Improvement Of Dual-fuel Engine Combustion Performance And Investigation Of Abnormal Combustion Mechanisms

The natural gas-diesel dual-fuel engine with the Otto cycle mode can directly meet the NO_xrestriction requirements of the International Maritime Organization(IMO)Tier III emission standard,and has developed rapidly in recent years.In order to reduce NO_x emissions and avoid the knocking combustion region,lean burn is widely used in modern natural gas engines.However,lean burn leads to the problems of low combustion rate and high HC emission.In addition,When the lubricating oil droplets enter into the cylinders,they may auto-ignite and cause abnormal combustion such as pre-ignition,then threatening the safety of the engine.Focusing on the above two core problems,this thesis studied the influence of multiple injections of pilot diesel on the performance improvement of a dual-fuel engine through the dual-fuel engine experiment,and investigated the abnormal combustion phenomena caused by lubricating oil.In order to quantitatively analyze the influence of lubricating oil entering the cylinder,a three-dimensional CFD model of the engine was established,and the process of abnormal combustion caused by lubricating oil was simulated,and the influence factors was thoroughly analyzed.At the same time,the effect of changing the injection timing of pilot diesel on suppressing abnormal combustion was also studied.The main work and conclusions of the paper are as follows:(1)The experimental was conducted on a dual-fuel engine with natural gas injection in the intake port to study the influence of two times of pilot diesel injection on combustion performance,and to reveal the influence of the injection timings of pilot diesel injections on combustion performance.The results show that diesel pre-injection can significantly reduce the ignition delay,and increasing the pre-injection amount at any pre-injection time will increase the peak heat release rate of the engine.However,too close interval between pre-injection and main injection will cause violent combustion,resulting in too high peak heat release rate,over high pressure rising rate and vibration noise,which is harmful to the long-term stable operation of the engine.Using reasonable pre-injection timing and pre-injection amount can significantly improve the thermal efficiency of the engine,reduce the peak pressure rise rate,the heat release rate and the CO emissions.(2)According to the experimental study of abnormal combustion caused by lubricating oil entering the cylinder,it was found that lubricating oil can significantly increase the cylinder pressure and the peak heat release rate of the engine.The CAD model of the test engine was drawn and the CFD numerical model was established.The CFD simulation software was used to quantitatively analyze the abnormal combustion phenomenon caused by lubricating oil entering the cylinder.The results show that for a naturally aspirated dual-fuel engine,it is difficult for lubricating oil to cause pre-ignition,but the lubricating oil vapor can enhance the reacting activity of the natural gas mixture,so that the peak heat release rate and the peak cylinder pressure increase significantly,resulting in rough combustion.The sooner the lubricating oil enters the cylinder,the more total evaporation,and the more dangerous it is.In this case,delaying the pilot diesel injection time can reduce the maximum cylinder pressure and heat release rate to a certain extent and inhibit rough operation.However,for a turbocharged dual-fuel engine,the lubricating oil droplets that enter the cylinder will become the ignition core and ignite the mixture before the pilot diesel injection,greatly advancing the start of combustion,increasing the peak cylinder pressure and pressure rising rate.In addition,the numerical simulation results show that once the turbocharged dual-fuel engine appears pre-ignition,the control of pilot diesel injection on combustion is completely lost,and the load must be reduced in time to ensure engine safety.