Simulation Study On Combustion And Emission Characteristics Of Coal-based Synthetic Diesel Fuel And Its Mixed Fuel

At the moment when automotive energy and environmental issues are becoming increasingly prominent,based on China's lack of oil,natural gas,and coal-rich energy structures,the development of automotive alternative fuels and the diversification of automotive energy sources will help us to get rid of our dependence on non-renewable petroleum fuels and achieve transportation energy transformation upgrading and ensuring energy security are vital.As a high-quality,high-efficiency clean petroleum alternative fuel,coal-based synthetic diesel has become more mature in production technology and has a good prospect in the application to internal combustion engines.This paper proposes the corresponding model fuel for the coal-based synthetic diesel used in the test,and builds the corresponding chemical reaction kinetics mechanism based on the model fuel,and then couples the mechanism with the three-dimensional simulation model to explore the different boundary conditions and composition ratios for coal.The effects of combustion and emission of synthetic diesel and its blended fuels provide basic research basis for exploring alternative fuels for internal combustion engines to achieve efficient and clean combustion modes.The main contents and conclusions of the study are as follows:Based on the physical and chemical characteristics and composition of coal-based synthetic diesel for the test,two model fuels,one-component and two-component,were constructed for it.The n-decane chemical reaction mechanism was determined for the one-component model fuel.For the two-component model fuel?mass fraction:88.99%n-dodecane/11.01%iso-octane?,the n-dodecane chemical reaction mecha-nism and the iso-octane chemical reaction mechanism are selected as the basic mechanism.After the simplification and fusion are completed,the mechanism of NO_x and PAH formation was determined and added.After further simplification,a simplified mechanism of n-dodecane/iso-octane/NO_x/PAH containing 132 components and 895 steps was finally obtained.Combined with the test data provided in the literature,in the flame retardance period,laminar flame propagation velocity,main component concentration and homogeneous compression ignition engine of 100%n-dodecane and 100%iso-octane,The simplified mechanism of alkane/iso-octane/NO_x/PAH was verified,and it was found that the comparison results were good,and even the prediction effect for some cases was better than the original basic mechanism.Then,the fuel chemical reaction mechanism was coupled with a three-dimensional simulation model,and compared with the cylinder pressure curve data obtained by burning coal-based synthetic diesel in the actual bench test.It is found that the prediction effect of two-component model fuel is better than that of single-component model fuel.In the study,the dimethyl ether/ethanol chemical reaction mechanism was further determined,and the simplified mechanism was integrated with the simplified mechanism of n-dodecane/iso-octane/NO_x/PAH.Finally,a simplified mechanism of n-dodecane/iso-octane/dimethyl ether/ethanol/NO_x/PAH containing 168 components and 1250 reaction steps was obtained.Compared with the experimental data and the simulation data of the original basic mechanism,it is found that the mechanism has a good prediction effect on the flame retardation period,laminar flame propagation speed and homogeneous compression ignition engine of 100%dimethyl ether and 100%ethanol.The three-dimensional simulation model of the simplified mechanism of n-dodecane/iso-octane/NO_x/PAH was coupled to complete the simulation calculation of coal-based synthetic diesel fuel under different boundary conditions.The research results show that:Advance the injection timing within a certain range,which can increase the maximum burst pressure in the cylinder and the average peak temperature in the cylinder when burning coal-based synthetic diesel,shorten the flame retardation period and the combustion duration,advance the main heat release time,improve the indication thermal efficiency and reduce the quality of Soot emissions,but the quality of NO_x emissions will increase.Increasing the injection pressure can increase the premixed combustion ratio,increase the peak pressure in the cylinder,the average temperature in the cylinder,and the heat release rate,shorten the flame retardation period and the combustion duration,and advance the main heat release time,but the degree of improvement of the indicated thermal efficiency is limited.The quality of NO_x emissions has increased significantly while the quality of Soot emissions has dropped significantly,making the"trade-off"relationship between NO_x and Soot even more obvious.Decreasing the intake oxygen concentration means increasing the CO₂ concentration,which can reduce the overall pressure in the cylinder and the average temperature in the cylinder,reduce the chemical reaction rate,delay the ignition time and the main heat release time,extend the flame retardation period and the duration of combustion,and reduce the indication thermal efficiency.In terms of emissions control,it was found that the introduction of a small proportion of EGR when burning coal-based synthetic diesel can achieve a significant reduction in NO_x emission quality while ensuring power and thermal efficiency when the increase in Soot emission quality is acceptable.Combining the completed simplified mechanism of n-dodecane/iso-octane/dimethyl ether/ethanol/NO_x/PAH with a three-dimensional simulation model,the effects of different fuel component ratios on the combustion and emission characteristics of coal-based synthetic diesel mixed fuels were explored.The study found:Whether blending ethanol or dimethyl ether into coal-based synthetic diesel fuel,it will improve the combustion characteristics of small amount of pre-mixed combustion and large diffusion combustion when burning coal-based synthetic diesel,which will help increase the pre-mixed combustion ratio.Both the burst pressure and the peak of the heat release rate have increased,the peak mass of OH radicals produced by combustion in the cylinder has increased,and the oxidation activity has increased.Blending dimethyl ether will shorten the flame retardation period and the duration of combustion.The main heat release time is advanced and the indicated thermal efficiency is increased.The addition of ethanol significantly shortened the duration of combustion,but the flame retardation period was too long and the main exothermic time was too late,resulting in a decrease in the indicated thermal efficiency.In terms of pollutant emissions,the addition of ethanol or dimethyl ether effectively reduced the Soot peak mass in the cylinder and the quality of Soot emissions outside the tank.Among them,the role of ethanol is most obvious,but ethanol will also significantly increase the quality of NO_x emissions,while dimethyl ether will reduce the increase in the quality of NO_x emissions.Increasing the blending ratio of dimethyl ether in the coal-based synthetic diesel/dimethyl ether mixed fuel has little effect on the overall pressure in the cylinder,which will slightly reduce the peak value of the average temperature in the cylinder,and gradually reduce the flame retardation period and the combustion duration.The main exothermic moment continues to advance and the indicated thermal efficiency is gradually increasing.The increase in the dimethyl ether blending ratio weakens the"trade-off"relationship between NO_x and Soot.The quality of Soot emissions continues to decrease,while the quality of NO_x emissions increases first and then decreases with small changes.Studies have found that blending a small amount of dimethyl ether with coal-based synthetic diesel can effectively reduce Soot emissions quality on the premise of ensuring dynamic performance and a small increase in NO_x emissions quality.