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Old August 19th, 2006, 04:23   #4
TDIMeister
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Default Part 2: Definitions and Terms

First of all, a few definitions and boundaries of the compressor map graph are in order.


The X-axis on the graph is corrected air flow in pounds per minute (lb/min, LBM, etc.), which you can obviously read. What is important to recognise is that this is a MASS flow rate, NOT a VOLUME flow rate (e.g. measured in CFM) which you may encounter in some other maps from different manufacturers. This distinction is vitally important, and although I won’t get into why, it suffices to say that a MASS flow rate is the ONLY useful parameter for your purposes. The term “corrected” basically corrects the inlet conditions with standardized reference conditions from which the maps are derived. There might also be consideration of the conversion between stagnation (total) pressure and static pressure (what you would read in a gauge) across the compressor, as well as inlet temperature. Most turbo manufacturers will state the reference ambient conditions if you know where to look for it, but it is usually 25C (298K) and 100 kPa. For your purposes, you don’t need to care about this further, except to reiterate that corrected MASS flow rate is important.

The Y-axis denotes the pressure ratio (PR), that is, the ratio of the pressure at the outlet and inlet of the compressor. Again, in more detailed maps, there maybe a footnote if the PR being illustrated is “total-to-total,” “total-to-static,” or “static-to-static,” etc. In most cases the difference between them are relatively small, and if not otherwise specified, you can assume, as in this case, static-to-static, which is more useful and intuitive since that’s what you measure with a gauge. Most people will go even further to simplify, treating PR as simply the absolute boost pressure in bars. This is not exactly correct, because there are pressure losses both at the inlet and outlet of the compressor at either point of measurement (i.e. some distance up- and downstream of the compressor itself, taking into consideration pressure losses in the air box, plumbing, intercooler, etc.), and dynamic and temperature effects. For example, if you are running at a boost pressure of 2 bars absolute, you can expect the actual compressor PR to be somewhat higher than 2. This is fine as an estimation, but just be aware of the distinction.

Now, with that out of the way, we look to the actual map curves themselves. The left-most boundary of the map is called the surge region. Surge is a not-well understood phenomenon, even by researchers in the field of turbomachinery – how much more murky it is then, for laypeople! This would be a whole other long missive, which I won’t get into. As far as you and I are concerned, it suffices that you would want to avoid operating in surge as much as possible.

The right-most boundary of the map is called the choke region. This is a bit more intuitive, as there is a maximum amount of a fluid you can pass though a hole of a given size before the flow becomes sonic and you can’t (normally) flow any more. Note here that the lines of constant RPM (the set of curves that start almost perfectly horizontal on the left side of the map, labelled 120000, 140000, 160000, etc.), drop off quite steeply near the choke region. The physical explanation of this is that as the compressor approaches choking flow, the RPM increases rapidly, as does the PR, but the mass flow doesn’t follow proportionately. A more simplified analogy: if you blow through a thin straw, at a certain point you can blow as hard as you can, but you won’t get appreciable more mass flow of air.

Next you note a bunch of teardrop-shaped loops and truncated curves going in the direction up and to the right with labels 76%, 75%, 70% and 65%. These are your efficiencies; the closed loops are what are colloquially called “efficiency islands.” The apparently open curves are actually just truncated loops outside of the normal operating range of the turbocharger.
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Last edited by TDIMeister; April 5th, 2008 at 06:24.
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