Browse our archive of questions and answers below or to access our old forum database of over 10000 questions and answers please visit the old Ask@ forum.

 Engine Efficiencies Last Post 21 Feb 2013 02:41 PM by Niel. 4 Replies. Sort: Oldest First Most Recent First
 Prev Next You are not authorized to post a reply.
 Author Messages
Peter-27

New Member
Posts:3

 06 Feb 2013 11:18 AM I have a little confusion about units. Working out volumetric efficiency: (Va+g + nR) / (Vs x N) That's the formula, and when using Va+g in l/s i'm getting the right answer. However in part C /(of the same question), thermal efficiency: Pb / (Vg / Qhv) I need to use Vg in m^3/s to get the right answer. These are ratios so obviously units need to cancel, but for the themal efficiency, the units don't cancel. So i'm guessing the problem is that. Can anybody help please?
Niel

Basic Member
Posts:193

 07 Feb 2013 02:09 PM Efficiency is a unit less number therefore the units must cancel or our are doing something incorrectly. To make sure we are thinking the same way could you please define all your terms and provide the units being used for each. Niel Leon engineering.com
Peter-27

New Member
Posts:3

 09 Feb 2013 06:21 AM Posted By Niel Leon on 07 Feb 2013 07:09 PM Efficiency is a unit less number therefore the units must cancel or our are doing something incorrectly. To make sure we are thinking the same way could you please define all your terms and provide the units being used for each. Niel Leon engineering.com Volumetric efficiency: (Va+g + nR) / (Vs x N) Va+g = Voulme of air + Volume of gas (l/s) nR = Number of cycles for power (standard number, no units) Vs = Swept volume (l/s) N = Rotational speed of crankshaft (revs per sec.) Thermal efficiency: Pb / (Vg / Qhv) Pb = Brake power (W) Vg = Volume of gas (m^3/s) Qhv = Heating value of fuel (J/m^3) Not sure on some of the units though, that's what is causing the issue.
Peter-27

New Member
Posts:3

 19 Feb 2013 01:12 PM I have solved the problem now; Heating value of fuel should be in kJ not J.
Niel

Basic Member
Posts:193

 21 Feb 2013 02:41 PM Glad you found the solution. Always - ALWAYS check units carefully. Niel
 You are not authorized to post a reply. Engineering Questions and Answers --Chemical/Material Engineering --Civil/Structural Engineering --Electrical Engineering --Mechanical Engineering --Student Questions --General Engineering --Puzzles Community Support --Jobs