Comparative study of combustion modes in internal combustion engines using CFD simulations

Internal combustion engines did not become a practical reality until the 1860s. Because of wide-spreading markets of internal combustion engines, researches and developments of IC engines, maybe less fundamental, have continued ever since to improve engine efficiency and exhaust emissions. In fact, both of SI and CI engines have their own advantages and disadvantages. Although CI engine has higher compression ratio which results in a higher thermal efficiency and better fuel economy than SI engine, diesel engine operates in more limited speed range and produces higher levels of nitrogen oxides (NOx ) and particulate matter (PM) emissions. Nowadays, decreasing of energy resources, high fuel price and environmental protection led the researchers to design the engines which maximize power and fuel economy while minimizing exhaust emissions. In order to achieve these goals some advanced combustion strategies such as Low Temperature Combustion (LTC), Homogeneous Charge Compression Ignition (HCCI) and Premixed Charge Compression Ignition (PCCI) have been introduced. Advanced combustion concepts are receiving increased attention for the reason that they can be a practical solution used in piston engines to meet the increasingly stringent emission regulations. In addition to the need for advanced combustion models to be utilized in future engines, the ability of using alternative fuels such as methane in these concepts is indispensable. Computational Fluid Dynamics (CFD) tools provide a detailed description of the fluid flow and transport phenomena occurring in the system, employing simplified kinetic mechanisms in order to develop current high computational effort. Noticeable advantages of CFD make it a promising way to realize flow patterns that are difficult, expensive or impossible to study using experimental techniques. Indeed, in parallel with novel combustion concepts and alternative fuel usage, 3D computational fluid dynamics simulation of in-cylinder spray formation/propagation, combustion and pollutant formation processes is receiving increasing attention during engine and combustion system development. Nowadays, CFD has been successfully defined for the calculation of fluid flow, mixture formation and combustion in internal combustion engines as a supplementary tool to in-cylinder pressure analysis, combustion diagnostics and optical mixture formation. In this study, commercially available AVL FIREr CFD code is used to perform the simulations. Boundary fitted computational grids are generated by Engine Simulation Environment Diesel (ESE Diesel) tool, then in-cylinder processes are simulated via Workflow Manager tool. The combustion chamber of engine is considered as a closed thermodynamic system and the governing equations for unsteady, compressible, turbulent flow and thermal fields are solved from the intake valve closing (IVC) to exhaust valve opening (EVO). In the first part of current study, simulated SI engine is validated by available experimental test results. To understand the grid sensitivity of multi-dimensional engine CFD, five different grid resolutions are adopted and compared. The selected mesh represents a compromise between computational time and accuracy, but is found to give adequately grid independent results. In favor of assessing the predictive accuracy and reliability of the CFD simulations, it is critical to perform validation studies over various engine operating conditions. In this regard, in-cylinder pressure, heat release rate, specific fuel consumption, brake mean effective pressure, brake power and emissions predictions are compared with the measured experimental results at three different engine speeds of 1500[rpm], 1660 [rpm] and 1800[rpm]. The results of the simulation are in good qualitative and quantitative agreement with the corresponding experimental data. Then a comprehensive parametric study of individual influences of exhaust gas recirculation, equivalence ratio and spark timing on the performance and exhaust emissions of the simulated gasoline engine is investigated at a fixed engine speed of 1500 [rpm]. In order to carry on the study, additional simulations are done by applying the natural gas (methane) as an alternative fuel and the performance and emission characteristics of CNG engine are compared with those of validated gasoline engine. Since the primary focus of this research is the examination of different combustion strategies for reducing NOx and soot emissions along with engine performance improvement, in addition to conventional natural gas fueled SI engine, a low temperature combustion strategy known as Homogeneous Charge Compression Ignition (HCCI) is developed by using natural gas as fuel. In order to prepare a reasonable comparison, the natural gas fueled HCCI engine is considered to operate under the same load of verified gasoline engine. Additionally in the second part of this research, first a conventional single cylinder diesel engine under full load condition at engine speed of 1500[rpm] is simulated with AVL FIREr CFD code, and then the individual and combined effects of EGR and engine load on combustion and engine-out emissions are studied. Moreover, in order to achieve the partially premixed compression ignition and lower temperature combustion of diesel engine, the effects of multiple injection strategies including injection ratio and first injection duration on performance and exhaust emissions of diesel engine are investigated.