Air consumption !

[racingt]

Ok guys this is off the wall but I wondered how much air the engine used. Don't ask why!
I guess it's 200cc every rpm. Or at 14000 rpm, 2800 litres of air per minute.
that's huge!
Am I correct?
Off to check the air filter now......

[RoninZX-10R]

Yep, that looks exactly correct to me...a 400cc engine draws in 0.4lt every 2 revolutions of the crank, so just as you say 200cc every rpm. 0.4 x 14,000 - 2,800lt/min (that's a hell of a lot!!!)

[phil x]

Yes, thats a theoretical maximum at WOT, in reality, the cylinder will not completely fill at wide open throttle & obviously the engine will draw significantly less air at lower revs

Phil

[Kurlon]

It's worse than that, under some conditions the cylinders can fill MORE than what their displacement would imply. That's why proper sized airboxes and variable length intake runner setups are becoming more and more import aspects of engine design.

The easier way to figure it out is to reverse the numbers based on HP. Because you know how much fuel you have to mix with a given volume of air to get a proper burn (14.2 / stoichiomatic Air/Fuel ratio) and how much energy you get from a set volume of fuel... if you know the HP you can determine the volume of air needed to generate that much punch.

CFM = HP x 1.62

[racingt]

Whoa, hold on kurion!
You are way to advanced for me!
Key point seems to be that with a fixed stroke length, the compression ratio will vary according to volume of air in the cylinder. I can understand phils point that the cylinder may not fill completely but how does it overfill without using a supercharger? By overfilling, the air must be pressurised I guess, so is this achieved by funnelling the air down the airbox, is this what ram air meant.? Is the pressurising achieved by the pressure gained by forward motion? So assuming compression ratios vary between say 6 and 14 to 1, just as example numbers, how is a effective burn pattern created? Everything is a compromise isn't it?

[Kurlon]

The easiest way to picture it is think of a pipe organ. A given pipe length will resonate at a set frequency, a little input, lots of output.

Next is to not think of air as a fixed density medium, but one that can have dense and thin areas, and those pockets can travel as waves. Aka, we're talking 'bout sound. :D

Think of the intake tract as a tuned pipe. The length is from the back side of the intake valve to the opening of the intake horn in front of the carb. When the intake opens air starts rushing down the pipe to fill the vacuum made by the piston dropping down the cylinder. This air has momentum, it doesn't just instantly start filling, it's got to get up to speed first. As the cycle continues the intake valve shuts. The air that has just now gotten moving doesn't suddenly stop, it slams against that intake valve making a dense pocket as it piles in. This dense pocket then ricochets back up the intake. Like a tuned pipe, when the dense wave hits the end of the tube, a portion of it reverberates back down the tube.

Now, if we're at the right RPM, just as that dense pocket is coming down the pipe, the intake is opening for a fresh charge. Rather than just waiting for the air to loiter in, the cylinder gets a blast of nice, dense air. Close the intake behind it, restart the cycle and you've just filled that cylinder with more air than it would normally draw on it's own. Machines with variable length intake tracks tune the intake lengths to maintain this effect over larger RPM ranges than a fixed setup could provide.

If you didn't have any sort of intake passage, just a valve exposed to the atmosphere, there wouldn't be any momentum effect to capitalize on, or anything to reflect a wave back at the right time. Your 150cc cylinder for example might only be drawing 75cc of air each pull?

Cam overlap can have an impact as well. By keeping the intake and exhaust open at the same time you can use the pulse of outgoing spent gas to help draw in a fresh charge even faster, this is precisely the effect two strokes with expansion chambers rely on.

All this is achieved without a 'ram air' setup or any form of supercharging. It just requires proper selection of intake shape and length, airbox size to act as a secondary resonator to help boost the effect, cam timing, exhaust design, etc. You can dial them to make crazy power at a given RPM, or spread the tuning for less peak but a broader curve, etc.

[28hodge]

Is this the reason why you ended up with gp bikes having 14 plus gears??

essentially their intake tracks were tuned to provide power at a very specific rpm range and it was in this range that they could harness the resonance obtained from the wave of dense air??

i think some bikes of that era made all their power in like a 500rpm range or something ridiculous? and as there was no ruling on the number of gears allowed the teams took full advantage to make good horsepower over a very small range.

Loving this though, really intresting stuff.

[Le_Blur]

I was having a similar conversation with a friend regarding 'scavenging' in two-stroke engines (the use of resonant pulses to evacuate the chamber of exhaust gases and draw fresh air in) and he reckons teams were tuning exhaust lengths to within a millimetre either way of the fundamental wavelength at the desired RPM. I think that's where the idea behind the 'pig-tail' exhaust section on the RC211V came from.

[mo haggs]

nicely broke down kurlon, have some karma.

[racingt]

Thanks kurion, I'm beginning to understand why the power curve has a curve, not a straight line. Also comprehending the futility of trying to improve what Honda created!
Great xplanation, I've followed mo and posted you some karma!