The elasticity of air..another theoretical consideration

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.

Wow, my head hurts from reading that. You really should go out and ride your bike and stop thinking. For the greater good. :?

The bike is in bits right now so I have time to ponder the unverse as it pertains to motorcycles.

At anything less than full throttle the only thing that matters as far as creating power is maintaining a good fuel mixture.

When the throttle is closed enogh to create manifold vacuum then the air is less dense so there are fewer molecules per volume of air. And less fuel to go with those molecules.

So still the question: At partial throttle openings, is there less than a full displacement amount of air in the cylinder?

powerdoc said:

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? .

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While we are pondering this, ask yourself does green cheese really exist, and is it good for you ? !

The answer of course is that a petrol engine, a Norton for example, rarely if ever takes a full lungfull, and mostly nothing like it. Unless you are a real hoon - or racer or stoplight dragger. (Have we ruled anyone out yet ?)
At idle, it is only drawing a very small measure of air - thats what the 'throttle' is for, to throttle it. Unless you are giving it FULL THROTTLE, with ZERO VACUUM in the inlet tract, its still not taking a full lungfull either.

And, having had a vacuum gauge fitted (as standard, in a car), the number of times you get zero vacuum and full throttle is rare - and scary fast, watch where you are going and not the vacuum gauge.
"I don't know where we are going, but we are getting there awful fast".

P.S. This also means that if your engine has a quoted compression ratio of 10:1, unless you have the throttle wide open and that zero vacuum, you are not really compressing 10 somethings into 1 either. At cruise its only only doing a few somethings (at most) into 1. Bikes that pobble around may never have achieved their 10 somethings into 1 in their whole lives...

I use the word somethings loosely...

Diesels are different.
Ask on a diesel forum...

Cheers.

We dont have that trouble if we get it supersonic , its Liquid . :lol: :shock: wonderous substance , the atmosphere .

powerdoc said:

The bike is in bits right now so I have time to ponder the unverse as it pertains to motorcycles.

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Time to ride another bike for the greater good. If you don't have one, find one - QUICK!

Your argument sparks a slight interest.

I suppose at some point a squirt gun with gas and a can of compressed air may produce the effects you speak. Although the possibility exists, the practicality does not. Throttles do not live at any point, although they may dwell at cruising speeds, this also is not fixed.

An initial opening, of which I believe you speak would be almost impossible to measure due to the infinite amount of variable, octane, barometric pressures, humidity level, and then there are the internal variables, inertia, compression ratio's, valve spring tension, on and on and on.

Again, the possibility may exists.

calbigbird said:

powerdoc said:

The bike is in bits right now so I have time to ponder the unverse as it pertains to motorcycles.

Click to expand...

Time to ride another bike for the greater good. If you don't have one, find one - QUICK!

Click to expand...

Anything that makes us think about making the bikes better IS for the greater good. A basic understanding of what the bits do never hurts.

powerdoc said:

So still the question: At partial throttle openings, is there less than a full displacement amount of air in the cylinder?

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Look at it this way. The volume is there when the piston is down. It is full of evenly spaced molecules of air. If the pressure in the cylinder is low then there are less molecules present.

Now does the pressure in the cylinder after the induction stroke ever measure as high as ambient. It will at low speeds although a percentage of the charge will be lost back out the intake valve as the piston moves up away from BDC.

It can at high speeds if the tuning of the intake system is correct. Usualy this can only happen over a limited range of speeds where the system is "in tune". Modern performance engines and even a well tuned Norton can end up with pressure above ambient due to the supercharging effects of the pulse tuned intake system. Jim

powerdoc said:

So still the question: At partial throttle openings, is there less than a full displacement amount of air in the cylinder?

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Yes! with no tick over set.desending a hill the engine now becomes a vacuum pump, No air.No fuel, No bang! Marval at the atmosphere holding you back! There is 14 psi of negitive displacement...look at a Jacobs engine brake ,using the atmosphere to hold back a 44 ton artic :!:

Jacobs make several different types of engine brake. Some of them close a valve in the exhaust system and the exhaust backpressure slows the motor. They will stop a herd of elephants.

Another type, [the one that makes all the noise on tractor trailer rigs]. Operates by raising an exhaust valve slightly off its seat. The work that is done pumping the air in and out through that restriction is what slows the motor.

Kind of like the power it takes to pump the air in and out of a crankcase that doesn't have a reed breather. Jim

powerdoc said:

So still the question: At partial throttle openings, is there less than a full displacement amount of air in the cylinder?

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This is actually a trick question ?

Since nature abhors a vacuum, whatever air is in there will be perfectly evenly spread around - eventually.

Discuss all you want as to what a "full displacement of air" actually means.
Besides which, by now its fuel/air vapour, an explosive mix...

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

Yes , I was going to mention that . ! whos Phil Irvine . :wink: EVERYBODY has read ' Tuneing for Speed ' , though I dunno about these computor types . Was compulsary for 10yr olds , in N.Z. :shock:

Now , the R.R. Merlin gets 150 Lbs THRUST through the rear ducted exhaust manifolds . Exhaust Pressure , so mathematically were around 15 Lbs , running hard . :lol:

Good call Graeme. Phil irving also wrote "Motorcycle Engineering" which gives an excellent discussion of all the basic parts of a motorcycle - particularly british bikes, since thats where he worked and had experience with. Think its still available new today, and they appear used now and then, usually well thumbed through. Easy to read, but gets quite technical where it needs to be, if thats what you need to go into.

Yes another good book worth reading.
And if you can get hold of any of his articles he wrote as "Slide Rule" instead of his real name. These are worthy of anyones time to read also.

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...

Dude! A vacuum exists and something fills it. All that matters is what you fill it with!