15th International Multidimensional Engine User’s Meeting at the SAE Congress 2005,April,10,2005 Detroit, MI

Advanced modeling of GDI and DI Diesel Engines: Investigations on Combustion and High EGR level Marc ZELLAT, Driss ABOURI, Thierry CONTE and Bharat RAGHUNATHAN CD-adapco Group The development of CFD methodology for Internal Combustion Engine represent a particular challenge because of many complex features and phenomena, perhaps more than any other widely-used mechanical device. Understand the processes taking place in the combustion chamber and the correlation between parameters and therefore a way to support the design is today essential to explore new solutions, reduce the cost and improve the development efficiency. 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 DI Diesel and GDI Engines operating with very high level of Exhaust Gases Recirculation (Higher than 30%) 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 enable to compute combustion for operating conditions with large EGR amount in the GDI and DI Diesel engines has been completed. Predictions have been compared with extensive data from a DI Diesel Engine in production over a wide range of operating conditions. The results show that the combustion model used in combination with a two steps auto-ignition model gives realistic Heat Release History as well as emission prediction. 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 with a special emphasis on DI Diesel engine combustion and GDI engine operating at part load with high EGR level. 2

THE COMBUSTION MODEL: ECFM-3Z

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 GDI 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) are computed and modified with the help of a mixing model.

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Figure 1: Principle of ECFM-3Z Model

The laminar flame speed id 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. For new low emissions combustion concepts such as HCCI engines or DI Diesel Engines wit pilot-injection, cool flames may occur. The main auto-ignition delay is strongly dependant on thermodynamic conditions (temperature and pressure). Therefore the heat release observed during the cool flame will affect the global auto-ignition delay. Base on this conclusion, a new double delay autoignition model, based on tabulated temperature profiles issued from complex chemistry, is proposed as illustrated in figure 2.

Total Auto-Ignition delay

Cold flame delay

Heat Release

Figure 2: Temperature profile during auto-ignition from complex chemistry calculation

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DI DIESEL COMBUSTION: MODEL VERIFICATION

Several engine configurations were chosen to validate the described spray and combustion modeling. The displacement range varies from 0.5 liter/cylinder to 2 liters/cylinder. Investigations will be shown in this paper for a typical European Automotive Direct Injection Diesel Engine with a displacement of 0.5 liter/cylinder with an eight holes injector and relatively compact bowl.

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3.1

Full load Operating condition

Two different engine speeds were computed for this engine and global quantities were compared to experimental data. A comparison between computed and measured in-cylinder pressure is presented in Figures 3 and 4, in order to assess the accuracy of the computational model with respect to the prediction of global cycle averaged cylinder quantities.

Figure 3: Comparison between calculated and measured in-cylinder pressure: 100% load, 1500 rpm

3.2

Figure 4: Comparison between calculated and measured in-cylinder pressure: 100% load, 4000 rpm

Part load Operating condition

It is well known that increasing the EGR level is a good solution for reducing NOx emissions in DI Diesel engines. However, compromise between NOx emissions and the other important pollutant, which is the soot, is also well known. Pilot-injection (or Post-injection) is then used to decrease the soot level. The calculated global quantities of pollutant data serve as the basis for further assessment of the spray and combustion models behavior under engine operating condition parameter variations. This enables the study of the influence of injection timing, %EGR, etc on soot and NOx formation for four different operating conditions. (C.f. Table 1).

OP 1 OP 2 OP 3 OP 4

RPM

Injector

Pilot injection

1500 2000 1500 2000

Injector 1 Injector 1 Injector 2 Injector 2

Yes Yes Yes No

Load (%) 20 40 20 40

EGR (%) 40 10 40 10

Table 1: Part load Operating conditions

Figure 5 and 6 show also results at EVO of relative soot and NOx formation trends obtained for this engine for the four different operating conditions. One can observe that trends are very well predicted.

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O.P. 3

O.P. 4

O.P. 4

Experiment

O.P. 3 Experiment

Star-CD - ECFM3Z Star-CD - ECFM3Z O.P. 2

O.P. 1

O.P. 1

O.P. 2

Figure 5: Comparison of NOx at EVO – Normalized Results based on OP1

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Figure 6: Comparison of Soot at EVO – Normalized Results based on OP1

GDI-SI COMBUSTION

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 GDI-SI engine under full load operating conditions for combustion understanding and optimization as well as under part load operating conditions. 4.1

THE ENGINE CONFIGURATION

The computed is a typical 2 liters GDI-SI Engine, under production. Because of the importance of the flow structure and mixture distribution within a GDI Engine, the full process of Intake, Injection, Compression and Combustion is computed at each simulation. Figure 7 shows the mesh used for a full cycle simulation.

Figure 7: View of computational grid

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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 8 and 9 for the full load 6000 rpm and figures 10 and 11 for the full load 2000 rpm. The overall agreement is rather good including the combustion speed for the two engine speeds full load operating conditions

Figure 8: Comparison between calculated and measured in-cylinder pressure: full load-6000 rpm

Figure 9: Comparison between calculated and measured Rate Of Heat Release: full load-6000 rpm

Figure 10: Comparison between calculated and measured in-cylinder pressure: full load-2000 rpm

Figure 11: Comparison between calculated and measured Rate Of Heat Release: full load-2000 rpm

4.3

MODEL VALIDATION – Part load – 1500 rpm, 3bars BMEP

High fuel efficiency and low emission levels make today’s Gasoline Direct Injection engine an attractive power source for automotive applications. However, since the engine operates at part load with an overall lean fuel/air mixture, no general after-treatment system is yet available. To facilitate future engine development improved knowledge of basic combustion process is needed. This includes mixture preparation and the use of high level of EGR. The ECFM-3Z is a good candidate for challenging this high level of complexity in combustion modeling, however as the model works with the assumption that the combustion of fuel occurs in a premixed regime, a good formulation of the laminar flame speed is an essential part of the model. In this work, the experimental correlation of Metghalchi and Keck is extrapolated to mixture with high level EGR. The new formula is tested under realistic engine geometry and operating conditions. The engine is the GDI engine investigated at full load in the previous section and the operating conditions are now 1500 5

rpm, 3 bars BMEP. The injection timing is at 80 CA before TDC making this load highly stratified. Two different EGR levels were investigated: 20 % and 40 %, the load is being maintained. The computed and measured in-cylinder pressures and corresponding rate of heat release are represented on figure 12 and 13 for 20% EGR and the 40% EGR respectively. The overall agreement is rather good including the combustion speed for the operating condition with the high level of EGR.

Figure 12: Comparison between calculated and measured in-cylinder pressure and R.OH.R: 20% EGR

Figure 13: Comparison between calculated and measured in-cylinder pressure and R.OH.R: 40% EGR

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CONCLUSIONS Presented in this paper are a new combustion model, the ECFM-3Z model, based on a flame surface density transport equation and a mixing model that can describe inhomogeneous turbulent premixed and diffusion combustion. The combustion model is coupled with improved burned gas chemistry that allows CO, soot, and NOx formation calculations. Corresponding validation data and application for several engine configurations (DI-DIESEL and SI Engines) and operating conditions are shown with a special highlight on GDI-SI engines and DI Diesel engines operating at part load with high level of EGR. 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] Baritaud, T., Poinsot, T., Baum,M. “Direct Numerical Simulation For Turbulent Reacting Flows.” Editions Technip

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