Research On Combustion Strategy For Diesel/Natural Gas Dual Fuel Engine

With the increasingly stringent emission regulations and energy crisis, diesel-natural gas dual fuel engine has become a hot research topic because of its excellent power, good economy and low emissions. Under the premise of not changing the mechanical structure and the exhaust emission treatment device of the original diesel engine, how to improve the economy,reliability and emission of the dual fuel engine is the main topic of this paper. Research method of combining experiment and numerical simulation is employed. Considering the combustion characteristics under different load areas and the unsolved problems in diesel-natural gas dual fuel engine, the research of combustion strategy is carried out for optimizing combustion performance and reducing emissions. The specific study contents are as follow:(1) In order to deeply understand the combustion process of diesel-natural gas dual fuel engine, the combustion mechanism model of diesel-natural gas dual fuel engine was established.This model is built by coupling a multidimensional model and a simplified kinetic model. Complex flow and spray phenomena in the engine cylinder are simulated by the multidimensional model, and chemical combustion processes is simulated by the simplified kinetic model. Comparing the calculation results of combustion mechanism model and the test results, it can be concluded that the combustion model predictions of ignition time, cylinder pressure and emissions are in good agreement with the experimental results. Hence, this combustion model can be used in the research of combustion strategy for finding high-efficiency and low-emission combustion modes.(2) In this paper, the n-heptane and methane are chosen as the reference fuel of diesel and natural gas respectively, and the study of constructing reduced kinetic model of diesel/natural gas is developed. Finally, the dual fuel reduced kinetic model which contains 77 species and136 reactions is established. The model highlights the effect of chemical interaction between the fuels on the ignition time, and improves the prediction accuracy. Compared with the detailed reaction kinetics model, the simplified model can simulate the process of heptane combustion, methane combustion and heptane and methane mixed gas combustion well and accurately predict the ignition time and the concentrations of important inter mediate products and final products.(3) Considering the poor reliability and the high NO_x emission of dual fuel engine in heavy-load working conditions, the influence of lean burn strategy, diesel substitution rate(DSR) control and multiple injection strategy on the combustion performance and emissions of engine are researched. It shows that the increase of excess air coefficient can effectively reduce the NO_x emission, but it will lead to the decline of torque and the engine economy becomes worse.Meanwhile,the DSR control can effectively improve the combustion efficiency, but the maximum rate of pressure rise is likely to exceed 1 MPa/?CA security value. Besides, proper pre-injection quantity and pre-injection interval can reduce the maximum pressure rise rate and improve the reliability of the engine, but it cannot reduce the NO_x emission. Combining the advantages of above methods, the paper proposes a new combustion control strategy called multiple injection lean burn substitution rate control(MILBSRC). After calibrating the control parameters, MILBSRC combustion control strategy can effectively reduce the maximum rate of pressure rise, piston surface temperature and NO_x emissions and realize the multi-object optimization of dual fuel engine.(4) Considering the high emission of unburned HC(uHC) of dual fuel engine under lowload working conditions, part premixed compression ignition(PPCI) combustion strategy is developed. Appropriate pre-injection quantity can effectively reduce uHC emissions and improve the combustion performance of dual fuel engine. In this paper, PPCI is chosen as the combustion strategy, and the mode switching surface is optimized under the constraints of uHC emission and DSR. After optimization, the application scope of co-combustion in small load is expanded. Considering the large fluctuation of the speed during the mode switching transition process, the paper combines the cerebellar neural network(CMAC) feedforward control and PID feedback control to achieve the rapid inhibition of abnormal fluctuation of speed, and compared with the pure PID controller, the CMAC-PID can achieve better dynamic performance.(5) The parameters of multi-injection strategy, the MILBSRC combustion strategy and the PPCI combustion strategy obtained from simulations are embedded into the new-generation independent-developed dual fuel engine active co-combustion control system to carry out bench verification. Validation results show that the simulations are in good agreement with the bench experiments. It indicates that the active co-combustion control system can realize the lean combustion, diesel multiple injections and PPCI control mode. The optimal combustion strategies of different load working conditions are implemented.