Combustion Flame Speed

[SteveS]

In Superbike racing there is no flat squish/quench and it's common practice to run Cosworth pistons that are carefully machined to match the curvature of the pentroof 4v head which incorporates radiused/contoured pads. With the exception of the flat areas on the bottom of the valve reliefs, the chamber is more or less one entire squish/quench zone. A profile of the combustion chamber would reveal an "arc" stretching across the bore with the thickness matching the squish clearance. Separately, a special thanks to Darin for taking time to post such interesting material.

[moper]

One thing I haven't seen mentioned is that the squish area is not limited to the flat areas of the head and piston. The squish can continue up the side of the dome if required. Another point is an ill fitting dome can disrupt the expected squish action. The squish can be directed by dome shape and clearance.

A few years ago, Ray Barton was using that same concept (at least I beleive he was on that idea anyway) and was hand fitting pistons to chambers on the S/S Hemis. I'm pretty sure he wasnt looking at wet flow pics such as Darin posts, but workling off Widmer's ideas on 4valve, and modifying for the central spark plug 2 valve hemi design. I'm guessing that's one of the reason he was making so much more power for a few years around that time. Cool stuff people..good reading..

[putztastics]

When I saw the grooves people are machining into the chamber I thought about that vortex there on that side of the chamber because it is really big a on a 23° head!

Darin, what did you think about the vortex/groove interaction??? Were you curious enough to try a groove on the wet flow bench?

The grooves are supposed to have a positive effect on detonation, it could be true as the 11-1 iron head 360 I used in groove testing is running at sea level on pump gas with no pinging. I didn't think it would.

It may be the small vortex causes air/fuel separation, and eliminating this air/fuel separation has a positive effect on detonation. This reminds me of a letter to the editor of Popular Science after an article on Smokey's hot vapor engine. The letter writer was an engineer and said it was interesting that Smokey's use of total vaporization had the effect of raising the octane rating of the fuel implying the unvaporized fuel in the charge increased detonation.

Or is that vortex messing up the quench effect causing an increase in detonation?

Does the ratio of liquid fuel to vapor fuel in the chamber even have any effect on detonation?

Liquid fuel doesn't burn, we don't want it in the chamber but when it's there what does it do?

One of the most interesting details that came up in the grooves discussions was the possibility of the increasing compression pressure causing fuel to condense out of vapor in the quench area near TDC. Or does the flame front get there fast enough to prevent that?

[beth]

One of the most interesting details that came up in the grooves discussions was the possibility of the increasing compression pressure causing fuel to condense out of vapor in the quench area near TDC. Or does the flame front get there fast enough to prevent that?

The flame front will never reach the outer limits of the squish before TDC because the flow is in the opposite direction. Condensation depends on the temperature of the squish pads. If any fuel condenses it is ejected during squish if the piston is close enough to the head.

There is not much talk about squish after TDC. A low pressure area is formed to pull various mixtures back toward the cylinder wall and these effects need to be considered also, not just during combustion but during the intake phase. Theres lots going on in a combustion space.



edit: used a wrong word

[Darin Morgan]

When I saw the grooves people are machining into the chamber I thought about that vortex there on that side of the chamber because it is really big a on a 23° head!

Darin, what did you think about the vortex/groove interaction??? Were you curious enough to try a groove on the wet flow bench?

The grooves are supposed to have a positive effect on detonation, it could be true as the 11-1 iron head 360 I used in groove testing is running at sea level on pump gas with no pinging. I didn't think it would.

It may be the small vortex causes air/fuel separation, and eliminating this air/fuel separation has a positive effect on detonation. This reminds me of a letter to the editor of Popular Science after an article on Smokey's hot vapor engine. The letter writer was an engineer and said it was interesting that Smokey's use of total vaporization had the effect of raising the octane rating of the fuel implying the unvaporized fuel in the charge increased detonation.

Or is that vortex messing up the quench effect causing an increase in detonation?

Does the ratio of liquid fuel to vapor fuel in the chamber even have any effect on detonation?

Liquid fuel doesn't burn, we don't want it in the chamber but when it's there what does it do?

One of the most interesting details that came up in the grooves discussions was the possibility of the increasing compression pressure causing fuel to condense out of vapor in the quench area near TDC. Or does the flame front get there fast enough to prevent that?

Holy crap, i never thought of that. Why do the simple things always escape us. I will try and test the groove on the quench pad but I need anouther camera like the one I had. maybe I can find one on e-bay.

[putztastics]

Hey HEY HEY Darin's gonna try a groove!

The groove might trip up the votex on the flow bench....but I'm not sure it will. I think the groove works during quench flow, closer to TDC.

One of the most interesting details that came up in the grooves discussions was the possibility of the increasing compression pressure causing fuel to condense out of vapor in the quench area near TDC. Or does the flame front get there fast enough to prevent that?

The flame front will never reach the outer limits of the squish before TDC because the flow is in the opposite direction. Condensation depends on the temperature of the squish pads. If any fuel condenses it is ejected during squish if the piston is close enough to the head.

There is not much talk about squish after TDC. A low pressure area is formed to pull various mixtures back toward the cylinder wall and these effects need to be considered also, not just during combustion but during the intake phase. Theres lots going on in a combustion space.



edit: used a wrong word

Yeah there is a lot going on ... I wonder how much of the total quench flow is left at 30-35 before TDC.

I wonder if the groove acts like a spark plug for the quench area, if it helps speed up the burn in that area around TDC.

What causes the clean spots at the outer edge of quench areas? Is is wet fuel wash? Is it no burn because of a lean mixture?

If a groove can supress detonation (like removing the vortex did) wouldn't that be a logical reason to use a groove?

[beth]

I wonder how much of the total quench flow is left at 30-35 before TDC.

The quench has not even started yet at 30 degres btc. There is a delay between ignition and flame kernel. I am saying the flame kernel cannot get to the outer areas of the squish band before TDC because the flow is outward toward the chamber.

[putztastics]

How much does the piston move up between 30 before TDC and TDC?

[beth]

That varies with stroke, alignment of bore to crank centerline and rod ratio. Roughly 1/6th of the stroke

[automotive breath]

How much does the piston move up between 30 before TDC and TDC?

http://home.earthlink.net/~scloughn/id21.html

Look down at the bottom of this article; Neels van Niekerk has a series of drawings from a simulation of the combustion process.

[beth]

How much does the piston move up between 30 before TDC and TDC?

http://home.earthlink.net/~scloughn/id21.html

Look down at the bottom of this article; Neels van Niekerk has a series of drawings from a simulation of the combustion process.

Nice illustrations but some of his assumptions are incorrect.

[automotive breath]

Nice illustrations but some of his assumptions are incorrect.

The thing that interests me most is the illustration that is presented, regardless of its accuracy. I find it interesting to break down a process that happens in a fraction of a second into frames so that they can be analyzed indivually. This could prove to be key to someone interested in developing modifications that are designed to improve the combustion process.

[Unkl Ian]

Darin: what lift % are you doing your wet flow tests at ?



Thanks.

[automotive breath]

Darin, If you decide to try this on a running engine with 45% squish area, it would likely be best to cut two grooves one on each side of the combustion chamber the way Jessie did on the 360. They work best if you cut the groove deep into the cavity contouring the bottom of the groove deep into the chamber roof.

[putztastics]

That varies with stroke, alignment of bore to crank centerline and rod ratio. Roughly 1/6th of the stroke

4.00bore X 3.51 stroke 6.25 rod 6100 rpm

.300 from 30° BTDC to TDC

The piston slows down quite a bit near TDC

Peak piston speed is 5800fpm piston displacement per degree is 324cc

@ 30° BTDC speed is 3484fpm piston displacement per degree is 61cc

@ 20° BTDC speed is 2433fpm piston displacement per degree is 28cc

@ 10° BTDC speed is 1242fpm piston displacement per degree is 7cc

I wonder when peak quench flow occurs.