In order to improve power density, the majority of diesel engines have intake manifold pressures above atmospheric conditions. This allows for the introduction of more fuel, which results in more power. Except for a few applications, these engines receive charged air from a turbocharger. The turbocharger develops boost by converting exhaust gas energy into power. This power is then used to compress the intake charge.The medium- and heavy-duty engine markets have both stringent regulatory targets and customer demand for improved fuel efficiency. Two approaches used to meet fuel efficiency targets are downspeeding and downsizing. Until now, the industry has adapted to the turbocharger lag experienced during a transient acceleration event. This performance deficiency is severely exaggerated when the displacement and speed of an engine are reduced. The solution proposed to improving fuel economy, while maintaining equivalent performance, is supercharging.Eaton's TVS® supercharger is a positive displacement pump with significantly improved efficiencies over prior models. Unlike a turbocharger, which relies on exhaust gas energy to create boost, a supercharger is directly coupled to the engine. Therefore, the supercharger speed increases proportionally with engine speed. This greatly reduces the lag that is present in a turbocharged engine. Addition of a supercharger, as this paper will show, dramatically improves low speed engine performance. This enables greater engine downspeeding that was previously not feasible with a turbocharged engine.This paper explores the fuel economy benefits of combining a supercharger with a turbocharger on a diesel engine. The fuel economy and performance improvements enabled by supercharging are dependent on application, drive cycle, and engine displacement. It also reviews the simulation results of three distinct engine sizes and applications exercised over vehicle appropriate drive cycles. The effect of boosting system configuration, downspeeding, exhaust gas recirculation (EGR) configuration, and supercharger clutching on fuel economy were examined. All of the simulations provided a fuel economy benefit over the baseline turbocharger-only engines. Fuel economy was improved, while maintaining or improving vehicle performance over the baseline. Efficiency improvements in excess of 14% were demonstrated over a transient drive cycle.