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The power of three: BorgWarner builds world’s first three-stage turbo system for diesels

Turbocharging technology has come a long way in recent years, doing much to improve the power output of smaller engines and generally helping to keep fleet fuel economy numbers up. But now BorWarner has introduced a new turbocharging system which represents a major leap forward for the world of turbocharging. Making its first appearance in the BMW M550d xDrive Sedan and Touring, the X5 M50d Touring and the X6 M50d is a 3.0-liter inline-six using the world’s first three-stage turbo system for diesels.

The system uses two smaller high-pressure BV45 variable turbine geometry turbochargers and one large B2 low-pressure water-cooled turbocharger. The idea is for each turbocharger to spool up at a different engine speed, and therefore more effectively keep a high volume of air flowing into the engine at all times. Compressed air allows for a better burn from the fuel, effectively getting more power out of the same amount of fuel.

The problem which many early turbo systems suffered from was that a single turbo can provide compressed air across only so much of the power band. This meant that engines would either bog down at higher revs or (more usually) suffer from “turbo lag” when revs were low and the turbo wasn’t yet up to speed. Much of this has been improved in recent years as twin turbos operating in sequence made for a more progressive application of boost, as opposed to the on/off nature of older single-turbo systems. A three-stage system is an evolution of this same idea, all three turbos keeping air flowing at whatever the engine speed.

BorgWarner says the system provides almost 25 percent more power and eight percent better fuel economy when compared to the older two-stage system used in cars like the BMW 740d (125.5 horsepower per liter of displacement versus 100.5 horsepower per liter of displacement for the old system). The engine produces 381 horsepower and 546lb-ft of torque, more than enough to do justice to the M badges on the cars. Diesels haven’t been very popular with road-going performance cars in years past, but as companies like Audi and Peugeot continue to look to diesel for their motorsports applications, the idea of a diesel in a high-end performance machine makes more sense. Where this technology will go next isn’t specifically known yet, but if the benefits are as good as BorgWarner claim, we’re sure to see a lot more of it.