Flow Through Prechambers

[smutts]

Wandering through all the literature on diesels, I haven't seen anyone working on smoothing the flow from cylinder to swirl chamber, isn't there any gains in pumping losses to be had here? Or are there plenty of horror stories of others heroic failures?
Or is that the entire point? to only allow some air in, so that the initial injection causes a dirty sooty cooler burning in the prechamber, then this rich hot half burned stuff blows into the cylinder to burn up in the rest of the air? Curious.

[Rabbit on Roids]

Wandering through all the literature on diesels, I haven't seen anyone working on smoothing the flow from cylinder to swirl chamber, isn't there any gains in pumping losses to be had here? Or are there plenty of horror stories of others heroic failures?
Or is that the entire point? to only allow some air in, so that the initial injection causes a dirty sooty cooler burning in the prechamber, then this rich hot half burned stuff blows into the cylinder to burn up in the rest of the air? Curious.

i bet its more along your second idea. cause i know old honda civic engines had a small separate barrel in the carb setup to run super rich, and the little chamber with the spark plug in it would get all that rich air/fuel. then it would ignite that and shoot it into the combustion chamber.

[Turbinepowered]

The small port is also intended to deliberately produce turbulence and swirl (thus, swirl chamber) to promote better mixing and less soot. The throttling effect of the pressure's slowed travel from the swirl chamber also reduced diesel slap and knock noise, for more acceptable characteristics for a passenger diesel.

[smutts]

My thoughts are that at 23:1 compression, that air is getting a bit thick and dense, if one thinks of carburettor bellmouths, there is a radius to help the air that is travelling to that bellmouth from all directions at once, to turn into one single jet down the pipe. I'm just interested why prechambers don,t have the sharp edges of the passage radiused. Or is it simply iconel is hard as hell and it's not worth the bother to machine it.

[truckinwagen]

Or is it simply iconel is hard as hell and it's not worth the bother to machine it.

probably mostly the truth right there.

inconel work hardens really bad, so they probably designed the precup to take as little machining as possible to save their tooling.

-Owen

[NintendoKD]

ooh, I like this discussion, trying to learn a little about improving design myself.  I want to take advantage of as many good characteristics as possible.

[Turbinepowered]

Technically, none of our VW diesels have prechambers. They're all "swirl chamber" diesels.

Radiusing and smoothing and other things are intended to reduce turbulence; while I don't doubt that perhaps saving the tooling from the ravages of work-hardened inconel was a factor, why would you reduce turbulence when you're trying to generate it? 

[NintendoKD]

I am thinking of adding a rifling type design into the cups that I make to add turbulence the spray pattern is somewhat of a flattened cone shape which will push the atomized fuel downwards against the sides of the chamber which will have a rifling to it to give the entire process a sort of twist moving downwards and back inwards, natural compression occurs in a vortex, so the hope in this design is an increase in performance from passive movement into a more effecient turbulent design.  I know it is hard to visualize, but maybe someone with cad can help me here?

[Turbinepowered]

If you're pushing fuel toward the walls of the chamber, don't forget about cold starts. You won't start if you shove cold fuel against cold walls with cold air, and you'll get wet chamber walls and some horrible combustion when it finally does start, when the engine is trying to burn that fuel.

[NintendoKD]

hmm interesting....... I wonder, there has just got to be a way

[Turbinepowered]

hmm interesting....... I wonder, there has just got to be a way

Yeah... it's called Direct Injection.

Have you seen how crazy some of those pistons look?

[NintendoKD]

That I understand, but there has to be a more efficient swirl chamber design, something that I have been missing, a piece of the equation, or I could just be chasing the dragon

[Turbinepowered]

That I understand, but there has to be a more efficient swirl chamber design, something that I have been missing, a piece of the equation, or I could just be chasing the dragon

Chasing a dragon. There is undoubtably a better design, but I imagine you are going to be slamming your  head hard against diminishing returns, agonizing your way through designs and equations and experiments... for no perceptible improvement on an engine that, honestly, has an extraordinarily crude control system.

[NintendoKD]

but that's the fun part

[gldgti]

i'm sure we've been here before, but i'll re-iterate succinctly a the history -

this swirl chamber thingy is really not a bodged up VW bit of crap - its got a hell of a lot of research behind it and for along time before VW picked it up - mercedes were using it just beforehand aswell, and both VW and merc got it from Recardo (we're talking 70's). Now, Recardo were and still are the world (no bull) leaders in combustion modelling and research.

The swirl chamber operation is actually quite efficient and elegant, and really the only disadvantage we see is from the heat lost through the extra surface area. Apart from this it all works mavallously well.

A couple of points -

1) we dont really need to aid flow into the swirl chamber - the idea is only to have a small amount of air in there anyway, since if you have more combustion in the prechamber, you will lose more heat through its walls and ultimately have a decrease in efficiency.
2) when combustion front is expanding out of the swirl chamber, the piston is basically at TDC. Its very important that the flow out of the chamber is directed properly. Ideally, we want and unbiassed flow (coming straight out of the hole in the precup) so that the pressure is initially evenly distributed across the piston crown. this is why:
>the injector is located centrally in the swirl chamber;
>the swirl chamber outlet port is simmetrical and central to the piston;
> the depressions in the pistion crown are symmetrical.
   
opening up the outlet of the prechamber will cause the combustion front to decellerate more gradually - this is bad for turbulence in the "true combustion chamber" which ofcourse is the cylinder itself.