Study on spark assisted compression ignition (SACI) combustion with positive valve overlap at medium-high load

The spark-assisted compression ignition (SACI) is widely used to expand the high load limit of homogeneous charge compression ignition (HCCI), as it can reduce the high heat release rate effectively while partially maintain the advantage of high thermal efficiency and low NOx emission simultaneously. This research is focused on the SACI combustion using positive valve overlap (PVO) strategy to optimize the gasoline engine performance at medium-high load. The influences of PVO configurations on gas exchange and combustion process are investigated and analyzed, based on the classification through exhaust gas re-breathing way. It is shown that the PVO formed by late exhaust valve closing (LEVC) is beneficial to achieve higher iEGR and higher inhomogeneous in-cylinder distribution than that formed by early intake valve opening (EIVO). Compared with EIVO strategy, the SACI combustion employing LEVC strategy is apt to achieve a relatively high in-cylinder thermal state but with deteriorated pumping loss. The results also reveal that the load of SACI combustion is mainly controlled by PVO and eEGR, and affected by the combustion timing, under stoichiometric operating condition. Within the acceptable combustion stability range, small PVO combined with high eEGR is recommended for better fuel economy at the same engine load. The rules of adjusting SACI combustion with PVO are also concluded, according to the analysis of ideal demand and actual management of iEGR and eEGR. Based on that, the fuel efficient strategy for SACI combustion at medium-high load is identified. Using the optimized PVO strategy, stable SACI combustion is achieved in a wider load range, with significant improvements on fuel economy, pumping loss and NOx emission. (C) 2012 Elsevier Ltd. All rights reserved.