A multi-physics, multi-scale investigation of the piston ring pack

It is essential for the automotive industry to improve efficiency and mitigate frictional losses in IC engines. About 20-25% of these frictional losses arise as a result of the piston ring pack-liner assembly. By reducing the friction, there is potential to improve fuel consumption and emissions. This paper conducts a multi-physics, multi-scale investigation for the piston ring to cylinder liner conjunction, analysing the fundamental tribology, asperity interactions and boundary conditions. A 2D hydrodynamic model has been created based on Reynolds equation for a piston ring – cylinder liner conjunction. The model uses a finite difference method, calculating friction and the minimum film thickness over a 4-stroke engine cycle

Influence of boundary conditions on starvation of piston ring conjunction

It is important to determine realistic inlet boundary conditions to correctly predict lubricant film thickness and generated frictional power losses in all tribological conjunctions. This is also true of piston compression ring as well. A 2D hydrodynamic solver using Reynolds equation is to analyse the differences between predicted conditions with a flooded inlet and that arising from a more realistic determined zero-reverse boundary condition for lubricant flow post inlet wedge stagnation point. The case of a cylinder of a 4-cylinder 4 stroke gasoline engine, running at the engine speed of 1500rpm is considered. The results show that with a more detailed and realistic inlet boundary a significant reduction in the minimum film thickness is predicted which leads to increased friction throughout the engine cycle

Surface characterization of a real-world cylinder liner subject to deposition from combustion

This paper investigates the effects of combustion product deposition using a cylinder liner taken from a C-segment passenger vehicle run for 105,000 miles. Using a novel methodology of Atomic Force Microscopy and X-ray Photoelectron Spectroscopy the pressure coefficient of boundary shear strength of asperities and the nature of the depositions along the liner is considered to predict the boundary friction of a piston ring pack. Results show that the combustion depositions create localized values of the pressure coefficient of boundary shear strength of asperities at top dead centre, mid-stroke and bottom dead centre, increasing ring pack friction by 50 N in the combustion stroke per engine cycle

An investigation into the oil transport and starvation of piston-ring pack

In order to accurately predict the lubricant film thickness and generated friction in any tribological contact, it is important to determine appropriate boundary conditions, taking into account the oil availability and extent of starvation. This paper presents a two-dimensional hydrodynamic model of a piston ring pack for prediction of lubricant film thickness, friction and total power loss. The model takes into account starvation caused by reverse flow at the conjunctional inlet wedge, and applied to a ring-pack, comprising a compression and scraper ring. Inlet boundaries are calculated for an engine cycle of a 4-cylinder, 4-stroke gasoline engine operating at 1500rpm with conditions pertaining to the New European Drive Cycle (NEDC). The analysis shows that the two main sources of starvation; firstly due to a physical lack of inlet meniscus and secondly due to reverse flow at the inlet wedge, significantly affect the prevailing conditions from the generally assumed idealised boundary conditions. Such an approach has not hitherto been reported in literature

A study into the effects of cylinder de-activation on the tribological performance of piston ring-liner conjunction

A study into the effects of cylinder de-activation on the tribological performance of piston ring-liner conjunctio

Boundary friction characterisation of a used cylinder liner subject to fired engine conditions and surface deposition

In cylinder friction contributes as a primary source of parasitic dissipations in IC engines. For future engines to become more efficient, with enhanced fuel economy and increased power output, accurate prediction of new designs is required over the full lifetime of an engine. The work carried out presents use of a local pressure coefficient of boundary shear strength of asperities value, taking into account the localised effects of surface texture, coating and surface deposition. XPS spectra analysis was also carried out to identify the surface depositions as a result of combustion, not previously taken into account during piston ring pack simulation. Friction was shown by simulation to drop by up to 30% between the compression and combustion stroke as a result of using a carriable coefficient of boundary shear strength of asperities. It was found that piston varnish on the liner corresponded to higher values of the pressure coefficient of boundary shear strength of asperities, therefore showing the importance of using real system components run under representative operating conditions or numerical analyses

Atomic force microscopic measurement of a used cylinder liner for prediction of boundary friction

Accurate simulation performs a crucial role in the design and development of new modern internal combustion engines. In the case of piston rings, simulations are used to effectively predict generated friction and power loss of proposed designs. These are consequences of viscous shear of a thin lubricant film, likewise boundary friction caused by direct interaction of piston rings with the cylinder liner/bore surface. The most commonly used model for determining boundary friction is that of Greenwood and Tripp. The model requires the pressure coefficient of boundary shear strength of asperities from the softer of the contacting surfaces as an input. This parameter needs to be measured. The paper describes the process of measurement using an Atomic Force Microscope (AFM), both for a dry surface and that wetted by the presence of a lubricant layer. For realistic results, the investigated specimen is a used, tested engine cylinder liner where boundary active lubricant additives are bonded to its surface as well as combustion products. This approach is as opposed to the previously reported works using new flat surfaces with base oil or partially formulated lubricants, and has not previously been reported in literature. The results show that for used cylinder liners, the measured boundary shear strength of asperities varies according to location along the stroke. Results are reported for the Top Dead Centre, Mid-stroke and Bottom Dead Centre locations. The measurements are subsequently used with 2D Reynolds Solution for a top compression ring-liner contact, where it is found that accurate localised predictions of generated friction and power loss can be made instead of the usual average value approach reported in literature

Elastodynamics of piston compression rings

The piston ring pack accounts for a disproportionate amount of the total engine frictional losses. The frictional behaviour of piston rings is significantly affected and governed by its flexible dynamics. The dynamically changing shape of the ring determines its contact geometry with the cylinder liner and hence affects the frictional losses. The compression ring undergoes a multitude of complex motions during the engine cycle prescribed by the gas pressure, contact reaction, ring tension, friction between the ring and its groove and inertial forces that excite a plethora of the ring’s modal responses. This adversely compromises the functionality of the ring through a number of undesired phenomena such as ring flutter, twist, rotation and jump. Therefore, a prerequisite for improving the prediction of tribological conditions is an accurate determination of the ring’s elastodynamic response. This paper presents a methodology to directly solve the governing differential equations of motion for different forms of beam cross-section, where the shear and mass centres are not coincident, typical of the complex cross-sections of a variety of different piston compression rings. Combined numerical and experimental investigations are undertaken to determine the dynamic behaviour of the compression ring

Results of measured data from atomic force microscope on ring pack performance

Frictional losses of an IC engine include 40-50% contribution due to piston assembly-liner conjunction. Reduction of friction would improve fuel efficiency and decrease harmful emissions. Therefore, it is important to accurately predict the frictional losses due to viscous shear of a thin lubricant film as well as boundary friction, generated by the direct contact of real rough contiguous surfaces. Greenwood and Tripp model is used to evaluate the contribution due to boundary friction. The model requires the determination of pressure coefficient of boundary shear strength of asperities, ς, which is analogous to the asperity coefficient of friction. This should be determined through measurement, using Atomic Force Microscopy (AFM) in Lateral Force Mode (LFM). The value of ς is dependent on the combination of surface and lubricant as a system. Boundary active lubricant additives adsorb or bond to the surface asperities and affect the value of ς. The value of this coefficient also alters with the evolution of interacting surfaces through the process of wear as well as any degradation of the lubricant. The approach can be used to create a database of such values for different lubricant-surface systems, in particular for piston-liner interactions

Effect of cylinder de-activation on the tribological performance of compression ring conjunction

The paper presents transient thermal-mixed-hydrodynamics of piston compression ring-cylinder liner conjunction for a 4-cylinder 4-stroke gasoline engine during a part of the New European Drive Cycle (NEDC). Analyses are carried out with and without cylinder de-activation (CDA) technology in order to investigate its effect upon the generated tribological conditions. In particular, the effect of CDA upon frictional power loss is studied. The predictions show that overall power losses in the piston-ring cylinder system worsen by as much as 10% because of the increased combustion pressures and liner temperatures in the active cylinders of an engine operating under CDA. This finding shows the down-side of this progressively employed technology, which otherwise is effective in terms of combustion efficiency with additional benefits for operation of catalytic converters. The expounded approach has not hitherto been reported in literature