Adaptability of piston skirt coatings on the tribological performance of heavy-duty diesel engine under low viscosity lubricant

Purpose The purpose of this paper was to improve the frictional wear resistance properties of piston skirts caused by the low viscosity lubricant by studying the tribological performance of three novel coating materials. Design/methodology/approach Comparative tribological examinations were performed in a tribological tester using the ring-block arrangement under two viscosity lubricants, the loading force was applied as 100 N, the speed was set to 60 r/min and the testing time was 180 min. Findings Under low viscosity lubricant, the friction coefficient and wear of the three coatings all increase, and the friction coefficient and wear of the PTFE coating are the largest, while the MoS2 coating has the lowest friction coefficient and wear. Under low viscosity lubricant, the friction coefficient of the MoS2 coating is 2.1%–5.4% and 20.0%–24.3% lower than that of the SiO2 and PTFE coating, respectively. The friction coefficient and wear fluctuation rate of the MoS2 coating is the smallest when the lubricant viscosity decreases, which indicates that the MoS2 coating has excellent stability and adaptability under low viscosity lubricant. Originality/value To reduce the piston skirt wear caused by low viscosity lubricant in heavy-duty diesel engines, the friction and wear adaptability of three novel composite coating materials for piston skirts were compared under 0 W-20 low viscosity lubricant, which could provide a guidance for the application of wear-resistant materials for heavy-duty diesel engine piston skirt.

Less wear on the piston skirts of internal combustion engines

A significant proportion of mechanical losses in internal combustion engines accounted for mechanical losses in the cylinder-piston group. Depending on the operating modes of the internal combustion engine, contact interaction in the piston-cylinder pair is possible, which leads to wear of the working surfaces of the resource-determining elements and a decrease in the operational life of the power unit as a whole, in connection with which the reduction of friction losses in the internal combustion engine elements and the piston - cylinder liner coupling in particular is relevant. Both domestic and foreign researchers are engaged in the solution of the above described problems, various profiles of pistons, methods of calculating the parameters of the oil layer are proposed, but the practical state of the issue determines the relevance of research in this direction. The paper considers the possibility of reducing the wear of piston skirts by reducing the contact surface in conjugation and providing an oil film in the friction zone, regardless of engine operating conditions. This opportunity is realized by forming a certain macro profile on the working surface of the piston skirt. The formation of the macrorelief was carried out by means of surface plastic deformation, with the reciprocating movement of a spherical tool on the machined surface.

The Effect Of Reference Plane On Values Of Areal Surface Topography Parameters From Cylindrical Elements

In this paper distortion of surface topography measurement results by improper selection of the reference plane is taken into consideration. The following types of surfaces from cylindrical elements were analyzed: cylinder liners after plateau honing, cylinder liners with additionally burnished oil pockets and turned piston skirts. Surface topographies of these elements after a low wear process were also studied. In order to obtain areal surface topography parameters, the form was eliminated using cylinders and polynomials of the following degrees: 2, 3, 4, 6, 8, 10 and 12. Parameters of surfaces after form removal were compared. After analysis of results the reference elements for each kind of surface were recommended. A special procedure was proposed in order to select the degree of a polynomial. This method is based on surface topography changes with increase of polynomial degree. The effect of improper form elimination on measuring uncertainty was studied.

Shear Heating of High-Viscosity Grade Lubricant in Piston Skirts: EHL at Idling Speeds in Initial Engine Start Up

A few initial cold engine start up cycles at low idling speeds do not prevent wear due to the absence of a fully established elastohydrodynamic lubrication (EHL) film between the piston skirts and the cylinder liner. It happens when the thermal loading due to combustion may be ineffective initially, and shear heating becomes significant as a result of the sliding motion of the piston. This study models the 2-D piston skirts EHL at the idling speeds in the initial engine start up by using a high-viscosity grade engine oil and incorporating the shear heating effects. The 2-D heat transfer equation is used with no source term effects to study the temperature changes and their effects on the viscosity of a Newtonian lubricant at the different idling speeds in the initial start up of an internal combustion engine. The 2-D Reynolds equation is solved numerically to generate the hydrodynamic pressures as the function of 720 degrees crank rotation cycle. Under the flooded lubrication conditions the inverse solution technique is employed to generate the hydrodynamic pressures in the EHL regime. The numerical analysis at the two different idling initial engine start up speeds is presented based on the 2-D heat equation having adiabatic conduction and convective heat transfer with no source term effects. Viscous dissipation coupled with the piston motion, the pressure fields generation, the temperature effects on the viscosity of the lubricant and the subsequent oil film thickness profiles in the contact region are examined. The influence of the low-temperature shear heating on the hydrodynamic and EHL film thickness at the time of initial engine start up are investigated. This study suggests that by using a high-viscosity grade oil in the idling speed engine start up the film temperature rises non-uniformly due to shear heating in the hydrodynamic and EHL regimes. The low temperature rise affects the pressure and temperature dependent oil viscosity, and the secondary transverse eccentric displacements of the piston. Resultantly, the piston skirts lubrication is affected despite the initial engine start up at the idling speeds.

Modeling EHL of Rough Piston Skirts Using Different Viscosity-Grade Lubricants in Initial Engine Startup

An engine lubricant plays a significant role in preventing adhesive wear of the rough interacting surfaces of the piston skirts and the cylinder liner. A fully established elastohydrodynamic lubricating (EHL) film, appropriate viscosity oil and the fairly rough interacting surfaces prevent wear during normal engine operation. The absence of an EHL film and inappropriate viscosity lubricant fail to minimize wear in the initial engine startup. This work considers a fairly viscous Newtonian engine lubricant to model the rough piston skirts hydrodynamic and EHL in the initial engine start up. The isotropic surfaces of the skirts and the cylinder liner having different roughness amplitudes are considered in the basic lubrication models. The flow factors are introduced in the 2-D average Reynolds equation, which is solved numerically to generate the hydrodynamic pressures. The inverse solution technique is used to develop the basic EHL model of the rough surfaces. The secondary piston dynamics and the contact geometry of interacting surfaces are incorporated in the basic lubrication models. The profiles of the piston eccentricities, secondary velocities, film thicknesses and pressures are generated as the function of 720 degrees crank rotation cycle. The study is extended to develop the lubrication models for the low and high viscosity grade engine lubricants separately. The simulation results are analyzed and compared with those of the basic lubrication model. The results show that the different lubricant viscosities alter the secondary displacements of the sliding piston and affect the lubrication of the rough interacting surfaces. The comparative analysis leads to optimize the use of appropriate viscosity-grade engine lubricant for a few low speed initial engine startup cycles.

Modeling Shear Heating in Piston Skirts EHL Considering Different Viscosity Oils in Initial Engine Start Up

A fully established elastohydrodynamic lubricating (EHL) film between the piston and the liner surfaces during normal engine operation minimizes piston slap and prevents adhesive wear. Wear cannot be prevented in the initial engine start up due to the absence of EHL film. During normal engine operation, thermal loading due to combustion dominates piston skirts lubrication. However, in a few initial cold engine start-up cycles, shear heating affects the lubricant viscosity and other characteristics considerably. This study models 2D piston skirts EHL by incorporating shear heating effects due to lubricant flow between the skirts and liner surfaces. The hydrodynamic and EHL film profiles are predicted by solving the 2D Reynolds equation and using the inverse solution technique, respectively. The temperature distribution within the oil film is given by using the 2D transient thermal energy equation with heat generated by viscous heating. The numerical analysis is based on an energy equation having adiabatic conduction and convective heat transfer with no source term effects. The study is extended to low and high viscosity grade engine oils to investigate the adverse effects of the rising temperatures on the load carrying capacity of such lubricants. Numerical simulations show that piston eccentricities, film thickness profiles, hydrodynamic and EHL pressures visibly change when using different viscosity grade engine lubricants. This study optimizes the viscosity-grade of an engine lubricant to minimize the adhesive wear of the piston skirts and cylinder liner at the time of initial engine start up.