By January 28, 2012 Read More →

BMW’s new six-cylinder diesel engine with three turbochargers.

The first of the new BMW M Performance Automobiles are powered by a 3.0-litre six-cylinder in-line diesel engine. Fitted in the BMW M550d xDrive, BMW M550d xDrive Touring, BMW X5 M50d and BMW X6 M50d, the 2993cc engine generates maximum output of 280 kW/381 hp. No less imposing is its peak torque of 740 Newton metres (546 lb-ft), all of which the driver will find on tap as low down as 2000rpm. Instantaneous torque development maintained into the upper reaches of the rev range is the hallmark feature of the new unit, underpinned by an array of innovative and unique technical wizardry. For the first time in a BMW, the engine is powered by a trio of turbochargers. Their arrangement and interplay strategy are the key to the engine achieving its exceptionally high efficiency.

The coordinated interplay of turbochargers of different sizes endows an engine with instantaneous responsiveness at low revs and allows charge pressure to be increased according to need when the engine is placed under greater loads. BMW uses this concept in the six-cylinder in-line diesel engines with BMW TwinPower Turbo technology fitted in models including the BMW 535d, BMW X5 xDrive40d and BMW X6 xDrive40d. And now the company is set to become the world’s first carmaker to present a diesel engine which expands the principle of multi-stage turbocharging to incorporate a third turbocharger. The engine developed for the BMW M Performance Automobiles will, for the first time, see two comparatively small high-pressure chargers working with a larger low-pressure unit. The integration of an additional high-pressure turbo increases the engine’s capability when it comes to generating charge pressure, a key ingredient in taking the engine’s power output to the next level.

One of the two small turbos is activated at engine speeds just above idle. Its low moment of inertia allows it to respond without delay to the slightest movements of the accelerator and therefore supply the combustion chambers with compressed air at an early stage. As revs increase, the flow of exhaust gas also reaches the larger turbocharger, which announces its arrival with the engine spinning at just 1,500 rpm. Working together with the small charger, it ensures that the impressive peak torque of 740 Newton metres (546 lb-ft) is generated at this low engine speed and maintained up to 3000rpm.

To further increase the performance of the large turbocharger, a greater volume of exhaust gas is required at around 2700 rpm. If the driver calls up additional power, a vacuum-modulated exhaust flap instantly opens up another supply route, allowing extra exhaust gas to flow past the already active high-pressure charger to the large low-pressure turbo. The third turbocharger – integrated into this bypass line – also has a low moment of inertia and variable compressor geometry, which allow it to spring into action as soon as the exhaust flap opens. The result is additional charge pressure, generated by two sources at the same time. The large turbocharger is able to deliver its full output, while the second small turbo builds on the effect of its two active colleagues by supplying even more compressed air to the combustion chambers. This arrangement allows the turbocharging system to drive the engine with forceful and sustained thrust to its maximum output of 280 kW/381 hp, which it notches up between 4,000 and 4,400 rpm. The maximum engine speed of the new diesel powerplant is 5,400 rpm.

In order to ensure that charge pressure is developed as effectively as possible, not to mention efficiently, both the exhaust flow and supply of fresh air to the turbos and the channelling of compressed air into the combustion chambers is regulated with maximum precision. If the large turbocharger is spinning at particularly high speeds, a vacuum regulator opens a wastegate valve to relieve the pressure and so avoid unwanted exhaust backpressure. The supply of fresh air is also controlled according to need by means of pneumatically activated flaps. For example, at low revs a bypass flap ensures that the air is channelled directly to the high-pressure charger, which spins into action very early. At less than 2700rpm a change-over flap keeps the air away from the third turbo, which is not yet active, to prevent unnecessary fluctuations in pressure. Indirect charge air cooling enables the temperature of the air compressed by the three turbos to be reduced to the optimum level for increasing engine output. Both the main radiator positioned immediately in front of the combustion chambers and the intercooler behind the low-pressure charger are supplied by a low-temperature water circuit with separate electric pump.

The BMW M Performance Automobiles lace this driving pleasure with additional acoustic appeal. Through sound design technology, the natural engine sound is specifically enhanced to suit the driving situation at the time and sent into the interior via the audio system, ensuring an authentic reproduction of the new engine’s unmistakable straight-six soundtrack. The system’s digital signal processing technology exchanges data directly with the engine management, allowing it to base its precise transmission of the engine’s timbre on the revs at any one time, the amount of torque being produced and the car’s speed over the road. Accentuating the straight-six diesel’s turbine-like, sporty and sonorous tones adds to the emotional appeal of the driving experience, especially in the BMW M550d xDrive when SPORT or SPORT+ mode is activated using the Driving Experience Control switch. This gives the driver an impressively accurate impression of how hard the engine is currently working and of the power being unleashed, and provides an even clearer window into the car’s responses on the road. Controlling the audio system in this way also ensures that the engine sound can be enjoyed to equal effect from every seat.

 

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