Sequential turbo surge line calculation?

[junglerc]

Hello,

I am having trouble visualizing the compressor surge line in a sequential turbo application, both with equal size primary and secondary compressors as well as different sized compressors.

With equal size compressors I could plot them as two in parallel, but something tells me this is not correct. Wouldn't the boost provided by the primary compressor provide additional drive torque and shift the surge line left? How far left? Or is the separation of flow within the compressor the limiting factor?

With different size compressors I have no idea... help?

 

[TDIMeister]

Every compressor manufacturer publishes their standard for air flow correction, and it takes the form

m_corr=m*(T1/T_ref)^0.5/(P1/P_ref).

T1 and P1 are the temperature and pressure at the compressor inlet, respectively. T_ref and P_ref are reference temperature and pressure standardized by the manufacturer. Garret has them as 298K and 1 bar (absolute), respectively. Other manufacturers may use different reference values, and some manufacturers display the X-axis of their maps in terms of a volume flow rate instead of mass flow (with a corresponding variation of the correction factor formula).

In a sequential setup, the second stage compressor inlet will see inlet air temperature and pressure above ambient, having being already compressed in the first stage. The correction above will correct your mass flows to plot the points on the map to the proper intersection of corrected mass flow and pressure ratio.

 

[izzmus]

You're describing a series setup, not sequential.

 

[TDIMeister]

Well, the OP should clarify that. In a true sequential setup, only one of the turbochargers (the small one) is operating first before the second one comes on stream. This is not how I interpret the OP's description. Contemporary systems may be either parallel or series. The setup employed in the BMW _23d and _35d has both turbochargers working in series and then at higher revs the small turbo is disabled with a bypass flap and only the large turbo works by itself. This too can be considered a sequential system.

 

[junglerc]

I am not asking about series compound compressors,I understand that application.

I am not clear on parallel sequential, especially with different sized compressors.

Thanks,

 

[Lou Scannon]

Well, for a given operating point, you have to determine what fraction of the engine's mass flow is going through each turbo. You also need to know the inlet conditions at each turbo, as TDIMeister pointed out. From there you simply plot the result on the respective compressor maps.