Design Of Highly-strengthened Diesel Forged Steel Piston

With the increasingly serious global energy and environmental crises,the combustion engine,as the largest consumption of petroleum,must improve its emission level at the time of improving its mechanical efficiency and heat efficiency.To achieve the above objectives,improving the engine specific power and compression pressure could be the attentive focuses.As the heart of engine,piston withstands huge thermal and mechanical load from combustion.The traditional aluminum piston can only bear 20MPa at maximum of the engine peak compression pressure after the optimization of its structure and material property.However,the maximum peak compression pressure of the engine can reach 25MPa,or maybe 30MPa in the future.Due to its material strength limit,the traditional aluminum piston is far from enough to meet the engine development requirements.So it's urgent to develop a new kind of piston that can meet the performance requirements of highly-strengthened engine.In order to meet the above requirements,the research work of steel piston has been launched at home and abroad.Steel piston has the following advantages:lower thermal expansion coefficient,lower thermal conductivity,lower thermal loss,higher strength,lower emission;in addition,it can bear higher compression pressure and temperature.Combining with the domestic and foreign current situation of the development of forged steel piston,a new kind of highly-strengthened diesel forged steel piston was designed in this study.The piston crown and skirt were connected as a whole by a weld bead of piston crown and the step structure of ring land.This kind of piston has the characteristics of high structural strength,low friction and wear,light weight and high reliability.It can meet the usage requirements of engine with compression pressure over 20MPa.The bench tests show that the newly developed engine,assembled with steel piston,exhibits stable performance,high efficiency and good economy.Beyond that,the engine indicators such as oil consumption and blow-by are improved to some extent compared with the engine assembled with aluminum piston.This article mainly introduced the piston integral structure design,FEA(finite element analysis),machining process,surface treatment process and highly-strengthened diesel steel piston test validation.Firstly,lightweight design concept was adopted in the highly-strengthened diesel forged steel piston by the way of low compression height design,constant strength thin wall,heat flow type inner cavity structure,large volume inner cooling channel structure and piston assembly lightweight design.By designing smaller fit clearance,optimizing piston pinhole profile and skirt profile,employing phosphating treatment and spraying molybdenum disulfide on the skirt,the engine noise reduced,especially at cold start.The piston assembly friction and wear reduced as well and the engine mechanical efficiency improved.By adopting these measures,the engine performance indexes and economic indexes improved to some extent.Piston three-dimensional model was drawn according to the design proposal.Then finite element analysis was carried out on the piston in order to evaluate if the structural strength of each part could meet the design requirements.This article mainly analyzed the piston temperature distribution,the stress and deformation of the piston under maximum compression pressure,maximum side force and maximum inertia force.Fatigue safety factors of the piston under the above conditions were also analyzed by fatigue analysis software.Dual positioning counterbore design and groove-excircle integral machining technology were adopted to manufacture the highly-strengthened diesel forged steel piston.The adoption of these technologies could not only ensure the machining precision but also reduce the positioning error and the error caused by second clamping.Piston phosphating process and skirt sprayed with molybdenum disulfide not only improved the pinhole lubrication and the engine cold start performance but reduced the engine friction and wear.The highly-strengthened forged steel piston passed the welding seam strength test,the piston temperature field test,the minimum scuffing test and the 2000h endurance test validation successfully.