Piston Top Shape and Wet Flow?

[DaveMcLain]

I was just wondering if the shape of the piston top either a dish, dome or flat top could have a significant effect on the wet flow into the cylinder... At first it seems like it would always be out of the way but at the beginning of the induction cycle its right up there for a while too. Could and or does it tend to deflect fuel up toward the chamber in some cases or does it tend to collect fuel that would otherwise spray onto the cylinder wall?

[panic]

IDK, but during overlap it's half of the chamber shape, and any surface irregularity interrupts cross-flow.

[DCal]

Well I suspect that this will be many pages long but I have some piston experience that may be of some help. Because the inlet is started by the last of the exhaust cycle it gets some pull in that direction and then it has to get around the intake valve, were OK to this point but now there's a huge dome in the way. I know the piston is moving downwards at this point and ignition IS going to happen but you may not be getting the best burn possible. We've proven this time after time that once you've achieved adequate compression the rest is just bragging rights and you've narrowed up the tuning window as well. A SBC can get a .600 dome but almost every time a .400 dome will make the same or more power and with less timing and maybe less jet.
I have a customer that builds Cosworth v8s which has an ugly little dome in the very center, we borrowed from our 4 valve Power Sports technology and pushed the dome out to the chamber walls and then adjusted for CR with a 6 inch radius bowl in the center, so that there is a dome with a radiused dish in the center. This got the fuel further away from the plug and I think it kept the mixture more "excited" especially with the tighter dome to chamber.And more uniformly spread across the piston top.

[mag2555]

I think a dish has its benefits , but more so with Intake valve inclination angles of 15 degrees or less.

[Juho_]

I'd think a large (tall) dome would eventually cause a restriction near TDC, when the inlet valve is open and mixture flowing to cylinder.
And the air-fuel mixture would be way slower to burn since the dome is getting on the way of flame after mixture near spark plug is ignited.

Not sure, how to explain it any better.. hope you'll understand - english is not my primary language.

In my opinion a flat top or slightly dished piston would be the best compromise. If combustion chamber volume can be kept low enough without ruining it's shape to achieve a high compression ratio, which you are probably looking for with a domed piston.
Low valve angle will help keeping the combustion chamber volume at bare minimum.
With low valve angle , when inlet and exhaust valves are both same time open, the mixture can escape to exhaust ports easier, since there'll be less space between the valves than with a greater valve angle.

Just some thoughts.

I might be wrong tho.
There are many B and K series Honda engines that are capable of producing more than 150hp per liter naturally aspirated, and have a huge dome on the pistons to achieve a static compression greater than 14:1 (often with E85 or methanol).
They do rev very high, and piston speed can be quite extreme like many engines do have that are capable of producing very high BMEP.

[piston guy]

Dave,
It's not only about the piston. Chamber shape ( as DCal was getting at) has an affect as well. Spark plug placement is important too. Piston top shapes "can" manipulate air flow to some degree and they can also enhance combustion too. There isn't "one" answer to your question unless you get more specific as to the head type .

BTW it was GREAT to see DCal ( and his brothers) last week at PRI. Now that DCal is on the east coast I hardly ever get to see him. "A little" piston experience is OVER 40 years experience by my math.

[MadBill]

Without too much trouble, dome/dish & valve relief flow impact could be evaluated on a flow bench:
o Mock up a block with a crank, timed-in cam, a rod and one of the pistons to be evaluated.
o Fit a head with an intake valve and checking spring and plug the exhaust guide to prevent leakage. Mount to the block and add rocker and pushrod. (Intake manifold and carb/TB optional)
o Set up the bench with a 3" vacuum hose to suck on the exhaust port.
o Flow the head at say 10 crank degree angular increments through the relevant range, optionally with the exhaust valve in place and also actuated by the cam.
o Compare/develop flow with different mocked up domes, with variable valve openings and fixed piston position, etc.

[pdq67]

I have often wondered what would happen if the way early 265" SBC engines, "sparkplug trough", head was used with a way long extended reach sparkplug tip to get the tip out of the trough? Do to the, "double-quench", design here is all.

Of course this would have to be done with a head that at least had ports as big as a -462 head or bigger.

My pair of 1956, 265", sparkplug trough, -306's cc'd at 118 to 120 intake port size so they are WAY SMALL!

pdq67

[Pro-SeriesFab]

Wasn't that the idea behind widmer's roller-wave pistons?

[DaveMcLain]

I was just wondering about how much of the total amount of fuel used must enter the cylinder very early in the cycle as soon as the valve opens much at all if the pressure in the chamber is low enough. It seems like with the wave action that's happening in the intake that it would be stacked up against the back of the intake valve and then spray in all around its periphery. Does it do this? If so it seems like the fuel would not tend to be in the area just behind the valve until much later and it seems like what would spray in would have a tough time getting put back into the mix. Does a sharp edge at the seat, on the edge of the valve etc help performance in this area even if airflow stays the same?

[LSP]

Ditto what DCal and piston guy said, gains are there to be had.

[hoffman900]

Ditto what DCal and piston guy said, gains are there to be had.

Hard to disagree with those guys considering their combined experience.

[LSP]

Ditto what DCal and piston guy said, gains are there to be had.

Hard to disagree with those guys considering their combined experience.

Yep, have learned a lot from both of them.

[DCal]

I forgot that the main point about the Cosworth dome change was that the motor picked up 12 hp which is significant for a motor that's been around for decades.

[David Redszus]

Piston dome shape contributes to several aspects of combustion from filling, to flame speed, to piston temperature, to scavenging.

While it is tempting to flow a domed piston at max air flow, (which occurs at/near max piston velocity not max valve lift), the piston is very far down the cylinder and not likely to have much effect on flow.

However, during overlap, the piston is very close to the valves and could affect flow dynamics. A further consideration is that a moving piston will cause a squish velocity motion which could have a considerable impact on flow before and after TDC.

Sometimes simple question have very difficult answers.