| In order to actively cope with environmental pollution and global climate change,the emission regulations of marine engine NOx and PM emissions at China and abroad are being tightened year by year.Existing diesel engines need to take a series of measures to meet the ship emission regulations,which will significantly increase the ship operating costs.Compared with traditional diesel fuel,LNG(liquefied natural gas)as the engine fuel has the advantages of cleanliness,high efficiency and economy,etc.Therefore,LNG has attracted worldwide attention.When LNG is applied in compression ignition engines,the pilot diesel oil and natural gas(gasified LNG)are injected directly into the cylinder through a specially designed injection device.Therefore,this type of engine is also called the high pressure direct-injection LNG engine with micro-diesel pilot ignition(HPDI engine).This type of engine can not only maintain the equal power with the diesel engine,but also significantly reduce pollutant emissions.It has broad application prospects in the marine propulsion device.In this thesis,a three-dimensional model for the HPDI LNG engine ignited with the pilot diesel was developed via the CONVERGE software according to the geometrical parameters of the combustion chamber and the operating parameters for the engine.The characteristics of dual-fuel high-pressure injection,combustion and emissions of the HPDI engine were analyzed.On the one hand,the influence of fuel injection timing,natural gas injection duration,natural gas injection timing,diesel injection duration and diesel injection timing on the characteristics of spray,combustion and emissions of the engine was discussed.On the other hand,the global sensitivity analysis method was used to get deep understanding on the main factors affecting the macro performance most and the influence of those factors on macro performances.The results are as follows:As the fuel injection timing was advanced,the increase rate of liquid length in the free development stage of diesel injection,the peak combustion pressure and NOx emissions increased,while the stability value of the sauter mean diameter,CO and unburned HC emissions decreased.Besides,PM emissions first decreased and then increased with the advance of fuel injection timing.As the duration of natural gas injection decreased,the disturbance of natural gas jet to diesel fuel spray enhanced and the peak combustion pressure,heat release rate and NO_x emissions increased,while CO and unburned HC emissions decreased.PM emissions decreased first and then increased with decrease in duration of natural gas injection.As the natural gas injection timing advanced,the time of diesel spray effected by natural gas and the ignition time of natural gas advanced,and meanwhile,the peak heat release rate increased.When the natural gas injection timing was further advanced,the phenomenon of two-staged combustion disappeared.At the same time,with the advance of natural gas injection time,CO and unburned HC showed the trend of decreasing first and then increasing.NOx emissions showed the trend of increasing first,then decreasing,and then suddenly increasing,while the reverse holds for the PM emissions.As the diesel injection duration decreased,the slope of spray penetration increased,and the breakage time of diesel droplets advanced,and then the stable value of droplet average diameter decreased.The ignition time advanced and the peak value of diesel combustion heat release rate increased with the decreased of diesel injection duration,while the peak value of heat release of natural and the emissions of CO,unburned HC and NOx decreased.PM emissions increased first and then decreased with the decrease in the duration of natural gas injection.With the delay of diesel injection timing,the time of diesel spray effected by natural gas,the ignition time of natural gas,and the corresponding time of maximum pressure advanced,while the ignition time of diesel delayed.As the diesel injection timing delayed,the peak heat release rate of natural gas combustion was reduced,while the maximum pressure in cylinder increased.When the injection time of diesel fuel was further delayed,the phenomenon of two-stage combustion disappeared and even misfire occurred.At the same time,with the delay of diesel injection timing,unburned HC and NO_x emissions showed a trend of first rising and then decreasing,and CO and PM emissions showed a trend of first rising,then falling and then rising again.The global sensitivity showed that the fuel injection time is the most influential parameters on the maximum liquid length of diesel injection,Δ,CA 90-0 and NOx,PM,CO and CH4 emissions;The diesel injection duration is the most influential parameters on the minimum sauter average diameter of diesel droplets and CA 5-0;The natural gas injection duration is the most influential parameters on CA 50-0.At the same time,the influences of those factors on the corresponding macro performance were represented via the least squares fitting method.The conclusions were consistent with the result described in Chapter 3.The influence of operation parameters on the macro performance of the HPDI engine can be illustrated more clearly via the global sensitivity analysis. |
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