Large-Eddy Simulations of Motored Flow and Combustion in a Stratified-Charge Direct-Injection Spark-Ignition Engine

Download or read book Large-Eddy Simulations of Motored Flow and Combustion in a Stratified-Charge Direct-Injection Spark-Ignition Engine written by Samuel Kazmouz and published by . This book was released on 2020 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: Stratified-charge, spray-guided, spark-ignition, direct-injection operation offers efficiency improvements to conventional engines used in light-duty vehicles. However, cycle-to-cycle variability (CCV) impedes extracting the full efficiency potential of such advanced engine operation modes. In this dissertation, multi-cycle motored and fired large-eddy simulation (LES) results of an optically-accessible single-cylinder four-valve direct-injection spark- ignition engine, called G4VDI, are presented and compared to experimental results. The main objective is to investigate the root causes of CCV in stratified-charge engines. For motored operation, four sets of 60 consecutive LES cycles, with different operating conditions, are compared with experiments. LES is able to capture the wave dynamics of the ports and the in-cylinder pressure with a difference of 0.12%-2.5%, compared to experimental results. The LES velocity fields are compared with particle-image velocimetry measurements at six cutting planes. Based on the local and volume-averaged structure and magnitude indexes, it is found that LES is able to reproduce key flow events and capture large-scale in-cylinder flow structures, especially in high tumble/swirl conditions. Using proper orthogonal decomposition, LES shows that high tumble/swirl conditions produce low CCV flow fields. CCV of in-cylinder pressure ranged between 0.13% and 0.23%. For fired operation, and using the thickened flame model (TFM), 20 consecutive LES cycles of a homogeneous-charge engine operation mode are presented followed by spray-characterization in four different ambient conditions. These results lay the foundation for two stratified-charge engine operation modes, in which 20 and 35 consecutive LES cycles are compared with experiments, respectively. TFM-LES is extended for partially premixed flames and is able to reproduce experimental in-cylinder pressure (0.5%-10%), cyclic variability (20.5%-22.7%) in global and local quantities, local fuel vapor distributions, and heat release curves for homogeneous and stratified burn. Tuning TFM to reduce the burn rate increases the tendency to produce misfires, as well as the levels of CCV. Correlation analysis done on the stratified-charge LES results suggests that the influence of the early burn on the subsequent flame development is more subtle for stratified combustion compared to homogeneous combustion, that is the local conditions at the spark plug when the flame starts propagating are more influential than the conditions at spark timing, and that the injection event creates velocity conditions which might be favorable or unfavorable for the combustion event. The main contributions of this dissertation are extending TFM to highly stratified spray combustion, showing that LES can reproduce experimentally measured flow and combustion behavior in a realistic engine, including CCV, and analyzing LES to provide new insight into CCV and misfires of stratified-charge engines.