Benefits of tapered quench for NA applications?

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BCjohnny
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Re: Benefits of tapered quench for NA applications?

Post by BCjohnny »

frnkeore wrote: Fri Mar 12, 2021 7:53 pm
frnkeore wrote: Thu Mar 11, 2021 1:15 pm Does this tapered quench area, have to be parallel, blowing up, towards the chamber? Can it be only on the piston, blowing down, into the piston recess?
No one has a opinion on this?
Yes, FWIW

There's obviously a quench, or squish effect going on, hence the term, but I don't think it's a singular effect

It also needs to be seen as a 'tapered burn' that, through it's innate timing effect, allows a more controlled complete burn that lessens spiking (detonation) of otherwise marginal combustion events

So the angle has as much, if not more, to do with this timing event than any particular direction to squish the mixture

As some open chambers can be prone to detonation by relatively 'normal' combustion into the end gas space, 'quenching off' that area with a timed, tapered, continuation of the main combustion event allows a more complete burn without detonation or blowing the unburnt charge out of the exhaust

The evolution of the combustion space as the piston descends allowing a more complete burn without spiking

And then you have to factor in the effect of engine speed ...... low speed 'pinking' and such, where flame speed outruns piston descent

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Re: Benefits of tapered quench for NA applications?

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frnkeore wrote: Fri Mar 12, 2021 7:53 pm
frnkeore wrote: Thu Mar 11, 2021 1:15 pm Does this tapered quench area, have to be parallel, blowing up, towards the chamber? Can it be only on the piston, blowing down, into the piston recess?
No one has a opinion on this?
I think it should technically be pointed upwards, as most of the chamber is above the block/head interface and that's usually where you want the mixture motion, plus you want any extra volume to be as close to the plug as possible, and it might de-shroud a valve the tiniest bit. But if all you have to do the work is a cartridge roll you could get just the burn control effect by rolling a piston on the bench and carving the edge of the dish down. That's actually what I was planning on doing since I thought about how hard it would be to get it right in the head with my lack of fancy tools

Hell, I might do both that *and* grooves in the head, eventually

Check out the thread in the Advanced section where I asked about detonation resistance. In it Erland Cox shares his experience with the Cosworth turbo motors where he used a 7 degree slant on a piston to eliminate detonation.
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Re: Benefits of tapered quench for NA applications?

Post by tinnitus »

The cylinder head developer that helps me mentioned not too long ago he’s seeing the same benefits in NA engines as seen in boosted and nitrous engines. A Wider tuning window for peak performance is the biggest real world benefit. I’ve only run mine boosted but it doesn’t seem as picky about timing and fuel as I thought it may be.
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Re: Benefits of tapered quench for NA applications?

Post by David Redszus »

The squish velocity of a wedge head must be fairly high since the combustion chamber shape is mediocre.
Below are piston positions below deck height at various crankshaft angles.

Assuming a piston at deck height as TDC = 0"
at 8 deg ATC = .020"
at 12 deg ATC = .045"
at 16 deg ATC = .080"
at 20 deg ATC = .125"
plus squish clearance distance.
The very small clearance does not produce tumble but does allow a directional swirl due to squish.

Maximum squish (in either direction) occurs at approximately 10 degs; the space available for
combustion is quite small. As the piston moves downward, the quench volume becomes greater
and at some point combustion becomes possible in the quench zone.

The outward squish velocity (ATC) is assisted by combustion pressure.
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Re: Benefits of tapered quench for NA applications?

Post by BLSTIC »

David Redszus wrote: Thu Mar 18, 2021 3:25 pm The squish velocity of a wedge head must be fairly high since the combustion chamber shape is mediocre.
Below are piston positions below deck height at various crankshaft angles.

Assuming a piston at deck height as TDC = 0"
at 8 deg ATC = .020"
at 12 deg ATC = .045"
at 16 deg ATC = .080"
at 20 deg ATC = .125"
plus squish clearance distance.
The very small clearance does not produce tumble but does allow a directional swirl due to squish.

Maximum squish (in either direction) occurs at approximately 10 degs; the space available for
combustion is quite small. As the piston moves downward, the quench volume becomes greater
and at some point combustion becomes possible in the quench zone.

The outward squish velocity (ATC) is assisted by combustion pressure.
I did a spreadsheet of that (simple one with sine wave motion and assumed 1mm piston to head), with a relatively narrow squish band and 4 degree taper it can start burning 4 degrees earlier. With a squish section like you guys and your parallel valves have, it could easily be a lot more
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Re: Benefits of tapered quench for NA applications?

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BLSTIC wrote: Thu Mar 18, 2021 5:44 pm
I did a spreadsheet of that (simple one with sine wave motion and assumed 1mm piston to head), with a relatively narrow squish band and 4 degree taper it can start burning 4 degrees earlier. With a squish section like you guys and your parallel valves have, it could easily be a lot more
How is this effected by timing? Would you use more or less advance?

Thanks
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Re: Benefits of tapered quench for NA applications?

Post by David Redszus »

69427 wrote: Thu Mar 18, 2021 7:09 pm
BLSTIC wrote: Thu Mar 18, 2021 5:44 pm
I did a spreadsheet of that (simple one with sine wave motion and assumed 1mm piston to head), with a relatively narrow squish band and 4 degree taper it can start burning 4 degrees earlier. With a squish section like you guys and your parallel valves have, it could easily be a lot more
How is this effected by timing? Would you use more or less advance?

Thanks
Flame speed is principally determined by chamber turbulence which consists of laminar flame speed and
squish velocity. Laminar flame speed does not change very much at a given piston speed.

Squish velocity is a function of clearance, squish area and piston speed. Combined, they contribute heavily
to total flame speed and therefore to required ignition timing. Higher turbulence levels require less ignition
advance. Since turbulence varies with piston speed, so must ignition timing.

Higher squish velocity will shorten the ignition delay period, resulting in a more consistent and stable
combustion pressure curve and a smoother running engine.

But these are not the only variables that will affect the combustion process and timing.
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Re: Benefits of tapered quench for NA applications?

Post by BLSTIC »

69427 wrote: Thu Mar 18, 2021 7:09 pm
BLSTIC wrote: Thu Mar 18, 2021 5:44 pm
I did a spreadsheet of that (simple one with sine wave motion and assumed 1mm piston to head), with a relatively narrow squish band and 4 degree taper it can start burning 4 degrees earlier. With a squish section like you guys and your parallel valves have, it could easily be a lot more
How is this effected by timing? Would you use more or less advance?

Thanks
I'm not qualified to answer that. It was just a function I made to try visualise the "area available to burn" aspect. I *expect* It could technically allow the end gas area to start burning earlier and at a lower combustion pressure, which would reduce the tendency for that area to detonate. It probably wouldn't effect the overall burn rate much before that point (this effect is largely after TDC). I think the answer depends on how far from MBT you were before
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Re: Benefits of tapered quench for NA applications?

Post by BradH »

BradH wrote: Thu Mar 04, 2021 1:54 pm Interesting how my post from almost 2 years ago came back from the dead...

I think someone above asked this question, but didn't see an answer: What's involved in machining a taper into the quench-side of a combustion chamber?

I suppose I could do my version of the BES "fire grooves" that I showed earlier in the thread... :wink:
.
FWIW, I picked up a used Wiseco piston of the same config & application as the CP I showed previously to see if I can DIY "fire grooves" into the piston quench area. It'll never be put into a running engine and only cost me $30. For that I can f**k it up six ways from Sunday and not think twice about hacking on it.
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Re: Benefits of tapered quench for NA applications?

Post by wkuran »

Powertrip wrote: Mon Apr 15, 2019 1:22 pm The Toyota paper discussed a directed squish, not a tapered quench if I'm reading that paper correctly.
On page 5 in the Toyota paper, Development of Toyota 1ZZ-FE Engine (https://drive.google.com/file/d/1hMznDx ... share_link), the authors describe the combustion chamber as a “TAPER SQUISH COMBUSTION CHAMBER – The squish area formed by the piston top and cylinder head bottom surface has been tapered by being inclined along the cylinder head combustion chamber wall (Fig. 10).”
Figure 10. Taper Squish Combustion Chamber.png


Working with the definition that taper means to reduce the thickness from one end to the other, I find it confusing when the authors say “tapered by being inclined,” because I don’t understand how angling something leads to it being tapered. Therefore, I can only assume that the squish area is tapered. To be certain, I would need to disassemble one of these engines and measure the angle of those two surfaces.

I hope that their squish area is tapered, and that the angle is deliberately selected to direct the squish towards the spark plug, because those are the two attributes of squish that hold the most promise with sidevalve engine chamber design.

Piston TDC.png


This sketch shows the piston at TDC. At 5000 rpm, with the squish area equal to 37% of the piston area, the maximum squish velocity (calculated) is about 40 m/s and occurs 8 degrees BTDC.

The squish direction is towards the valves rather than the spark plug. Changing the squish direction is not easy since it requires redesigning the head and piston.

I agree with Vannik (memberlist.php?mode=viewprofile&u=11007), a ST forum member, that “a 1 degree taper usually leads to better results as a parallel clearance can lead to trapped end gas with piston rocking, leading to trapped end gas and more pre-ignition / detonation.” Angling the squish zone of this sidevalve chamber by one degree reduces squish velocity by about 10 percent. The reduced velocity can be made up by reducing the quench height or by increasing the squish area.

Creating turbulence in the chamber (the goal of squish) requires work. That work comes from the squished A/F mixture in the form of kinetic energy. I don’t know how much is required to achieve the desired level of turbulence. Nor do I know the desired level of turbulence. However, I am convinced that a sidevalve chamber, because of it length, requires more energy than an OHV chamber to get the job done.

Kinetic energy includes the effect of squish clearance and squish area. A tight squish (less than 0.020”) may sound impressive but the velocity stream may not have the necessary energy if the area is not considered as part of the design. Large squish areas on sidevalve engines is easy to achieve because the piston is not over the valves.
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