Turbocharger: Description and Operation
11 05 04 (098)
2-Stage Turbocharging
E60, E61, E83, E90, E91, E92 / M57TU TOP, M57TU2 TOP
Introduction
The M57TU TOP diesel engine is redesigned.
The M57TU2 TOP with 2-stage turbocharging is used as the most powerful diesel engine.
1 small and 1 large turbocharger are used for 2-stage turbocharging.
The M57TU2 TOP can be recognised by the changed arrangement of the turbochargers.
The increase in power output is 10 kW/14 hp.
In addition, the M57TU2 TOP now has injectors with piezo-elements.
The M57TU2 TOP is distinguished by its extraordinary power output characteristics.
- High torque at low engine speeds: Good response with impressive pulling-away dynamics
- Usable rpm range extended upwards: High output yield at low engine speeds
The power output is 210 kW/286 hp.
Peak torque of 580 Nm is achieved at 1750 rpm.
In 1st, 2nd and reverse gears, torque is limited to 500 Nm.
The 2-stage turbocharging is new for a series-production vehicle. It is familiar from high-power engines in boats.
The maximum boost pressure for the entire system is 2.95 bar. The maximum turbine speed is 205,000 rpm for the small turbocharger and 150,000 rpm for the large turbocharger (physical limits). The actual turbine speeds are lower.
A further advantage of the 2-stage turbocharging is the use of robust turbochargers.
The M57TU2 TOP complies with the EURO 4 exhaust emissions standard. The engine is equipped with a diesel particulate filter for this purpose.
Brief description of components
The most important components for the 2-stage turbocharging are:
- 1 small turbocharger
The size of the turbocharger used has a decisive influence on the operation of the engine (power and response characteristics).
Because of its small mass, which are accelerated by the exhaust gas, a small turbocharger is extremely responsive. Boost pressure is built up very quickly when the throttle is opened. This means that the engine responds with much less delay.
- 1 large turbocharger
A large turbocharger has different characteristics.
When the throttle is opened, the exhaust gas has to accelerate a large mass.
The turbocharger, and thus also the engine, therefore react more sluggishly.
However, if more final output is needed, i.e. high boost pressure and air flow rate, the larger cross-sectional area of the turbocharger makes it more efficient.
For the 2-stage turbocharging, the two turbochargers are arranged in series. The exhaust gas first drives the small turbine, then the large one. On the fresh-air side, the large compressor compresses the fresh air first, then the small compressor.
- Turbine control valve
The turbine control valve controls the distribution of the exhaust flow between the small and large turbines. The turbine control valve is pneumatically adjusted by a diaphragm can. The turbine control valve is variable in its range of adjustment. An electropneumatic pressure converter charges the diaphragm can with a partial vacuum.
- Compressor bypass plate
The compressor bypass plate allows the small compressor to be bypassed on the air side. The compressor bypass plate is pneumatically adjusted by a diaphragm can. The compressor bypass plate is either fully open or fully closed. An electric valve charges the diaphragm can with a partial vacuum.
- Wastegate valve
Once nominal rating has been achieved, the wastegate valve opens to prevent excessively high boost pressure. Part of the exhaust gas is fed through the wastegate valve past the large turbine. The wastegate valve is pneumatically adjusted by a diaphragm can. The wastegate valve is variable in its range of settings. An electropneumatic pressure converter charges the diaphragm can with a partial vacuum.
- DDE control unit
The DDE has been further developed for activation.
The DDE control unit also activates the above actuators for 2-stage charging.
The M57TU2 TOP has like the M57TU TOP no electrical air flap control.
In addition, the M57TU2 TOP has injectors with piezo-elements.
System function
When designing a turbocharger, a compromise must always be found between good response characteristics and a final output.
The turbochargers used by BMW have variable turbine geometry to reduce this drawback of single-stage turbocharging, although they are unable to eliminate it completely.
In 2-stage turbocharging, the two turbochargers work hand-in-hand.
Operation at low load and low speed:
The lines bypassing the small compressor and the small turbine are sealed. The exhaust gas first drives the small turbine, then the large one.
On the fresh-air side, the large compressor compresses the fresh air first, then the small compressor.
The small compressor does the main part of the compression work.
This ensures very good response characteristics. This situation is supported by the fact that the large compressor precompresses the fresh air.
Operation at medium load and medium speed:
As speed increases, the large turbocharger becomes gradually more important. The engine reaches its peak torque of 580 Nm from 1750 rpm.
The turbine control valve splits the exhaust flow between the two turbines. The variable setting of the turbine control valve allows the different operating ranges to be realised. At the same time, the boost pressure is regulated.
Operation at high load and high speed:
The small turbine and small compressor are bypassed when the compressor bypass plate and turbine control valve are fully open.
This allows the large turbocharger to deliver large amounts of air without the small turbocharger restricting the airflow.
The compression work is then done entirely by the large compressor. The engine reaches its maximum power output of 210 kW/286 hp at 4400 rpm.
Notes for service staff
Service staff should note the following points:
- General note:
- Diagnosis: ---
- Encoding/programming: ---
Subject to change.