Study On Combustion And Heat Transfer Characteristics Of High Compression Ratio Diesel Engine And Matching Law Of Oil-Gas-Chamber

Increasing the efficiency of heavy-duty diesel engines and reducing CO₂ emissions are the main challenges facing the international internal combustion engine industry in the present and even for a long period of time in the future.From the perspective of thermal cycling,the compression ratio has a significant effect on the thermal efficiency of the internal combustion engine.Increasing the compression ratio can significantly improve the thermal efficiency of the internal combustion engine.However,under high compression ratio conditions,the process of oil and gas mixing,combustion,heat transfer,etc.in the cylinder of the internal combustion engine will change.These changes will restrict the efficiency of the internal combustion engine and even deteriorate.Defining the law of variation is a prerequisite for the development of a high compression ratio combustion system.In this paper,a three-dimensional numerical simulation model is established based on CONVERGE software for a heavy-duty diesel engine,and the model is calibrated by using the measured cylinder pressure,combustion heat release rate and emission data.Firstly,based on the established simulation model,the influence of compression ratio on diesel engine energy balance,combustion and heat transfer process is studied.The results show that as the compression ratio increases,the throat radius of the combustion chamber gradually decreases.At the same nozzle angle,the distance from the oil beam to the wall of the pit is shorter and the position is more upward,so the movement to the squish area the oil bundle is more,causing the equivalent ratio of the squeezing zone to be too high,the combustion loss is increased,and the thermal efficiency is lowered.In the case of high compression ratio,more fuel enters the squeezing zone,resulting in increased combustion loss and reduced diffusion combustion speed.In order to solve the problem,a stepped combustion chamber profile is proposed in this paper.The results show that the stepped combustion chamber is beneficial to form a double vortex structure,and the over-mixed gas jet from the wall jet to the squeezing zone is redirected back to the pit area to promote oil and gas mixing,reduce the proportion of over-mixed gas in the cylinder,and increase the speed of the diffusion combustion stage.On this basis,the throat radius,depth,and main-sub-combustion chamber volume ratio of the stepped combustion chamber are optimized.The results show that the thermal efficiency achieved maximum value when the volume ratio of the main-sub-combustion chambers is 90:10 at different throat radius.The paper studies the matching law of the nozzle angle and the shape of the combustion chamber,the number of nozzle holes and the eddy current ratio,as well as the influence law on the combustion and heat transfer process in the cylinder.The results show that as the angle of the nozzle increases,the equivalent ratio of the squeezing zone gradually increases,and the equivalent ratio of the pit area decreases gradually.The maximum combustion efficiency can be obtained when the angle of the nozzle is in the middle position.At the same time,as the depth of the combustion chamber gradually increases,the angle of the nozzle should be gradually reduced to obtain the best indicative thermal efficiency.As the vortex ratio increases,the intensity of the vortex in the cylinder increases,indicating that the thermal efficiency gradually increases.When the vortex ratio is increased,the number of nozzle holes should be appropriately reduced to maintain the optimum eddy current intensity in the cylinder and improve the thermal efficiency of the engine.The research results can provide theoretical support for the development of high compression ratio engine combustion system.