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Piston area ?

Piston area ?

Piston area ?

Consider 2 engines with a 90mm bore.
One has a flat piston
The 2nd has a spherical piston

Will the one with the spherical piston make more torque because there is more piston area ? Or are you supposed to use projected ( so bore) area for this calculation ?

RE: Piston area ?

No. use projected area.

RE: Piston area ?

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.

RE: Piston area ?

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.

RE: Piston area ?

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.

RE: Piston area ?

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.

RE: Piston area ?

The piston with the larger surface area eats up more heat, and represents a loss of power.

RE: Piston area ?

Not necessarily- flat top pistons are not the ultimate solution.

RE: Piston area ?

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... https://www.jepistons.com/blog/dish-dome-flat-top-...

RE: Piston area ?

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

RE: Piston area ?

Quote (Hemi)

This is the first time I came across an aftermarket piston supplier being cited as an authority on combustion recipe fundamentals

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. 😂

RE: Piston area ?

The ideal combustion chamber is spherical for its low surface area to volume ratio. Other factors dictate departures from that shape.

A true "hemi" would have flat piston crowns BTW.

je suis charlie

RE: Piston area ?

Quote (gruntguru)

A true "hemi" would have flat piston crowns.

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.

RE: Piston area ?

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.


RE: Piston area ?

Those are Ferrari pistons which are the least flat I've seen in F1. Regardless, they're still pretty flat compared to what I call a "domed piston."

RE: Piston area ?

Quote (RodRico)

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.
My interpretation is that a spherical combustion chamber compresses the gas into a sphere and a hemispherical chamber compresses the gas into a hemisphere - which would require a flat piston crown.

je suis charlie

RE: Piston area ?


The ideal combustion chamber is spherical for its low surface area to volume ratio. Other factors dictate departures from that shape.

I was waiting for someone to tell us what the rest of the chamber looked like, but if flat tops are ideal for all fuel and ignition systems then the other surfaces’ shape must not matter either. 😜

RE: Piston area ?

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: http://www.geckomotorcycles.co.uk/magento/4tw-yama...

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: https://www.snowmobile.com/manufacturer/ski-doo/in...

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.

RE: Piston area ?

95 pounds? That bronze insert adds a lot of weight.

RE: Piston area ?

Everything in the UK seems priced heavily. smile

RE: Piston area ?

We're talking about piston engines in which compression and expansion result from the linear motion of a piston. An ideal spherical combustion chamber is only spherical at the instant of minimum volume. At other times, it's a cylinder with hemispherical ends. In contrast, a cylindrical combustion chamber formed by flat surfaces is always a cylinder. I decided to explore the two mathematically.

I defined the minimum volume of a cylinder having flat ends with a bore of 4 inches and a minimum clearance of 0.1 inches. I then calculated the surface area and volume of the cylinder at both the minimum volume and at a volume 10x larger. Next, I defined a spherical combustion chamber having the same minimum and maximum volumes and calculated the surface area. The result is below.

At min volume, the spherical chamber bore is only 31% of the cylindrical chamber with only 19% of the area.
At max volume, the spherical chamber height is 932% of the cylindrical chamber height with 97% of the area.

Heat transfer is a function of temperature difference, surface area, and time. The spherical chamber starts with less surface area, but its stroke is 9x larger and it has about the same surface area at maximum volume. The amount of heat transferred thus depends on how quickly combustion occurs; it will lose significantly less heat during very rapid HCCI combustion but its performance will be less impressive in a diesel cycle. It also has only 31% of the ring circumference, but for any given RPM, the rings are sliding much more quickly due to a 932% longer stroke, so ring friction (which dominates FMEP) will likely be higher at a given RPM.

It's important to note the above discussion relates to an ideal spherical chamber that can't actually be built. First, there must always be some minimum clearance between the piston and head to account for mechanical tolerances, and this means the chamber can't be perfectly spherical. Furthermore, the edge of the piston can't be infinitely thin as in a hemisphere due to loads and the need for ring lands. These practical considerations move the spherical chamber closer to the cylindrical chamber.

In the end, I suppose a spherical chamber may make sense for a long stroke HCCI engine running at low RPM, but I think it's a poor choice for high revving, high horsepower, engines using traditional combustion.

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