| Due to environmental pollution and energy crisis, development and utilization of gaseousfuel, such as CNG etc, have become the cosmopolitan research topic. At present, CNG is oneof the widely studied gaseous fuel all over the world, which is known as the most promisingalternative fuel in the21stcentury by its characteristics, such as high octane rating, worldwidereserves, low cost and low emission. By utilizing the characteristics of natural gas,CNG/diesel dual fuel engine can maintain high thermal efficiency, improve use economy andeffectively control the exhaust emission such as NO and Soot, which has promisingdevelopment prospects.The purpose of this paper is to study on the effect of diesel injection characteristics oncombustion and exhaust pollution of a CNG/diesel dual fuel engine with high compressionratio and CNG substitution rate. Using software STAR-CD, the combustion process ofCA6DL2CNG/diesel dual fuel engine has been numerical simulated at1335rpm,75%loadcondition. The CNG substitution rate under this condition is61.53%, the injection timing andpulse width are718°CA and2.64°CA, besides, the top dead center is at720°CA. Andthen, basing on this condition, the simulation has been done by varying diesel injectionprogram, including injection timing (716,720,722°CA), injection pulse width (1.96,3.16°CA) and two-stage injection of diesel (the proportion of second-stage injection is about24.2%and4.5%). The effect of injection characteristics above on the combustion processhave been analyzed from the aspects of macroscopic quantity and microscopic phase-fields.Through the simulation analysis, some conclusion can be draw. The combustion of dualfuel engine can be divided into three parts, including the period of diesel premixedcombustion, the period of diesel diffusion combustion, the main part of duration of naturalgas. The third one was starting after730°CA, and at that time, the curve of pressure ofcombustion in cylinder has been decreased.When the diesel injection timing was delayed, the phase of combustion of total fuel incylinder would be lagged, the duration of diesel diffusion combustion would be increasing,while it is advantage for the initial combustion of natural gas. Under the condition ofinjection timing of722°CA, the ignition delay of diesel was increasing by7%and the oneof natural gas was decreasing by11%, but the second peak value of combustion pressure was decreasing by23%, the phase of it was delayed by5.75°CA; the CA50of total fuel incylinder was delayed by5.84°CA when it was compared to the condition of injection timingof716°CA.When the diesel injection pulse width was shortened, the combustion rate of diesel wasincreasing, as well as the average turbulence in cylinder, which could enhance the rate ofcombustion of total fuel in cylinder. Compared to the condition of injection pulse width of3.16°CA, under the condition of1.96°CA, the second peak value of combustion pressure incylinder was increasing by13%, the phase of it was advanced by2.5°CA, the maximumpressure rise rate was increasing by38%, the phase of it was advanced by2.25°CA, theaverage turbulence in cylinder was increasing by more than15%after750°CA.The amount of ignition point under the swo-stage injection mode was larger than thesingle-stage mode, and this tendency is more obvious by increasing the quantity of secondinjection, which could shorten the ignition delay and the period of combustion of diesel andnatural gas. Compared to the condition of single-stage injection, when the second-stageinjection quantity among the total quantity of diesel was24.2%, the ignition delay of dieseland natural gas were decreasing by12%and5%, the duration of combustion of them weredecreasing by46%and19%, the second peak value of combustion pressure in cylinder wasincreasing by11%, the maximum pressure rise rate was increasing by99%, the rate of heatrelease was increasing by58%. |
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