Reduction of fuel consumption without compromising performance or drivability is a central goal of engine development. Downsizing and downspeeding by means of turbocharging has already proven to be a viable solution to this challenge. Applying conventional, singlestage charging systems allow downsizing ratios of 30-40%. But, if more aggressive downsizing is pursued, reduced low-end torque and slower transient response can cause performance and drivability to suffer. This paper describes the application of two-stage turbocharging to extend the downsizing ratio potential to 60%. Successful execution of the concept involves intensive optimization of the high- and low-pressure stages via simulation of both stationary and transient operation. Special design features are proposed to ensure optimal functionality and durability and to minimize heat loss. The optimized concept obtains a specific power of 113 kW per liter, while still producing 26.4 bar brake mean effective pressure (BMEP) at 1500 rpm. Low-end torque to 30 bar BMEP is feasible but requires additional optimization of the combustion system and a move away from the typical flat torque curve with a steady build-up associated with turbocharged engines. Applied to a 2.0 L engine in a mid-size sport-utility vehicle (SUV), the two-stage turbocharging technology has attained a fuel consumption improvement of about 10% on the combined US FTP 75 / Highway drive cycle versus a baseline 5.3 L naturally aspirated engine.
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