I am currently finishing up a program that will plot points on a compressor map. In an attempt to make the program even more accurate I am adding some code that will allow my program to recognize which efficiency island of the map a certain point has fallen into. This brings me to a question:
To plot a point on a compressor map we're told to find the pressure ratio and the mass flow rate. To find the mass flow rate we first need to find the Density Ratio of the air. The equation for the Density Ratio includes the temperature of the air exiting the intercooler, which is directly related to the efficiency of the compressor.
Every book I've read on the subject says to calculate the temperature of the air exiting the compressor by using the maximum efficiency of the compressor (i.e. 78% for the T04E50.) That seems fine except for the fact that once the mass flow rate is found, the point on the compressor map where the mass flow rate and the pressure ratio meet might fall onto an efficiency island which is less than 78% efficient.
For example:
Compressor map used: T04E50
Displacement: 151.82 cu in.
Atmospheric pressure: 14.7psi
Ambient temperature: 85F
@3300 RPM, 95% volumetric efficiency, 8psi boost
Compressor pressure ratio: 1.61
System pressure ratio: 1.54 (1psi loss across intercooler core)
Temperature exiting compressor: 182F (78% efficiency)
Temperature exiting intercooler: 114F (70% efficiency)
Density Ratio: 1.46
Volumetric flow rate: 138
Mass flow rate: 14.69
On the map, a pressure ratio of 1.61 and MFR of 14.69 puts the point in the 72% efficiency island. This would mean that my density ratio and mass flow rate are all wrong.
At this point, should I re-calculate the temperature of the air exiting the compressor (and subsequently, the temp exiting the intercooler, density ratio and mass flow rate) using 72% as the compressor efficiency? Doing so would change my values to:
Temperature exiting compressor: 191F (72% efficiency)
Temperature exiting intercooler: 117F (70% efficiency)
Density Ratio: 1.45
Volumetric flow rate: 138
Mass flow rate: 14.59
The mass flow rate changed by 0.1. While this isn't a large number I'm sure there are other examples where the difference may be more substantial. Perhaps the new
PR / MFR would put the new point onto the 70% efficiency island and the process would have to begin agian.
My question is, is there any merit to me adding this to my program? Would I be better off just making the user aware of which efficiency island each point falls onto but not re-calculating everything every time the efficiency island does not match the efficency used to calculate the air temperature exiting the compressor?
Compressor maps
Linear Mode
