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Currently, lubrication analysis of piston ring is generally done under engine rated operating condition. However, the engine (such as the vehicle engine) does not always operate in rated operating condition, and its operating condition changes frequently in actual use. In addition, the lubrication status of piston ring is generally assumed as the flooded lubrication or a certain form of poor lubrication in most of the lubrication analysis.

In this paper, based on the equations about the flow rate of lubricating oil and the variation of control volume, the flow model of lubricating oil in the piston ring-cylinder liner conjunction is established. The lubrication analysis of piston ring for a four-stroke engine under different engine operating conditions is done, in which the lubricating oil at the inlet of piston ring is considered as the lubricating oil attached on the relevant location of cylinder wall after the piston ring moves over at the previous stroke.

There is remarkable difference for the lubrication characteristics of the piston ring under different engine operating conditions. The worst lubrication status of piston ring may not take place under engine rated operating condition.

In this paper, based on the measured engine cylinder pressure, the lubrication analysis of piston ring for a four-stroke engine under different engine operating conditions is done in which the lubricating oil supply condition at the inlet of piston ring is considered. The results of this paper are helpful for the design and research of engine piston ring-cylinder liner conjunction.

A free-piston engine (FPE) is an unconventional engine that abandons the crank system. This paper aims to focus on a numerical simulation for the lubricating characteristics of piston rings in a single-piston hydraulic free-piston engine (HFPE).

A time-based numerical simulation program was built using Matlab to define the piston motion of the new engine. And a lubrication mode of piston rings was built which is based on the gas flow equation, hydrodynamic lubrication equation and the asperity contact equation. The piston motion and the lubrication model are coupled, and then the finite difference method is used to obtain the piston rings lubrication performances of the FPE. Meanwhile, the lubrication characteristics of the new engine were compared with those of a corresponding conventional crankshaft-driven engine.

The study results indicate that compared with the traditional engine, the expansion stroke of the HFPE is longer, and the compression stroke is shorter. Lubrication oil film of the new engine is thicker than the traditional engine during the initial stage of compression stroke and the final stage of the power stroke. The average friction force and power of the hydraulic free piston engine are slightly lower than those of the traditional engine, but the peak friction power of the FPE is significantly greater than that of the traditional engine. With an increase in load, the friction loss power and friction loss efficiency decrease, and with a decrease in equivalence ratio, the friction power loss reduces, but the friction loss efficiency decreases first and then increases.

In this paper, only qualitative analysis was performed on the tribological difference between conventional crankshaft engine and HFPE, instead of a quantitative one.

This paper contributes to the tribological design method of HFPE.

No social implications are available now, as the HFPE is under the development phase. However, the authors are positive that their work will be commercialized in the near future.

The main originality of the paper can be introduced as follows: the lubrication and friction characteristics of the new engine (HFPE) were investigated and revealed, which have not been studied before; the effect of the HFPE’s special piston motion on the tribological characteristics was considered in the lubrication simulation. The results show that compared with the traditional crankshaft engine, the new engine shows a different lubrication performance because of its free piston motion.

Piston ring dynamic problem plays an important role in the lubricant characteristics of a reciprocating engine, which lead to engine wear and the increased consumption of lubricating oil. A cavitation analysis of the piston ring lubrication with two-dimensional Reynolds equation has rarely been reported owing to the complex working condition. The purpose of this study is to establish a precise model that can provide guidance for the design of the piston ring.

In this paper, a cavitation model and its effect on the piston ring lubrication was studied in a simulation program based on the mass-conserving theory which is solved by means of the Newton–Raphson method. In this study, some models such as mixed lubrication, asperity contact, blow-by/blow-back flow and cavitation have been coupled with the lubrication model.

The established model has been compared with the traditional model that deals with cavitation by using the Reynolds boundary condition algorithm. The cavitation zone, pressure distribution and density distribution between the piston ring and the cylinder have also been predicted. Studies of the changing trend for the pressure distribution and the cavitation zone at few typical crank angles have been listed to illustrate the cavitation changing rule. The analysis of the results indicates that the developed simulation model can adequately illustrate the lubrication problem of the piston ring system. All the analyses will provide guidance for the oil film rupture and the reformation process.

A two-dimensional cavitation model based on the mass-conserving theory has been built. The cavitation-forming and -developing process for the piston ring–liner lubrication has been studied. Non-cavitation occurs in the vicinity of top dead center and bottom dead center. The non-cavitation period will be longer in the vicinity of 360° of crank angle. The density distribution in the cavitation zone can be obtained.

The purpose of this paper is to study the real-time change of surface roughness at different small regions of piston rings during running-in process. Meanwhile, the effects of real-time change of the rough surface topography on the lubrication and friction of piston rings are investigated.

An uneven wear model has been developed to research the running-in behavior at the different small regions of piston rings. The model is verified by comparing the simulation results with the experimental results on a reciprocating friction and wear test rig.

This research shows that the wear process of piston ring surface is uneven during running-in. At most time of the operating cycle except the vicinity of top dead center and bottom dead center, the minimum oil film thickness ratio increases while the friction force and power loss decrease after the running-in period.

Through this research, the running-in behavior of piston rings is investigated in detail. The interaction between the running-in and the lubrication and friction of piston rings is understood more deeply.

Cavitation in piston-ring lubrication is studied as part of the performance of piston-ring assemblies. Cavitation degrades performance in engineering applications and its effect is that it alters the oil film pressure, generated at the converging-diverging wedge of the interface. Studies tried to shed light to the phenomenon of cavitation and compare it with cavities that have been identified in bearings. The paper aims to discuss this issue.

Lubricant formulations were used for parametric study of oil film thickness (OFT) and friction providing the OFT throughout the stroke and LIF for OFT point measurements. Lubricant formulation affects cavitation appearance and behaviour when fully developed.

Cavitation affects the ring load carrying capacity. Different forms of cavitation were identified and their shape and size (length and width) is dictated from reciprocating speed and viscosity of the lubricant. A clear picture is given from both techniques and friction results give quantifiable data in terms of the effect in wear and cavitation, depending on the lubricant properties.

Engine results are limited due to manufacturing difficulties of visualisation windows and oil starvation. Therefore, full stroke length sized windows were not an option and motoring tests were implemented due to materials limitations (adhesive and quartz windows). Lubricant manufacturer has to give data regarding the chemistry of the lubricants.

The contribution of cavitation in piston-ring lubrication OFT, friction measurements and lubricant parameters that try to shed light to the different forms of cavitation. A link between viscosity, cavitation, shear thinning properties, OFT and friction is given.

This study aims at proposing an approach for optimizing the shape of the top piston ring face for minimum friction force using an inverse method. The shape of the top piston ring face determines the amount of oil distribution in the interface of the ring and liner. Therefore, the shape has a significant impact on the tribological performance of this interface.

The shape of the ring face is represented by a polynomial function and is based on the load analysis of the ring. The optimization of the shape was performed using the Sequential Quadratic Programming method. The minimizing of the friction parameter at the interface was considered during the solving process to obtain an optimum ring shape.

The optimized high degree of the shape of the ring face could lead to a reduced friction parameter. The proposed method could be applied for the tribological design and optimization of the piston rings.

There still need effort to investigate the effect of design parameters (e.g. property of lubricant)on the optimization of the ring face.

The subject matter is important and the method has practical value.

The aim of this paper is to develop a technique to measure the oil film thickness between piston ring and liner throughout the stroke, without impairing the surface properties of the piston ring and liner. Mechanical properties of the piston ring, like ring stiffness, are also not altered. Effect of variation in bore on the movement of piston ring can be studied with the proposed technique.

The gap Hmin between the cylinder liner and the piston ring is formed due to the hydrodynamic pressure generated by the presence of oil film between piston ring and liner. This gap can be inferred by measuring the movement of the inner surface of piston ring with reference to a sensor mounted on the piston at a fixed distance from the piston ring. The piston ring is connected to the sensor through reasonably rigid member. The underlying assumption here is that there is no elastic deformation of the piston ring due to the hydrodynamic pressure. The fundamental sensor to measure oil film thickness used in this setup is a set of strain gauges.

It is possible to measure oil film thickness by the proposed arrangement for the entire stroke without changing the surface properties. Mechanical properties of the piston ring, like ring tension, are not affected. The results possibly provide the correct picture of the piston ring movement throughout the stroke. The measurement at near zero speed can give information on the movement of the piston ring due to hydrodynamic action and to the variation in the bore. The measurement is not affected by engine vibrations. The proposed technique can be helpful in validating the theoretical models proposed in the literature.

The measurement is possible only in unfired condition. However, this attempt can be considered as the basis to measure OFT in fired condition with necessary improvements. It is not feasible to measure quantity of lubricant/extent of lubricant on leading or trailing edge of piston. Effect of temperature on the oil film thickness cannot be studied as the engine is not fired. It is assumed that the piston ring does not pass through elasto‐hydrodynamic lubrication regime. Debris/worn out particles in the oil may affect the indicated oil film thickness at local points.

This study aims to compare cavitation shapes between the simulating test rig and the engines to strengthen the findings that were first observed in the simplified experiments. Different forms of cavitation were identified, and their shape and size (length and width) were dictated from reciprocating speed and viscosity of the lubricant. Cavitation degrades performance in engineering applications and its effect is that it alters the oil film pressure.

Lubricant formulations were used for parametric study as well as different operating testing parameters in a simulating test rig and single cylinder engines with visualisation windows. An algorithm was used for extracting cavitation data from imaging, and comparison was made.

Similar phenomena at the simulating test rig and the engine were investigated and compared. The effect of different operating conditions was assessed along with the variations produced from the parametric lubricant study.

Engine results are limited due to manufacturing difficulties of visualisation windows and oil starvation. Firing tests are another difficult challenge as the modified section pressure is under more pressure and the window view is affected by combustion process. Limited pictures can be captured before cleaning is required. A lubricant manufacturer has to provide data regarding the chemistry of the lubricants.

The effect of cavitation in piston ring lubrication along with variable operating and lubricant parameters is further studied with quantification of cavitation results through image processing. These forms of cavities are affected by lubricant properties and operating conditions. A link between viscosity, cavitation, shear thinning properties, oil film thickness (OFT) and friction is given.

The purpose of this paper is to present a measurement technique wherein the film thickness is measured in unfired condition for entire stroke length but without impairing the original condition of piston ring, liner and lubricant, i.e. non-invasively. Film thickness is measured at different speeds up to 500 rpm. The measurements are initially carried out at near zero speed followed by speeds mentioned above. Measurement highlights the combined effect of variation of bore diameter and ring face profile on the film thickness.

The film thickness is measured with the help of a set of strain gauges. Four strain gauges are mounted on a sufficiently elastic steel strip which is mounted in a simply supported condition. This assembly of strain gauge is mounted on small rectangular bracket. A cutout is made in the piston to accommodate the bracket. A pin bearing a slot of size sufficient enough to accommodate the piston ring on one side is fixed between the piston ring and the strain gauge assembly. This ensures the transfer of the movement of the piston ring on to the strain gauge. The deflection of the strain gauge is pre-calibrated against a sufficiently accurate dial gauge. Hence any radial movement of the piston ring is sensed by the strain gauge assembly. A data logger unit is connected to the strain gauge output to log the data at every crank angle. A rotary encoder is connected to the crank shaft, to have the correlation of the strain gauge output with the crank angle.

The technique is capable of measuring oil film thickness for entire stroke at low speeds in unfired engines. The effect of variation in bore diameter on the oil film thickness is significant and hence such measurement can enlighten the path for research to reduce friction. The experimental results of the oil film thickness are in good agreement with predicted values, particularly in the forward stroke (BDC to TDC).

The methodology is not suitable for fired engines as on date but can be taken up as a future work with necessary modifications. It does not take into consideration the effect of elasto-hydrodynamic lubrication.

It can be used to measure OFT between piston ring and liner in unfired engines and reciprocating compressors also.

It can help to indentify the areas of research so that the friction between piston ring and liner can be reduced thus increasing efficiency of the engine and reducing fuel consumption and emissions.

The work presented is a part of PhD work under progress at S V National Institute of Technology, Surat, India. The setup is in the college premises and the experiments are conducted on the same.

This paper aims to propose a literature review of the main physical phenomena considered by previous studies focusing on the modelling and the numerical simulation of frictional behaviour of piston rings, in the first section. In the second section, the more recent technical papers and patents about piston ring pack are briefly discussed. They deal with novel materials, innovative manufacturing methods and modified shape for improving frictional, stability and blow-by behaviours.

This review paper aims at covering last period technical efforts about engine piston ring pack friction reduction through novel materials and manufacturing methods as well as new surface profiles according to the last outcomes of multiphysics numerical simulation.

The paper type is “literature review”. The findings of the authors of papers and patents are described.

This review paper proposes a survey of recent papers and patents on piston rings topic.