Surging in a turbocharge happens when a reversal of gas flow through the turbo charger compressor happens. When an engine cannot use up all the air delivered by the turbo compressor, the pressure inside the scavange reciever increases rapidly. This air will be blown back through the turbo compressor through the diffuser ring. It can cause imbalance of the rotor shaft and vibrations in the system which is not at all good for the plant.
Turbocharger characteristic curve is a plot between pressure ratio and air flow through the turbo compressor. This curve is defined for each RPM of the turbocharger and shows how the turbocharger will behave under such RPMs. Turbocharger will have different efficencies under different pressure ratios for same RPM.
As the efficency of the turbocharger increases for the same RPM, the turbocharger reaches more and more near to the surge point. Surge point is a praticular pressure ratio at which a turbocharger will surge under a particular RPM.lets say a T/C starts surging at a pressure ratio of 2.0 for an RPM of 15000. The surge point for 15000 RPM can be said as 2.0 bar under this condition. Above 2.0 pressure ratios, the T/C mass is defintiley going to surge, be it 2.1 or 200 for 15000 RPM.
If u draw a line connecting all the pressure ratios or surge points for all the T/C RPMs possible, u get what u call the SURGE LINE .See the image below for more clarity.
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