Brain Teaser

[holeshot]

According to this link

http://encarta.msn.com/encnet/refpages/RefMedia.aspx?refid=701500647

the density of air is about 1.3 kg/m3, which, according to my trusty converter calculator, is equivalent to 0.08115635 lbs/ft3.

Doing the math-

15lbs x ft3/0.08115635 lbs = 184.8 ft3 (the volume of air you'd burn with 1 lb of gasoline at a ratio of 15:1)

All hypothetical of course.. :confused:

 

[WoodsRider]

Originally posted by jmics19067
... at 100% volumetric efficiency...

That's what puzzles me. A naturally aspirated engine is roughly 80% efficient. Now while the 2-cycle gasoline engine typically uses crankcase vacuum to draw the air-fuel mixture through the carburetor, the rotating crankshaft assembly then forces the incoming air-fuel charge into the cylinder like a Blower (mechanically driven supercharger). A blower-scavenged engine is roughly 130% efficient. Would the true efficiency be somewhere in the middle or at either end of the spectrum? :confused:

Rich - For a guy that steers clear of the woods you've definitely heard my bike a time or two. ;)

 

[jmics19067]

Originally posted by Rich Rohrich



jmics - are you sure you didn't double the air consumption ? :eek:

well actually I did double the air consumption for a two stroke engine beings that I had a four stroke equation in front of me < holley handbook for knitwits>

four stroke equation is ( cubic inch displacement divided by 2) x (rpm divided by 1728) x volume efficiency = cfm

cid rpm
__ X ___ x ve = cfm

2 1728


I wrote out the four stroke formula two different ways to try and explain myself proper.
Now with the two stroke having a gulp of air every rpm compared to the four stroke I presumed the formula would look like

(cid x rpm x ve)
_____________ = cfm
1728

By getting rid of " divided by two" from the first equation I am hoping that I am putting the four stroke and two stroke intakes on an equal playing field.

The other thing I was concerend with is that if there is 25.4 mm in an inch would 2.54centimeters cubed equal one cubic inch

heh funny thing when I was in school math came so much easier to me but I couldn't stand it now I look at it like a puzzle <solve for Y.....> but is so much harder keeping everything in order< one too many knocks on the head I guess> sheeesh the effort it took me just to try and write this down legibley <atleast I hope since I am using a keyboard:)> was tiresome.

 

[jmics19067]

Originally posted by Rcannon
Since there is not a dumb forum, I will ask this here. How much volume would 15 lbs of air take up?? Would this be a 12 x12 room full?? More? less??

I am not sure but I believe air pressure at sea level is 14 psi which means for a given square inch there is 14 lbs of air directly on top of it.

which makes me think what is sea level since the atlantic an pacific oceans are actually two different hieghts. How come there is constant rough seas at the bottom of South America and yet the oceans never level out? what would of happened if the French completed the Panama Canal?

 

[Rich Rohrich]

Originally posted by jmics19067
well actually I did double the air consumption for a two stroke engine beings that I had a four stroke equation in front of me

It was kind of a loaded question. :confused: Your answer is correct I was just hoping you'd explain how you came to the final conclusion which you did quite nicely. THANKS :thumb:

 

[Rcannon]

This may not be that complicated. Look at a goggle lens an hour after riding . This is without pressure pulling the dirt in.

And..judging by Rich's picture he is the type you would want on your side in a barroom brawl. Rich, did you play football??

 

[holeshot]

I couldn't see where the 1728 came from at first, but it's the conversion from cubic inches to cubic feet. 1 cubic foot = 1728 cubic inches.

 

[jmics19067]

Originally posted by Rich Rohrich


It was kind of a loaded question. :confused: Your answer is correct I was just hoping you'd explain how you came to the final conclusion which you did quite nicely. THANKS :thumb:

No, thank you sir!!! It is always a pleasure to learn something new and you have done quite a bit of teaching from me just reading your posts. I would like to thank Okiewan for setting up an excellent web site . I would also like to thank someone else for correcting me on a very long winded run on sentence when I first starting checking out this site,but I can't remember if it was Jaybird since we where talking about chains. Again thank you.

Between you three people <and the rest of the people on this site> you have kind of forced me to start thinking again in a fun way.

 

[Chief]

Originally posted by jmics19067


I am not sure but I believe air pressure at sea level is 14 psi which means for a given square inch there is 14 lbs of air directly on top of it.

HMMMMMM That's what I usually run in my tires .............so if I lived on the beach I could trash my compressor :) :) :)

 

[Rich Rohrich]

Originally posted by Rcannon
Rich, did you play football??

NAH, at my high school I was way too small :scream:

 

[Rcannon]

Yea Rich, are'nt you number 53 for the Oakland Raiders???

 

[WoodsRider]

jmics - I have a different formula, but roughly the same result.
cu. in. displacement(*2 for 2-cycle)*RPM*Volumetric Efficiency/3456

Where as Volumetric Efficiency = 0.8 for Naturally Aspirated, 1.3 for Blower Scavenged and 1.7 for Turbocharged.

Therefore at a contstant engine speed of 8,000 RPM a 250, 2-cycle would require 70.6 CFM of air @ 100% efficiency, 56.5 CFM of air @ 80% efficiency and 91.8 CFM of air @ 130% efficiency.

 

[jmics19067]

Originally posted by Chief


HMMMMMM That's what I usually run in my tires .............so if I lived on the beach I could trash my compressor :) :) :)

but only if your beach was in a vacuum which would make jetting a nightmare

Originally posted by WoodsRider
jmics - I have a different formula, but roughly the same result.
cu. in. displacement(*2 for 2-cycle)*RPM*Volumetric Efficiency/3456

if I am reading your formula correct;

cid(2) x rpm x ve
______________ = cfm
3456

it is the same actually only manipulated by doubling everything
cid(2) cid
______ = _______
3456 1728

The difference between the four stroke and two stroke is intake happens twice as often on a two stroke. Where you times the numerator by two I divided the denominator by two. heh just because I can actually remember the proper names I am also going to throw in ; when dividing fractions you are actually multiplying its recipricol
X 1 X 2
_ divided by _ = _ times _
2 2 2 1

please excuse my rudimentary ramblings but I am amusing myself. After 20 years of greasing trucks and fixing flats I am surprised my brain isn't total mush.

 

[jmics19067]

arrrgh now that just didnt look good lol