The elasticity of air..another theoretical consideration

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rpatton said:
GRM 450 said:
Powerdoc, Phill Irving wrote a book called Tuning for Speed, it may still be available.
It's old but very good and still "mostly" applicable today, (allowing for modern materials and machining).
Worth getting a copy.

graeme
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Scientific Design of Exhaust and Intake Systems by Smith and Morrison is a lot like Tuning for Speed. It's none the less accurate given the methodology is ancient. Fitting reading while working on a Norton.Time warp. When you're working on your modern bike you can download plug and play apps for intake and exhaust lengths, airbox volume, snorkel lengths...
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Phillip H Smith also wrote The Design and Tuning of Competiton Engines. It's far more readable that his Scientific Design of Exhaust and Intake Systems ( I have both books) and has practical advice on engine building. It's like my bible for race engines.
 
But, hey, is it available for the Kindle????? Will look for the titles, thanks.
 
Since so many of you chimed in on my last theorectical consideration of single vs dual carbs, think about this: Is each barrel on each aspiration stroke, regardless of throttle opening, sucking the full displacement of that barrel. or is the air elastic enough that at partial throttle openings there isn't truly that full displacement sucked? If you suck the full displacement regardless of throttle opening, then you're very lean at partial throttle if you are stochimetrically correct with your mixture at full throttle. Since the cylinder sucks what it can, unless supercharged, it would seem to me that at partial throttle the effects of cross-portal interference with flow would be negated, and because of the maintenance of flow across the venturi the single carb would, at part throttle, would work better.
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Elastic air/gas has a number of effects. Its the throttle and ports and valve restriction of slow idle piston speed that limits the amount to compress. If you could burn the leaness as you correctly pointed out, to limit heat power, you could idle at WOT like a diesel does. The slow piston and valve action would allow time for full filling to compress. To do a valid compression test you must hold WOT or skews the test lower.

You can't say anything universal but for single cylinders with single carbs as the firing order and valve timing in mulit cylinders can push and shove carb and manifold flow all over the place and right back out the carb. In olden days with small carbs ~400 cfm, they had to use several carbs to feed the big engines but set up linkage so only one small carb handled the starting and idle conditions. Its possible to set up all carbs to start and idle on but way more tedius and still less effective.

In our 360 twins as intake closes it can bounce back and elastic compression wave beyond the carb spray tube to pick up fuel on way out then again on way in as the next piston sucks, which over richens. On over lapped cam/valve opening near TDC the rising piston can shove a pressure wave right out the carb beyond air filter. Slow mo video of this is impressive.

On single carb/throat per jug set ups, the over all throat area must be rather larger than a single carb feeding multiple jugs at hi rpm. Single carb can flow many pulses on each rev or even constant so can make up for the shorter single draw limits of a single throat per jug configuration.

Another elastic phenomenon is at hi rpms the piston smacks the gas at super sonic speeds and creates a hi density layer on the crown that the head chamber does not see until piston arrives. This can make the mix as thick as honey to stifle tumble and swirls. Two valve and staggered opening/size 4 valves have advantage of more swirl that's less stifled at TDC. Also principle Singh Grooves use, channels to squirt flame jets into far reaches of stagnant area and then suck em right back at piston moves away with sudden low pressure layer to fill.

Try to sleep on this tib bit, there is not enough volume of combustion products/vapors to give much power, its mainly the pressure of heated N2 that gives us the torque we crave.
 
On single carb/throat per jug set ups, the over all throat area must be rather larger than a single carb feeding multiple jugs at hi rpm. Single carb can flow many pulses on each rev or even constant so can make up for the shorter single draw limits of a single throat per jug configuration.

Hobot,Why would the throat of a single carb need to be bigger. The carb only feeds one cylinder at a time. The is no continuous flow between intake events on a twin at any RPM. Continuous flow does not begine to happen until there are at least 6 cylinders feeding from one carb.

Of course a single carb can be bigger and still work. That is because the fact that the intake events happen twice as often and that will help keep a single carb delivering fuel. Jim
 
By the scientific definition of elasticity, air is not at all elastic. Elasticity is the property of a material to return to it's original shape and volume after being distorted in some way. Air is very deformable but will not return to it's original shape, form or volume without an external force causing it to do so.
 
Would the substitution of the word 'inertia' (of a moving column of air) make a dour old scotsman happier ? !

Interesting point though, that had not gone un-noticed.

P.S. I apologise unreservedly if anyone was offended by my sense of humour....!
 
powerdoc said:
SteveMinning said:
By the scientific definition of elasticity, air is not at all elastic. Elasticity is the property of a material to return to it's original shape and volume after being distorted in some way. Air is very deformable but will not return to it's original shape, form or volume without an external force causing it to do so.
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http://www.nd.edu/~powers/ame.20231/webster1965.pdf
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I can't account for the use of a term in the 17th century. In my post I said "by the scientific definition" because that's the definition used in physics, strength of materials and other engineering courses (yes, I took them).

Webster's first definition is close to the one used in physics:

http://www.merriam-webster.com/dictionary/elasticity
 
Maybe the important thing is that air behaves like a gas. It follows the laws of fluid dynamics, but it isn't a liquid. It easily compresses and expands, and it that sense it is elastic. Liquids do too, but not so you'd notice.
 
At high speeds, like in the inlet tracts, the textbooks say that it fully behaves like a liquid.

And with the fuel vapour in it, perhaps we should no longer keep calling it 'air' ?
 
Somewhat tongue in cheek response on my part. While I was composing the first post, I realized that the term elastic referred to the ability of returning to it's exact shape after deformity. But, after all, we sometimes call rubber bands elastic bands, only the rubber material itself could be elastic but the band itself would not return to it's exact shape. But, if it's not elastic, is it "plastic" or just "deformable" or "ability to expand and contract in response to changes in STP variations". I think most everyone knew of what I spoke.
 
Hey, in a 850 Commando, the 'air' (fuel vapor really) keeps returning to that exact 414cc cylinder shape. A few thousand times a second even. Whats the problem ?
 
Rohan said:
At high speeds, like in the inlet tracts, the textbooks say that it fully behaves like a liquid.

And with the fuel vapour in it, perhaps we should no longer keep calling it 'air' ?
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Maybe as regards to flow, but since we're talking about intakes, I was thinking about the effects of wave front propagation and timing. They are examples of high and low pressure regions. They do reflect the compressibility and expandability of air. Liquids do it too, but not like this.
 
At near supersonic speeds, air (fuel vapor) is for all intents and purposes a liquid.
Ask any supersonic jet pilot...

Water in a garden hose knows about pressure and vacuum induced flows, and water hammer (and tsunamis) would suggest that pressure waves equally exist in liquids.

But we get your drift...
Cheers.
 
I do love physics; you think the designers of Nortons had any inkling of it?
 
powerdoc said:
I do love physics; you think the designers of Nortons had any inkling of it?
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No !
From all accounts they just tried it, and if it worked went with it.
Same as all makers and tinkerers and racers (and dreamers too) back then.
Although a few like Ricardo explored the whys and wherefors.

If you watch the history of engine development over the centuries, it took absolutely decades and decades to figure out the inlet valve needed to be larger than the exhaust valve.
This is utterly fundamental to good performance - but took near 80 years to figure out....
 
powerdoc said:
I do love physics; you think the designers of Nortons had any inkling of it?
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Perseverance, dedication, Heart, stiff competition, luck, inspiration (JLN) and as for physics goes, ole Albert was just coming to light or should i say the speed of light.
 
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