Abstract
Keywords - turbocharging, two-stage, HCCI, Diesel engine, modelling
The paper describes the use of a wave action model to obtain turbocharging specifications in an automotive Diesel engine operating with HCCI combustion. In order to enlarge EGR available zone, and benefit from HCCI operation at partial loads, high intake manifold pressure is needed and, therefore, the use of a two-stage turbocharging system is mandatory. The study evaluates, by performing convenient 1D computer simulations, the engine behaviour regarding: exhaust available energy, turbines characteristics (VGT or with waste-gate), compressors matching and secondary exhaust valve lift potential concerning incylinder residual gas fraction. Basically, although the process is solved by trial and error, maximum power operating point is used to select the low pressure turbocharger and EGR demands at partial load conditions define the high pressure turbocharger.
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 leads to choose a large compressor. Concerning VVA strategy, secondary exhaust valve lift does not increase the in-cylinder residual gas fraction. Instead it balances internal and external EGR and changes the in-cylinder temperature, which is interesting for HCCI control purposes.
The described methodology is appropriate at early stages of engine development to save time and costs and when experimental information is not available.