Turbocharger compressor maps 1

[tmorgan4]

Hi,

I am having a difficult time wrapping my head around a question regarding turbocharger compressor maps. If anyone could provide an explanation it would be very much appreciated!

Turbocharger compressor maps essentially plot 4 variables on a 2D plot - mass flow, pressure ratio, rotor speed, and efficiency.

Many compressor maps show what I would expect to see. For a given rotor speed the mass flow increases as pressure ratio decreases (inversely proportional). If you follow a line of constant rotor speed, you can pick a specific mass flow OR pressure ratio and only ONE unique point exists. This is what I would expect. See map1.jpg (attached) for an example.

On other compressor maps the mass flow and pressure ratio both increase as we follow a constant rotor speed line starting at the surge line and moving to the right. This means we have two different mass flow rates for the same pressure ratio while holding a constant rotor speed! How can this be?

See map2.jpg (attached). If we look at the green dots both points have a pressure ratio of ~2.45 and a rotor speed of 111981 rpm yet the left point has a mass flow rate of 19 lb/min and the right point is flowing 34 lb/min. What is causing the difference in mass flow rate between the points? Efficiency?

Thanks in advance for any clarification.

http://files.engineering.com/getfile.aspx?folder=853764d9-dc1a-4515-a8d3-454dc4a06e87&file=map1.jpg

http://files.engineering.com/getfile.aspx?folder=fa2a1215-b316-40c8-a8a6-6fec127719ad&file=map2.jpg

 

[SomptingGuy]

Part of the problem is that some compressors can't be trusted to sit at the right place on a map, when there are equally valid choices. The haven't read the manual.

- Steve

 

[Lou Scannon]

It's simple. The compressor is not running independently of the boundary conditions. If combined with positive displacment machine, such as a reciprocating engine, for instance, the volumetric flow rate downstream of the compressor will be largerly govererned by the positive displacement machine.
Regardless of the boundary conditions, like any contour map, in general, there is no fundamental requirement for the contours to be monotonic with respect to any of the axes.

"Schiefgehen will, was schiefgehen kann" - das Murphygesetz

 

[SomptingGuy]

What I was alluding to is that the operating point moves continuously around the map. It does not jump to a "best" point. Shaft speed is a dependent variable in all this and may settle to the same value with various combinations of the independent variables. In reality, the independents are only engine speed and fuelling and their contribution to air flow, pressure ratio and turbo shaft speed are very complex. There is much feedback between all these dependent variables too. And that's before anyone has added EGR and/or wastegate/VGT control systems.


- Steve

 

[btrueblood]

"See map2.jpg (attached). If we look at the green dots both points have a pressure ratio of ~2.45 and a rotor speed of 111981 rpm yet the left point has a mass flow rate of 19 lb/min and the right point is flowing 34 lb/min. What is causing the difference in mass flow rate between the points? Efficiency? "

Yes. Alternatively, you could state that the compressor is operating in a partially "stalled" condition when operating away from the best efficiency line.

 

[btrueblood]

I think Steve's and hemi's posts are assuming that you are talking about the combined performance of a turbocharger, but I was assuming that the plots you showed are correctly described by you as compressor maps only, i.e. the compressor performance measured with its shaft being directly driven by some controlled motor.

 

[turbomotor]

Hi Tmorgan4,

I think if you track down a good turbomachinery textbook and read the chapter on "Energy transfer between a fluid and a rotor", or something similar that explains the Euler Equation as applied to turbomachines, the answers to your questions will make more sense. If you are not familiar with dimensional analysis (Buckingham Pi Theory), you may want to read up on that, also.

Many books out there explain things well, but my favorite is a book by Shepherd called "Principles of Turbomachinery". Very practical explanation of how these things actually work, yet detail enough to facilitate quantitative performance analysis.

Based on your questions, I think you will enjoy the reading.

Dick

 

[Lou Scannon]

Compressor maps are generated on sophisticated test stands with intricate control and measurement of all pertinent parameters. In this case, the volumetric flow rate, inlet/outlet pressures, and power being fed into the shaft are deliberately manipulated in order to manoeuver the compressor to any desired point within its operating range, thereby mapping out its performance.
The point is, the compressor map is what it is, there is nothing mysterious or freaky about there being coordinates of identical pressure ratio and speed at two different flow rates.

"Schiefgehen will, was schiefgehen kann" - das Murphygesetz

 

[gruntguru]

I think you would agree it is feasible to produce a compressor with a horizontal characteristic (a range of possible flow rates at a given shaft speed and pressure ratio) over some of its operating range. "Map 2" shows several sections where there is the case.

For the speed line to dip to the left is unusual because this is an unstable operating condition (surge). That is what the surge line is for - all the speed lines fall if continued to lower flows. Left of the surge line is not plotted because this is a no-go zone - the instability will wreck the compressor if operated continuously in surge. In some cases (like Map 2) the manufacturer may allow operation in an area where the surge is "mild".

To visualise surge, think about gradually restricting the discharge at constant speed, starting at the right of the map, moving the operating point to the left and increasing the pressure as you go - this is stable. If you continue to the point where the pressure starts to fall, the reduction in pressure produces a further reduction in flow through the restriction - leading to a further reduction in flow - leading to . . . . . and so on. The flow goes to zero or thereabouts, as does the pressure and suddenly we have a situation where flow can restart, to fill the low pressure volume between the discharge and your restrictor - and the then the whole cycle starts again. This repetitious cycle is "compressor surge"

Engineering is the art of creating things you need, from things you can get.