| As the vehicle population increase rapidly.the global energy crisis and environmental pollution Increasingly serious,exploring alternative fuel become the main researching direction of modern internal combustion engine. Recently, natural gas as an abundant alternative fuel has been attracted by many researchers owning to its advantages such as low emission,economy, security. Natural gas was stored as CNG or LNG, CNG engines have poor storage facilities and short range mileage, LNG liquid storage technologies save the storage space and improve the driving range. LNG car now mainly adopt LNG vaporization outside the cylinder and apply pre-mixed gas, or inject LNG directly into the intake manifold. Which always have the questions about the lower pneumatic efficiency, uneven mixture and so on.The energy in LNG vaporization can be used to reduce the cylinder temperature, as well as increase the driving range further if LNG liquid spray to achieve. In order to simulate the LNG liquid injection process, a LNG liquid spray model is set up with AVL-Fire, after refining the grid of spray area, Establish the LNG fuel-Characteristics data bank from liquid phase to gas phase in Fortran. Simulate the three-dimensional LNG liquid injecting process. The main contents of the policies can be summarized as follows:Flash boiling occurs in the LNG liquid injection process, after reading a great number of literatures on relevant research field, this paper introduces the basic principles of flash boiling spray, The simulation result shows that the flash boiling phenomenon appears during the LNG spray process,2℃A crank angle after injection, the liquid phase LNG rapidly boiled into a gas. within 4℃A crank angle, the SMD of LNG dropletreduced from 150μm to 35μm.after the subsequent evaporation process, the SMD can reach to 5μm extremely.when the back pressure is 1 atm, injection pressure is1MPa, results show that the spray penetration increases with the enhancement of nozzle opening pressure, but not so significantly, the penetration distance keeps 62mm/60mm when the injection pressure is 2/4MPa. LNG spray can still keep the characteristic in a high injection pressure when it was at a lower pressure.When the fuel injection pressure is 2MPa, and the environment pressure is 1bar/1.5bar/1.8bar respectively, the penetration of LNG spray is 60mm/50mm/34mm, from this data we can see the back pressure affects the LNG spray characteristic obviously. At the same time, we do the research about the influence of LNG spray and environment temperature to the spray characteristic. When the fuel was 110K/170K, the LNG spray penetration and SMD data float no more than5%, while the background temperature affects the result obviously, when the temperature changes from 25℃to 45℃, the time that LNG spray droplet completely vaporized decreased by 20 crank angle. At the same time. The investigation about the influence of different factors on the flow field, velocity field, and concentration field when LNG liquid injection.Analysis showed that the cylinder's temperature dropped significantly, the maximum temperature is 718K at TDC, the average temperature is about 600IK, the injection timing affects mixture formation process obviously, the earlier injection timing, the more uniform mixture, when the injection timing is 80℃A BTDC, the mixture has a good quality, at TDC the equivalence ratio is 0.6, while the injection time pushed back, the mixture at TDC stratified obviously.when LNG spray injected at 320℃A crank angle, the mixture at TDC with equivalence ratio 1.2 max,0.3 min. the organization of chamber shape and The angle which between spray axis and chamber axis Swirl ratio influenced the LNG spray droplet evaporation and the mixture uniformity. When LNG injected at 80℃A BTDC,130°ngle leads to a better mixture (equivalence ratio 0.6), that is 0.72 when 90°angle. And the swirl ratio also influenced the mixture formation process,but not so obviously. When the simulation took the compression ratio as 12, the max-equivalence ratio of mixture in cylinder is 1.2, that is 0.78 as in the combustion of 14 compression ratio. LNG liquid spray can take a larger conpression ratio as its faster evaporation, and the temperature in cylinder at TDC is suitable for ignition... |
