Subaru EJ25 combustion chamber to do / not to do

[ptuomov]

you did read this?
http://www.motortecmagazine.net/the-fut ... e-engines/

we found some squish helped even at 50 lbs on alky 2valve motor

Yes. Interesting idea. Seems like a counterproductive thing to do, though, with high boost and pump gas. However, "seems like" does not equal "it is". Has anyone run a poly quad head on a high boost pump gas Subaru engine?

The two valve motors are completely different animals...

[ptuomov]

I don't think it has to do with the gas.
More to do with the heat, density, and turbulence generated in the charge. It will show up in how much ignition advance you need. Since you did a test engine already, you should have some idea of how much timing it needed, and how that compares to other engines.

My logic (correct or not) is that with high boost and pump gas, you don't want to speed up the burn too much. You want to keep the burn slow enough such that the peak cylinder pressure doesn't get too high while the average pressure over the power stroke still remains high. Also, I think one would want to keep the cycle-to-cycle variability in the early burn to the absolute minimum. How to do that? Dump port and small squish area? I'm fine with the residual tumble pushing the early flame to the exhaust side as long as that happens very consistently.

In terms of the test engine, we have high octane race gas for an experiment to see what it would really want in terms of timing (not knock constrained).

[modok]

Boosted Vw's and Hondas have found Widmer's pistons work very well pushing the limits of knock boosted as well as they do NA, in fact, maybe better. The subaru heads tend to have very generous intake valve size compared to displacement, so it would appear IMO that they would work GREAT.

[ptuomov]

Boosted Vw's and Hondas have found Widmer's pistons work very well pushing the limits of knock boosted as well as they do NA, in fact, maybe better. The subaru heads tend to have very generous intake valve size compared to displacement, so it would appear IMO that they would work GREAT.

What shape piston for high boost and pump gas? There are many designs on the website.

[Leftcoaster]

What shape piston for high boost and pump gas? There are many designs on the website.

Those which concentrate unswept volume around the spark plug and exhaust valves, albeit observing conventional squish, unshrouding the intakes as possible and minimising piston domes

Not possible to see, as from memory neither piston crowns nor moulds are posted adjacent to the matching combustion chamber - - the devil is in the site's text

And check how and why they inhibit low lift intake flow - -

[modok]

I'm not an expert on it, but usually knock does happen most often at the edges of the chamber by the intake valves, and looks to me like they fill-in those areas and makes them more like quench areas, putting more volume by the plug and exhaust vavles and less at the far side of the intakes, and it actually works. Many other designs, blue flame, may-fireball, sure does work, and for the same reasons.
Of course, that means intake valve clearance at TDC can be a problem, but if you don't NEED a lot of overlap then it's quite practical. Short stroke, big valves, turbo......you don't need much overlap.
Don't tell the subaru guys. Putting a turbo on that glass house should be discouraged

[ptuomov]

What shape piston for high boost and pump gas? There are many designs on the website.

Those which concentrate unswept volume around the spark plug and exhaust valves, albeit observing conventional squish, unshrouding the intakes as possible and minimising piston domes

Not possible to see, as from memory neither piston crowns nor moulds are posted adjacent to the matching combustion chamber - - the devil is in the site's text

And check how and why they inhibit low lift intake flow - -

The so called "roller wave" piston? I like those shapes, although for different reasons than what the text says. I think that with four valve head, you'll get a tumble motion. The tumble motion lights up the charge from the spark towards the exhaust valves like a bonfire in a storm. So it makes sense to make room for it. The roller wave shape may also enhance the tumble motion earlier in the cycle.

This all seems to produce a very fast burning chamber. Great for a normally aspirated motor, but what about a high boost pump gas motor?

[Leftcoaster]

The so called "roller wave" piston? I like those shapes, although for different reasons than what the text says. I think that with four valve head, you'll get a tumble motion. The tumble motion lights up the charge from the spark towards the exhaust valves like a bonfire in a storm. So it makes sense to make room for it. The roller wave shape may also enhance the tumble motion earlier in the cycle.

This all seems to produce a very fast burning chamber. Great for a normally aspirated motor, but what about a high boost pump gas motor?

Tony Knight has provided valuable insights on this very thread - - while no doubt there are others, prolific contributors Bjorn Dieldock and Erland Cox have previously mentioned similar projects and might respond if alerted to yours

[wyrmrider]

you want a nice even gas to burn and resulting burn pattern
Tumble alone does not give you this
even with a stratified charge- which can be a good thing and what you get with DI only
you end up with rich spots and leans spots
you can get incomplete burn and "explosions" at the same time

some swirl either homogenizes the mixture or hides the problem
best clue is your piston tops and combustion chamber flame patterns
I have NOT done a high boost 4V but several different 2V
I would think port inj with a DI at the last moment would work best
Interesting thread ptuomov
maybe the Polyquad with separate injectors for each port

[ptuomov]

I'm not an expert on it, but usually knock does happen most often at the edges of the chamber by the intake valves, and looks to me like they fill-in those areas and makes them more like quench areas, putting more volume by the plug and exhaust vavles and less at the far side of the intakes, and it actually works. Many other designs, blue flame, may-fireball, sure does work, and for the same reasons.
Of course, that means intake valve clearance at TDC can be a problem, but if you don't NEED a lot of overlap then it's quite practical. Short stroke, big valves, turbo......you don't need much overlap.
Don't tell the subaru guys. Putting a turbo on that glass house should be discouraged

I believe knock happens on the intake side of a four valve head for the following reason. The tumble motion drives the flame to the exhaust side. Exhaust side then burns first. Knock is ignition of the unburned end gas. Since the exhaust side burns earlier because of the tumble motion, the unburned end gas has to be on the intake side. It's like the old joke about why do you rob banks having the answer because that's where the money is.

[user-23911]

It seems to happen on both sides (intake as well as exhaust) as well as by the pin.
I've got quite an extensive collection of broken pistons from many years of breaking things.

[4vpc]

I'm not an expert on it, but usually knock does happen most often at the edges of the chamber by the intake valves, and looks to me like they fill-in those areas and makes them more like quench areas, putting more volume by the plug and exhaust vavles and less at the far side of the intakes, and it actually works. Many other designs, blue flame, may-fireball, sure does work, and for the same reasons.
Of course, that means intake valve clearance at TDC can be a problem, but if you don't NEED a lot of overlap then it's quite practical. Short stroke, big valves, turbo......you don't need much overlap.
Don't tell the subaru guys. Putting a turbo on that glass house should be discouraged

I believe knock happens on the intake side of a four valve head for the following reason. The tumble motion drives the flame to the exhaust side. Exhaust side then burns first. Knock is ignition of the unburned end gas. Since the exhaust side burns earlier because of the tumble motion, the unburned end gas has to be on the intake side. It's like the old joke about why do you rob banks having the answer because that's where the money is.

Not really, consider how would tumble motion mean the the end product ends up at a certain place? It's like playing Roulette or stopping an empty washing machine and expecting it to stop in the same place every time.
The roller wave pistons endeavour to push the whole of the mix over to the exhaust side where it's hotter, it isn't a fix, just a minor improvement. Has anyone ever done a proper scientific back to back test on them?
That aside, what happens is that raw fuel is left around the inlet valves, this is the cooler side of the port and that is the cause of det. Put some water in your mouth and try to blow it out in a mist, it will curl around and coat your lips, this is what wetflow testing or similar will show you.
Then you have to have a deep understanding of detonation and it raises the question; does it actually damage the areas which are the cause? I believe it does, but maybe not exclusively. This is why DI in petrol engines represent a massive step in so many important aspects of the engine.
The above observations are based on a 4 valve per cylinder motor.

[ptuomov]

I'm not an expert on it, but usually knock does happen most often at the edges of the chamber by the intake valves, and looks to me like they fill-in those areas and makes them more like quench areas, putting more volume by the plug and exhaust vavles and less at the far side of the intakes, and it actually works. Many other designs, blue flame, may-fireball, sure does work, and for the same reasons.
Of course, that means intake valve clearance at TDC can be a problem, but if you don't NEED a lot of overlap then it's quite practical. Short stroke, big valves, turbo......you don't need much overlap.
Don't tell the subaru guys. Putting a turbo on that glass house should be discouraged

I believe knock happens on the intake side of a four valve head for the following reason. The tumble motion drives the flame to the exhaust side. Exhaust side then burns first. Knock is ignition of the unburned end gas. Since the exhaust side burns earlier because of the tumble motion, the unburned end gas has to be on the intake side. It's like the old joke about why do you rob banks having the answer because that's where the money is.

Not really, consider how would tumble motion mean the the end product ends up at a certain place? It's like playing Roulette or stopping an empty washing machine and expecting it to stop in the same place every time.
The roller wave pistons endeavour to push the whole of the mix over to the exhaust side where it's hotter, it isn't a fix, just a minor improvement. Has anyone ever done a proper scientific back to back test on them?
That aside, what happens is that raw fuel is left around the inlet valves, this is the cooler side of the port and that is the cause of det. Put some water in your mouth and try to blow it out in a mist, it will curl around and coat your lips, this is what wetflow testing or similar will show you.
Then you have to have a deep understanding of detonation and it raises the question; does it actually damage the areas which are the cause? I believe it does, but maybe not exclusively. This is why DI in petrol engines represent a massive step in so many important aspects of the engine.
The above observations are based on a 4 valve per cylinder motor.

I don't think so, but then again it's not like I have looked inside a running engine. I'm mostly just thinking out loud. Don't misread any excessive degree of confidence to my writings.

I have tried to read a lot of research on cycle-to-cycle variability. The new research uses large eddy simulations and camera instrumented test engines. The only problem is that almost all of the very latest stuff is about gasoline direct injection motors, but there is some port injected research as well.

I agree that there's a lot of variability between cycles, and by one Japanese study even the small intake and exhaust squish pads cut that down a lot. They also speed up the burn, which may not be desirable for a high-boost pump gas motor.

A number of recent articles (mostly French) show cycle to cycle variability in the flame travel for the standard four-valve head that burns towards the exhaust. This both with LES and video, they have a database of experiments now. All the cycles, both fast and slow, burn towards the exhaust.

There's also studies that measure the onset of the knock. For the standard four valve head with the spark plug at the roof like in normal production engines, all the knock events come from under the intake valve edges. When the standard plug is replaced with a very long experimental plug that has electrodes close to the piston, now all the knock events move to under the exhaust valves.

All this research points towards a large part of the tumble motion systematically and reliably persisting even near TDC, with the gas flowing from the intake side towards the exhaust side near the combustion chamber roof and from the exhaust side towards the intake side near the piston. I would post the diagrams from my phone, but this site doesn't know how to automatically resize attachment images so no photos of diagrams from the papers.

There's another study about tumble and piston dish shapes. The now-standard dish piston generates a stronger tumble than either flat top piston or a GDI dish piston. That makes sense, and I'd guess from that research that the roller wave piston also generates a strong tumble.

I don't have any obvious reasons to think that detonation only damages areas that are close to the point where the knock starts. You don't have any obvious reasons to put much weight on what I think! Anyone with an internet connection can read those papers and instantly exceed my expertise on the area.

[user-23911]

Expertise comes from examining the end result.

Once you know what that is.

Then you can come up with a theory as to why.

[ptuomov]

Expertise comes from examining the end result. Once you know what that is. Then you can come up with a theory as to why.

Then end result is that the vast majority of knock events in a four-valve pentroof head are initiated on the intake side and the vast majority of pre-ignition events are initiated near the spark plug. That's what the optical, pressure measurement, etc. tests say.

I am very much in agreement with you that the factory engineering for Japanese and German cars is usually top notch, so I do read their technical papers about measurements of the "end result" and their theories about why. For example, I found this piece on knock and pre-ignition in high-boos motors very informative: https://www.iav.com/sites/default/files ... en_ans.pdf