Marc ZELLAT, Driss ABOURI, Thierry CONTE and Riyad HECHAICHI CD-adapco

Transcription

1 16 th International Multidimensional Engine User s Meeting at the SAE Congress 2006,April,06,2006 Detroit, MI RECENT ADVANCES IN SI ENGINE MODELING: A NEW MODEL FOR SPARK AND KNOCK USING A DETAILED CHEMISTRY IN STAR-CD Marc ZELLAT, Driss ABOURI, Thierry CONTE and Riyad HECHAICHI CD-adapco Over the past, substantial advances have been made in the modeling of combustion in SI Engines. The progress can be attributed to the development of new models, which explicitly take into account the fact that combustion largely occurs in the wrinkled flame regime. Examples are the Weller Wrinkled Flame model, the G-equation Model and the ECFM-3Z an extension of the Coherent Flame Model. If these models are well established now to describe the normal turbulent flame propagation, all need to be completed by a Spark model and a Knock model, especially to help engine engineering developers to optimize the combustion process to meet with the special requirements for modern engine. The aim of the present paper is to describe the recent development of the general multi-purpose code STAR-CD in the field of Internal Combustion Modeling with a special emphasis on SI Engines operating at full load and low speed. An enhancement of the new combustion model developed in the GSM (Groupement Scientifique Moteurs including IFP, PSA and RENAULT) and implemented in STAR-CD, The Extended Coherent Flame Model-3Z (ECFM-3Z) is described first. This model is completed with a Spark model which takes explicitly into account the arc and kernel creation, the transfer of the energy to gas as well as with a knock model based on complex chemistry calculated using the conditional un-burnt temperature and connected to main combustion. Predictions have been compared with extensive data from a SI Engine in production at full load. The results show that the models give realistic Heat Release History, Indicated Power, as well as Knock Onset. 1 INTRODUCTION The development of CFD methodology for IC engine design represents a particular challenge due to the complex physics and mechanics, perhaps more than with any other widely-used mechanical device [1]. Improved understanding is essential to explore new solutions, reduce costs, and improve development efficiency. Although substantial advances have been made in all areas (turbulence, spray modeling, combustion, numerical methods, parallel computing, pre- and post-processing, etc.), there are numerous additional requirements to be met for it to become a design tool. The aim of this paper is to describe the recent developments in the multi-purpose CFD code STAR-CD [2] in the field of IC engine modeling on SI engine operating at full load with a special emphasis on the spark creation and knock onset. 2 THE COMBUSTION and AUTO-IGNITION MODELS The ECFM-3Z model is a combustion model based on a flame surface density transport equation and a mixing model that can describe inhomogeneous turbulent premixed and diffusion combustion. This model is an extension of the ECFM [3] combustion model, previously implemented in STAR-CD and extensively validated for SI engine applications. The idea is to divide the computational domain taking into account the local stratification. In the mixed zone, standard ECFM is computed, with an improved version of the post-flame chemistry model in the burned gases and an auto-ignition model in the unburned gases. The evolution of the mass included in the 3 mixing zones (Figure 1) is computed and modified with the help of a mixing model. 1

2 Figure 1: Principle of ECFM-3Z Model The laminar flame speed is computed from the fresh gases state properties using the experimental correlation of Metghalchi and Keck. From this correlation, the laminar flame speed is linearly extrapolated to zero for very rich or very lean mixture. The pre-reactions in the end gas exhibit a quite complex phenomena such chain propagation, degenerating branches, the appearance of cool flame. The heat release observed during the cool flame will affect the global auto-ignition delay and consequently the Knock Onset. Based on this conclusion, a double delay auto-ignition model issued from complex chemistry is proposed. The mechanism contains about 550 species and composed with more than 2000 reactions. Obviously it is out of the range of a CFD code to solve and transport al these species. Therefore tabulation has been adopted for the main quantities as illustrated in figure 2. The table is created by varying equivalence ratio, pressure, temperature and EGR level. A combination of the tables and the transport of one intermediate specie is then enough to predict auto-ignition and Knock Onset. Total Auto-Ignition delay Cold flame delay Heat Release Figure 2: Temperature profile during auto-ignition from complex chemistry calculation 2

3 3 THE SPARK MODEL: AKTIM AKTIM (for Arc and Kernel Tracking Ignition Model) is a model dedicated to Spark Ignition simulation [4]. This model is fully coupled with the combustion model ECFM. This model simulates the spark creation and convection (elongation and rupture). The flame kernels evolutions are computed using the energy transferred by the spark to the gases, thermal losses and local gases thermochemical properties and turbulence. Kernels are convected by mean flow and turbulence. The Lagrangian method is used to compute the spark and kernels. Many effects such as electrical circuit characteristics (see figure 3), multiple breakdowns, flow convection (see figure 4) at the spark plug (downstream, cross-flow or upstream configuration) etc. are taken into account in this model. Figure 3: Simplified coil / Inductive ignition system Figure 4: spark convection at spark plug 4 HIGH PERFORMANCE SI ENGINE: MODEL VERIFICATION The combustion in SI engines is initiated by an electric discharge for the spark plug. The ECFM- 3Z main concept is based on flame density transport equation. Therefore, one can notice that flame surface density can be initialized at some point to start the combustion. A correlation based on experimental results is used to describe the spark model. The ECFM-3Z was used and applied with success in several GDI-SI Engines [3]. However, these applications were limited to part load engine operating conditions. The objective is to test and validate the model in a SI engine under full load operating conditions for combustion understanding as well as under knocking conditions for Engine performance optimization. 3

4 4.1 THE ENGINE CONFIGURATION The computed configuration is a typical 2 liters SI Engine, under production. Because of the importance of the flow structure, turbulence level and mean flow intensity at Spark Timing, the full process of Intake, Injection, Compression and Combustion is computed at each simulation. Figure 5 shows the mesh used for a full cycle simulation. Figure 5: View of computational grid: Complete geometry (left) and combustion chamber (right) 4.2 MODEL VALIDATION Full load / 6000 RPM and 2000 RPM The computed and measured in-cylinder pressures and corresponding rate of heat release are represented on figure 6 and 7 for the full load 6000 rpm and figures 8 and 9 for the full load 2000 rpm. The overall agreement is rather good including the combustion speed for the two rotational speeds full load operating conditions. Figure 6: Comparison between calculated and measured in-cylinder pressure: full load-6000 rpm Figure 7: Comparison between calculated and measured Rate Of Heat Release: full load-6000 rpm Figure 8: Comparison between calculated and measured in-cylinder pressure: full load-2000 rpm Figure 9: Comparison between calculated and measured Rate Of Heat Release: full load-2000 rpm 4

5 4.3 MODEL VALIDATION FULL LOAD 2000 RPM: INFLUENCE OF SPARK TIMING To assess to Knock model, the engine was operated first with an iso-octane fuel (RON/MON 100/100). The effect of spark timing on the brake torque was then evaluated. A comparison between the Indicated Mean Effective Pressure (IMEP) and the brake torque, scaled using the measured Maximum Brake Torque (MBT) is presented in Figure 10, with the corresponding In-cylinder pressure history. The figure shows that the model capture very well the spark timing for MBT as well as the effect of the spark timing on the brake torque. Figure 10: In-cylinder Pressure: Advanced, MBT and Retarded spark timing Effect of Spark Timing on Brake Torque at 2000 rpm Full Load 4.4 KNOCK ONSET: FULL LOAD 2000 RPM When the engine operates with the fuel characterized by its RON/MON 95/95, knock occurs for a spark timing before MBT as indicated in figure 10. The 25 CA BTDC spark timing was simulated using a gasoline fuel characterized by its RON/MON of 95/95. Figure 11 shows the pressure history obtained from this analysis and compare it to the one obtained from the analysis without knock (RON/MON 100/100). Figure 11: Pressure History with and without knock This figure shows clearly the appearance of a sudden increase in cylinder pressure history, demonstrating the presence of knock under these conditions which was confirmed from experimental data. Finally, knock can be visualized by a three dimensional plot of the intermediate specie and/or the creation of flame surface density taking place in end gas as it is shown in figure 12. The figure shows the 5

6 Knock Onset taking place under exhaust valves, resulting from the forced flame convection due to the relatively high tumble created during the intake. Figure 12: Knock Onset Location RPM Full Load. Spark Timing 25 CA BTDC (RON/MON 95 95) 5 CONCLUSIONS Enhancements of the new combustion model developed in the GSM (Groupement Scientifique Moteurs including IFP, PSA and RENAULT) and implemented in STAR-CD, The Extended Coherent Flame Model-3Z (ECFM-3Z) are presented. The model is completed with a New and Original Spark model that takes into account explicitly the arc and kernel creation, the transfer of the energy to gas. For knock prediction a model based on complex chemistry calculated using the conditional un-burnt temperature is presented. The latter is connected to main combustion. The proposed models are valid for homogeneous and stratified charge (GDI) included high EGR level. The knock model takes into account the end gas in-homogeneity as well. Predictions have been compared with extensive data from a SI Engine in production at full load. The results show that the models give realistic Heat Release History, Indicated Power, as well as Knock Onset. 6 REFRENCES [1] Gosman, A.D. State of the art of multi-dimensional modeling of engine reacting flows. Oil & Gas Science Technology, Vol. 54 (1999) [2] STAR-CD V3.15 PROSTAR & es-ice are Trademarks of CD-adapco Group [3] Duclos, J.M., Zolver, M., Baritaud, T. 3D modelling of combustion for DI-SI engines. Oil & Gas Science and Technology, Vol.54 (1999) [4] Duclos, J.M., Colin, O. Arc and Kernel Tracking Ignition Model for 3D Spark-Ignition engine calculations. Comedia