BorgWarner is supplying the three-stage turbocharging technology for BMW’s M Performance diesel engine. (Earlier post.) Exclusively developed for BMW M Performance automobiles, the engine powers the M550d xDrive Sedan and Touring, X5 M50d Touring, and X6 M50d models.
The diesel delivers maximum output of 280 kW (381 hp) and a maximum torque of 740 N·m (546 lb-ft). Compared with the 3.0-liter six-cylinder diesel engine in the BMW 740d boosted by a BorgWarner regulated two-stage (R2S) turbocharging system, the BMW M550d xDrive Sedan with BorgWarner’s three-stage turbocharging technology increases power output nearly 25% and improves fuel economy 8% while meeting Euro 6 emissions standards.
The three-stage turbocharged diesel engine breaks new ground with its specific output of 93.6 kW per liter of displacement compared with 75.2 kW per liter in the two-stage version.
BorgWarner’s tri-turbo system consists of two small BV45 high-pressure variable turbine geometry turbochargers integrated with one larger B2 low-pressure water-cooled turbocharger. By integrating an additional high-pressure turbocharger, the engine’s capability to generate boost pressure reaches a new level compared with two-stage turbocharging systems.
The three turbochargers are activated successively at different engine speeds. The first high-pressure turbocharger starts at engine speeds just above idle. With increasing revs, the larger low-pressure turbocharger comes into play. To further increase performance, the second high-pressure stage turbocharger comes on line as well with all three turbochargers working in harmony. For effective and efficient boost pressure, the exhaust flow and the supply of fresh air is regulated with maximum precision.
At particularly high speeds of the low-pressure turbocharger, the wastegate valve is opened by a vacuum actuator to avoid backpressure, while pneumatically activated flaps precisely control the supply of fresh air. Indirect charge air cooling optimizes the compressed air temperature in all three turbochargers to increase engine output.