Yes, generally a turbo stall is a compressor stall.

 

Effectively, the condition is that the compressor blades stall aerodynamically similar to an airplane wing at high angle of attack.

This occurs when the pressure differential across the compressor is too large and/or the airflow through the compressor is too slow (relative to the design of the compressor) – which essentially creates a high relative angle of attack between the fast moving angled compressor blades and the slow moving air.  Note the compressor blades are effectively airfoils (even in a circumferential compressor as in our tubro’s).

 

This can be caused by relatively high compressor back pressure and/or low availability of inlet air (often a combination of both).  For example: rapidly closing the throttle at high altitude, before the controller/waste gate has the opportunity to react and/or the momentum of the turbo to slow down. Upper deck pressure builds and flow through the compressor slows.  If the upper deck pressure is high enough the pop-off value will release, but the compressor can stall at an upper deck pressure below this if the flow is slow enough.  The sonic nozzles on a typical IVP setup tend to help alleviate this, by creating another outlet for the upper deck pressure, and by keeping some flow moving, which reduces the demands on the responsiveness required from the controller/waste/gate and overall turbo system.

 

Note, a “normal” turbo compressor is not designed to be able to maintain much of a pressure differential without air flow – the turbo compressor is an open blade system which is the opposite of a roots type blower (supercharger), which has sealed rotors.  The Turbo compressor isn’t effective at just “beating” the air, rather it is designed to “move” the air.  The closer the speed of the air is matched to the speed/angle of the compressor blades, the lower the relative angle of attack between them.  When the air slows down, the relative angle increases, until the flow separates and the compressor stalls.

 

When the compressor stalls, the relative high pressure upper deck (outlet) air momentarily flows backwards through the compressor and goes out of the inlet, (even tough the blades are still spinning fwd), which lowers the pressure differential across the compressor and creates an opportunity for the compressor to start pumping/moving air again (which it’s otherwise inclined to do whenever it’s spinning).  The stall (momentary/explosive backflow) is generally bad for the compressor (high stress on blades, shaft, etc), and the entire intake system.

 

Compressors can be designed to minimize/avoid this, but at the expense of overall performance/efficiency (no free lunch).  The common engineering design/specification document for a compressor is called a compressor map (ref http://en.wikipedia.org/wiki/Compressor_map - this link is oriented to axial gas turbines, but the core concepts are the same).  A compressor map diagram depicts the relationship between flow and pressure differential for a given compressor design.  Normal operation is conducted/charted below (and to the left of) the surge/stall line running diagonally from lower left to upper right.  Above the surge (stall) line is where the flow breaks down and compressor stalls (combination of too much pressure/too little flow for the given design).

 

A compressor stall can lead to an enriching of the mixture, due a sudden loss/reduction of upper deck pressure and flow.

 

The compressor stall itself can sound like backfiring.  The momentary/explosive backflow through the still spinning compressor can/will make a “bang”, like the sudden outflow from popping a party balloon.

 

Google on if you want to know more.

 

 

Brent,