| The harmful emissions of dual-fuel engines using natural gas as the main fuel are significantly lower than those of conventional diesel engines.Therefore,the promotion and application of dual-fuel engines is of great significance for solving environmental pollution and energy problems.And the shape of the combustion chamber has an important impact on the engine's fuel atomization,gas mixture formation,and combustion processes.So the use of a reasonable combustion chamber structure is essential for improving the performance of dual fuel engines.In this thesis,the 135 single-cylinder engine is used as a prototype machine,and uses three-dimensional CFD software FIRE to simulate and calculate the working process of three different shapes of combustion chamber on the 135-cylinder dual fuel mode.The results show that when the double-layered flow combustion chamber is operated in the dual-fuel mode,the pilot diesel fuel injection is small,the injection duration is short,and the diesel fuel spray penetration is short.The pilot diesel fuel has burned before it realizes double-layer flow.The fuel combustion performance has no significant effect.At the same time,the double-layered flow combustion chamber operates slowly in the dual-fuel mode,and the post-combustion is severe.The center of gravity of the entire combustion process is about 10°CA after the TDC.The turbulent intensity gradient of the double-layered flow-combustion chamber is small,and the overall strength is small.There is no obvious strong turbulence area in the fuel injection area,which is disadvantageous for the diesel to rapidly ignite natural gas.The necked-type combustion chamber has a relatively strong squish flow,and the turbulence intensity in the crater region is high,which is favorable for the rapid ignition of natural gas by diesel fuel and the combustion speed is fast.However,with the acceleration of the combustion speed,the maximum temperature in the cylinder rises and the NO_x emission increases.NO_x in the necking type II combustion chamber is 11.8%higher than that in the deep?combustion chamber,and 44.7%higher than that in the double-layered flow combustion chamber.The overall soot emission level of the dual-fuel mode is relatively low.Due to the high combustion temperature of the necked combustion chamber,the later stage of soot is oxidized to a large extent,and the emission level is the lowest.The soot emissions of the double-layered flow combustion chamber are significantly higher than those of other combustion chambers.The?combustion chamber's performance is between the necking type and the double-layered flow combustion chamber.At 75%load,the mixture gas concentration is lower,the combustion rate is reduced,the effect of necking type combustion chamber on the combustion speed is reduced,but the combustion rate is still the fastest,and the double layered combustion chamber is the slowest.Calculations also have found that a moderate reduction in speed at 75%load can make natural gas combustion more adequate.By increasing the amount of pilot diesel and reducing the replacement rate,the disadvantage of slower combustion at lower loads can be improved.Comprehensive comparison of the effects of these three types of combustion chamber on combustion performance revealed that for premixed natural gas,the use of a large turbulent combustion chamber is beneficial to improving the distribution of pilot diesel and improving the combustion speed of natural gas;the combustion rate of double-layered combustion chamber is slow.There is no obvious effect on the combustion of premixed natural gas.Calculations show that the necked combustion chamber has better combustion performance in the pure diesel and dual fuel modes,and the low load performance in the dual fuel mode is inferior to the high load performance. |
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