Turbocharging, function

Charging serves to make an appropriate boost pressure available to the engine in every operating phase and thus to increase engine performance and engine torque. For exhaust-gas turbocharging the flow energy of the exhaust gases are used to drive the turbocharger. The turbochargers with variable turbine geometry draw in fresh air at the respective supercharger inlet and feed it supercharged via the respective supercharger outlet, the charger air pipe, the charge air cooler and the charger air distribution pipe, to the individual engine cylinders.

 Turbocharger charging is controlled via the boost pressure control.

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The engine has two turbochargers, which drive the respective compressor turbine wheel, through which the exhaust gas flows. The turbines ate fitted with an adjustable turbine geometry. It is therefore possible to adjust the flow cross-section of the guide vane in relation to the momentary engine operating point. This takes place via modification of the flow ratios using the axially rotatable, mounted guide vanes between the spiral housing and the turbine wheel. At low engine speeds, the flow cross-section is reduced by closing the guide vanes on the emission side. The speed of the turbine wheel is thereby increased on the fresh air side and the boost pressure is increased. At higher engine speeds, the increased opening of the guide vanes enlarges the flow cross-section.

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Modification of the flow cross-sections from "slightly open" to "completely open" is caused by changing the angle of incidence of all guide vanes on the adjusting ring. This creates the conditions for a variable "nozzle effect", or the variable nozzle turbine (VNT).

The electronic engine control determines the respectively optimal opening angle of the guide vanes. A performance map is stored in the electronic engine control, in which an optimal boost pressure is assigned to each engine operating point. Adjustment of the guide vanes is performed by the left boost pressure positioner (Y100) and the right boost pressure positioner (Y100/1). These electrical actuators offer a higher actuator dynamic and via position feedback, the option to equalize the delivery rate of both turbochargers in conjunction with the left exhaust back pressure sensor (B60/1) and the right exhaust back pressure sensor (B60/2).

The entire exhaust power is always utilized due to the dynamic adjustment. The turbochargers with variable turbine geometry provide:

 a higher boost pressure even in the lower engine speed range

 a reduced exhaust back pressure in the top engine speed range

 An increased torque as a result of better cylinder charge

 An improved dynamic boost pressure buildup

In comparison with its predecessor, the vane geometries of the compressor turbine wheel and turbine wheel have been developed further. This achieves improved efficient and a higher, reliable charger rpm. Thus a higher engine performance with a reduced, specific fuel consumption is possible. One other new innovation is the 3rd generation VNT mechanism. A special feature here is the thermal decoupling of the guide vane from the turbine housing, which guarantees functional safety under high thermal loading.