Charging, function

W09.00-1017-79

General

Charging on the engine takes place by means of two series-connected exhaust gas turbochargers (1, 2) of different sizes, which operate at different compression stages. The smaller turbocharger for the high pressure stage (2) already operates at low engine speeds. The larger turbocharger for the low pressure stage (1) comes more and more into use for increasing engine rpms. The combination of the differently sized exhaust gas turbochargers (1, 2) makes it possible for the ground current of the exhaust gas (E) to be used optimally over the entire rpm range, so that the optimum boost pressure can ultimately be reached.

With the help of the boost pressure regulator (Y636), the boost pressure can be regulated electronically. The electronic boost pressure control:

 allows setting of the optimal boost pressure over the whole load range

 Takes into account the influence of other systems, which engage in the exhaust gas and charge air current, such as exhaust gas recirculation, charge air cooling, engine brake, camshaft adjustment

 Takes into account numerous parameters, for example temperature and pressure of the intake air, fuel injection data, fuel grade, valve timing, etc.

 improves the transient response during charging

 Improves the engine torque development over the entire rpm range, especially at low engine speeds

 reduces the fuel consumption

 lowers the pollutant emission

 increases the engine power

The boost pressure regulator (Y636) is connected via its own CAN bus to the engine management (MCM) control unit (A4). The engine management (MCM) control unit (A4) evaluates all control-relevant data with the aid of characteristics maps and transmits appropriate messages to the boost pressure regulator (Y636), which actuates the boost pressure control flap (15) over a linkage. Dependent on the position of the boost pressure control flap (15) either the whole exhaust (E) is led to the turbine of the turbocharger of the high pressure stage (2) or part of it is diverted to the turbine for the turbocharger of the low pressure stage (1). The turbine of the turbocharger of the high pressure stage (2) is exclusively driven by direct fed in exhaust (E). The turbine of the turbocharger of the low pressure stage (1) is driven by the part of the exhaust (E) diverted over the boost pressure control flap (15) and by the remaining portion which leaves the turbocharger of the high pressure stage (2).

For low engine rpms the mass flow of the exhaust (E) is small. In this case only the smaller turbocharger of the high pressure stage (2) is directly driven by the mass flow of the exhaust (E). Thus an optimal boost pressure is already built up at low engine rpms. The mass flow of the exhaust (E) then increases with an increasing engine rpm. An ever greater part of the exhaust (E) is now directly led to the larger sized turbocharger of the low pressure stage (1). Its turbine already takes over virtually the whole turbine work at medium engine rpms.

Function

The two exhaust gas turbochargers (1, 2) each consist of a turbine and a compressor that are attached to a joint shaft (9, 13). The exhaust (E) flows over the turbine wheel (8) and causes this to rotate. This rotational movement is transferred to the compressor wheel (10) over the shaft (9). The exhaust (E) is subsequently driven over the turbine wheel (12) which drives the compressor impeller (14) over the shaft (13). The compressor impeller (14) of the low pressure stage compacts the intake air (B) cleaned by the air filter. The pre-compressed intake air is led into the compressor of the high pressure stage over a pipe (11) and is further compacted there by the compressor wheel (10). The compacted and heated charge air (C) is now present at the outlet of the compressor of the high pressure stage. The heated charge air (C) passes over a charge air pipe to the charge air cooler (4). In the charge air cooler (4) the compacted air is cooled whereby the air density of the fresh gas filling and thus the output of the engine is increased. The cooled charge air (D) subsequently passes over the charge-air distributor (5) into the individual cylinders.

Boost pressure control (over wastegate)

The engine management (MCM) control unit (A4) determines the current boost pressure with the aid of the charge air pressure and temperature sensor (B616). If the boost pressure is too high, it transmits an appropriate message to the boost pressure regulator (Y636), which opens the boost pressure control flap (15) over a linkage. The wider the boost pressure control flap (15) is opened more or less exhaust (E) is diverted over a bypass on the turbine wheel (8) tom the turbine of the turbocharger of the low pressure stage (1). Since only part of the exhaust gas (D) hits the turbine wheel (8), the acceleration is not so strong anymore and the boost pressure drops.

Turbocharger protection function

The engine management (MCM) control unit (A4) detects the temperature of the heated charge air (C) at the outlet of the compressor of the high pressure stage over the charge air temperature sensor in the compressor housing (B614). In combination with the current rotor speed and the load condition of the engine, the engine management (MCM) control unit (A4) recognizes whether the boost pressure regulator (Y636) is actuated and whether the injection must be adapted in order to protect the turbocharger of the high pressure stage (2) against an excessively high thermal load.