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Pre combustion chamber design research

Rockabillyrat

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Where did you get all this ?
It took months of reading several books and hundreds of research articles to figure out the combustion process on and IDI. I probably have the biggest private library on IDI thermodynamics then anyone at the moment. And I didn't do it alone, n8in8or and myself have been working on this together for some time now.

But the two biggest pieces of information came from "The high-speed internal-combustion engine" by Ricardo and Hempson. And Thermodynamic analysis of indirect injection diesel engine operation by Kamel. Very hard stuff to read, most of it at engineering level but I made sense of the important parts.
 

EWC

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So you think the precup design has to do with after the fuel has been ignited ?
 

Rockabillyrat

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The cup is responsible for a a few things. It changes the velocity of air entering the chamber during the compression stroke. And atomizes the fuel leaving the chamber during the power stroke. The size of the throat effect the pressure differential so different size cups will have different effects on this process. Also the physical shape of the cup directs air to swirl inside of the chamber and directs the flame front at the piston dishes creating a swirl in the main chamber. The cup is part of the injection process.
 

Rockabillyrat

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Pressure differential created at the throat. It's like putting your figure at the end of a garden hose.
 

Rockabillyrat

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It swirls in the chamber creating the turbulence needed to distribute the fuel for ignition. The high velocity swirl is why an IDI has a shorter delay period than a DI engine.
 

EWC

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No doubt the is quite a lot of air movement . What else is happening ?
 

Rockabillyrat

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Well after ignition is started at the chamber walls. The swirl creates a "thermo mixing" and the remaining air is mostly consumed in the chamber. At that point the piston reaches TDC and the chamber starts reverses flow into the main chamber.
 

EWC

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And how does the ignition start ? What's the one thing missing ?
 

Rockabillyrat

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Super heated air from compression and the high velocity swirl in the cup start the ignition process. As well as the chamber walls once they are up to temperature.
 

EWC

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Right , heated air from the compression . The walls and the precup have nothing to do with the ignition as far as temp . You can start a cold engine without glows or the precup being hot as long as you have good compression . The air in the cylinder is compressed and forced into the precup by the 2 small circles and the angle at the side of the piston directly under the precup . The angle directs the air up into and thru the opening in the precup . This heated air then fills the precombustion chamber where the atomized fuel , from the injector , is then injected . The heated air starts the combustion cycle and this cycle continues until the injector closes . There is a reason it is called a precombustion chamber . After the fuel is ignited , it doesn't matter where it goes as long as it gets out of the chamber . So now we have ignited fuel coming back out of the chamber , pushing down on the piston . Gasoline , when ignited , has about 2000 psi in the cylinder . I would expect diesel has more as the BTU's are higher and we have more compression . The 2 little circles and the angle are not noteworthy in this respect . There is no delay with an IDI but because it has a prechamber , as opposed to DI , we need more advance not less than DI .

I have seen a lot of people posting about how a precup works and most get it wrong . I'm not talking about the size of the opening , just the principle of how it works .
 

Rockabillyrat

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The heat from the chamber walls and cup do aid in the ignition process. Sure a engine can start on compression alone. But there is a reason an IDI sounds like rocks in a tin can until in builds heat in the chamber walls.

Yes the piston dishes direct air at the chamber opening. But they also create a dual swirl in the chamber by redirecting the flame front. I posted pictures of this earlier in this thread already. Its is a critical part of the combustion process.

Yes it's a precombustion chamber. But during longer duration periods fuel is still being injected after TDC and the pressure differential atomizes the fuel entering the main chamber.

And DI needs more timing because it has a longer delay period. It need more time to start the ignition process so it need to be injected earlier.

Your really close to fully understanding this process. I suggest reading the two books I referred to earlier.
 

EWC

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I remember reading those pages on the theory about precups when you posted them a while ago . First sentence is a red flag for me . Only 40% of the swept volume is in the prechamber ? Where is the other 60% ? Remember this is at TDC . Not happening as you have stated because of the pressure differential . Don't buy the theory that the circular cuts in the piston create separate flame fronts either . Best way to describe the combustion process is pure chaos .

Not sure the chamber and the precup aid in the ignition of the fuel either as the temperature is approximately running temp . Now the precup itself is probably hotter as this has combustion gases flowing thru it and the metal used is designed to handle this temp . The reason you hear the rattle when started is the advance is on . Disconnect that and the rattle goes away .

There is no pilot injection on the 6.2/6.5 . With the pintles you have pictured , I can see varying fuel but no pilot injection .

Either way I think you should try what you believe to be the best for you .
 

Rockabillyrat

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The two books i posted the other day have better information than the ones I posted months ago. Either way none of the books and research papers I have found specifically talk about the 6.2 and 6.5. So the 40% ratio of cup volume to compressed cylinder volume is common spec for single cylinder IDI engines used on generators. You have to read through these papers and take only what applies to what your working with. Our ratio is different on a 6.5 and I do plan on measuring it.

Wrong, again the two dishes make a dual swirl in the chamber. Its not a theory, it's a fact. Here is a few pictures to prove it. The first one is of a real engine that was modified to allow researchers to view the combustion event.
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Clearly a duel swirl in that picture.

This next picture (sorry for the screen shot since I'm jot a member of this forum) shows what happens when you put an IDI piston in backwards. The flame front went right across the piston with very little mixing in the cylinder. Like I said the piston dishes are critical to the combustion process. Its not chaos, it's a engineered combustion event and every part has its purpose. That is why I don't agree with machining the pistons on an IDI.
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All IDI engines use a delay type nozzle. They limit fuel at initial pintle opening. Its not exactly pilot injection. But that combined with a two stage combustion process almost mimics pilot injection. VW went as far as machining a small dish at the end on the pintle and they referred to it as pilot injection on their later injectors for the.

I'm not going to repeat myself on the topic of heat from the cup and chamber walls and their effect on the ignition process. Its stated over and over again in multiple books and research papers. Do the reading and rethink your opinion.
 

n8in8or

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Here is a diagram of the testing rig and a photo of the rig itself to help explain what is in those pictures. Each of the pictures of the actual flame front shows a shot of the prechamber (the smaller round portion at the bottom of the picture) and of the main combustion chamber (the larger portion at the top where you can see the dual swirls) at the same moment in time so you can see the relationship of what is happening during combustion.

Diagram of testing rig.

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Photo of testing rig.

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This is the rig Ricardo used to do his testing and the development of the Ricardo (what he called the “Comet”) piston design that works in harmony with the swirl type precombustion chamber.
 

Will L.

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A friend and I tried flat topped pistons years ago and did not like the results. Then swapped them out for the ones from Kennedy that are similar to the ones from TSP and had great results. Keep in mind, until recently I was always a higher compression is better guy. My buddy wasn’t. Now thinking back about it, maybe the deeper cut pattern in the pistons keeps air flow good or maybe improved it? Or possibly it lessened it but the gains made by lowering the compression offset the slight loss.

At work there is a couple hydro engineers, one that specializes in flow characteristics and helps gubmint agenecies across the country. Maybe I can drag him in for an opinion as to the changes the cuts make?

The things I learned and saw running Kennedy’s pistons was proof in the pudding I wanted lower compression, but it had to be done right. That’s why I got so excited when Chris showed his piston top. All the wazoo builds done by peninsular for unky sams spec ops hmmwvs had those pistons, and sfter getting the ip and turbo banks threw in they were all running numbers near Nate’s level. Wether whoever first started the design of the pistons engineered it out or swung in the dark and hit a home run, idk. Just happy they work well.

As to the backwards ran pistons- we tried it just to see one time- haha- frickin halarious! It wont hurt anything but I do suggest doing it in summer not winter so you can get it to start easier! That or hack up a set of precups (which I also tried several times) and you’ll quickly know something is critical in the flow.

I read stuff in a class before playing with these, so I just went “yeah, flame front isn’t flowing right”.
The pics in the book showing preliminary combustion in the precup really tells the story. And simply flowing up under compression, and the flame front flow on ignition obviously are near mirror images so it seems easy to see.

Also someone mentioned the knock under advanced timing. Yes the advance increases that knock, but having played with piston to wall clearances A LOT, and the wrist pin allowance...I can tell you the flame front making the piston rock over too soon is a huge part of that knock. But so does too much ignition in the precup and not enough on the piston top. Less quantities of more volatile fuel will show it.

I feel for Rockabillyrat (is RR ok like WW is war wagon?haha) because he is using a not exact term that has been redefined since it first came out to describe what’s happening. And it doesn’t sound right, if you don’t get what’s happening. I read the first time he said it and have been thinking on a better description, and failed. It is like there is 2 seperate ignition events. Some of the fuel sprays into the cylinder -just look at injector nozzle with a head off through precup port and you can see some will. While the rest hits the much hotter precup blatung around in there and getting lit. The convergance of the fuels in ignition ontop of the piston: when it is perfect, the knock is minimal. When the precup ignites a bit too soon and the delay ignition on top of the piston occurs it really klacks.

When a di diesel has too early timing, you hear the difference? That di sound is piston/rod shock. You have to imagine subtracting that sound from the idi klack.

Hope I helped somebody understand a little better. If I muddied the waters, I apologize.
 

Rockabillyrat

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A higher compression engine has better thermo efficiency. But less available air at TDC. Lowering the compression does the opposite. It's a give and take kinda thing. Both Gm and Ford made compression changes by changing the volume of the piston dishes. This make sense to me because the idea is to increase the available air at TDC. The piston dishes are the first area to make contact with the flame front. So it seems like a better area to machine than the whole piston top. But this is all just my opinion. The only way we will know for sure is to see what effect the machine pistons have on combustion inside of the chamber. And that takes equipment we just don't have. If only we knew someone that could simulate them on a computer....

Will L. I would like to hear what your engineering buddy has to say on the subject. Oh and RR is just fine. I'm used to people shortening my name. My last name gives everyone problems!!!
 
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