Charging Pressure Control: Notes

The charging pressure (Boost) of the turbochargers is directly dependent on the exhaust flow that enters the turbines and it determines the speed of the turbocharger. Both the speed and the mass of the exhaust flow are directly dependent on the engine speed as well as the engine load. The Digital Motor Electronics controls the charging pressure through the wastegate valves. The wastegate valves are operated by vacuum units and are controlled by the DME through vacuum solenoids (EPDW).

The vacuum is generated using the permanently driven vacuum pump of the engine and stored in two vacuum reservoirs. It is ensured that these consumers do not have a negative influence on the function of the power brake booster by using a two stage vacuum pump.

The wastegate valves can influence how much of the exhaust flows through the turbine wheel. Once the charging pressure has reached the desired level, the flap of the wastegate valve starts to open and a portion of the exhaust flow is routed past the turbine wheel. The decreased exhaust flow through the turbine prevents the speed of the compressor from increasing further.

In full load operation, the N74 engine works with an excess boost pressure of up to 0.7 bar in the intake manifold.