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Mercedes two-stage turbocharging in new diesel units

The new diesel unit – see the news story here – draws the air it needs to breathe from not one but two turbochargers, marking the first ever instance of two-stage turbocharging in a series-manufactured passenger car diesel engine from Mercedes-Benz.

The aim of this concept is to eliminate the inherent drawbacks of a single-stage turbocharger. These include, for instance, the moment of inertia of a large turbocharger, which drivers may perceive as sluggish start-off characteristics (turbo lag). What’s more, it is virtually impossible to reconcile good start-off abilities and maximum power along with low fuel consumption even at full throttle when deploying just a single-stage turbocharger.

The compact-sized module for the new two-stage turbocharging concept consists of a small high-pressure (HP) plus a large low-pressure (LP) turbocharger. Both comprise a turbine and a turbine-driven compressor, and are connected with one
another in series:

• The HP turbine has a diameter of 38.5 mm and is positioned directly in the exhaust manifold. The flow of exhaust gases flows through this turbine first, causing it to rotate at speeds of up to 248,000 revolutions per minute.

• Integrated into the HP turbine housing is a bypass duct, which can be opened or closed by a charge-pressure control flap triggered by an actuator. If the duct is closed, the whole exhaust stream flows through the HP turbine, meaning that all of the energy contained in the exhaust gases can be directed towards propelling the HP turbine only. In this way, the optimum charge pressure can be built up at low rev speeds.

• As the engine speed increases, the charge-pressure control flap opens to prevent the HP charger from becoming overloaded. A portion of the exhaust stream now flows through the bypass duct to relieve the load on the high pressure stage.

• Downstream from the HP turbine, the two exhaust gas streams join up again, and any remaining exhaust energy drives the 50-millimetre LP turbine at a maximum speed of 185,000 revolutions per minute.

• To protect against overload, the LP turbine also features a bypass duct, which is opened and closed by means of an actuator-controlled flap known as the "wastegate".

• Once the engine reaches medium rev speeds, the HP turbine's charge pressure control flap is opened so wide that the HP turbine ceases to perform any appreciable work. This allows the full exhaust energy to be directed with low losses into the LP turbine, which then does all of the turbine work.

The two compressors are likewise connected in series and are in addition connected to a bypass duct. The combustion air from the air cleaner first flows through the LP compressor (diameter 56.1 mm) where it is compressed as a function of the LP turbine's operating energy input. This pre-compressed air now passes into the HP compressor (diameter 41 mm) that is coupled to the HP turbine, where it undergoes further compression – the result is a genuine two-stage turbocharging process.

Once the engine reaches a medium rev speed, the HP compressor can no longer handle the flow of air, meaning that the combustion air would heat up too much. To avoid this, the bypass duct opens to carry the combustion air past the HP
compressor and direct to the intercooler for cooling. In this case, the charge pressure control flap is completely open too, meaning that the HP turbine is no longer performing any work. This is the equivalent of single-stage turbocharging.

The benefits of this elaborate, needs-driven control of the combustion air feed with the aid of two turbochargers are improved cylinder charging (for high output), meaning abundant torque even from low rev speeds. Besides this, fuel consumption is lowered too.

The upshot of this as far as the driver is concerned is harmonious driving characteristics with zero turbo lag, good torque delivery over the entire rev band, tangibly superior performance, plus better communication between engine and accelerator.

For the full news story on the new diesels click here

For the detail about the fuel injection system click here