Getting The Wrong Answer 2

[mmm5288]

I cannot figure out why MathCAD 14 is out putting the wrong answer.

I am doing a simple Power calculation using the formula:

P = BMEP * V.s * N/2 where

N = 4400rpm = 73.333rev/s

V.s = 2L = .002m^3

BMEP = 22bar = 2200000Pa

The correct answer is 161.3kW and MathCAD is giving 1014kW.

I uploaded the file for you to see.

Thanks for any help.

 

[mmm5288]

Forgot to add file.

http://files.engineering.com/getfil...a-477d-8e2d-86bf63785601&file=Question_1.xmcd

 

[jhardy1]

I don't know the formula or its derivation, but 1014 / 161.3 = 6.286 = 2 pi.

It looks like a confusion over appropriate units in the formula - the fundamental unit of cyclic frequency is rad/s = 2 pi x cycles/s. If you enter

N = ?

into Mathcad, it will return 460.767 1/s (i.e. rad/s), NOT 73.33 rev/s.

Hope this helps!

 

[sibeen]

I'm with jhardy1, in that I don't know your formula, but it appears to be you should be converting to rads.

You have N/2, so as rad = 2 pi, placing N/(4*pi) in your equation will give you the correct answer.

 

[mmm5288]

Thank you jhardy1 and sibeen for your response. The formula is a UK one for their HARE vehicle. That's why it is so different from what normally is seen.

I never took into consideration what MathCAD does to the revolutions and rad/s. It has been some time since I had to do any calculations involving them.

Once again thanks for the help.

 

[Spiry]

you must to be care about this type of confusion