Engine downsizing and down-speeding are essential to meet future US fuel economy mandates. While turbocharging has been a critical enabler for downsizing, transient boost response performance remains a concern even with variable geometry turbochargers. This slow build-up of boost and hence torque is commonly referred to as turbo-lag. Mitigation of turbo-lag has, therefore, remained an important objective of turbocharger performance enhancement research. A regenerative, hydraulically assisted turbocharger is one such enhanced turbocharging system that is able regulate the turbocharger speed independent of the available engine exhaust energy. With external power available on the turbocharger shaft, the engine performance and emissions can be managed during both transient and steady-state operations. The key to fully utilizing the ability of such an assisted turbocharger depends on the energy recovered from turbocharger shaft and/or vehicle driveline. Energy available from the turbocharger shaft is dependent on the engine exhaust gas energy. Energy recovered from the driveline depends on vehicle braking energy. A previously developed high-fidelity 1-D simulation of a diesel engine with a regenerative-hydraulically assisted turbocharger is used to investigate the energy availability for a medium duty diesel engine over standard driving cycles. The study shows that the energy recovery from turbocharger shaft is limited and driveline energy recovery is necessary for achieving fuel economy benefits on the order of 4%.

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