaha TZ250 Grand Prix racing motorcycles looks like this:
The pistons are plain flat top, not having to accommodate valve reliefs, and the port angles take care of directing the scavenging flow.
Slightly different:
That is a lower revving, longer stroke, direct-injection, turbocharged, lower compression two-stroke. Pistons are still flat. But the combustion chamber in the head is much deeper and the injector is dead-center pointing straight down with the spark plug off to the side a little - probably in the interest of keeping the direct-injected fuel in the vicinity of the spark plug for easier ignition.
As soon as you have to accommodate valves in the head, can't do it that way any more.
Two-stroke engines are unconstrained by having to accommodate intake and exhaust valves. The cylinder head of the last generation of Yamaha TZ250 Grand Prix racing motorcycles looks like this:
The pistons are plain flat top, not having to accommodate valve reliefs, and the port angles take care of directing the scavenging flow.
Slightly different:
That is a lower revving, longer stroke, direct-injection, turbocharged, lower compression two-stroke. Pistons are still flat. But the combustion chamber in the head is much deeper and the injector is dead-center pointing straight down with the spark plug off to the side a little - probably in the interest of keeping the direct-injected fuel in the vicinity of the spark plug for easier ignition.
As soon as you have to accommodate valves in the head, can't do it that way any more.
TugboatEng
Marine/Ocean
95 pounds? That bronze insert adds a lot of weight.
BrianPetersen
Mechanical
Everything in the UK seems priced heavily. 
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