Experimental And Numerical Study Of Natural Gas HCCI Engine With Controlled Hydrogen And DME Fuel Additives

During the last decade an alternative to conventional internal combustion engines, i.e. spark-ignited and Diesel engines, has been under investigation by an ever increasing number of research groups. This alternative process is called Homogeneous Charge Compression Ignition (HCCI). That engine tests have elucidated that it has a great potential to reduce significantly the harmful emissions, and it is expected to reduce fuel consumption as a comparison with conventional engines operation. HCCI is defined as the process in which a homogeneous mixture of air and fuel, diluted with excess air as well as combustion products, is compressed under such conditions in which the auto-ignition occurs near the end of the compression stroke, followed by a combustion process that is significantly faster than conventional Diesel or Otto combustion.This study concerns the numerical and experimental investigation of HCCI engine combustion process. The principal aim of the work underlying the thesis has been using and developing the simulation tools that can contribute for understanding and further development of CNG HCCI engine process. Since HCCI combustion is strongly dependent on chemical kinetics, most of the work presented achieved with the capabilities of the detailed oxidation mechanisms which can describe the oxidation of CNG fuels with and without hydrogen or/and DME additives.Two models, with varying levels of complexity and corresponding calculation times, have been developed, validated, and used in this analysis. These simulation tools discussed are the so-called single-zone (CHEMKIN) and multi-zone (KIVA-3Vr2) models. The detailed reaction mechanisms are implemented in these models and a detailed-chemistry approach is used to predict the auto-ignition timing and the rate of combustion. Whereas the zero-dimensional (single-zone) model is based on the assumption of a perfectly homogeneous mixture, while the 3D-CFD/chemistry coupling model is more sophisticated since multiple zones are used to represent the mixture inhomogeneity in the combustion chamber of complex engines geometry. Modeling of HCCI engine requires balanced approach that captures both fluid motion as well as low and high temperature fuel oxidation. A fully coupled 3D-CFD and chemistry scheme would be the ideal HCCI modeling approach, but is computationally very expensive. As a result, modeling assumptions are required in order to develop tools that are computationally efficient, yet maintain an acceptable degree of accuracy.The major accomplishments and findings from this research can be recognized and traced by summarizing the sequence of collecting data from the results as follows:1- A zero-dimensional, thermodynamic model with detailed chemical kinetics and cylinder wall heat transfer correlations have been used to study the detailed oxidation mechanism of pure CNG and CNG/hydrogen in HCCI engine. This mechanism made up of 325 reversible elementary reactions among 53 species. The mechanism was numerically investigated at different operating and geometry conditions of HCCI engine during the time period in which both intake and exhaust valves are closed. In addition, an extended hydrocarbon oxidation reaction mechanism including 81 species and 362 elementary reactions has been used to simulate the combustion and emission behaviors of HCCI engine with DME fuel. Also, a detailed chemical kinetic mechanism considers 99 species and 477 reversible elementary reactions has been used to simulate the autoignition and combustion of CNG/DME fuel mixture with the effect of 10 and 20 % of hydrogen mole fraction.2- The gas fuel injection and its interaction with in-cylinder flow field and combustion behaviors of an HCCI engine with port fuel injection were numerically investigated by using numerous capabilities of multi-dimensional computational fluid dynamic (KIVA-3VR2) code coupled with a detailed chemical kinetics mechanism consisted from 314 elementary reactions among 52 species. The combustion process have been simulated successfully with the optimal hydrogen dose related to the specified operating conditions which give the highest thermal efficiency under stable working conditions of pure CNG and CNG with 20% of hydrogen blend. In addition, KIVA-3V is used to investigate the mixing process in HCCI engines prior to combustion, particularly for operation with liquid DME fuel. The calculations of complex chemical-kinetics/turbulent interaction, was carried out using a reduced oxidation mechanism of DME that mainly contains 145 elementary reactions among 27 species.3- Engine operating maps based on a series of simulation runs in the case of pure CNG fuel with and without hydrogen blends in addition with the effect of DME fuel additives into the engine missfire and knocking limits at different engine loads have been concluded. These tasks were achieved to find the optimum operating conditions of the HCCI engine fueled mainly by CNG with the minimum number of the laboratory engine tests.4- Experimental studies have been conducted in a commercial single cylinder diesel engine to investigate the basic performance characteristics of HCCI engines combustion and emissions. The CHEMKIN and improved KTVA-3Vr2/chemistry model results were validated using the HCCI engine experimental data. The predicted ignition timing by different model agree well with experimental data at different engine load since homogeneous charge was obtained with varying hydrogen or/and DME additives into the CNG/air mixture.