| Homogeneous charge compression ignition (HCCI) is a combustion strategy which can yield improved efficiency and NOx emissions over traditional combustion strategies. However, HCCI often produces higher hydrocarbon emissions, and control of load and combustion timing presents an engineering challenge. In residual-effected HCCI, recently burned exhaust gas is used to accomplish the required dilution and sensible energy rise for reliable compression ignition. In this work, residual-effected HCCI was studied using exhaust reinduction and exhaust retention, enabled by the use of fully-flexible variable valve actuation (VVA).; To determine the extent of feasibility of residual-effected HCCI by exhaust reinduction, HCCI was achieved at low compression ratio using a combination of late intake valve closing and late exhaust valve opening strategies. Load and timing control were demonstrated without the use of a throttle, thereby avoiding a pumping work penalty. HCCI showed a modest efficiency benefit and a significant reduction in NOx emissions but with increased hydrocarbon emissions relative to a comparable homogeneous charge spark ignition (HCSI) process on the same engine. An ideal cycle model indicated that the efficiency benefit of HCCI over HCSI, after accounting for reduced throttling losses, is due to decreased combustion duration and increased dilution. The model suggests that residual-effected HCCI efficiency will improve further with strategies that achieve the minimum possible initial temperature rise.; The flexibility of the valve actuation system allowed exploration of other, more innovative valve strategies. The results suggest the best way to achieve and control HCCI using the minimum degrees of freedom. A minimum of two degrees of freedom are required to independently control load and phasing, with the most promising control inputs as variable intake valve lift, variable intake valve closing (at elevated compression ratio), and air dilution. A zero-dimensional combustion model including comprehensive kinetics and heat transfer was used to determine the physical basis for residual-effected HCCI limits at low load, high load, and air dilution. The model results indicate that practical residual-effected operation has greater sensitivity to reactant concentrations due to the high level of overall dilution. Model analysis suggests explanations for the experimentally observed limits of residual-effected operation. |
