F2008-06-186
Sequential Two-Stage Turbocharging Specifications for an Automotive HCCI Diesel Engine
The paper describes the development of a new boosting system for a HCCI diesel combustion engine. The objective of the work is to settle down the turbocharging specifications in an automotive Diesel engine operating with HCCI combustion at partial load. Due to both high EGR requirements and reasonable performance in these conditions, high intake manifold pressure is needed and, therefore, the use of a two-stage turbocharging system is mandatory. 1D models are a suitable tool to provide the specifications compatible with air and EGR requirements. The study evaluates, by performing convenient computer simulations, the engine behavior regarding: exhaust available energy, high or low pressure EGR system, turbines characteristics (VGT or with waste-gate), compressors matching and secondary exhaust valve lift potential. Main technical conclusions of the study are detailed. The high pressure turbine should be a variable geometry turbine due to the high EGR rate requirements at partial engine load that lead to a rather small turbine, incompatible with full load operation. The low pressure turbine could be a fixed geometry turbine. Related to the compressors specifications, special attention should be paid to the low pressure compressor map since the high air mass flow requirement may lead to choose a large compressor, inappropriate for transient operation. Secondary exhaust valve lift does not increase the EGR+IGR rate. Instead it balances internal and external EGR and changes the in-cylinder temperature, which is interesting for HCCI control purposes. Finally, a wave action model is used to provide specifications of a two-stage turbocharging system in an automotive HCCI Diesel engine, including EGR loop architecture and secondary exhaust valve lift strategy. The methodology is appropriate at early stages of engine development to save time and costs and when experimental information is not available.
Session: Charging Systems