BMW X5: Exhaust Gas Turbocharging
The turbocharger is driven by the engine's exhaust gases, i.e. exhaust gases under pressure are routed by the turbocharger turbine and in this way delivers the motive force to the compressor, which rotates on the same shaft.
It is here that the induction air is precompressed in such a way that a higher air mass is admitted into the engine's combustion chamber. In this way, it is possible to inject and combust a greater quantity of fuel, which increases the engine's power output and torque.
The turbine and the compressor can rotate at speeds of up to 200,000 RPM. The exhaust inlet temperature can reach a maximum of 1050ºC. Because of these high temperatures, the N54 engine's turbochargers are not only connected with the engine-oil system but also integrated in the engine-coolant circuit.
It is possible in conjunction with the N54 engine's electric coolant pump even after the engine has been switched off to dissipate the residual heat from the turbochargers and thus prevent the lube oil in the bearing housing from overheating.
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Fig. 38: Identifying Exhaust Gas Turbocharging
- Compressor
- Cooling/Lubrication
- Turbine
Bi-turbocharging
Utmost importance is attached to turbocharger response in the N54 engine. A delayed response to the driver's command, i.e. the accelerator-pedal position, is not acceptable. The driver therefore must not experience any socalled "turbo lag".
This requirement is met in the N54 engine with two small turbochargers, which are connected in parallel.
Cylinders 1, 2 and 3 (bank 1) drive the first turbocharger (5) while cylinders 4, 5 and 6 (bank 2) drive the second (2).
The advantage of a small turbocharger lies in the fact that, as the turbocharger runs up to speed, the lower moment of inertia of the turbine causes fewer masses to be accelerated, and thus the compressor attains a higher boost pressure in a shorter amount of time.
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Fig. 39: Identifying Turbocharging Components
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Boost-Pressure Control
The boost pressure of the turbochargers is directly dependent on the flow of
exhaust gas which reaches the
turbocharger turbines. Both the velocity and the mass of the exhaust-gas flow
are directly
Load Control
Load control of the N54 engine is effected by means of the throttle valve and
the waste gate valves.
The throttle valve is the primary component in this process. The wastegate
valves are actuated t
Turbocharger Diagnosis
There are three "Golden Rules" to adhere to when diagnosing concerns on the
N54 engine:
1. When diagnosing cases of smoke from the exhaust, it is important to avoid
unnecessary replacement of
the tu
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Workshop Hints On Cylinder Leakage Testing
When performing cylinder leakage tests, the following tips might be helpful:
Remove all spark plugs to allow easier rotation of the engine. (If this test is
done after a compression test, the
plugs should already be out).
Perform the leakage test on all cylinders, not just the problem
cylinder.
Instructions For Compressor Replacement
IMPORTANT: Compressors with plastic belt pulleys:
Avoid impacts/knocks to plastic belt pulley (caused by tools,
contact with
base).
Return faulty compressors in their original packaging only.
IMPORTANT: When starting up a new compressor for the first time, it is
absolutely essential
to carry
