Friction in Piston Ring - Cylinder Liner Contacts

With today’s striving towards reduction of fuel consumption it is moreimportant than ever to understand the function of different componentsin the internal combustion engine. There is a need to develop and usetools to investigate and predict the result of specific design changesmade on the components. In this work, the mechanics and the tribologyof the power cylinder unit and more specifically the operation of thepiston rings was investigated both numerically and experimentally. Theobjectives of the numerical part of this thesis were to develop simulationtools that can be used to quantify design changes to the TLOCR andthe cylinder liner. Such as the dimensions of the ring itself but also ringtension, running land profile, out of roundness of the cylinder liner andsurface texture of dimple type applied on the cylinder liner. Numericalsimulation models were developed and used to investigate operation of atwin land oil control ring (TLOCR). TLOCR are typically used in heavyduty diesel engines (HDDE). The TLOCR plays a very important role inthe engine since it is supposed to distribute the correct amount of oil onthe liner to lubricate the other rings. It is important that the TLOCRdoes not leave too much oil on the liner for the two top rings since itcould lead too high oil consumption. In a HDDE the piston assemblyis the largest contributor to frictional losses where the piston ring packaccounts for the major part of this. The oil control ring is the largestcontributor to frictional losses in the piston ring pack therefore makingit very interesting to study from a fuel consumption perspective. One ofthe models developed in this work accounts for the tribological interfaceof the TLOCR against the cylinder liner and piston ring groove as wellas the elastic deformation of the ring and the ring dynamics within thepiston ring groove. The actual ring cross section was modelled in orderto account for the full three dimensional elastic deformation of the ring.By solving all of these problems as a coupled system, the entire operation of the oil control ring can be understood in a better way than earlierand this opens up new optimisation possibilities for the TLOCR. Sincethe cylinder liner in an engine will always have some deviation fromperfectly round this is important. The full ring is modelled in order toaccount for out of round cylinder liners. The model can therefore beused to investigate the effect on oil distribution by reduced ring tensionwhich will affect the frictional losses of the system. It was found that thereduction in tangential force on the TLOCR with kept sealing capability,enabled by reduced out-of-roundness, could result in friction reductionof 40 % at mid-stroke.Because of the complexity, a multi-physics model of this type introducesdifficulties with convergence. Especially when implementinga mass conserving cavitation model and solving for the reversal of thepiston ring. Implementation and numerical verification of a mass conservingcavitation model was therefore performed. A method for dealingwith the convergence problem close to reversal was implemented and discussed.A model considering texture, of dimple type, on the cylinder linerwas also developed to find dimple dimensions optimal for reduced fuelconsumption. Since the dimples are modelled in a deterministic manner,only a periodic section of one land of the TLOCR was considered. Themodel takes mixed lubrication and inertia of the ring into considerationand a mass conserving cavitation algorithm were implemented. Theresults from the model predict friction reduction of approximately 40 %at mid-stroke speeds.The objective of the experimental part of this thesis is to developa novel test method for evaluation of piston ring friction at realisticspeeds. The test-rig was designed and constructed during the first halfof the project. The rig was designed so that standard HDDE productionpiston rings and cylinder liners could be easily mounted, and so thatpiston ring friction could be measured without influence from pistonfriction. Both cylinder liner and oil tank were equipped with heaters,where the oil supply resembles that in an actual engine. Repeatabilityof the results was found to be good. Influence on friction by differentcylinder liner surface roughness and coatings were investigated. Variouspiston ring designs, ring tension and coatings were also investigated. Itwas found that friction of the TLOCR could be reduced with close to50 % with kept sealing capability by reduced ring tension and differentgeometry on the lands in contact with the cylinder liner. The numerical simulation model governing the full cross section of the TLOCR wasvalidated against measured piston ring friction data at a number ofdifferent speeds and excellent correlation was found