Tribological optimisation of the internal combustion engine piston to bore conjunction through surface modification

Internal combustion (IC) engines used in road transport applications employ pistons to convert gas pressure into mechanical work. Frictional losses abound within IC engines, where only 38- 51% of available fuel energy results in useful mechanical work. Piston-bore and ring-bore conjunctions are fairly equally responsible for circa 30% of all engine friction - equivalent to 1.6% of the input fuel each. Therefore, reduction in piston assembly friction would have a direct impact on specific performance and / or fuel consumption. In motorsport, power outputs and duty cycles greatly exceed road applications. Consequently, these engines have a shorter useful life and a high premium is placed on measures which would increase the output power without further reducing engine life. Reduction of friction offers such an opportunity, which may be achieved by improved tribological design in terms of reduced contact area or enhanced lubrication or both. However, the developments in the motorsport sector are typically reactive due to a lack of relative performance or an ad-hoc reliance, based upon a limited number of actual engine tests in order to determine if any improvement can be achieved as the result of some predetermined action. A representative scientific model generally does not exist and as such, investigated parameters are often driven by the supply chain with the promise of improvement. In cylinder investigations are usually limited to bore surface finish, bore and piston geometrical form, piston skirt coatings and the lubricant employed. Of these investigated areas newly emerging surface coatings are arguably seen as predominate. This thesis highlights a scientific approach which has been developed to optimise piston-bore performance. Pre-existing methods of screening and benchmarking alterations have been retained such as engine testing. However, this has been placed in the context of validation of scientifically driven development. A multi-physics numerical model is developed, which combines piston inertial dynamics, as well as thermo-structural strains within a thermoelastohydrodynamic tribological framework. Experimental tests were performed to validate the findings of numerical models. These tests include film thickness measurement and incylinder friction measurement, as well as the numerically-indicated beneficial surface modifications. Experimental testing was performed on an in-house motored engine at Capricorn Automotive, a dynamometer mounted single-cylinder ‘fired’ engine at Loughborough University, as well as on other engines belonging to third party clients of Capricorn. The diversity of tests was to ascertain the generic nature of any findings. The multi-physics multi-scale combined numerical-experimental investigation is the main contribution of this thesis to knowledge. One major finding of the thesis is the significant role that bulk thermo-structural deformation makes on the contact conformity of piston skirt to cylinder liner contact, thus advising piston skirt design. Another key finding is the beneficial role of textured surfaces in the retention of reservoirs of lubricant, thus reducing friction

Investigation of reciprocating conformal contact of piston skirt-to-surface modified cylinder liner in high performance

The article presents detailed analysis of the conforming contact between a piston and cylinder liner in a high-speed racing engine under extreme operating conditions owing to high loads and operating speeds in excess of 19 000 r/min, resulting in a high sliding velocity of 42 m/s. The analysis indicates contact forces generated in the order of 2.5 kN. The contribution due to fluid film lubrication is found to reside in iso-viscous rigid or elastic regimes of lubrication, which is insufficient to form a coherent lubricant film during some parts of the cycle, such as at top-dead-centre (TDC). The article shows that at combustion, 95 per cent of the contact can remain in boundary or mixed regimes of lubrication. Piston skirt surface modification features are used in conjunction with an electrolytically applied composite coating, Ni[SiC]p to produce advanced cylinder liners to remedy the situation. Detailed numerical analysis shows that significant improvement is achieved in the regime of lubrication condition

Investigation of reciprocating conformal contact of piston skirt and ring-pack to cylinder liner under transient condition

This paper presents an investigation of piston lubrication combining the ring pack in order to study if sufficient lubricant film is formed during reversal at TDC and combustion

In-cylinder friction reduction using a surface finish optimization technique

The paper describes the importance of reducing frictional losses in internal combustion (IC) engines, thereby improving engine efficiency. One of the main sources contributing significantly to engine friction is the interaction between the piston compression and oil rings and the cylinder bore/liner. Improving the tribological performance in these conjunctions has the greatest potential for performance improvement in the IC engine. Traditionally, the approaches used to tackle this problem have relied heavily on empirical engineering judgement. These have resulted in many inconclusive studies, involving a large number of alternatives, including the introduction of cylinder liners with surface modification work and/or with special coatings. This paper highlights a fundamental investigation of surface modification and coating and its impact on frictional performance. The study combines numerical and experimental approaches. Very good agreement is found between the conclusions of numerical predictions and those of engine test bed work

Tribology of piston skirt conjunction

Frictional losses in the piston skirt to cylinder liner conjunction account for approximately 2.5% of the energy supplied to the modern car [1]. These losses are contributed by viscous shear of the lubricant and asperity interactions on the contiguous surfaces. However, for most of the piston cycle the regime of lubrication is dominated by elastohydrodynamics or hydrodynamics. Hence, friction due to viscous shear is dominant. Most idealistic analyses employ a “cold” piston skirt shape and use either a measured profile or by approximated polynomials as the input shape [2-4]. In reality, however, pistons are subject, not only to contact forces, but also thermo-mechanical distortion. These are as the result of thermal expansion of the piston as well as its global mechanical deformation in situ. They alter the pistonliner conjunctional gap. The piston structure is designed in such a way as to prevent gross localised wear in service by means of skirt profile and structural stiffness modification [5]. Considering the combined effect of global as well as local deformation of the skirt under the influence of contact force, it is vital to take into account the effect of shape and rigidity of both the piston and liner structures in an integrated thermoelastic and elastohydrodynamic analysis. This approach is more representative of the in situ “hot” skirt condition as noted by McClure [6]. This paper shows the significant differences observed in the generated pressures, film thickness and friction by comparing “cold” piston profiles; disregarding large scale global deformation and “hot” thermo-elastically deformed skirt conjunctions with representative skirt stiffness

Characterisation of automotive bore material and coatings using atomic force microscopy [Abstract]

Characterisation of automotive bore material and coatings using atomic force microscopy [Abstract

Combined numerical and experimental investigation of the micro-hydrodynamics of chevron-based textured patterns influencing conjunctional friction of sliding contacts

Reciprocating and low-speed sliding contacts can experience increased friction because of solid boundary interactions. Use of surface texturing has been shown to mitigate undue boundary friction and improve energy efficiency. A combined numerical and experimental investigation is presented to ascertain the beneficial effect of pressure perturbation caused by micro-hydrodynamics of entrapped reservoirs of lubricant in cavities of textured forms as well as improved microwedge flow. The results show good agreement between numerical predictions and experimental measurements using a precision sliding rig with a floating bed-plate. Results show that the texture pattern and distribution can be optimised for given conditions, dependent on the intended application under laboratory conditions. The translation of the same into practical in-field applications must be carried out in conjunction with the cost of fabrication and perceived economic gain. This means that near optimal conditions may suffice for most application areas and in practice lesser benefits may accrue than that obtained under ideal laboratory conditions

Asperity level tribological investigation of automotive bore material and coatings

Choosing in-cylinder surfaces is complex. A well-chosen surface has low friction and wear. Conversely, poor oversight often leads to premature failure through wear and scuffing. Typically cylinder bore surfaces are selected experientially. This paper demonstrates the use of Atomic Force Microscopy in LFM mode, characterising typical cylinder bore materials and coatings. The approach uses integrated LFM with continuum contact mechanics. It evaluates the real contact area and effective elastic modulus of the surface, including the effect of any reactive surface film. Surface energy and shear strength, as well as the coefficient of friction in nanoscale interactions are also determined. These properties are measured for 6 cylinder bore materials, including for composite Nickel-Silicon Carbide and DLC, used for high performance engines

The measurement of liner - piston skirt oil film thickness by an ultrasonic means

The paper presents a novel method for the measurement of lubricant film thickness in the piston-liner contact. Direct measurement of the film in this conjunction has always posed a problem, particularly under fired conditions. The principle is based on capturing and analysing the reflection of an ultrasonic pulse at the oil film. The proportion of the wave amplitude reflected can be related to the thickness of the oil film. A single cylinder 4-stroke engine on a dyno test platform was used for evaluation of the method. A piezo-electric transducer was bonded to the outside of the cylinder liner and used to emit high frequency short duration ultrasonic pulses. These pulses were used to determine the oil film thickness as the piston skirt passed over the sensor location. Oil films in the range 2 to 21 μm were recorded varying with engine speeds. The results have been shown to be in agreement with detailed numerical predictions

Fractal characterisation of inhomogeneous rough surfaces

Load bearing conjunctions are never perfectly flat. They are covered by surface features, which may be either unintentional roughness inherent in the manufacturing process or a combination of such roughness with intentionally introduced surface texture. In either case, only a small proportion of load bearing surfaces are in contact and carry load. This depends on the surface topography, material properties and contacting conditions. Simple surface roughness characterisation parameters such as Ra, Rp etc. although commonly used do not give an adequate description, particularly where surfaces are deliberately textured. Furthermore Imaged surface topographies commonly exhibit features below the resolution of the imaging apparatus. We demonstrate the application of a fractal geometry analysis to honed internal combustion engine cylinder liners, imaged by Atomic Force Microscopy and Confocal Laser Scanning Microscopy. This method enables anisotropic surfaces to be characterised by five parameters which can then be used to generate model surfaces whose characteristics follow very closely those of the measured surface. Analysis of the structure function allows the determination of length scales and roughness features relevant at a given contact length. The reconstruction of anisotropic surfaces is demonstrated. Results from modelled contacts between these surfaces reveal the likely asperity contact geometry to be used in modelling contact and friction in these interfaces