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combustion chamber shape + detonation/preignition

Hello All,

I wonder if anyone here has opinions on the effects of combustion chamber shape on detonation or preignition in an engine, especially with a turbocharger. Goran B. (beepbeep) was telling me in another thread that the 930 combustion chamber is not as good as something like a modern 4 valve pentroof chamber like you see in a 959/996TT/956/911GT1/saab/volvo/toyota/bmw/etc. Is this true, and if so, why? I realize that sharp edges are bad but the 930 chamber doesn't have any, really.

In the case of porsche engines, the 2 valve hemispherical combustion chambers are in aircooled heads, but the 4v pentroof combustion chambers are in water cooled heads. To be fair, let's assume that the cooling is equal. For example, the 2 valve/cyl BMW heads and pistons make a similar combustion chamber to a 930.
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Old 07-29-2004, 01:52 PM
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Here's the quote from beepbeep from another thread:

Quote:
Speaking about chamber design, domed pistons are usually big no-no when it comes to knock resistance. You want your combustion chamber free of sharp edges and slightly spherical. This is little easier to achieve when you have four small valves instead of two big ones.
Well, what if you have flat pistons in a Hemispherical 2 valve combustion chamber? Are slightly dished pistons in a shallow pent roof somehow better?

Furthermore, an SC piston for example has a pretty gentle dome. Can the squish near TDC cause any (just slightly early) pre-ignition?

One big thing is that if you have a single plug in a hemispherical 2 valve chamber, you have to run more ignition advance than you would in a pent roof 4 valve with center plug. And if I remember correctly, more advance can cause detonation on a turbo engine. can anyone clarify this a little more?

Too bad stephen kaspar doesn't hang out here, he might have some ideas.
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Old 07-29-2004, 02:06 PM
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Here's another factor I thought of: A typical pentroof chamber like a 959 chamber has some squish area on 2 sides. a flat topped piston in a bore-sized hemispherical chamber has almost no squish. This may make a difference to preignition/detonation, but I don't know enough about the effects of squish.

Some 2 valve non-hemispherical chambers have really massive squish areas. Perhaps A Quiet Boom would have an opinion here too.

...And people say I talk to myself...Bah!
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Old 07-29-2004, 02:27 PM
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My understanding is that it is a few factors:

1) Size: 4 valve pente-roof combustion chambers are very compact and often can use flat top pistons. So they only take up a band across the center +/-25% of the cylinder diameter. The outer half of the cylinder is the squash zone that promotes turbulance in the cylinder and thus reduces the tendency to knock. Hemi heads on the other hand are a rough hemi-sphere covering the full diameter of the cylinder. So the distance is further from the spark plug to the edge of the combustion chamber at TDC. So where a 911 head may have volume of about 68 or 70 cc's, a pente-roof head from a similar sized cylinder may be only 25 cc's.

2) Shape: Since pente-roof heads have less volume, it is possible to get the desired CR without a dome on the piston which keeps the combustion chamber compact. In the worst case 911 (a 2.0 with S pistons or worse 906 pistons), the combustion chamber's shape is more like an upside down bowl (just the bowl, not the area inside the bowl which is taken up by the piston dome). So at TDC, the flame front has a much longer and contourted distance to travel.

3) Valve size: Hemi heads generally have 1 large intake valve (which is often heavily shrouded by the piston at TDC), which flows a lot of air, but generally doesn't promote much turbulance. 4 valve heads on the other hand are often not shrouded since the piston doesn't have a dome, and the flow through the two intake valves generates turbulance.
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Last edited by jluetjen; 07-29-2004 at 03:11 PM..
Old 07-29-2004, 03:09 PM
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What John said and spark plug location.
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Old 07-29-2004, 06:00 PM
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Thanks, John!

I will respond in more detail later, but... does the pure distance that the flame front has to travel affect detonation or preignition? Perhaps through ignition advance? Can anyone describe this in detail?
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Old 07-29-2004, 06:15 PM
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I'll try to answer my own question.

knock is mainly a function of pressure, temperature, and mixture at any given point in the cylinder. For performance, you would like to fire the plug such that you have peak cylinder pressure just after TDC to maximize your average combustion pressure during the burn. However, the earlier you fire the plug before TDC, the more the temperature and pressure ramps up before the piston has a chance to go down. This may cause knock.

Also, turbulence in the combustion chamber promotes even mixture, which keeps a lean physical space in the cylinder from knocking (autoigniting before the flame front gets to it).
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Old 07-29-2004, 07:55 PM
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Kind of makes you want to have a diesel, if you're thinking about turbos.
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Old 07-29-2004, 07:56 PM
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John Erb at silvolite has some very informative articles on the KB-pistons website.
take a look:
http://kb-silvolite.com/article.php
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Old 07-29-2004, 08:17 PM
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Thank you, John. I'll read some of the stuff over there.
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Old 07-29-2004, 08:49 PM
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Re: combustion chamber shape + detonation/preignition

Quote:
[i]
I wonder if anyone here has opinions on the effects of In the case of porsche engines, the 2 valve hemispherical combustion chambers are in aircooled heads, but the 4v pentroof combustion chambers are in water cooled heads. To be fair, let's assume that the cooling is equal. For example, the 2 valve/cyl BMW heads and pistons make a similar combustion chamber to a 930. [/B]
Unfortunately cooling will never be equal. The benifit the Hemi has with straighting out of the inlet track and the large valve area it provides , tends to be somewhat inefficent by the large chamber surface area it creates. This allows heat loss from the compressed mixture reducing efficiency, and because of complications in flame travel and componant conduction, will promote the erratic state of the end gas temperature thusly detonation. This is the cooling in question, charge cooling and or heating as it is in this case and end gas temperature jumping from the igniton point over the middle of the mixture not the normal propagation. Hence Twin ignition and refduced lead time.

As well as the previous points the other very good posts have stated
The Pent Roof chamber does not have as an issue surface area in regards to temperature + the benefit of valve curtain area especially at low lift conditions, and thermal efficency nearly 5% greater. Efectively transferring more energy that was wasted by the Hemi to expansion in the 4 valve.
Turbo Charging follows the same suit, thermodynamically, as an aspirated motor and has the complexity of elevated inlet temp to exacerbate the equation even more, as the need for a Twin Plug system will become tantamont.

you guys have great posts, sorry if I screwed up the Quote part I am new to this.

Best regards

Last edited by racing97; 07-31-2004 at 02:28 PM..
Old 07-31-2004, 09:27 AM
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Racing97,

I’m not trying to flame or argue but just learn.

You said: “…erratic state of the end gas temperature thusly detonation.”

My lame understanding of detonation is that primarily detonation occurs as the cylinder pressures increase and parts of the unburned mixture, that are not at the flame front, explode (detonate) and not burn in a controlled manner.

I had never considered that unburned end gasses would detonate as the cylinder pressure was dropping.

Please help me learn something new. This is a very complicated PV=nRT, chemical, thermal, and mechanical process.

I’m going to restrict my consideration to 2-valve, air-cooled, high compression (9:1 to 12:1 CR), 911 engines. There are interesting things to learn from similar turbo engines that are applicable to normally aspirated engines – and vice versa.

I think I understand why twin ignition, starting two flame fronts at 26 degrees BTDC compression is better than single plug, starting a single flame front at 36 degrees BTDC compression. Are there other benefits? Any disadvantages (other than complexity and cost)?


Best,
Grady
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Old 08-01-2004, 10:46 AM
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Grady, from what I have read the peak cylinder pressure should be in between 17 to 20 degrees atdc. If detonation occurs the pressure starts advancing towards tdc and if it is bad enough btdc. With single plug I suspect that cylinder pressures are higher at tdc or right before because of the 36 degrees that it requires to achieve the peak cylinder pressures at the17 to 20 deg. atdc. Those higher tdc pressures set off the detonation. I would also suspect that with twin plug and higher compression you would see less tdc pressures than with single plug and lower compression.
Its always apleasure talking with you, let me know if I am full of poo.

Aaron
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Old 08-01-2004, 11:29 AM
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http://www.flymanaged.com/detonation.htm
very good and to the point
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Old 08-01-2004, 12:05 PM
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The image in this post is way beyond being able to be interpreted as detonation damage. What I see is the result from ring & piston failure. The “hammered” periphery of the piston is from broken parts in the “squish” close clearance area between the piston and head. Note the entire top of the piston is “peppered” with dents from parts flying around in the combustion chamber.
Could this problem have been initiated by detonation or pre-ignition? Certainly.
It could be from other causes also. Too lean mixture in an aircraft engine is my first choice. Note the seizing on the piston skirt.

I agree with most of what Graves has to say. Some is very aircraft specific and not necessarily applicable to automotive applications. One important point is that the detonation shock waves can be additive in certain places.

The area above the compression ring land (the part of the piston just above the compression ring) has combustion (hot) gasses on three sides of it. The top is obvious. The compression gasses must travel past the side of the piston and press on the top and inside of the compression ring for it to seal it properly to the cylinder and piston. This happens every compression (combustion) stroke.

Add to this poor design (too much deck clearance) and poor piston shape (where there should be additional proper squish), detonation can occur above the ring land. This is right at the weakest and highest temperature point. The piston is deformed down toward the compression ring. The ring is no longer free to move in the ring land. Hot combustion gasses pass the ring. The piston melts, the ring breaks, and all hell beaks loose. It is probably pieces of ring that did damage in this image.

I would be hard pressed to tell if this failure was from detonation (low octane fuel, too much ignition advance), over temperature (too lean or other), or simple parts failure (parts used too long).

Best,
Grady
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Old 08-01-2004, 03:30 PM
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Grady
(My lame understanding of detonation is that primarily detonation occurs as the cylinder pressures increase and parts of the unburned mixture, that are not at the flame front, explode (detonate) and not burn in a controlled manner.)

(I had never considered that unburned end gasses would detonate as the cylinder pressure was dropping.)

First of all I claim Pole Postion in the Lame department.
I'm sorry if I led you to believe Cyl. press was dropping (I did not find that in my post)
Quite the contrary you and I are stating the same thing (parts of the unburned mixture, that are not at the flame front, explode (detonate) and not burn in a controlled manner.) I call it end gas, because the combustion process has all ready begun. Normal combustion pressure will under these conditions will almost double uncontrollably and try to drive the piston back down the bore before it gets to TDC. Once again the pressure is rising.

The point to the original post is that the Hemi chamber with its larger surface area can be very sensitive to this condition and as a result it can hinder the production of power when tunning to avoid such conditions.
This and other disscusions are great, unfortunately The Ideal gas laws such Carnot are what is studied in schools and the Otto cycle are applicable in the studies of heat transfer, but it is hard to apply Adiabatic and Isothermal conditions, which is where the text books try to go.
I sure some members of this board are a great deal more versed on this than I

Best regards
Old 08-02-2004, 08:03 AM
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