A Study On Thermal Efficiency Improvement Of Diesel Engine By Low Heat Rejection Combustion Coupled With Turbocompound System

With China’s carbon peaking by 2030 and carbon neutrality by 2060 approaching,commercial vehicles,as a significant carbon emitter,contribute more than 50% of carbon emissions with 20% of the market holdings,and the pressure to reduce carbon is increasing.Although more industry trends judge electrification/hydrogen fuel cell as the future direction of vehicle carbon reduction,commercial vehicle diesel engine,as the main means of production,is still the most dominant commercial vehicle power in the near to medium term due to its high-power density,low operating cost and reliable and durable characteristics,and remains the focus of commercial vehicle carbon reduction.Therefore,as the main power of medium and heavy commercial vehicles,automotive diesel engines urgently need to improve energy efficiency further and reduce carbon emissions.The energy generated in an internal combustion engine is mainly converted into three parts: work,exhaust energy and heat transfer.50% BTE can achieved for the modern commercial vehicle heavy-duty diesel engine by the use of high-efficiency clean combustion,advanced air management,high-pressure common-rail fuel injection,low friction,and other advanced technologies.Further improving the efficiency of diesel engines has become increasingly difficult.The use of low heat rejection technology to reduce engine heat dissipation from the engine and enhance the utilization of exhaust energy,thus improving the total engine energy utilization efficiency,is one of the important directions to further improve the brake thermal efficiency(BTE)of diesel engines.In this paper,low heat rejection(LHR)combustion technology in the cylinder is studied for a heavy-duty exhaust turbocharged diesel engine to reduce heat dissipation in a cylinder and improve indicated thermal efficiency(ITE)and exhaust energy.The matching technology of turbocharger and power turbine is studied to increase the recovery and utilization rate of exhaust energy.The coupling of LHR combustion and turbocompounding technology is studied to improve the brake thermal efficiency of commercial vehicle diesel engines.The research in this paper provides a reference for the engineering application of high efficiency and LHR combustion system design and optimization,turbocompounding system matching etc.,which will promote the sustainable development of diesel engines for CN VI emission heavy commercial vehicles.The main research contents and related conclusions of this paper are as follows.(1)Analysis of the energy-saving path and potential of low heat rejection diesel engine combustion systemBased on the first law of thermodynamics,this paper studies the energy distribution law of in-cylinder combustion and unifies the full-text combustion energy analysis method.Based on the second law of thermodynamics,this paper carries out the analysis of the exergy utilization of LHR diesel engines.Based on the second law of thermodynamics,the analysis of the LHR diesel engine’s heat and power utilization is carried out.Through simulation analysis,the technical paths and ideas of using thermal insulation coating,combustion optimization,water temperature enhancement,etc.to improve the exhaust energy grade;using a power turbine to recover the exhaust pressure energy;using intercooler temperature control to reduce the pumping gas loss,reduce the exhaust heat retention radiation loss;using waste heat recovery to recover the exhaust remaining exergy,gradually improve the total energy efficiency of LHR diesel engine,which provides theoretical guidance for the research of this paper.(2)Research on "thermal swing insulation coating."To reduce the in-cylinder heat dissipation by using thermal insulation coating,this paper systematically carried out research on the thermal insulation coating modeling method,thermal insulation coating characteristics and its influence on the thermal efficiency of diesel engines,and ideal thermal insulation coating characteristics,thermal insulation coating preparation and engine test.To realize the rapid simulation of heat transfer characteristics of the thermal insulation coating,a 1D CHT model is established in this paper.Compared with the heat transfer model of traditional Conjugate Heat Transfer(CHT)calculation and the actual measurement results in the cylinder,the 1D CHT model can accurately simulate the heat transfer of the combustion chamber wall by refining the boundary layer mesh to more than 10 layers.The results show that the in-cylinder indicated thermal efficiency increases by 0.4% when the top wall of the piston increases from 575 K to 775 K.Compared with steel or aluminum piston,the use of coating material with thermal swing coating(TWC),such as Si RPA,has a better energy-saving effect through low heat dissipation;this paper establishes the Do E model with different In this paper,Do E models with different coating characteristics are established,and the effect of coating characteristics on the piston and the bottom of the cylinder head on the engine economy is studied using response surface analysis,and the ideal coating characteristics are optimized.The analysis results show that the piston and cylinder head coated with the ideal coating can achieve a low heat dissipation combustion chamber and increase the in-cylinder indicated thermal efficiency of the engine by 4.6%.The analysis also shows that the piston top coating has a more significant effect on the energy saving of an LHR diesel engine compared with the cylinder head bottom coating;the better the following of the coating surface temperature and the fluctuation of the in-cylinder gas temperature,the more significant the energy-saving effect is,which theoretically proves that the thermal swing is more effective than thermal barrier coating.Theoretically,it is proved that the thermal swing coating is more suitable for LHR engines.In this paper,two thicknesses of thermal insulation coatings were prepared by using the plasma spraying method.By comparing with the heat balance results of the original engine,the actual coatings made the average indicated mean effective pressure(IMEP)in the cylinder increase by 1.3%,and the exhaust temperature of the full load speed of the engine increased by 10～20℃,which verified the low heat dissipation effect of thermal insulation coatings and improved the exhaust energy quality.The study based on the characteristics of the thermal insulation coating provides a good guide and reference for the development of the actual coating and its application in LHR diesel engines.(3)Optimization study of LHR combustion systemTo realize the LHR combustion system with thermal insulation coating,the optimization study of the LHR combustion system is systematically carried out in this paper,taking into account the thermal efficiency,emission and low heat dissipation performance.In this paper,A open-type combustion used in diesel engines was chosen as the initial combustion chamber.Firstly,the influence of single factors such as fuel injection parameters,swirl ratio and their combinations on the thermal efficiency of diesel engines was studied,and arrive at the optimization direction of LHR combustion system with small surface volume ratio,low swirl and large flow nozzle.Secondly,a merit-function of combustion system with the goal of improving the BTE of diesel engines was established,and the research on the optimization method of low heat rejection and high-efficiency combustion system was carried out.Finally,through the optimization test of the combustion system with coating,an LHR combustion system suitable for the original engine was developed.The test results show that although the thermal efficiency of the developed LHR combustion system is low due to the need to consider the low heat dissipation requirement,the original efficiency can be achieved,and the LHR effect with the increase of discharge temperature can be obtained by optimizing the timing of the main injection,add and adjust the timing of pre-injection and post-injection,and increasing the injection pressure.The optimization direction and evaluation optimization method of the LHR combustion system carried out in this paper has good reference significance for low heat loss combustion development,and the experimental study of low heat loss combustion system carried out verifies the development method and effect of low heat loss combustion system.(4)Research on the matching theory and strategy of turbocompounding systemTo make full use of the energy of exhaust,optimize the distribution of exhaust energy in the turbocharger’s turbine and power turbine,and solve the contradiction that the increase of exhaust back pressure caused by the increase of power turbine leads to the decrease of diesel engine energy efficiency,this paper carries out the research on the theory of turbocompounding system and matching optimization strategy.This paper firstly analyzes the mathematical expression of the turbocompounding system and establishes the theory and method of turbocompounding system matching based on the mass flow coefficient(MFP).Secondly,with this MFP as the variable,the effect of the turbocharger’s turbine,power turbine and transmission ratio on diesel engine economy in the turbocompounding system is simulated and analyzed.The study shows that the matching rules of turbocharger’s turbine and power turbine are opposite at different operating speeds of diesel engine.The diesel engine needs a combination of a large turbocharger’s turbine and a small power turbine to improve the economy at low speed;the opposite combination is needed to improve the economy at high speed.The impact of a turbocharger’s turbine on diesel engine economy is more noticeable compared with a power turbine,and the energy-saving potential obtained is more significant;Third,for the characteristics of turbocompounding systems more suitable for long-distance commercial vehicles,the simulation and experimental research of fix geometry turbocharger(FGT),variable geometry turbocharger(VGT)and wasted-gate turbocharger(WGT)matching the power turbine with MFP as the primary variable is carried out purposely,and the matching of turbocompounding system is established methods and processes.(5)Integration and experimental study of LHR combustion coupled turbocompounding systemThis paper builds a diesel engine with LHR combustion coupled with a turbocompounding system.Systematically experiments were carried out on LHR combustion concepts such as improving the compression ratio,raising the intercooler temperature,and adding the thermal insulation coating to achieve the total energy utilization efficiency of the engine and giving the technical solutions and paths for energy saving of diesel engine based on LHR combustion.A heavy-duty diesel engine was optimized,and the thermal efficiency was improved by 3%,which verified the effectiveness and efficiency of the method of LHR combustion coupled with a turbocompounding system to improve the engine economy.This paper systematically researches the critical technologies of LHR diesel engine with thermal insulation coating,LHR combustion system optimization and turbocompounding system matching,integrates in-cylinder LHR combustion and outcylinder turbocompounding system,realizes the theory and method of LHR combustion coupled with turbocompounding system to improve the brake thermal efficiency of diesel engine,and the process and method in the study provide a good reference for the engineering application of this technology,and gives a good contribution to the diesel engine to achieve the low carbon emission.