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Basic automotive: why is a straight-six "perfectly balanced"?
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Re: Basic automotive: why is a straight-six "perfectly balanced"? -
04-27-2007
, 09:17 AM
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In article <59c686F2k6r4lU2 (AT) mid (DOT) individual.net>, forty <cforteNO (AT) SPAMgmail (DOT) com> wrote: brafield (AT) hotmail (DOT) com wrote: What is it about (all?) straight-sixes that renders them perfectly balanced, as most authorities seem to say? These have somewhat useful explanations with some animations: http://auto.howstuffworks.com/question366.htm http://www.answers.com/topic/flat-engine Because you need cartoons to understand things. |
Super-Intelligent-and-Stronger-Than-Batman-Superman-and-Spiderman-Put-Together
Committee on Making the World Right:
All college courses are hereby stricken from the use of illustrated text
books, graphical web content, PowerPoint (unless the content is strictly
text), and any and all multimedia content in their pedagogy.
..
..
..
..
All too easy, YFI
--
forty
“To embrace an extreme, one must first let go of reason.”
#12
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What is it about (all?) straight-sixes that renders them perfectly balanced, as most authorities seem to say? Does a V-12 have the same perfect-balance architecture, or does that depend on the V-angle, and if so, why? Lastly, about 40 years ago a standard first-year college text was Rouse's Automotive Eng --- does it still exist or is there a better alternative for someone who never passed a sci/eng course in his life? |
Firstly it's nothing to do with firing order or how evenly spaced that is.
Plenty of engines with perfect balance have uneven firing intervals. It is,
as some have mentioned, to do with masses travelling in one direction being
balanced by similar masses travelling in the opposite direction but it's
rather more complicated than that.
Take an inline 4 cylinder engine with a 180 degree crank. Pistons 1 & 4 are
at TDC while 2 & 3 are at BDC. There are equal masses travelling in opposite
directions at all times so primary balance is perfect but there's a
secondary effect. Due to the geometry of the crank/rod system pistons move
away from TDC faster than they move away from BDC. Only with an infinitely
long conrod would the piston motion be truly sinusoidal reaching peak piston
speed at 90 degrees ATDC.
So there is a larger force acting on Piston 1 at TDC than on its balancing
Piston 2 at BDC at the same time. This leads to to a secondary out of
balance shaking force that can't be corrected within the basic crank/rod
system.
Now contrast a flat 4 opposed inline engine - a Boxer engine. In this case
the crank is the same but when Piston 1 is at TDC, Piston 2 is also at TDC
but on the opposite side of the engine bay. Now both pistons are moving at
the same speed at all times and the secondary shaking force mentioned above
does not exist. Unfortunately what you get in its place is a secondary
'moment' but that's getting too complicated to go into.
Long story short - with an inline straight 6 both primary and secondary
shaking forces and moments sum to zero which an inline straight 4 can't
achieve. If you want to go deeper into the maths then I suggest The Internal
Combustion Engine In Theory And Practice Volume 2 by Charles Fayette Taylor.
You'll do yourself a big favour by not even attempting it though. It's not
quite what I'd call bedtime reading.
Dave Baker
Puma Race Engines
#13
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In article <cbk233ttam6pgu8ctvf5nssqejv9upnk2c (AT) 4ax (DOT) com>, a_Frank <fajp (AT) notthis (DOT) optushome.com.au> wrote: Not sure, never worked with, nor thought about them. At a guess, a V shape will introduce some more vibration, since the cylinders are not firing upwards, but i'm not sure if that even flies.. BMW Boxer motors and Goldwing engines don't fire upward. Nice and smooth. They are also not "V". |
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Regards, Frank
#14
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a_Frank wrote: On Thu, 26 Apr 2007 22:22:40 +0100, Phil Newnham <pnewnham (AT) yahoo (DOT) com wrote: brafield (AT) hotmail (DOT) com wrote: What is it about (all?) straight-sixes that renders them perfectly balanced, as most authorities seem to say? Put simply it's because everything balances out - if there is a mass moving in one direction on a 6 cylinder engine, then somewhere there is a mass moving in the opposite direction at the same speed. AIUI It isn't Unlike in other configurations, where one piston or pin remains stationary, until the active piston is done with it's bang.. ;-p Well, a piston can be said to be temporarily stationary at top dead centre, but no, the point is that in other configurations there are always out of balance loads at some point during the cycle, whereas in a 6 cylinder inline or flat engine, or a V6 with a 60 degree bank angle, there aren't (apart from the actual bang, AIUI). But I'm not an engine designer and I am aware that wikipedia isn't the best source. I'd look in my vehicle engineering book but it's at uni and I'm not. I know and you put it well, i was just picking on your wording up |
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I remember when Audi started marketing their 5 cylinder here, they said it was the most balanced engine configuration. To which the guy i worked with at the time said "WTF?". Audi dropped that claim shortly after the release, so i'm guessing a few more people asked that very same question direectly to them. Wikipedia mentioned that several configurations that aren't perfectly balanced have been labelled perfectly balanced by the selective definition of what's included or not included. They can be well balanced. Fours and Vs use counter shafts to do |
cylinders, which was wrong.
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Regards, Frank
#15
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I know and you put it well, i was just picking on your wording up there. ;-) |
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They can be well balanced. Fours and Vs use counter shafts to do exactly that, but Audi made that claim with a clear reference to the 5 cylinders, which was wrong. |
secondary balance was sometimes selectively interpreted so that it
didn't include the moment that they knew was out of balance. Ask Dave
what they mean, he clearly knows more about it than me
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Phil
http://www.flickr.com/photos/tmc1979/
#16
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In article <59eev5F2juudrU4 (AT) mid (DOT) individual.net>, forty <cforteNO (AT) SPAMgmail (DOT) com> wrote: noone wrote: In article <59c686F2k6r4lU2 (AT) mid (DOT) individual.net>, forty <cforteNO (AT) SPAMgmail (DOT) com> wrote: brafield (AT) hotmail (DOT) com wrote: What is it about (all?) straight-sixes that renders them perfectly balanced, as most authorities seem to say? These have somewhat useful explanations with some animations: http://auto.howstuffworks.com/question366.htm http://www.answers.com/topic/flat-engine Because you need cartoons to understand things. This just in from the Sweeney Super-Intelligent-and-Stronger-Than-Batman-Superman-and-Spiderman-Put-Together Committee on Making the World Right: All college courses are hereby stricken from the use of illustrated text books, graphical web content, PowerPoint (unless the content is strictly text), and any and all multimedia content in their pedagogy. Don't cry. Well, cry a little. It's funny. |
http://i8.photobucket.com/albums/a37...ulosewonka.gif
--
forty
“To embrace an extreme, one must first let go of reason.”
#17
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brafi... (AT) hotmail (DOT) com> wrote in message news:1177608375.662863.205930 (AT) c18g2000prb (DOT) googlegroups.com... What is it about (all?) straight-sixes that renders them perfectly |
much for the replies; I think it's good for these basics to be
refreshed from time to time. Much appreciated. This is what the
Internet is so good for (and this time NO FLAMES!) ++++
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You'll do yourself a big favour by not even attempting it though. It's not quite what I'd call bedtime reading. Dave Baker Puma Race Engines |
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