Research On The Effect Of Gas Fuel On Combustion And Emission Characteristics Of Internal Combustion Engine

With increasing concerns on environment protection, the worldwide communities are bringing forward increasingly rigorous regulations for reducing toxic emissions from Internal Combustion Engines (ICE). As a result, improving fuel economy and reducing exhaust emissions are among the highest priorities for automotive industry and also remain as the most demanding subject for academia engaged in the research of combustion engines. Gas fuel can offer a promising perspective to enable the internal combustion engine to meet these challenges. In order to improve combustion process and decrease emissions, this paper studies the effect of gas fuel on performance and emission of ICE.Based on the physical and chemical characters of coal-bed gas, a model S195 diesel engine with swirl chamber has been modified to fuel with coal-bed gas. Detailed experiments have been carried out to investigate the combustion and emission characteristics of the engine operating with different grades of coal-bed gas. The experimental results show that the idling stability of coal-bed gas engine increases with the methane percentage increasing. Increasing compression ratio is helpful to enhance idling stability when ignition timing is constant. The combustion pressure difference resulting from the variation of methane content in coal-bed gas is not evident at low load condition, but it is evident at high load condition. The pressure rise rate increases with the methane percentage increasing when engine load condition is constant, and the combustion duration also reduces. The effect of inertia gas on combustion process is small when methane percentage reaches a certain level. The HC and CO emission decreases with methane percentage and engine load increasing, but it is reverse for NO emission.A quasi-dimensional 2-zone combustion model has been developed by using the MATLAB platform. The combustion model includes two sub-models, with the first calculating the turbulence intensity history and the second calculating flame parameters by random point method. Emissions of NO, HC and CO have been predicted using the numerical model based on an extended Zeldovich mechanism, wall quenching and incomplete oxidation theory. The effects of various components in coal-bed gas on the engine combustion process and emissions have been investigated, with a satisfactory agreement between the simulation and experimental data achieved.The influence of EGR ratio and hydrogen addition on performance and emissions of diesel engine has been studied. The experimental results show that the peak pressure and maximum rate of pressure rise reduces when EGR technique is adopted. EGR system can improve the operation stability. EGR system will result in engine output decreasing, fuel consumption increasing and smoke increasing. The peak pressure and maximum rate of pressure rise increases with the quantities of hydrogen increasing at all kind of load condition. It is concluded that induction of small quantities of hydrogen can significantly enhance the thermal efficiency and economics of ZS195 diesel engine. With hydrogen induction, NOX level increases, but smoke, HC and CO emissions reduce. In addition, hydrogen-enriched air is also used as intake charge in a diesel engine adopting exhaust gas recirculation (HEGR) technique. When HEGR technique is adopted, peak pressure reduces and the maximum rate of pressure rise and heat release increase at low load condition. But they are increase at high load condition. The HEGR technique can reduce NOX and smoke emissions simultaneously while the engine efficiency is improved.The chemical kinetics model of diesel fuel exhaust gas reforming has been developed using the CHEMKIN platform. The results show that the reaction profile of diesel fuel exhaust gas reforming was significantly influenced by the reformer inlet temperature with or without catalyst. The reforming reaction can start without catalyst until the reformer inlet temperature reach 900K, H2 and CO content in the reformer product is small. When the catalyst is adopted, the reforming reaction can start at the reformer inlet temperature of 600K, the reforming reaction rate is high, H2 and CO content increase. With the catalyst, the diesel fuel exhaust gas reforming reaction is endothermic, steam reforming reaction and water gas shift reaction are main reaction; partial oxidation reaction is secondary. Without catalyst, the reforming reaction is exothermic which the quantities of heat release are decided by the O2/C ratio. H2O/C ratio and O2/C ratio also influence the reforming reaction. The H2 and CO content of reformer product increases with the H2O/C ratio and O2/C ratio increasing. The optimum H2O/C ratio and H2O/C ratio is 1.5-2 and 0.3-0.5 respectively. Without catalyst, the H2 and CO percent in reformer product decreases with flow rate while the volume of reformer is constant. With the catalyst, the effect of Gas Hourly Space Velocity (GHSV) on H2 and CO percent is small while volume of reformer is constant. The structural parameter of reformer (length and volume) slightly influences the reaction profiles of diesel fuel exhaust gas reforming.Three-dimensional combustion model has been developed using FLUENT software. The effect of EGR, hydrogen addition and HEGR on performance of ZS195 diesel engine is studied. The modeling results are compared with the measured data obtained from a parallel experimental investigation, with a good agreement. The velocity vector was influenced by the fuel injection quantities, although the influence is small during the prophase of fuel injection and anaphase of combustion. The partial temperature, O2 and NO mass fraction decreases with the EGR ratio increasing. The local temperature, O and NO mass fraction increases with the quantities hydrogen addition increasing. The HEGR technique can reduce NO and smoke emission simultaneously.