They are known as the Lemda controller that provides additional force to overcome initial inertia of the turbocharger rotor. Have control airline and a valve which opens at low rpm for assistance. Once the turbocharger reaches a minimum speed the control air cuts off.

Thank You For Your comment, While i draw the diffuser in the diagram it is not marked or indicated. It was a small mistake at the time; a diffuser in turbocharger is located or produced in between the back plate of the compressor and the volute casing. While the majority of kinetic energy converted to pressure energy in the volute casing; small part of it get transformed in the diffuser itself. I hope i answers your query and will try to update the above image as soon as possible.

Both pulse and constant pressure turbocharger are two ways super charging is done in marine diesel engines. Now the aim for supercharging is to improve the power to weight ratio of the engine; thus improving its overall efficiency. Now for a six cylinder inline diesel engine you will have power stroke at different times for all 6 cylinders. IN pulse type supercharging the exhaust of individual units are directly sent to turbine blades via nozzles. This allow the turbine to be rotated at required rpm even from cold start; since the kinetic energy of such exhaust came in form of pulse; it is called pulse turbocharging. While this arrangement is cheap, utilize exhaust energy fully and can be used even at low speed without auxiliary blower; it increase the chances of blow-back, reduce life of turbine blades and require appropriate turbine position and engine synchronization. On another hand in constant pressure turbocharger exhaust gas end up in a common manifold before passing on to the turbocharger. This results in better overall efficiency, allows expansion in number of cylinders, simple design, increase the life of turbine blades. The only drawback is that it needs auxiliary blower as it naturally don’t provide enough exhaust at start to run turbine at required rpm.