Simulation And Experimentation On Combustion Process Of Biogas-Diesel Dual Fuel Engine

Up to the present, petroleum and nature gas are still the main sources of fuel for the internal-combustion engine. However, the reserves of petroleum and nature gas are reducing sharply. Moreover, greenhouse effect results in the frequent appearance of global catastrophic climate and the situation of environment are getting serious. So to searching a new substitute of energy for the engine is urgent. Biomass energy is considered as the reproducible energy that can be stored and transported, and its high efficiency of conversion and low pollution are more and more significant. The present situation and development trend of internal-combustion engine are briefly discussed. The developments of biomass energy, the technology and combustion modeling of dual fuel engine and the technology of biomass gasification are summarized. The re-equip technology and theory model of the dual fuel engine are developed on the basis research of predecessors. The foundation for the utilizing and development trend of biomass energy sources is given.Biogas is made from agricultural and forestry residues by a negative pressure downdraft gasifier. It is utilized as main fuel in a biogas-diesel dual fuel engine that is refitted from the ZH1115 direct injection diesel engine after being cooled and filtrated. The power and economic performances of a gas-diesel dual fuel engine may be worse because its air volume in cylinder is less than the diesel engine with the same displacement volume due to the volume of gas fuel in cylinder. The components and their performances data of biogas are analyzed. The calculation formula of power performance of the gas-diesel dual fuel engine is deduced. The calculated results are fitted in with the experimental ones. The tests at different loads and speeds are carried out for the diesel engine and the dual fuel engine. The mapping characteristics, combustion characteristics and specific emission characteristics of the diesel engine and the dual fuel engine are compared and analyzed. The effects of parameters such as load, engine speed and fuel supply advance angle on combustion process are studied. The results show that compared with diesel engine, the ignition time of dual fuel engine delays, maximum combustion pressure and maximum combustion pressure rise rate are lower and their corresponding phases are later, after combustion period is longer except at low speed and large load. The NO_x emission of dual fuel engine is lower remarkably than diesel engine.Based on the pilot fuel spray and mix, combustion chemical reaction kinetic, turbulent flow, combustion calculation theory, NO_x formation model, initial and boundary conditions and mesh creation, a three dimension combustion model of biogas-diesel dual fuel engine has been established. In the pilot fuel spray and mix model, discrete phase liquid droplets model has been used to simulate the movement of pilot fuel liquid droplets. Multi-injection solid cone injector source and Rosin-Rammler distribution have been used to distribute fuel injection. The breakup of pilot fuel liquid droplets has been simulated by Wave Breakup model. Ignition delay has been calculated by the formula deduced by Hardenburg and Hase. Turbulent flow has been simulated by Renormalization Group Theory (RNG)κ-εformula and near-wall region turbulent flow has been amended by wall functions. In combustion calculation, net production rate has been gained based on combining the laminar finite-rate model and the eddy dissipation model. Diesel combustion has been described by single step reaction, and biogas combustion by simplified chemistry mechanism and single step reaction of CO, H₂ and CH₄. In NO_x estimating model, thermal NO_x formation, prompt NO_x formation and fuel NO_x formation have been calculated by probability density function (PDF). In order to get more reasonable results, initial conditions are set to testing data. In calculating mesh, periodic boundary has been adopted. Because the injector has four even collocation nozzle holes, in order to save calculation time, only a quarter geometry model has been considered. Mesh creation has been carried out with hexahedral and cuneiform cell. The mesh regions swept out by the moving surfaces has been represented by dynamic layering zones.By comparing the computer simulation results with the experiment ones, it is shown that the three-dimension combustion model can be used to simulate the combustion process of diesel engine and biogas-diesel dual fuel engine and to evaluate performance parameters of the two engines. Then, the effects of main parameters such as the quantity of pilot fuel, the biogas component and quantity on the combustion process of dual fuel engine have been researched. The results show that with the quantity of pilot fuel decreasing, ignition time delays, maximum combustion pressure becomes lower, the corresponding phase gets later, the after burning is heavier and NO_x emission reduces. The higher the heat value of biogas is, the higher the maximum combustion pressure and the average temperature are, and the worse the NO_x emission is.