UNSTEADY EFFECTS ON MIXING AND COMBUSTION PROCESSES IN A REALISTIC IC-ENGINE BY USING LARGE EDDY SIMULATION

Abstract

Large eddy simulation (LES) based analysis of the unsteady variations of the in-cylinder flow and their effects on mixing and combustion processes in direct injection sparkignition engine is presented in this paper. The configuration under study represents a four-stroke internal combustion engine with variable tumble-system. The KIVA-3V code extended to LES has been used to perform 40 consecutive engine cycles in order to characterize cycle-to-cycle fluctuations of the flow field quantities. The impact of parameters of fuel spray injection on mixture preparation and combustion processes is pointed out. For this purpose nonreacting as well as reacting two-phase flows have been analyzed. INTRODUCTION Fuel spray injection is one of the key processes in direct injection spark-ignition (DISI) engines, which strongly affects the possibility to reduce emissions and specific fuel consumption. Among other conditions, unsteady in-cylinder flow generally leads to cycle-to-cycle fluctuations (CCF) of mixing as well as combustion processes. The LES method has proved to be a reliable tool in order to provide detailed information about highly unsteady phenomena of in-cylinder flows. Recent reviews of LES applications in IC-engines can be found, for example, in Thobois et al. (2007) and Vermorel et al. (2009). A discussion about ignition and combustion models was provided by Tan and Reitz (2006). The application of the AVBP code to the investigation of CCF in spark ignited IC-engine dealing with a homogeneous mixture is reported in the following recent references. Laget et al. (2011) carried out LES simulation covered nine cycles of a four cylinder IC-engine in order to investigate cylinder to cylinder influence and variability. An attempt to perform reactive LES in a realistic four-valve IC-engine using coherent flame model was reported by Vermorel et al. (2009). Analysis of cycle-to-cycle combustion variations was based on nine consecutive engine cycles. Cyclic combustion variability in a single cylinder IC-engine was investigated by Enaux et al. (2011). Multi-cycle LES of flow and combustion in a whole engine set-up covered 25 consecutive engine cycles. The issues of boundary conditions for LES applications are given by Pera and Angelberger, 2011. Thereby a statistical analysis over 15 consecutive engine cycles was used to compare LES results with particle image velocimetry (PIV) measured data. A transient flow field and spray structures inside an optically accessible DISI IC-engine were investigated using PIV imaging by Muller et al. (2011). One hundred consecutive cycles were saved every 1 crank angle degree (CAD) allowing a detailed investigation of cyclic variations. An interaction of the in-cylinder charge motion and the fuel spray injection along with a correlation between the in-cylinder flow and the subsequent spray momentum of different injector types were pointed out. Application of PIV to the investigation of the charge motion generation and evolution of CCF for various intake port designs was reported by Adomeit et al. (2011) while Heim and Ghandhi (2011) analyzed the bulk fluid motion and small-scale turbulence in a two-valve IC-engine. Application of the KIVA-3V code extended to LES in order to characterize CCF of the in-cylinder flow and the charge motion in a realistic DISI IC-engine was reported by Goryntsev et al. (2007a). A parallelization strategy based on variation of initial conditions has been used to simulate 40 full engine cycles. The impact of the velocity CCF on mixing processes and a qualitative analysis of the intensity of CCF below the spark plug was provided by Goryntsev et al. (2007b; 2009). The present paper focuses on detailed investigation of fuel-air mixture preparation and combustion processes under various spray boundary conditions in a DISI IC-engine using LES. The unsteady effects of in-cylinder velocity as well as mixing on combustion processes are pointed out. Finally a qualitative analysis of the intensity of CCF of in-cylinder pressure is provided. Multi-cycle LES covers 40 consecutive engine cycles of two-phase flow for each considered case. INVESTIGATED CONFIGURATION AND NUMERICAL METHOD A four stroke DISI engine with variable charge motion system (VCM) shown in Figure 1 was used for LES analysis. The compression ratio of the engine is 10.5, engine speed is 2000 rpm. The main parameters of the IC-engine as described by Pischinger et al. (2007) are shown in Table 1. CAD values are given relative to combustion top dead centre (TDC). The main parameters of fuel spray injection are summarized in Table 2.