The BMW M550d xDrive Sedan, the most fuel-efficient of the new series, accelerates from 0 – 100 km/h (62 mph) in 4.7 seconds and has a top speed of 250 km/h (155 mph). Average fuel consumption is 6.3 liters/100 kilometers (37.3 mpg US), with CO₂ emissions of 165 g/km. The vehicle meets Euro 6 exhaust standards.

Output and torque diagram for the new diesel. Click to enlarge.

M Performance TwinPower Turbo diesel with tri-turbo. The market launch of the first BMW M Performance Automobiles also marks the premiere of the most powerful diesel engine ever fitted in one of the brand’s models. With maximum output of 280 kW/381 hp, the 2,993 cc engine has a specific output of 93.6 kW/127.3 hp per liter of displacement. The maximum torque of 740 N·m (546 lb-ft) is on tap as low down as 2,000 rpm.

Playing a central role in this increase in power is M Performance TwinPower Turbo technology. 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.

The M Performance TwinPower Turbo technology—including the requisite charge air cooling—is integrated into a small space in the main unit. Its compact construction puts the engine in a position to meet future pedestrian protection stipulations, while the arrangement of the three turbochargers is also part of an intelligent system.

Both the exhaust inflow to drive the turbos and the supply of fresh air, plus the channelling of compressed air to the combustion chambers, have been designed to ensure that the three compression units work as a team as effectively as possible at all engine speeds. Efficiency is further optimized by the variable turbine geometry of the two high-pressure chargers, which allows them to react even more precisely to the driver’s power needs.

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 slight 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 peak torque of 740 Newton metres (546 lb-ft) is generated at this low engine speed and maintained up to 3,000 rpm.

To further increase the performance of the large turbocharger, a greater volume of exhaust gas is required at around 2,700 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 sustained thrust to its maximum output, which it achieves 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, 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 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 2,700 rpm 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.

Increased combustion pressure. Maximum combustion pressure in the engine has risen from the 185 bar of the most powerful diesel engine in the existing BMW line-up to 200 bar. As part of this development, the crankcase in the new 3.0-liter diesel engine features a novel tie rod concept for the assembly of the main bearing caps and cylinder head. The sintered main bearing caps are given extra strength by a central screw. Like the crankcase, the cylinder head is also subjected to a special high-pressure compression process.

This “HIPen” manufacturing concept sees the aluminium castings heated to solution annealing temperature and the casting pores created during manufacturing welded under high pressure. This process gives the finished component additional strength. A double diagonal bore ensures the interbore bridges have high thermal stability.

The geometry of the crankshaft and connecting rods has been further optimized and they are now made from higher-strength materials. Added to which, hub bushings and bowl rim remelting enhance the effect of the increase in piston compression height.

Fuel injection system. Higher pressure also raises the efficiency of the injection system. The injection system of the new six-cylinder in-line diesel engine has also benefited from further development. The upgraded system raises the injection pressure of the piezo injectors to 2,200 bar. During each power stroke, three pre-injections, one main injection and four post-injections of fuel take place.

An ultra-high-performance pump channels the fuel to the combustion chambers through a common rail made from forged stainless steel.

The output and capacity of the cooling system have been given another boost, too. An additional low-temperature circuit supplied by an electric water pump controls the temperature of the intercoolers. The exhaust treatment system includes a diesel particulate filter and oxidation catalytic converter, which is located close to the engine in the same casing. More efficient exhaust cooling, meanwhile, minimises the formation of nitrogen oxides. Standard-fitted BMW BluePerformance technology, which includes a NO_x storage catalytic converter, helps the new diesel engine powering the BMW M550d xDrive to meet the EU6 exhaust standard not due to come into force until 2014.

Eight-speed automatic. The configuration of the transmission management system for the BMW M Performance Automobiles promotes dynamic acceleration. The M-specification gearshift dynamics enable extremely rapid gear changes with an almost uninterrupted flow of power. The eight-speed Sports automatic transmission offers the driver two automated shift programs—D and S modes—as well as the option of changing gear manually (in M mode). The automatic gearbox is operated using an electronic gearshift lever on the centre console adorned with an M logo. Manual mode allows the driver to change gears sequentially using either the gearshift lever or the paddles on the steering wheel.

In keeping with M fashion, the right-hand paddle changes up a gear and the left-hand paddle is used for downshifts. If the driver activates manual mode using the gearshift lever, the transmission holds the gear selected until the engine’s revs hit the limiter. By not shifting up automatically in this mode, the gearbox gives the driver maximum control over the car when pushing the dynamic boundaries. The driver can also switch instantaneously from automatic gear changes to M mode with a nudge of one of the gearshift paddles; if M mode is selected in this way, the gearbox’s automatic shift-up function remains active. The transmission also restores automatic mode if the gearshift paddles are not used again following an upshift or downshift.