Help with Displacement formulas in Excel

[jeremyse]

Just another engineering student needing help with writing functions in "MS
EXCEL".
Particularly "Piston Displacement". I have the following formula, but I am
not able to get "Ms Excel" to understand it.
The formula is:
X=9.625in - (2.125in)(cos)-SQRT(56.25in^2)-(4.515625in^2)(sin^2)

I would greatly appreciate any help

Thanks,
Jeremy

[Nick Coleman]

Hi Jeremy,

Sorry, this non-engineering person can't make sense of the formula. Cos
of what value? Sin of what value? Is IN shorthand for inches, or is it
a variable name?

To get you on your way, type an equals sign followed by standard algebra
(without spaces), remembering that an asterisk * is the sign for
multiplication.

The F1 key will bring up a self-explanatory help window to help you
further.

Cheers,
Nick

[Andrew S. Franks]

Hello, Jeremyse,
I believe the displacement is determined by calculating the volume of the
cylinder swept by the piston. I believe the method used to calculate
displacement is to multiply pi by the square of the radius; that will give you
the area, sometimes referred to as the "bore." Then, multiply that area by
the length of the piston's travel, sometimes referred to as the "stroke."
Of course, there could be some question as to whether one uses the radius of
the _cylinder_, or the radius of the _piston_ . These two dimensions are
_slightly different_ , the difference being taken up in practice by the _piston
rings_ .

In the days of the muscle cars, some folks would tinker with these two
characteristics. An engine modified in this manner was said to have been
"stroked and bored."
Perhaps some automotive engineer can provide a definitive answer to the issues
you have raised.
I hope this has helped.
Sincerely,
Andrew S. Franks

[Fritz Meissner]

Shouldn't be any problem with that formula if you remember that the
constants mustn't have "in" (I asume this is inches and not a variable name)
after them, that cos and sin need to refer to an angle (e.g. if angle (in
RADIANS not degrees) is in A1 then cos(A1) etc) and all multiplications must
be explicitly shown by *. Formula should look like

=9.625-2.125*cos(a1)-sqrt(56.25^2)-(4.515625^2)*(sin(a1))^2

Could be simplified by noticing that sqrt(56.25^2)=56.25; also becomes a lot
more readable if you put the constants into separate cells and use named
ranges to give them (and the variables) sensible names - I find that this is
an absolutely necessary habit when writing engineering spreedsheets if you
want to have any chance of getting the right answers.

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