Analysis Of Ignition And Combustion Characteristics Of Diesel Engine Under Special Environments

The liquid oxygen solid carbon closed-loop diesel engine is suitable for use in underground and enclosed environments as it operates without competing air with workers,and solves the problem of asphyxiation and poisoning caused b y CO2 emissions.However,its ignition and combustion environment is unique(CO2/O2),and the high concentration of CO2 can lead to unstable ignition and combustion of diesel,making it prone to misfire in variable working conditions.Therefore,it is necessary to research the ignition and combustion characteristics of diesel under CO2/O2 atmosphere,which can better guide the application of liquid oxygen solid carbon closed-loop diesel engine.This study investigates the ignition and combustion characteristics of n-heptane as a surrogate fuel of diesel under different CO2/O2 atmosphere.A simulation model is developed based on actual constant volume combustion bombs,which considers the chemical effects of CO2 and updates the reaction kinetics parameters.A simplified mechanism for n-heptane,suitable for CO2/O2 atmospheres,was proposed.The visual constant volume combustion bomb experimental platform was constructed to verify the accuracy of the simulation results.The high-speed camera was utilized to record the ignition and combustion process at CO2 volume fractions of 35%,40%,50%,and 60%under initial conditions of 850 K temperature and 3 MPa pressure.Various parameters were obtained through image processing techniques,such as ignition delay time,ignition position,flame length characteristics,and accumulated spontaneous emission intensity.The impact of physical,direct chemical and third-body effects of CO2 on the ignition delay time was analyzed using the closed homogeneous reactor in CHEMKIN.Research indicates that as the volume fraction of CO2 increases,the ignition delay time also increases,and the ignition location moves away from the nozzle.At the volume fraction of CO2 is 60%,the maximum error between the simplified mechanism calculation and experimental results is 10.1%.At the volume fraction of CO2 is 35%,the ignition and combustion states are unstable.Moreover,as the volume fraction of CO2 increases,the peak temperature of the flame decreases and the OH production decreases.The average length initially increases and then decreases,while the average width increases and the cumulative spontaneous emission intensity decreases.The decrease in average length and cumulative spontaneous emission i ntensity becomes more significant when the CO2 volume fraction exceeds 50%,with a decrease of 47.8% and 34.4%,respectively.However,the temperature and OH production curves become smoother,indicating a more stable combustion state.At the volume fraction of CO2 is 60%,the obvious flame lift phenomenon occure after ignition,and n-heptane is not completely burned with an efficiency of only 87.9%.Sensitivity and reaction path analyses revealed that with the increase of CO2 volume fraction,the rate of OH generated from H2O2+M→2OH+M increases,and the effects of low-temperature chain reactions of n-heptane and H2O2 on ignition become more significant.The physical and direct chemical effects of CO2 increase the ignition delay time,while the third body effect shortens it.Moreover,the third body effect increases the rate of OH production and H2O2 decomposition by 40.6% and 40.4%,respectively,and promotes the reduced ignition delay time by 5.68 times.The chemical effects of CO2 become more prominent,with the ratio of chemical effects to physical effects increasing from 7.60% to 17.32%.