Brent,

Always appreciate your insight (please comment on text below if in error somewhere).

 

I do have Hugh MacInnes book and looked at your reference.  

Page 19 primarily refers to turbines not compressors (although both use A/R as a common specification dimension).

 

It was my understanding that, what is primarily changed between a .5 and .6 A/R compressor on our birds is the Radius (Area is essentially unchanged).

Ref Figure 2-15 in the book on page 19 (re-created below) - note the housing Radius changes, but the output cross sections area (and impellor) are essentially unchanged.

 

Area (A) is the cross-sectional area of the compressor output.  Since we all use the same silicon rubber connector sleeve and aluminum tubes that connect the compressor output to the intercoolers, we all have essentially the same “A” on our compressor outputs (with the exception of the slightly larger/longer compressor back plane throat on a higher “R” housing).  My understanding was a .5 A/R compressor housing was physically a bit larger because the radius of the scroll was increased (as shown in fig 2-15 and below as R2 in blue).

 

My understanding is that A/R of the compressor is not nearly as critical (sensitive) to performance as on the turbine.  As I understand, various A/R ratios on turbine housings are typically indicative of the Area changing (turbine inlet throat size).  Smaller A = more turbine power, but also more engine exhaust back pressure.  On the turbine, a smaller “R” (other things equal) is also a marginal impediment to mass flow (versus a larger R), but also allows the turbine to develop more power – due in-part to the angular velocity/momentum of the fluid entering the turbine (pin-wheel effect).  Thus, typically the turbine housing radius is as small as possible/practical (relative to the Area of the turbine inlet and diameter of the expeller).  Thus, turbine A/R ratio variation is typically determined by inlet Area geometry variations (R being held to the practical dimensional minimum for any configuration).

 

Likewise, it seems obvious that a compressor with a smaller A (outlet area) would generally flow a smaller volume and/or create/sustain a higher pressure ratio (other things equal and with other performance spec impacts) relative to a similar compressor with a larger A.  However, I’ve never fully understood “why” Radius and A/R ratio (with Area held constant as shown below and assumed in our installations) has much effect on compressor performance (apparently based on some other articles I’ve read, it doesn’t).  Most books/articles I’ve found (including Hugh MacInnes’s) don’t really address this.  I have others like Gordon Oates (Aerothermodynamics of Aircraft Engine Components), that are quite technically detailed – but primarily deal with axial flow (common) jet turbines.  Others like http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19930091893_1993091893.pdf deal with turbo/engine sizing assuming the performance of a set of available pre-designed compressors that are otherwise fixed.  Likewise, Charles Taylor (engine god http://www.amazon.com/Internal-Combustion-Engine-Theory-Practice/dp/0262700263/ref=sr_1_3?s=books&ie=UTF8&qid=1291739980&sr=1-3 ), does not do turbo design any real justice in his texts.  There are some other old NACA compressor design papers I have yet to read/research (now in process).

 

Perhaps the compressor R effect has to do with the rate of change in angular velocity/momentum of the fluid flow (from longitudinally, to circumferentially to laterally).  With a smaller “R”, the apparent rate of directional change of flow vector is higher, which perhaps/probably has subtle effects on the compressor map/performance.  If so, this would essentially be the appropriately opposite reason of why R is important on the turbine housing – the added effect of incoming angular momentum of the fluid flow imparted to the turbine blades (i.e. pinwheel effect), in addition to the classic expansion effect of the fluid through the turbine.  If so, I would perhaps expect a compressor with smaller R (larger A/R ratio) to generally have a slightly higher surge line (more pressure at lower flow rate, on the compressor map, i.e. higher pressure ratio – other things equal), due to the fact that the fluid flow vector is generally more aligned to the impellor movement (angle of attack) and would also have a harder time flowing 180 degrees backwards against/through the compressor impellor vanes (low R) vs just 90 degrees (higher R) (angles exaggerated for clarity).  This of course would likely/probably come at the expense of mass flow performance, no free lunch.

 

Also, I’m not aware if the Piper A/R=.5 compressor and std TCM-TSIO-550 A/R=.5 compressors use the same compressor impellors.  I understand the overall impellor dimensions are the same, but it seems the actual impellor vanes (often incorrectly referred to as the impellor Trim) could be different (in count and/or shape/profile).  It seems that would/could have a significant difference on the compressors performance characteristics.  More research needed???

 

 

 

 

 

Rick

 

 

 

Rick Writes:

<<<

For anyone who might not have caught/understood this…

I believe, the Mirage .50 A/R is a larger turbo/compressor housing (as expected).  A/R = Area/Radius of the output scroll.  Similar output connector area, larger scroll/housing radius. <snip>

Rick>>>>

Compressor housings for a given model of turbocharger are designed to be interchangeable and therefore have nearly the same R (Scroll Radius) so describing a turbocharger as "bigger" can be misleading. What is important to keep clear is that a higher A/R ratio (0.60) will cause the turbocharger to run slower and produce less boost (pressure ratio) than a lower A/R (0.50) which will cause the turbocharger to run faster and produce more boost. If you don't believe me then read page 19 of Hugh MacInnes' book "Turbochargers".

Regards
Brent Regan