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      05-04-2013, 12:02 PM   #45
Boss330
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Really interesting discussion. Some useful info on engine design:

http://www.epi-eng.com/piston_engine...y_contents.htm

Volumetric efficiency:
http://www.epi-eng.com/piston_engine...efficiency.htm

Quote:
For contemporary naturally-aspirated, two-valve-per-cylinder, pushrod-engine technology, a VE over 95% is excellent, and 100% is achievable, but quite difficult. Only the best of the best can reach 110%, and that is by means of extremely specialized development of the complex system comprised of the intake passages, combustion chambers, exhaust passages and valve system components. The practical limit for normally-aspirated engines, typically DOHC layout with four or more valves per cylinder, is about 115%, which can only be achieved under the most highly-developed conditions, with precise intake and exhaust passage tuning.
Chapter on torque:
http://www.epi-eng.com/piston_engine...on_engines.htm

Torque charts for a few engine designs:









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      05-04-2013, 01:32 PM   #46
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Quote:
Originally Posted by CanAutM3 View Post
You are correct, peak volumetric efficiency is reached at torque peak. I should have quoted 90-95% VE at torque peak instead of the 80-85% at power peak. It was a bit late at night when I posted...

Can you quote the source of the 106% VE for the S54B32? I am very surprised by this high number. Street cars with their intake and exhaust plumming, filters and catalytic converters have quite a lot of restrictions that hamper VE. There was another thread on the topic of VE, there were a lot of assumptions but not many hard facts.
Its in one of the technical handbooks I have in my garage, tech/spec and BMWs workshop-guidlines for the S54. Could not find it online though.
But heres a link to a different forum, if you scroll down the page you`ll find a guy called Mash3 stating the same number for the S54.

http://www.m3forum.net/m3forum/showt...=236058&page=3

Not the most credible source maybe, but the best i can give you her and now.
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      05-04-2013, 03:05 PM   #47
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This chart shows VE for the S54 engine:

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      05-05-2013, 09:41 AM   #48
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Quote:
Originally Posted by Boss330 View Post
This chart shows VE for the S54 engine:

A peak VE of 115% for a street car?

I am sorry, but I am not buying it

Further, something seems off with the chart. A VE curve should mirror the shape of the torque curve. In this chart, peak VE is achieved around 3500RPM while the S54 has its torque peak around 4900RPM...

Last edited by CanAutM3; 05-06-2013 at 05:44 AM.
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      05-05-2013, 03:53 PM   #49
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Quote:
Originally Posted by Boss330 View Post
Really interesting discussion. Some useful info on engine design:

Chapter on torque:
http://www.epi-eng.com/piston_engine...on_engines.htm

Torque charts for a few engine designs:









Thanks for sharing these graphs

The graph for the one cylinder engine reflects well the half revolution (from 360 to 540 degrees) power stroke and then the negative torque required for the compression cycle. There is also a small dip in the torque to push the exhaust gases out and then torque remains slightly negative to suck the charge in.

It is also interresting to see that on the 6 cylinder engine, even if the power strokes overlap, it is not sufficient to overcome the torque consumed by the compression strokes (still some negative torque). Only as of the V8 does the instantenuous torque always remain above zero.

Last edited by CanAutM3; 05-05-2013 at 04:02 PM.
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      05-07-2013, 05:43 AM   #50
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Quote:
Originally Posted by CanAutM3 View Post
It is true that only one cylinder spark plug "fires" at once. However the power stroke, the time positive cylinder pressure applies force on the piston, lasts about half a revolution. So for engines with more than four cylinders, power strokes will overlap.
This is why I said "sum of all forces" in an earlier post. But the pressure is still greatest at TDC and combustion typically occurs then (or near it at least). The force diminishes over time.

Quote:
The sledgehammer example is not really representative because it refers to an impact. If you have 100 fingers pushing on a object with 1lb each, it will yield the same force as a person leaning with his shoulder on the object with 100lbs.
The sledgehammer was an exaggeration to highlight the nature of the comparison, I revised it right after to tone it down. The point is that the force is applied all at once vs. over some interval of time.

Quote:
Since peak torque is measured for a given RPM, I still think torque of the engine pulses need to be averaged to properly reflect an engines abilities.
It is a summation of the torque made in each cylinder. If all cylinders fired at the same time, two engines with the same stroke and differing cylinder counts (and every thing else normalized) would clearly yield the same torque curve. This would also be the case if the force acting on the piston decreased at the same rate in both cases.

We are deep into all of the complexities that led to me revising my original response. To be more precise at this point would mean applying the math to properly model the system. Il'l have to take a look at the graphs that Boss330 posted to see if they cover what I am saying.
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      05-07-2013, 06:32 AM   #51
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Quote:
Originally Posted by mkoesel View Post
This is why I said "sum of all forces" in an earlier post. But the pressure is still greatest at TDC and combustion typically occurs then (or near it at least). The force diminishes over time.
Agreed on the fact that cylinder pressure is greatest around TDC. However because of the crank/rod/piston geometry, no torque can be produced at TDC. The cylinder pressure conversion to torque is maximized at the piston mid course where the "torque arm" is greatest. Peak instantaneous torque for a given cycle therefore happens somewhere past TDC.

T = P * A * S/2 * sin Teta

T = Torque
P = Cylinder pressure
A = Piston Area
S = Stroke
Teta = Crank position angle where TDC=0


Quote:
Originally Posted by mkoesel View Post
The sledgehammer was an exaggeration to highlight the nature of the comparison, I revised it right after to tone it down. The point is that the force is applied all at once vs. over some interval of time.

It is a summation of the torque made in each cylinder. If all cylinders fired at the same time, two engines with the same stroke and differing cylinder counts (and every thing else normalized) would clearly yield the same torque curve. This would also be the case if the force acting on the piston decreased at the same rate in both cases.

We are deep into all of the complexities that led to me revising my original response. To be more precise at this point would mean applying the math to properly model the system. Il'l have to take a look at the graphs that Boss330 posted to see if they cover what I am saying.
I think we agree on the majority of concepts. However I think we are not aligned on the definition of "peak torque".

My understanding of the torque output of an engine is the mean output of all the cylinder pulses for a given RPM, as would be measured on an engine dyno. Peak torque is the highest mean output over the entire rev band. IMO this definition is consistent with manufacturer published data and all published dyno charts. With this definition, cylinder count for given displacement, volumetric efficiency and stroke values should not have an impact (except for friction losses).

However, looking at instantaneous torque, I agree, a bigger piston will yield a greater momentary peak torque. As I mentioned previously, I don't think this number properly reflects the capabilities of an engine and it is not a number that is comonly used to descibe an engine characteristics.

Looking forward to read your interpretation of Boss330's graphs .

Last edited by CanAutM3; 05-07-2013 at 10:46 AM.
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      05-22-2013, 05:20 AM   #52
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How about them using Variable-geometry turbocharger/s its popular in diesels were the technology can cope with the lower exhaust temperatures.
But Porsche said they have solve the problem with the use of VGT on gasoline engines ,the 997 Turbo uses it and i guess the newer 991 Turbo to.

http://en.wikipedia.org/wiki/Variabl...y_turbocharger
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      05-22-2013, 07:49 AM   #53
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Quote:
Originally Posted by Holset View Post
How about them using Variable-geometry turbocharger/s its popular in diesels were the technology can cope with the lower exhaust temperatures.
But Porsche said they have solve the problem with the use of VGT on gasoline engines ,the 997 Turbo uses it and i guess the newer 991 Turbo to.

http://en.wikipedia.org/wiki/Variabl...y_turbocharger
BMW uses twin scroll turbos to get much of that VGT bang for MUCH less buck. Those blowers are EXPENSIVE.
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      05-23-2013, 06:11 AM   #54
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Quote:
Originally Posted by Needsdecaf View Post
BMW uses twin scroll turbos to get much of that VGT bang for MUCH less buck. Those blowers are EXPENSIVE.
Yeah i know they use twinscroll i wrote earlier in this thread that my guess is that the M3/M4 will use two turbos in sequential a smaller twinscroll and a bigger singlescroll.
I have read here and there that BMW is testing that type of setup and i think that would make sense as you can have a broad powerband.

I dont think the VGT turbos used in many diesel cars is that expensive but the ones used in the 997 Turbo may be so as the metal needs to cope with the higher temps.

So say that they use one VGT Turbo instead of two or three conventional they maybe able to cut cost and that could be a smart idea as a VGT turbo act as a smaller and bigger turbo in one for a broader powerband without the comlexity of two or three turbos that may be sequential controled.
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