Dual-Stage Turbocharger Matching and Boost Control Options

Diesel engines are gaining in popularity, penetrating even the luxury and sports vehicle segments that have traditionally been strongly favored gasoline engines as the performance and refinement of diesel engines have improved significantly in recent years. The introduction of sophisticated technologies such as common rail injection (CRI), advanced boosting systems such as variable geometry and multi-stage turbocharging, and exhaust gas after-treatment systems have renewed the interest in Diesel engines. Among the technical advancements of diesel engines, the multi-stage turbocharging is the key to achieve such high power density that is suitable for the luxury and sports vehicle applications. Single-stage turbocharging is limited to roughly 2.5 bar of boost pressure. In order to raise the boost pressure up to levels of 4 bar or so, another turbocharger must be connected in series further multiplying the pressure ratio. The dual-stage turbocharging, however, adds system complexity, and the matching of two turbochargers becomes very costly if it is to be done experimentally. This study presents a simulation-based methodology for dual-stage turbocharger matching through an iterative procedure predicting optimal configurations of compressors and turbines. A physics-based zero-dimensional Diesel engine system simulation with a dual-stage turbocharger is implemented in SIMULINK environment, allowing easy evaluation of different configurations and subsequent analysis of engine system performance. The simulation program is augmented with a turbocharger matching program and a turbomachinery scaling routine. The configurations considered in the study include a dual-stage turbocharging system with a bypass valve added to the high pressure turbine, and a system with a wastegate valve added to a low-pressure turbine. The systematic simulation study allows detailed analysis of the impact of each of the configurations on matching, boost characteristics and transient response. The configuration with the bypass valve across high pressure turbine showed better results in terms of both steady state engine torque and transient behavior.