Contact US

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Getting The Wrong Answer

Getting The Wrong Answer

Getting The Wrong Answer

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.

RE: Getting The Wrong Answer

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!

RE: Getting The Wrong Answer

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.

RE: Getting The Wrong Answer

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.

RE: Getting The Wrong Answer

you must to be care about this type of confusion

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members! Already a Member? Login


Low-Volume Rapid Injection Molding With 3D Printed Molds
Learn methods and guidelines for using stereolithography (SLA) 3D printed molds in the injection molding process to lower costs and lead time. Discover how this hybrid manufacturing process enables on-demand mold fabrication to quickly produce small batches of thermoplastic parts. Download Now
Design for Additive Manufacturing (DfAM)
Examine how the principles of DfAM upend many of the long-standing rules around manufacturability - allowing engineers and designers to place a part’s function at the center of their design considerations. Download Now
Taking Control of Engineering Documents
This ebook covers tips for creating and managing workflows, security best practices and protection of intellectual property, Cloud vs. on-premise software solutions, CAD file management, compliance, and more. Download Now

Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close