As automakers battle to reduce emissions, we're seeing more cars launched with one, two and occasionally three turbochargers. Until now, no-one besides Bugatti has been crazy enough to strap four turbos to the one engine, but BMW's new 750d will change that with its quad-turbocharged 3.0-liter inline six.
By adding another turbocharger to its M Performance diesel, BMW has managed to squeeze five percent more power from the same 3.0-liter displacement. The fact there's 294 kW (400 hp) available at 4,400 rpm is impressive, but it's nothing compared to the crazy torque being produced.
With a peak torque figure of 760 Nm (560 lb.ft), the 750d produces more twisting force than out-and-out performance cars like the V8 BMW M5 and twin-turbo Audi RS6. What's more, BMW claims there's 450 Nm (332 lb.ft) on tap essentially from idle, which should make for utterly seamless performance regardless of what gear you're in.
In spite of this remarkable performance, BMW's engineers have actually managed to shave eleven percent from the old car's combined fuel economy figure. With the right tire and wheel combination specced, the 7 Series will return 5.7 l/100 km (49.6 UK mpg), which corresponds to CO2 emissions of just 149 g/km.
So how has BMW managed to extract such impressive figures from such a big car? As you might have gathered from the headline, the quad-turbo setup plays a big part. Two compact variable-geometry turbos built into the one housing handle high-pressure demands by spooling up quickly and providing an instant boost.
In BMW's outgoing tri-turbo motor, these little turbines worked in tandem with one big low-pressure turbo, but that has been replaced by two smaller units for faster response.
Generally, only the two low-pressure turbochargers and one of the high-pressure turbines will be in action at any one time. Both high-pressure turbos are only called on if you floor the throttle from standstill, at which point the two bigger low-pressure turbochargers are bypassed to let the boost build quickly. It's thanks to this system the car will hit 100 km/h (62 mph) in 4.6 seconds from a standing start.
All of this boosting and bypassing is managed by BMW's latest generation of Digital Diesel Electronics controller, which is tasked with managing the activity of each turbocharger, the position of the variable-geometry turbine vanes, the position of the flaps that allow each turbine to be bypassed or called into action, the exhaust gas butterfly valve, the wastegate and intercooler.
And it has to manage all of those things every single time the driver puts their foot down, adjusting the way the system operates based on how hard the throttle has been pressed. As impressive as it sounds, we shudder to think about how much damage could be done to the engine (and your wallet) if one small cog in that complex mechanical dance isn't working properly.
Turbos aside, pressure also has a role to play in making the engine more efficient. Maximum combustion pressure has been increased from 200 to 210 bar (3046 psi), and the cylinder head and crankcase have been manufactured using a high-pressure compression process. Meanwhile, the piezo fuel injectors are operating at more than 2,500 bar (36,259 psi).
The engineers behind the 750d's engine have also introduced also a new five-layer head gasket, low-friction coatings on the cylinder bores and pistons made from an alloy of aluminum and silicon.
The 750d xDrive will be available in regular or long wheelbase guise when it launches in June. Although there's already a V12 petrol in the range delivering monstrous torque figures, the discerning limousine driver will likely be very tempted by the combination of power and efficiency offered by the new diesel.
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