Study Of Marine Diesel Engine Combustion System Parameters Coordination Control Based On Low-Temperature Combustion Technology

In order to meet the increasingly stringent emissions restriction,it is indispensable to improve the combustion and emissions technology of marine diesel engines.The trade-off between NO_x,soot and ISFC brings new challenges for the development and application of innovative techniques that could reduce the engine’s NO_x-soot emissions with the lowest possible fuel penalty.Based on the high efficiency and clean combustion technology route of low-temperature combustion diesel engine,this paper started from the control of in-cylinder combustion temperature(intake air humidification or exhaust gas recirculation)to reduce the NO_x emissions of marine diesel engines,adopted the technical measures of reducing the soot emissions by the advanced injection timing strategy or oxygen-enriched combustion technology,and cooperatively studied the combustion optimization path that simultaneously reduces NO_x-soot emissions without loss of power of marine diesel engines.The technical route to achieve Tier III emission standards was also been analyzed in this paper.In this research,numerical studies were conducted on a four-stroke supercharged intercooled marine diesel engine that meets Tier II emission standards under 75%loads at 1350 rpm by using AVL Fire code.The intake oxygen concentration varied from 21%to 24%in increments of 1%;humidity ratio ranged from 0 to 100%in increments of20%;EGR rate ranged from 0 to 25%in increments of 5%;the advanced SOI(start of injection)varied from 14.5°CA BTDC to 20°CA BTDC.In order to improve NO-soot emissions without serious penalty in power,two coupling strategies were studied,namely oxygen-enriched combustion(OEC)coupled with intake air humidification(IAH)and OEC coupled with exhaust gas recirculation(EGR).The result indicates that higher in-cylinder pressure and temperature,shorter combustion durations and lower soot emissions,higher indicated power and NO emissions are observed when only OEC is applied,while opposite trends are found when only using IAH or EGR.Analysis results reveal that the NO-soot emissions can be reduced to lower than that of the original engine without loss of power by optimized combination of oxygen concentration and humidity ratio.Reduction of NO and soot can be realized simultaneously and power loss is not exceeding 2%of the original engine by applying oxygen concentration combined with EGR rate.There are nine kinds of combinations of OEC coupled with EGR to meet the Tier III standard,while OEC coupled with IAH has only one combination to meet this regulation.In addition,based on combustion technologies of homogeneous charge compression ignition(HCCI)and low temperature combustion(LTC),the effect of IAH and EGR on combustion and emission characteristics of marine diesel engine at advanced injection timing were studied.The result indicates that lower in-cylinder pressure,temperature and NO emissions,higher ISFC and soot emissions are observed when only IAH or EGR is applied,while opposite trends are found when only using advanced SOI.The proper combination of different SOI respectively with humidity ratio and EGR rate can improve the trade-off relationship between NO and soot.Meanwhile,the increase in ISFC is improved by using advanced SOI under high EGR rate or humidity ratio.Compared with the advanced SOI coupled with EGR,advanced SOI coupled with IAH results in less loss of ISFC.Analysis results reveal that both above-mentioned coupling schemes can achieve low NO-soot emissions while ensuring that ISFC does not increase.There are ten combinations of advanced SOI coupled EGR that can achieve NO emissions to meet the Tier III standard,while advanced SOI coupled IAH has only one combination to meet this regulation.