Research On Improving Comprehensive Performance Of Marine Low-speed Diesel Engine With Small Cylinder Diameter Based On Combined Application Of EGR And WHR

With the continuous development of global trade,maritime transportation industry plays an important role in economic globalization.Marine low speed diesel engine plays an important role in the field of ocean-going ships because of its high reliability,thermal efficiency and low cost.However,marine diesel engines are still faced with high NOx emission and difficulties in improving thermal efficiency.Due to the increasingly strict emission laws and regulations and the strict restriction of energy efficiency index requirements,the work of energy saving and emission reduction for diesel engine has very important practical significance.The combined application of waste heat recovery technology(WHR)and exhaust gas recirculation technology(EGR)of marine diesel engine can not only improve the thermal efficiency of diesel engine,but also reduce the NOx emission.Therefore,in order to achieve the goal of energy conservation and emission reduction of marine diesel engine,MAN 6S35ME-B9.5 two-stroke low-speed diesel engine being regarded as the research object,this paper carried out the coupling mechanism and matching strategy of WHR and EGR,and propose the waste heat recovery scheme for EGR diesel engine and the control strategy of waste heat recovery system.The main work will be carried out as follows:(1)Three-dimensional modeling was carried out for the target models with Converge simulation software to find the EGR rate meeting the Tier III emission standard and analyze the impact of exergic flow in the cylinder.The results showed that NOx is easy to be generated at low equivalent ratio and high temperature.The increase of EGR rate can lead to the decrease of temperature in most areas of the cylinder,which has an obvious effect on the inhibition of NOx.When the EGR rate reached 25%,NOx emissions met Tier III standards.On the other hand,the negative effect of EGR rate on diesel engine cannot be underestimated.The results showed that the increase of EGR rate decreased the combustion reaction rate and the combustion duration side length.At 25% EGR rate,fuel consumption of the diesel engine was increased by 1.8g /k Wh and cylinder pressure is decreased by 6.8bar compared with EGR rate of 5%.In addition,with the increase of EGR rate,energy efficiency increased first and then decreased.Exergic exergy and exergic energy coefficient decreased first and then increased.(2)GT-Power simulation software was used to establish a one-dimensional simulation model of the target model,which was verified based on the experimental data,in order to simulate and calculate the Eco-EGR,ECA-EGR and Non-EGR operation modes of the diesel engine.Then,according to the thermodynamic analysis results of the target model and the operation characteristics of the dual-pressure steam Rankine cycle system,this paper carried out a matching design of the efficient steam Rankine cycle of comprehensive waste heat recovery and utilization and diesel engine,proposing a combined waste heat recovery system integrated with EGR and WHR.The results show that the output power of the turbine can reach257.76 k W when the diesel engine load is 100% in Tier II mode.In Tier III mode with 75%load on the main engine,the turbine can produce 175.5k W.In addition,the dynamic simulation model of the joint system was analyzed.The results showed that the waste heat recovery system has good response and transient performance when the diesel engine has sudden load change.(3)The influence of EGR rate change on the overall performance was analyzed.The results showed that the increase of EGR rate will lead to the increase of the power consumption of the blower in EGR system.Meanwhile,the power of the diesel engine decreased slightly and the fuel consumption increased in ECA-EGR mode.On the other hand,when the EGR rate was less than 25%,the engine exhaust temperature increased with the increase of the EGR rate,while when the EGR rate was greater than 25%,the engine exhaust temperature decreased with the increase of the EGR rate.In addition,the increase of EGR rate can also reduce the main engine exhaust flow rate.Then,this paper carried out performance optimization calculation for the joint system.Through optimization methods such as injection timing and intake and exhaust by-pass strategy,the power generation ratio of waste heat recovery system can reach 5.4% and the total thermal efficiency of the whole machine can reach 49.43% at 100% load and meeting the Tier III emission standard,which was 2.53% higher than the original ECA-EGR diesel engine.By changing the valve opening and load size,the main engine can be switched between ECA-EGR mode,Low-EGR mode and Non-EGR mode to improve the applicability of the ship.The research results in this paper have practical significance and provide reference for energy saving and emission reduction of low-speed machine.