To ask an engineering question please visit our forum site at Eng-Tips.

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?

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

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.

I have solved the problem now; Heating value of fuel should be in kJ not J.

Glad you found the solution.

Always - ALWAYS check units carefully.

Niel