Numerical Modelling of Fuel Injection and Stratified Turbulent Combustion in a Direct-Injection Spark-Ignition Engine Using an Open Source Code

Doktorsavhandling, 2014

Stringent regulations on the emission of pollutants, especially carbon dioxide (CO2), necessitate the development of advanced combustion technologies. Gasoline direct injection (GDI) into the combustion chamber of a Spark Ignition (SI) engine is a combustion strategy that is widely recognized as having the potential to improve fuel efficiency of internal combustion engines for passenger cars. In particular, the CO2 emissions of GDI engines could be 20-26% lower than those of equivalent port-fuel injected (PFI) SI engines. Unfortunately, the development of combustion systems for GDI engines is very challenging because the creation of an appropriate combustion system is a major challenge because they require very precise formation of an ignitable fuel-air mixture with a steep stratification gradient right between the ignition electrodes to permit efficient operation under a wide range of operating conditions. CFD simulations are widely used to increase the efficiency of engine R&D. This thesis aimed at development of a tool for numerically modelling of working process in a GDI engine. The work had two main objectives. First, an available CFD code should be applied to multidimensional GDI engine simulations. Second, the code should be developed by implementing advanced models of stratified turbulent combustion, followed by validation against experimental data. As far as a code is concerned, although there are several mature commercial CFD packages on the market, there is also a need for less expensive software. Consequently, there is growing interests from both industry and academia in the OpenFOAM open source CFD code, whose source code is freely available so that users can implement and test new models without paying licence fees. However, it had not been used to simulate combustion in a GDI engine when the work reported herein was begun. This thesis presents an assessment of OpenFOAM as a tool for the numerically modelling of fuel injection, spray breakup, evaporation, mixture formation, and stratified turbulent burning in the combustion chamber of a GDI engine. OpenFOAM was used to simulate hollow-cone sprays of gasoline and ethanol discharged by a piezo-controlled pintle-type injector. The liquid properties of gasoline were implemented in the code to enable the simulation of gasoline sprays. In addition, the implementations of various spray breakup models such as LISA, TAB, Reitz-Diwakar, and KHRT into the standard OpenFOAM package were checked and modified in order to more closely reflect their descriptions in the original papers. Liquid penetration and SMD calculated by simulations using the revised model implementations were compared to experimental data provided by my colleagues. These comparisons showed that the best agreement between the experimental data and simulations was achieved when using a combination of the uniform droplet size and KHRT models. This model was therefore used in all subsequent engine simulations. As far as modelling is concerned, the code’s modelling capabilities were enhanced by implementing and developing models relevant to the turbulent burning of stratified gasoline-air mixtures at the elevated temperatures and pressures associated with combustion in a GDI engine. More specifically, two relevant issues were addressed. First, a semi-detailed chemical mechanism for the combustion of a gasoline surrogate in air was developed and validated. The gasoline surrogate consisted of iso-octane, toluene, and n-heptane in volumetric proportions of 55%:35%:10%, respectively. The mechanism includes 120 species participating in 677 reactions. It was validated against experimental data on the ignition delay times and laminar flame speeds of different mixtures at a range of pressures and temperatures. The mechanism was then used to compute laminar flame speeds for gasoline-air mixtures at equivalence ratios of 0.2≤ϕ≤2.0, unburned gas temperatures of 298≤T_u≤800K, and pressures of 1≤P≤30 atm. The results of these calculations were approximated and the approximations were implemented into OpenFOAM for subsequent CFD modelling of stratified turbulent combustion in a GDI engine. In order to study stratified burning in a GDI engine, the Flame Speed Closure (FSC) model of premixed turbulent combustion was combined with a so-called presumed Probability Density Function (PDF) method that made it possible to account for the influence of turbulent fluctuations in local mixture composition on the local burning rate. The combined model based on the FSC and presumed PDF approach was implemented into OpenFOAM and the roles played by its various submodels were investigated in a step-wise fashion. Finally, the so-extended code was used to simulate a GDI engine burning a globally lean mixture. Good agreement between results computed by me and experimental data provided by my colleagues was obtained. In addition, the model’s sensitivity was investigated.