| HC and CO emissions are higher at low load for natural gas/diesel dual fuelengine and HC and CO emissions can be controlled by reducing the excessair coefficient. When excessive air coefficient is small, it will produce more NOXandNOXemissions can be reduced using EGR. This paper studies the influence of EGRon performance of natural gas/diesel dual fuel engine at low load and small excessair coefficient condition.The natural gas/diesel dual fuel engine test bench is established. On the basis ofexperiment, numerical simulation platform is built using3D simulation softwareSTAR-CD, and then the debugging and verification of the model are completed. Theboundary conditions are as follows, speed is1335r/min, the mass flow rate of diesel is1.42kg/h, the mass flow rate of natural gas is11.02kg/h, the injection pressure is70Mpa, the injection timing is7°CA BTDC, the injection timing of natural gas is315°CA BTDC and the excess air coefficient is1.4. The influence of EGR ratioon performance of natural gas/diesel dual fuel engine is studied by experimentalmethod. The influence of CO2mixing ratio on performance of natural gas/diesel dualfuel engine is studied by experimental method and simulation method at the sametime.The combustion process of the dual fuel engine can be roughly divided into thefollowing periods, ignition delay period of diesel, rapid burning period of diesel,ignition delay period of natural gas, post combustion period of diesel, rapid burningperiod of natural gas, post combustion period of natural gas. There is no clearboundary between adjacent combustion period and they affect each other and arecoupled with each other. Compared with diesel, combustion duration of natural gas ismuch longer. For natural gas/diesel dual fuel engine, the first peak of heat releaserate is mainly formed by premixed combustion of diesel and the second peak of heatrelease rate is mainly formed by rapid combustion of natural gas.With the increase of EGR ratio and CO2mixing ratio, the oxygen concentrationdecreases gradually due to the increasingly volume of dilution gas in the cylinder; themassand specific heat capacity of the refrigerantin the cylinder increases gradually,the heat capacity of the refrigerant in the cylinder increases gradually, the heatabsorbedof the refrigerant increases gradually, the temperature in the cylinderdecreases gradually. As a result,ignition delay period andcombustion durationperiodincrease gradually, burning quality becomes deteriorate gradually,NOXemission decreases gradually, HC and CO emissions show anincreasing trend. The study results show that: the initial ignition position of diesel islocated on the position between the oil beam central and peripheral air, because theconcentration of the mixed gas is suitable for combustion on that area; the initialignition position of natural gas is the same with diesel and natural gas is ignited bydiesel; there is a lot of NO at the bottom of the combustion chamber close to thecylinder wall and between the piston top and cylinder head; there is a lot of HC at thebottom of the combustion chamber close to the cylinder axis.When the EGR ratio is equal to11.5%, EGR ratio has little effect on thecombustion performance of the engine, NOX, HC and CO Emissions are relativelyideal. When the CO2mixing ratio is equal to8.4%, CO2mixing ratio has little effecton the combustion performance of the engine, NOX, HC and CO Emissions arerelatively ideal. Compared with the real EGR, the influence of CO2on combustionand emission performance of natural gas/diesel dual fuel engine is more significant. |
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