Research On Pre-ignition Mechanism And Suppression Strategies For Low-speed Two-stroke Dual-fuel Marine Engines

Otto cycle mode low-speed two-stroke dual fuel engine occupies a leading position in the low-speed dual fuel engine market,which can directly meet the harmful emission limit requirements of IMO phase 3 emission standard（IMO tier III）.However,the spontaneous combustion of cylinder lubricating oil leads to the pre-ignition of natural gas,which hinders its further improvement of power and economy,and threatens the safety of the main engine.In order to solve this problem,we must understand the mechanism of natural gas pre-ignition caused by cylinder oil spontaneous combustion,and master the pre-ignition suppression measures on this basis.At present,there is a lack of systematic and quantitative analysis on the influence law of many factors on pre-ignition in low-speed dual fuel engine,and the corresponding pre-ignition suppression strategy is also lack of pertinence.Focusing on these two basic problems,this paper carries out research from three aspects:revealing the chemical combustion promotion mechanism of natural gas pre-ignition caused by cylinder oil spontaneous combustion,quantitatively analyzing the influence law and corresponding control strategy of pre-ignition and exploring high-efficiency technical measures to inhibit pre-ignition.The specific works are as follows:First,shock tube experiment and detailed reaction kinetic mechanism analysis were carried out to study the spontaneous combustion characteristics of natural gas/diesel and natural gas/cylinder oil mixture.Then the chemical combustion promotion mechanism of natural gas pre-ignition caused by cylinder oil spontaneous combustion is analyzed,and the optimization path of dual fuel engine in natural gas/diesel normal combustion mode is revealed.Natural gas,diesel and lubricating oil were replaced by methane（CH₄）,n-heptane(n-C₇H₁₆)and n-hexadecane(n-C₁₆H₃₄).The results show that:（1）the increase of n-C₁₆H₃₄ concentration in CH₄/n-C₁₆H₃₄ mixture will significantly shorten the IDT.In addition,the comparison of the IDT of CH₄ mixed with n-C₁₆H₃₄ and n-C₇H₁₆ shows that the chemical combustion promotion amplitude of n-C₁₆H₃₄ is significantly stronger than that of n-C₇H₁₆.Under the same mixing ratio,the IDT can be further shortened by about 50%compared with n-C₇H₁₆.The results show that the increase of cylinder oil vapor concentration in the natural gas/cylinder oil mixture area in the combustion chamber will further aggravate the chemical combustion promoting effect of cylinder oi.Thus the objective tendency and risk of pre-ignition caused by cylinder oil spontaneous combustion in low-speed dual fuel engine are proved.Therefore,pre-ignition must be restrained from the physical aspects of optimizing engine design and control parameters and reducing the amount of cylinder oil entering the combustion chamber.（2）The study on the spontaneous combustion characteristics of CH₄ mixture also shows that mixing a small amount of n-C₇H₁₆ in CH₄ mixture will also greatly improve the activity of the mixture,and moderately increasing the content of n-C₇H₁₆ can further shorten the IDT.It can be seen that in normal combustion mode,pre-injection of diesel can significantly improve the reaction activity of natural gas-air pre-mixture.Moderately increasing the pre-injection amount will further shorten the IDT and improve the engine performance.Second,using the self-developed visual rapid compression machine,the influence law of multiple factors on natural gas pre-ignition caused by the auto-ignition of cylinder oil is studied,the quantitative analysis results are obtained,and the pre-ignition suppression strategy is formulated.The results show that:（1）the existence of local rich area of natural gas leads to the increase of pre-ignition flame propagation speed by about 10.7 times,the intensity of pre-ignition by about 30 times,and the tendency of pre-ignition by about 57%.Although the changes of MN of natural gas,compression temperature and pressure,single oil droplet size and other factors will slightly advance the occurrence time of pre-ignition,the influence on the propagation speed of pre-ignition flame,the intensity of pre-ignition and the occurrence tendency of pre-ignition is much weaker than that of local rich natural gas region.（2）Therefore,the priority control strategy to suppress pre-ignition is to improve the uniformity of pre-mixture and avoid the accumulation of cylinder oil in the local rich region of natural gas-air pre-mixture.After the design optimization meets the above requirements,the engine can get rid of the excessive dependence on the high MN of natural gas or the reduction of power,so as to solve the contradiction between pre-ignition suppression and engine performance improvement.In addition,the ignition delay period of small volume oil droplets is very short.If the accumulation of multiple small oil droplets may aggravate the tendency of pre-ignition,measures such as improving the structure of scavenging port should be taken to avoid the accumulation of oil droplets in the combustion chamber.Third,the research on fuel injection and mixture quality improvement of low-speed dual fuel engine is carried out by using three-dimensional CFD numerical simulation.The distribution laws of cylinder oil droplets and cylinder oil vapor in the combustion chamber and the concentration distribution law of natural gas-air pre-mixture are obtained through calculation.Based on this,the suppression measures of pre-ignition are studied.The results show that:（1）under the combined action of high-speed scavenging flow and large-scale eddy current,cylinder oil droplets and oil vapor are mainly distributed near the cylinder wall and below the exhaust valve.This leads to local cylinder oil accumulation.The distribution of high concentration oil vapor also coincides with that of high concentration natural gas-air pre-mixture.Thus the tendency of pre-ignition is aggravated.（2）In order to solve this problem,the natural gas injection time is advanced from 228°CA to 226°CA.The lower deflection angle of the natural gas injection valve increases from 0°to 10°.On the premise of strictly limiting the escape of natural gas,the local rich area of natural gas-air mixture that originally coincides with the distribution of oil vapor concentration area can be transferred to the outside of the oil vapor concentration area（cylinder bottom）.And its volume percentage in the combustion chamber is also greatly reduced from 18%to 5%,so as to effectively avoid local rich mixture and inhibit the occurrence of pre-ignition.