Which one of these combustion chambers you like the best?

[Brian P]

Is that BMW S1000RR piston designed to squish from the exhaust to intake side? Or what is going on there?

The flat areas between each pair of intake valves and exhaust valves are designed to squeeze air/fuel out from those areas and generate turbulence in that vicinity.

The piston dome approaches the head closely near the outer perimeter. Same intent as the above. But the reason for doing it at the piston rather than using a cloverleaf head is that doing it in the piston in these locations avoids shrouding charge motion on the sides of the chamber - most likely with the intent of enhancing whatever tumble they can get with such bore and stroke dimensions.

The piston dome is recessed into a bowl in the center because that's closer to where the spark plug is.

The whole intent is to get as much of the mixture as close to the spark plug as possible and generate small-scale turbulence in the outer reaches of the chamber so that the flame speed will be higher in the outer reaches once combustion gets going.

For what it's worth, my ZX10R has chambers somewhat similar to the BMW but the chamber is drawn in a little on each side (between the intake and exhaust valves) and the piston is flat in that area. The squish band is flat with the piston rather than being raised up following a raised piston dome the way the BMW does it.

edit: I found this better image of what the S1000R piston looks like ... actually, this is a Wiseco aftermarket piston, but same idea. The raised areas between the intake and exhaust valves, and the central dish, are readily visible.

[Brian P]

Is that BMW S1000RR piston designed to squish from the exhaust to intake side? Or what is going on there?

$10,000 question. Minimising tdc combustion chamber "dead" space around the intake valves while maximising it in the vicinity of the exhaust valves & spark plug is what Widmer/Endyn promote

Which gets me back to my earlier question in the thread whether the intention with the squish is to move unburned charger, burned charge, or burning charge?

Perhaps the Endyn approach is to move the cold, unburned gas to where the fire is. Perhaps the BMW approach is to move the fire to where the cold, unburned gas is. Questions, questions...

The intent is most certainly to generate turbulence in the unburned charge, so that once the flame front gets to it, the turbulence enhances the flame speed.

Due to the timing of the events, this will normally be occurring after ignition, just as the piston closely approaches TDC, so the combustion process will already be going when this happens, but the intent is to mix up the portion that has not yet burned but is just about to.

Once a given element of air/fuel mixture has burned, there is no purpose to having turbulence in it. If anything, you don't want much turbulence any more, because you don't want heat transfer any more at that point. (You want a little bit to encourage the last pockets - resulting from imperfect mixing - to finish the process.)

[Brian P]

Shouldn't we expect, over time, that all the chambers approach a more 'ideal' set-up and all start looking the same? I'm noticing a lot of similarities from the the purpose built race engines of 20 years ago and present with what's being offered by Honda and the bike manufacturers.

Yes and this is what has been happening.

There will always be detail diferences due to the differing objectives behind different engine designs, though. An engine designed for fuel economy will tend to have narrow bore, long stroke, therefore less of a "pancake" shape to the chamber, and be lower revving, so less need for small-scale squish turbulence and perhaps more reliance on tumble persisting into the combustion phase (possible, because the shape is not so flattened). An engine designed for outright horsepower will be like the S1000R, and ZX10R, and R1, and F1 cars, etc.

Still ... practically every modern engine design is using some variation of a pentroof 4-valve with central spark plug, with only minor detail differences.

Even the direct-injection engines are using pentroof variations, generally with a shaped dish near the center of the piston to guide the injection spray.

[ptuomov]

The whole point is to force stuff to get closer to each other... if one considers the fact that the flame kernel is (should be) already well underway before the point of tightest squish (TDC) and the piston almost stops dead for atleast +/- 20 degrees TDC it's easier to visualize. The problem comes when you have two flat (and worse still, wide)sections with 0.2mm running clearance, a rocking piston and the surfaces are colder than the surroundings.. a compact chamber which pins fuel droplets inside a virtually sealed off section of chamber - that is, sealed off from any extra heat besides the heat from pressuration - that can vaporize the fuel droplets, you have a powder keg with a lit fuse. Pour the gunpowder on the floor, it burns. Stuff it with no place to go, bang.

I reread what you said there, and I think one thing that I had not previously appreciated is piston rock. Piston rocks near the TDC. That rock is very small for me, given that I run aluminum piston in an aluminum cylinder, but it’s still there. It makes sense to taper the squish pad assuming the worst case scenario piston rock, whatever the trig functions give you based on the worst-case piston-to-bore clearance. That should prevent trapped volumes there.

[ptuomov]

...He runs 12,5:1 compression and the crowns look very ordinary. The combustion chamber is almost stock. I have tried to weld up chambers like that to something like this:

http://www.topplocksverkstan.se/bilder/ ... vRc-22.jpg

But I believe it is a lot of work for zero flow gain, possibly worse combustion but the same compression with a smaller dome. Everything has to be rounded off like in this chamber and there should be as little in the way as possible to interfere with charge motion.

Why do you say it’s “possibly worse combustion” when he can run a smaller piston dome? Is that statement based on an empirical fact? Because my instinct (which you can take to Starbucks and they’ll give you a free coffee for it, as long as you give them $2.20) says that it’s going to burn better with smaller dome, unless the dome is working some charge motion black magic.

[ptuomov]

edit: I found this better image of what the S1000R piston looks like ... actually, this is a Wiseco aftermarket piston, but same idea. The raised areas between the intake and exhaust valves, and the central dish, are readily visible.

That piston doesn't look at all like the one in the BMW diagram. The piston in the BMW diagram has this huger squish pad / crown on the exhaust side that at least by appearances should move the charge to the intake size. That Wiseco aftermarket piston doesn't look anything like the stock piston in that cutout from BMW.

That Wiseco piston side squish pads remind me of the air-cooled Porsche hemi pistons.

[justahoby]

...He runs 12,5:1 compression and the crowns look very ordinary. The combustion chamber is almost stock. I have tried to weld up chambers like that to something like this:

http://www.topplocksverkstan.se/bilder/ ... vRc-22.jpg

But I believe it is a lot of work for zero flow gain, possibly worse combustion but the same compression with a smaller dome. Everything has to be rounded off like in this chamber and there should be as little in the way as possible to interfere with charge motion.

Why do you say it’s “possibly worse combustion” when he can run a smaller piston dome? Is that statement based on an empirical fact? Because my instinct (which you can take to Starbucks and they’ll give you a free coffee for it, as long as you give them $2.20) says that it’s going to burn better with smaller dome, unless the dome is working some charge motion black magic.

I am with you on less dome, more squish.. Even with the plug straight above pointing down on it.

[Erland Cox]

I had to take a lot of material out of that chamber to get near the flow I had with the stock chamber.
I didn't think at the time that it would give a loss, I was hoping for a gain.
The only advantage is a smaller or no dome and a lighter piston and that is not bad at all.
I have a chance now to see the difference between a cloverleaf chamber and a more open one.
If you send me your email address I can send you pictures of the head I recently did, it is a one cylinder billet head for a Husaberg motorcycle.
It has very shallow valve angles so the problem is that the chamber is too small so the piston needs a bowl.
I am making another head with the possibility to take more material out of the chamber and I already ground out a lot in the first head.

Erland

[ptuomov]

If you send me your email address I can send you pictures of the head I recently did, it is a one cylinder billet head for a Husaberg motorcycle. It has very shallow valve angles so the problem is that the chamber is too small so the piston needs a bowl. I am making another head with the possibility to take more material out of the chamber and I already ground out a lot in the first head.

I am in the dish camp all the way, so that's very relevant for me. All these 928 engines that we play with end up with long enough strokes that part of the combustion chamber ends up in the piston dish. For example, here are some piston photos of the piston that has valve reliefs for the head #2:

IMG-20130606-00074.jpg

IMG-20130606-00073.jpg

IMG-20130606-00075.jpg

Even in a clean sheet design, you want a significant fraction of the combustion chamber in a piston dish if you can, no? Some dish is going to make for a more compact combustion chamber. I may sound like a one trick pony (or a pony with no tricks at all!) but I just think that if the flow is the same or enough, we just want the most compact combustion chamber that consists of both the head and the piston crown.

What is the best head for that piston dish, by the way? Thinking mostly from the perspective of combustion.

[Erland Cox]

A piston with a dish will be haevier than a flat top and the rings will run hotter if they are at the same heaight.
I like a chamber with the absolute minimum surface to volume area.
If a chamber with ridges forces you to run a dished piston I believe that a more open chamber and a smaller or no dish is better.
Believing and knowing are two different things but hopefully I can test two different chambers on that Husaberg.

Erland

[justahoby]

A piston with a dish will be haevier than a flat top and the rings will run hotter if they are at the same heaight.
I like a chamber with the absolute minimum surface to volume area.
If a chamber with ridges forces you to run a dished piston I believe that a more open chamber and a smaller or no dish is better.
Believing and knowing are two different things but hopefully I can test two different chambers on that Husaberg.

Erland

If you look at pistons like on the Kieth Black site you can see it is pretty much true. Dish and dome are both heavier than a flattop by a conderable amount in some application .
Now when you say you like minimal surface area in the chamber, do you like the chamber to be shorter/ wider for volume less squish? If you do then why?

[ptuomov]

A piston with a dish will be heavier than a flat top and the rings will run hotter if they are at the same height. I like a chamber with the absolute minimum surface to volume area. If a chamber with ridges forces you to run a dished piston I believe that a more open chamber and a smaller or no dish is better. Believing and knowing are two different things but hopefully I can test two different chambers on that Husaberg. Erland

If you are trading off piston weight and the compactness (say volume / surface area ^1.5) of the combustion chamber, wouldn't that suggest you want a least some piston dish in the design and never any dome? Flat top piston is the lightest, but you can increase the compactness of the combustion chamber by putting in a very small dish that is not going to increase the weight much.

If the domes, deep valve reliefs, ridges and the like are justified, they are only justified from flow perspective. Filling the heads with ridges etc. while running dished piston doesn't sound justified from combustion perspective. It seems that one would rather be better off shrinking the largest dimension of the combustion ovoid. Now that ignores the flow, by for example calling for valve shrouding or smaller valves so it's not practical for most cases.

[ptuomov]

"The combustion chamber of the YZF-R15 engine is a hemispherical 4-valve type and its shape is designed with the minimum area necessary for the two intake and two exhaust valves. At the same time it is designed with a minimum intake and exhaust valve angles to realize an optimum combustion chamber shape. As an important factor determining combustion efficiency, the surface area/volume ratio (S/V ratio) has been minimized as well. This combines with the effects of the fuel injection system to produce outstanding engine performance characteristics and contributes to more enjoyable running performance in long-distance riding in the suburban areas and running in the mid- to high-speed range."

Yamaha seems to really go for the Head #2 type. More Yamaha combustion chambers, some ported for more flow and less compact chambers:

[ptuomov]

Here is one more shot of the head #2, also a shot of the piston:

photo 1.JPG

IMG-20130606-00075.jpg

This is for about 750 cc cylinder with 102.5mm bore and 92mm stroke so a dish is needed.

[Ben D]

This has been a very interesting thread for me. Regarding the question of trading off squish with boost in a pent roof chamber, there has to be something said about exaggerating the scenario to arrive at an answer. There would be nothing more exaggerated on this topic than the 60-80 psi boost F1 engines of the 1980s. BMW ran a fairly conventional 1980's vintage pent roof combustion chamber in their engines, and they ended up with a shallow dish design with peripheral squish band.

bmw piston.jpg

While they would have had a need for a relatively low compression ratio, they also would have had to run large piston to bore clearances , promoting piston rock, as well as excessive fuel to keep the piston cool. Under these extreme conditons I can visualise the shallow dish would promote squish only where it was needed, while promoting mixture movement towards the spark plug and away from the cylinder walls. It would also reduce the chances of trapped volumes forming around the intake side of the piston due to piston rock. All of these features would contribute to lessoning the chances of preignition or detonation.