Fuel efficiency is one of the most important areas of automotive vehicle research and development today, with rising fuel costs, energy security and environmental concerns being at the forefront of customers and legislators minds. Heavy Duty Diesel Engines (HDDE) are the primary source of mechanical power generation in today’s trucks and buses and this is likely to continue for the foreseeable future. In the 2011 European Commission White Paper on transport, a reduction of at least 60% of greenhouse gas emissions from transport by 2050, with respect to 1990 levels, was called for. The report concludes that acting on vehicles’ efficiency through new engines, materials and design will help in the reduction of oil dependence, the competitiveness of Europe’s automotive industry as well as health benefits, especially improved air quality in cities. Therefore, the efficiency and frictional losses in a vehicles powertrain are areas of great interest. This thesis focuses on the Piston Ring to Cylinder Liner (PRCL) contact and the potential for improving its performance through the specification of an optimised cylinder liner surface texture. The PRCL contact is one of the biggest contributors to mechanical losses in a HDDE and so there is potential for large performance gains to be achieved through optimisation of this contact. This research has led to the development of a simulation tool capable of calculating the friction, lubrication regime, oil consumption risk and wear that occurs in the full ring-pack of a HDDE. Furthermore, the tool allows for the evaluation of the relative performance of different cylinder liner surface topographies. A mixed lubrication model, incorporating flow factors calculated using the homogenization technique, has been implemented to allow all regimes of lubrication to be considered. A mass-conserving cavitation algorithm, formulated as a Linear Complimentarity Problem, enables lubricant cavitation, fully-flooded or starved inlet conditions and the quantity of lubricant deposited on the cylinder liner surface to be modelled. The simulation tool is validated with both reciprocating bench tests and full single cylinder fired engine tests. The reciprocating bench tests measured both friction and film thickness and both showed good correlation with the predictions from the simulation tool. Simulations and experiments were conducted on four different cylinder liner variants and both ranked the frictional performance of the cylinder liner variants in the same order. A parametric study of honing depth, spacing and angle was undertaken using the developed simulation tool and the influence of these parameters on lubricant film thickness, friction, wear and oil consumption was investigated. The thesis concludes that a reduction in specific fuel consumption is achieveable through the optimisation of cylinder liner texture and outlines how this might be achieved.

The piston ring pack is the single greatest contributor to mechanical losses in a heavy duty diesel engine, accounting for 1.1–6.8% of the total losses. Therefore, the piston ring-cylinder liner contact is potentially the most rewarding area to study when attempting to reduce mechanical losses in a heavy duty diesel engine. In this work, four different heavy duty diesel engine cylinder liner variants have been tested to evaluate the lubricating conditions that occur when a section of top compression ring is reciprocated against them in a lubricated environment. Two of the cylinder liners were traditional grey cast iron and plateau honed with different honing angles, one had ANS Triboconditioning® applied and the last was plasma sprayed with a stainless steel and ceramic coating, then honed. An experimental test rig was used where friction and film thickness was recorded, by means of an ultrasonic technique. A numerical model was also developed to calculate the friction and film thickness. Comparisons are made between the simulation and experiment, and the four cylinder liner variants are also evaluated. It was found that both simulation and experiment could differentiate between all surfaces and the results from the model and experiment also correlated well with each other. A lower plateau average surface roughness, as exhibited by the ANS Triboconditioning® and plasma liners, led to a significant reduction in friction.

The piston ring–cylinder liner contact is a major source of the total parasitic losses in an internal combustion engine. The lubrication process of this contact highly influences the amount of friction, oil consumption and wear that occurs. In this work, a reciprocating test rig combined with an ultrasonic film thickness measurement system was developed and then used for tribological investigation of the piston ring–cylinder liner contact under idealised cold conditions. A special piston ring and cylinder liner holder were designed and five sensors were glued on to the back side of the liner specimen. Ultrasonic reflections captured by the sensors, used to obtain the film thickness, and friction were continuously recorded as the piston ring section reciprocated over the liner. Several experiments were performed at different speed and load conditions. Furthermore, a numerical model has been developed to predict film thickness and friction in all lubrication regimes. The experimentally measured film thickness and friction were compared with the output from the numerical model and good correlation was found. The parameters affecting the accuracy of the ultrasound measurements and numerical simulations of film thickness and friction are then discussed.

Vertical Scanning Interferometry (VSI) may induce optical artefacts in surface topography measurements. The influence of these optical artefacts on the calculation of Rk surface roughness parameters, contact stiffness and flow factors were studied. Two surface measurement techniques were used: Atomic Force Microscopy (AFM) and VSI. Calibration grids were used to make it easier to isolate the causes of these artefacts, while a real engineering surface was used to compare these two techniques in an industrially applied case. It was found that the optical artefacts have a large influence on all the roughness parameters, contact stiffness and flow factors calculated on the calibration grids. However, for the engineering surface the differences between AFM and VSI measurements were much smaller.

A numerical model has been developed to investigate the effect of cylinder liner honing angle on hydrodynamic lubrication between piston ring and cylinder liner. The Reynolds equation was solved in 2D with periodic boundary conditions. An artificial surface texture was generated, based on a real surface measured with white light interferometry. Cavitation was modelled with the Vijayaraghavan and Keith algorithm. Honing angles between 25° and 75° were investigated to find the effect of honing angle on film thickness.

In work presented last year at the 37th Leeds-Lyon Symposium on Tribology a semi-deterministic model of the piston ring-cylinder liner contact was developed. The honing grooves (texture) were considered on a global scale and the surface roughness was incorporated on a local scale using the homogenization mathematical averaging technique. However, as noted at the time, the contact throughout the full engine cycle was considered to be fully flooded. It was assumed that there was always enough lubricant present to fully flood the entrance to the contact. In reality this is unlikely to be the case, particularly during the midstroke of the cycle, as with high entraining speeds the predicted hydrodynamic films can be several microns thick and there is unlikely to be sufficient lubricant on the liner to form these thick films. Therefore, an oil availability model has been developed to track the quantity of oil available on along the length of the liner. The full ring pack, compression, scraper and oil control ring are modelled. It is assumed that the oil control ring always runs in the fully flooded condition and the scraper and compression ring have the oil available to them that they left behind on the previous stroke. In the previous work the honing grooves were modelled and the effect they have on film formation and cavitation investigated. However, their effect on the distribution and retention of oil on the liner could not be seen due to the fully flooded inlet condition. With the incorporated oil availability model the effect of different honing groove patterns can be further investigated.

Many simulations already exist to model the piston ring-cylinder liner contact; however, very few models have been used to investigate the optimum surface texture. An axisymmetric, time-dependent two-dimensional semi-deterministic texture-roughness model of the piston ring to cylinder liner contact with periodic boundary conditions and mass preserving global cavitation has been developed. The cylinder liner texture, generated by honing, was considered deterministically on the global scale, after an investigation comparing deterministic and homogenized solutions. The surface texture of a real cylinder liner was measured with white light interferometry and an algorithm developed to generate an artificial periodic texture representative of the real surface. The effect of cylinder liner plateau roughness has been incorporated on the local scale by homogenization of the Reynolds equation and calculation of flow factors from real surface topography. Using the homogenization technique to incorporate the effect of surface roughness leads to a more efficient solution than mesh refinement of the deterministic problem as the roughness does not need to be resolved on the global solution domain, allowing for significantly less degrees of freedom in the global problem. The lubricant boundary pressures have been calculated using results from a numerical ring-pack model and the lubricant viscosity has been adjusted based on the cylinder liner wall temperature. It was found from the result of a comparison between deterministic and homogenized solutions that surface texture should be modelled on the global and not on the averaged roughness scale as is the case with many previous investigations.

Different averaging techniques have proved to be useful for analyzing the effects of surface roughness in hydrodynamic lubrication. This paper compares two of these averaging techniques, namely the flow factor method by Patir and Cheng (P&C) and homogenization. It has been rigorously proved by many authors that the homogenization method provides a correct solution for arbitrary roughness. In this work it is shown that the two methods coincide if and only if the roughness exhibits certain symmetries. Hence, homogenization is always the preferred method.