Optimization of a turbocharger and supercharger compound boosting system for a Miller cycle engine

Abstract

This paper describes the design optimization of a compound boosting system consisting of a turbocharger and a supercharger for a 2.0 l four-cylinder Miller cycle engine which has a high expansion ratio of 12.0:1 and variable valve actuation. Various system configurations and supercharger sizes were evaluated numerically and experimentally to reduce the supercharger power consumption and the engine fuel consumption while maintaining the same engine torque performance in steady-state conditions. The supercharger–turbocharger boosting system with a V400 supercharger showed an average engine fuel consumption that was 2.8% lower in boosted conditions than did the turbocharger–supercharger boosting system with the same V400 supercharger; this was predicted by engine cycle simulations and verified by experiments. When the supercharger was placed upstream of the turbocharger, the supercharger inlet pressure was lower and the total mass flow rate through the supercharger was reduced, which reduced the supercharger power consumption and the bypass air flow. The turbocharger–supercharger boosting system with a smaller supercharger (R340 or V250) significantly improved the engine efficiency (by 3.3% or 5.0% respectively in comparison with the turbocharger–supercharger boosting system with a V400 supercharger), by reducing the mass air flow rates through the supercharger and minimizing the supercharger power consumption. The turbocharger–supercharger boosting system with a V250 supercharger achieved the lowest engine fuel consumption in full-load conditions of all the turbocharger and supercharger compound boosting system options evaluated for the 2.0 l Miller cycle engine on the basis of the simulation results. This study defined the optimal system layout and the optimal supercharger size for implementing the turbocharger and supercharger compound boosting system on a 2.0 l Miller cycle spark ignition engine to maximize the improvement in the fuel economy of the vehicle while maintaining the same torque performance.