Issue link: http://read.uberflip.com/i/588723
74 DIESELTECHMAG.COM DIESEL TECH NOVEMBER 2015 144826HypMax14h.indd 1 9/23/14 2:20 PM 151495HypMax14h.indd 1 2/17/15 1:21 PM The compressor map is generated in a controlled environment. One problem with using this information is that the compressor flowchart does not usually fit with the actual performance of the com- pressor when boosting an internal com- bustion engine for the following reasons. The geometry of the compressor inlet is rarely straight due to engine compartment configuration constraints; this is very important because it modifies the inducer aerodynamics. The outlet piping of the compres- sor also has a significant impact on the compressor performance. The outlet vol- ume of a test stand is much smaller than the total volume of the intercooler and intake manifold of an engine. Reciprocating internal combustion engines create pressure pulsations because of the sequential intake pro- cesses of the cylinders. The aforementioned reasons can create conditions that force the com- pressor to operate to the left of the "surge" line on the map, causing the compressor inducer to enter an area of flow instability, or compressor surge. Compressor surge is actually a system phenomenon caused by the summation of aerodynamic stall at the element (or blade) level of the stage. The stage, in the case of a turbocharger compressor, is the inducer part of the compressor wheel. The compressor inducer is the small diameter part of the compressor wheel (the area where the air enters the compressor). Bark Bark At the low end of the impact range, this instability may cause the turbo to mildly flutter. More extreme cases of flow instability will cause the turbo boost to fluctuate wildly and a "bark- ing" sound can be heard from the turbo itself. The flow instability will diminish once the turbo speed slows enough to move the operating point back into the stable region of the compressor map. The instability results from a situation where the air pressure after the com- pressor is higher than the compressor itself can maintain; the higher air pres- sure can result from the engine effects discussed earlier. It is possible for sev- eral elements (or blades) of a stage to stall without the entire stage stalling. When the entire stage does stall, the basic flow characteristics of the stage are no longer stable and the instability causes random fluctuations in flow and pressure. The higher air pressure causes flow along the blade to be so severely retarded that it can no longer follow the surface. TECH CORNER CORNER TECH CORNER TECH TECH TECH CORNER