Study On Stability And Control Technology Of Marine Lean Burn Natural Gas Engine

With the worldwide energy crisis and environmental pollution problems getting more and more serious,developing efficient and clean natural gas engine has been becoming a research focus and extensive choice in the field of marine engine.Lean burn technology can meet the requirements of TIER Ⅲ because of its effectively improving fuel combustion efficiency and reducing NOx emission.However,with the mixture becoming leaner,the cycle-to-cycle variations of natural gas engine become more and more serious,especially near the lean limit.As a result,partial combustion and misfire will make the engine exhibit unstable.Therefore,studying the combustion process and dynamic behavior of lean burn natural gas engine,revealing the relationship between operating parameters and combustion stability,determining the main factors on combustion stability and then effectively controlling them are of great significance to extend the lean limit,improve the fuel economy and reduce the emissions.In this paper,based on an electronic controlled natural gas engine converted by a 2135 marine diesel engine,the combustion stability and control technology of marine lean burn natural gas engine were in-depth studied.And the relationships between operating parameters and combustion stability were revealed,the main factors on combustion stability were determined.And then the combustion stability control was achieved by λ close loop control strategy,in-cylinder combustion close loop control strategy and delay self-feedback control strategy.The main research works were carried out as follows:First,we completed the software and hardware design of natural gas engine’s throttle drive control module,gas injection control module and ignition control module through the modular design.Some typical operating points were chosen in the marine engine’s propeller characteristic curve to analyze how λ,gas injection timing and ignition timing affect the lean combustion stability.In order to improve the stability of initial flame kernel,we further put forward the design and control method of double stage coupling discharge ignition system,through simulations and experiments.We analyzed how double stage coupling discharge ignition’s characteristics influences the stability of combustion.The results show that when the speed is constant,the optimal ignition timing increases with an increasing λ;at the condition of low speed and low load,the coefficient of cyclic variation increases first and then decreases;the influence of ignition timing decreases with the increasing speed and increasing load.The lean burn boundary is mainly affected by the λ and ignition conditions.By the way of controlling the timing and interval of double stage coupling discharge,we can effectively expand the boundary of lean burn.Compared with single discharge,the coefficient of cyclic variation of the double stage coupling discharge reduces from15.7% to 5.21% when λ is 1.85.Then,based on chaos theory and nonlinear dynamic analysis method,lean combustion of natural gas engine is analysed.According to reconstruct the phase space of the combustion system and calculate LLE,influence of boundary parameter on the dynamic characteristics of natural gas engine combustion is revealed,the results show that: the combustion system exhibits a quasi periodic behavior when mixture is thick.if λreaches to the boundary of lean burn,combustion system attractor is loose when IAA reaches to 60°CA or SAA is too low,meanwhile LLE is greater than 0 and shows an increasing trend.This indicates that the system is more sensitive to initial conditions;the combustion system shows obvious chaotic characteristics.At last,the characteristic parameters of combustion have low frequency oscillation characteristics,so combustion closed-loop and delayed self-feedback chaotic control strategy are used to combustion stability control.Compared with air-fuel ratio closed loop control strategy,IMEP and φpmax combined combustion control method IMEP and Pmax combined combustion control method can effectively improve the combustion stability of lean burn natural gas engine,delayed self-feedback chaotic control strategy further improve the combustion stability when λ>1.6.