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Engine basics => Engine set up tips => Topic started by: Gene Elliott on October 12, 2006, 10:41:48 AM
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I'm going to bear my ignorance here and ask if someone would explain the numbered designation for motors. What does the cubic inch displacement number actually measure? What does that refer to? And it's possible that the piston diameter on my OS 40 is a different size from the piston diameter of my Fox 40, right? Thanks for your input.
Gene Elliott
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Gene,
Displacement is merely the volume of liquid that would be displaced (forced out) in the cylinder if the piston was moved from the bottom of the stroke to the top. It's measured in cubic numbers, either centimeters or inches.
Our engines are very small compared to an automobile engine. The old small block Chevy engine displaced 283 cubic inches, spread over eight cylinders (roughly 35 cubic inches per cylinder). Our model aircraft engines are typically single cylinder and displace much smaller amounts of liquid. For example, a Super Tigre .51 engine displaces just over 1/2 cubic inch.
Displacement is calculated by taking the area of the cylinder ("PI R square" - remember that one?) and multiplying that number by the distance the piston travels up the cylinder (stroke).
Hope this helps,
Ron
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..."The old small block Chevy engine displaced 283 cubic inches, ..."
Come on Ron, we all know that they were 265 cu. in. or are you too young? <=
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..."The old small block Chevy engine displaced 283 cubic inches, ..."
Come on Ron, we all know that they were 265 cu. in. or are you too young? <=
Touche. ~> You are correct. Forgive my Senior moment. They started at 265 and eventually got bored out to 327 cubic inches. Great engine design.
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Touche. ~> You are correct. Forgive my Senior moment. They started at 265 and eventually got bored out to 327 cubic inches. Great engine design.
And on out to 350. :D There is a 400 small block Chevy! They are all of the same block as the 265.
Bill <><
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Yea tiny when compared to a caddy 500
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What a mill! We have some serious car enthusiasts here.
Ron, thanks for the good explanation. . ."the volume of liquid that would be displaced" . . .It seemed to me a curious way to measure the size of an engine, but the more I think about it - it does measure the size. I was a college Spanish teacher and math and sciences were not my cup of tea.
Thanks Ron,
Gene
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Hi Gene,
I guess you got the "displacement question" answered, so the other ones you asked are what I will dwell on. ;D
A "40' engine is four tenths or fourty hundreths of a cubic inch displacement, and an .049 is fourtynine thousandths of a cubic inch displacement. The new rule allowing .90 engines in stunt gives us almost a one cubic inch engine!
As to the pistons and such of one maker to the other, they can vary quite a bit in diameter since the "stroke" of the engine is the other 1/2 of the equation for measuring displacement. And, as you know, I do not know of a model airplane engine that displaces exactly what the engine has marked on it. For instance, a .40 might actually have a displacement of .3978.
Bill <><
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Hi Folks,
Technically, it is called the "swept volume", which is, as previously said, the bore area multiplied by the piston stroke. (i.e. bore dia squared multiplied by Pi, divided by 4, and that result multiplied by the maximum travel of the piston - keeping the units the same - inches or metric).
Oh, and the above multiplied by the number of cylinders if more than one.....
Happy motoring (or flying!)
Cheers
Harry
forgot to mention - original engines tend to be smaller to allow for first rebore, so the engine does not go above the original quoted capacity.
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ˇGracias hermano Bill!
Gene
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How about a 90 cubic inch V-12?
f~
Bob Z.
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How about a 90 cubic inch V-12?
f~
Bob Z.
What year did Ferrari make that one? Looks early late '50s-early '60s. ??? ??? ???
Bill <><
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Yea but my caddy is in my garage waiting to be put in a rat truck. f~ AP^
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Hi Gene,
I guess you got the "displacement question" answered, so the other ones you asked are what I will dwell on. ;D
A "40' engine is four tenths or fourty hundreths of a cubic inch displacement, and an .049 is fourtynine thousandths of a cubic inch displacement. The new rule allowing .90 engines in stunt gives us almost a one cubic inch engine!
As to the pistons and such of one maker to the other, they can vary quite a bit in diameter since the "stroke" of the engine is the other 1/2 of the equation for measuring displacement. And, as you know, I do not know of a model airplane engine that displaces exactly what the engine has marked on it. For instance, a .40 might actually have a displacement of .3978.
Bill <><
Hi Gene Hello Bill
Actually the new size aloows bigger than a 90, we can go to a .9153, or a 92 , rounding the size off.
Of Course don't forget to make up those 21 thou line LOL
Regards
Randy