Effects Of Nozzle Structure And Injection Timing On Intake And Combustion Characteristics Of Marine Gas Engine

There is a higher demanding of economic and emission properties of Diesel Engine along with the energy crisis and increasingly stringent emission regulation. Natural gas engine,although developing rapidly for the abundant fuel and good emission performance, is still confused by low flame speed in mixing gas, thus it’s essential to investigate how to generate mixing gas from which rapid flaming may benefit in later stage of compression. Gas injection timing could impact the sequence of natural gas entering cylinder, then effects generating of mixing gas. And the nozzle, which is one of the most important parts of fuel supply system,influences the homogeneity of engine intake flow. This paper,which is based on a natural gas engine reformed by 2135 marine diesel, has studied the influences of nozzle structure and injection timing on mixture formation and combustion and exhaust emission in the engine by numerical simulation.Three kinds of nozzle structure has been presented, then geometrical model including air throttle, intake manifold, nozzle, inlet duct, air valve and combustor has been established through three dimension cartography software named CATIA. AVL FIRE,fluid mechanics computation software, is used to discretization of the model and determining initial and boundary conditions. Finally the model is validated by reasonable turbulence, ignition, and emission models and calculation approach.The influence of intake,compression,combustion and expansion process,which have been simulated with vary gas injection timing, on the generating of mixing gas and flame propagation course has been analyzed particularly. And the result shows that delaying gas injection timing rationally may contribute to produce better hierarchical distributed mixing gas with higher and lower destiny near and far away from sparking plug respectively, which may increase flaming speed and then ameliorate engine performance. For the specific engine,the flaming speed of mixing gas is faster with 25% and 100% workloads when the timing is after thirty and forty-five degrees of top dead center respectively.The effect of intake, compression, combustion and expansion process calculated with vary nozzle structures on the homogeneity of engine intake flow and flaming characteristic has been investigated. And the result indicates that nozzle structure has discernible effect on the mixture uniformity in in-cylinder. Vertical injecting direction between natural gas and fresh air may enhance collision and mixture of the jet flow and fresh air and generate more uniform mixing gas before the gas injects in cylinder. For the specific engine, the distributing of mixing gas is almost the same with different nozzle structures at the ignition time. At last,the multi-orifice nozzle with thirty degrees bevel keeps higher turbulent energy in combustion process, which contributes faster flaming speed of mixing gas, thus is the superlative nozzle structure.This paper revealed the influence of the nozzle structure and injection timing on engine intake mixture and combustion in lean-burn condition, provided basic research data to optimize nozzle structure and injection timing.