Study On High-efficiency And Controllable Combustion Technology Of High Power-density Marine Diesel Engine

With the environmental deterioration and energy crisis becoming increasingly prominent in recent years,carbon emissions,especially CO₂,are drawing more attention than ever.For diesel engines,CO₂ emission is directly related to fuel consumption.As the main power source of domestic and international shipping,high power-density marine diesel engines usually operate under medium and high loads,which makes the impact of fuel consumption even more important.Fuel consumption is largely dependent on the in-cylinder combustion process.Therefore,the optimization of combustion system with the purpose of thermal efficiency improvement is an extremely important problem for high power-density diesel engines to be solved.Characterized by large scale in size,large injector orifice,high injection pressure and boundary conditions for combustion,the spray development characteristics and combustion process of high power-density marine diesel engine are very different from the existing theories based on common diesel engines.However,in view of the combustion characteristics of high power-density marine diesel engines with high intensification,the previous combustion optimization strategy is not applicable,the research on combustion control methods is still insufficient,and the development and design of combustion system basically depends on data accumulation and law research.Thus,in this thesis,based on a high power-density marine diesel engine,a forward designing method of combustion system was suggested,which starts from the ideal cycle to target heat release rate,and then to combustion process control strategy and the matching of combustion chamber and injection system,so as to realize efficient and controlled combustion.The method can greatly reduce the workload of combustion system optimization and designing.The main work of this thesis is concluded as follows:（1）The Sabathe-Miller cycle,as well as the definition of the degree of constant-volume combustion and degree of Miller were proposed,and then a zero-dimensional model was established to calculate the brake thermal efficiency.After that,a comprehensive optimization of intake air and combustion process was conducted by researching its influence law on brake thermal efficiency.The results showed that,the brake thermal efficiency could achieve a maximum value without consideration of mechanical constrains.For the practical diesel engine working process optimization,it is necessary to obtain the degree of Miller,which can ensure the excess air coefficient under the intake pressure limit provided by the supercharging system,and then determine the optimal compression ratio under the limits of explosion pressure and pressure rise rate,and then determine the corresponding optimal constant volume degree of thermal efficiency and ideal heat release process.（2）According to the optimal heat release derived by the Sabathe-Miller cycle optimization,the ratio of the premixed/diffusion combustion can be determined.And the combustion process is characterized by the double Wiebe function.The influence of heat release rate characteristic parameters such as start of combustion,combustion duration and combustion speed on engine performance are researched.Finally,the target heat release rate satisfying the optimal thermal efficiency of each typical load under the boundary limit of the model was obtained.The results show that the shape of heat release rate has significant influences on the pressure rise rate and thermal efficiency.With the delay of start of combustion,the maximum pressure-rise-rate decreases,but the thermal efficiency loss is obviously increase.With the increase of premixed combustion duration,the maximum pressure-rise-rate decreases and the brake thermal efficiency decreases slightly.With the increase of combustion quality parameters in the premixed combustion or diffusion combustion stage,the heat release rate in the corresponding stage changes from higher in the early stage to higher in the late stage,that is,the combustion rate changes from fast in the early stage to fast in the late stage.When premixed combustion and diffusion combustion are concentrated,the brake thermal efficiency is higher,but the pressure rise rate increases significantly.In general,the premixed combustion characteristic parameters have little influence on the overall combustion process,but have great influence on the maximum pressure rise rate.The diffusion combustion quality parameters play a decisive role in the combustion performance parameters.On this basis,in order to analyze the combustion process of high-power density marine diesel engines,the existing phenomenological random combustion model is optimized for the spray impingement process.The results show that the spray impingement significantly promoted the fuel-air mixing process and the heat release rate.After the validated by computational fluid dynamic model of spray and flame development.A combustion process dividing method is proposed,which divides the combustion process into before-collision combustion and after-collision combustion based on the spray impingement time point.（3）A forward design method of combustion system is proposed.Start with the target heat release rate,based on the theory of spray structure,the relation between the target heat release process and controllable parameters of diesel engine combustion system is established,so that the combustion process can be controlled by the design of the controllable parameters of the combustion system,such as the injection parameters and the shape of the combustion chamber.The results show that the fuel mass of the combustion stage before spray impingement can be expressed as a formula about the nozzle diameter and the injection pressure.For a bowl-bore diameter ratio of the combustion chamber,the controllable parameter scheme of the fuel injection system before spray impingement can be obtained by calculating the spray penetration distance at the time of spray impingement simultaneously.（4）The matching of fuel injection parameters and combustion chamber shape was studied based on the target heat release rate.By coupling the control method of pre-impingement combustion,a certain relationship between the parameters of injection system and bowl-bore diameter ratio of combustion chamber is established.Then,according to the theoretical basis that the heat release rate of diffusion combustion depends on the mixing speed of fuel and gas and the quality of combustible fuel,the depth of combustion chamber pits is constrained by the peak phase of combustion after wall collision.On this basis,the matching of piston pit radius and injection angle is studied by using experimental design method.The results show that:for the designed high power density marine diesel engine with cylinder diameter of 270mm,stroke of 390mm,connecting rod length of 740 mm and compression ratio of 16.2,under a certain injection angle,with the increase of bowl-bore diameter ratio,the explosion pressure of different schemes presents a monotonously increasing trend,while the fuel consumption first decreases and then increases.When the bowl-bore diameter ratio is 0.78,the fuel consumption reaches the minimum value.For a bowl-bore diameter ratio of 0.78,under a certain injection angle,with the increase of the pit radius within the range of 18mm～22mm,the fuel consumption decreases first and then basically remains unchanged,and the minimum value is basically achieved when the pit radius is 21mm.For a certain pit radius,with the increase of the injection angle,the fuel consumption generally shows a decreasing trend within the range of 141°～148°.Under the premise of ensuring the lowest fuel consumption and not exceeding the limit of maximum heat load and NO_x emission,the optimal combustion chamber scheme is as follows:caliber ratio 0.78,pit depth 25.1mm,pit radius 21mm,and injection angle 145°.（5）The design of the combustion system was validated by experiments,and the results showed that the combustion system design method in this thesis can realize the degree of constant volume,and also the main characteristic points and shape of the target heat release rate.Besides,the fuel consumption value calculated by the combustion system design method in this thesis is basically consistent with the experiments,within 3%error under four typical operating loads.The combustion system scheme in this thesis can significantly improve fuel consumption,and fuel consumption can be as low as 179g/k Wh at the most commonly used75%load.