M Priest

Automobile engine tribology : approaching the surface

There has been relentless pressure in the second half of the twentieth century to develop ever more fuel efficient and compact automobile engines with reduced environmental impact. From the viewpoint of the tribologist this means increasing specific loads, speeds and temperatures for the major frictional components of the engine, namely the piston assembly, the valve train and the journal bearings, and lower viscosity engine oils with which to lubricate them. Inevitably, this leads to decreasing oil film thicknesses between the interacting surfaces of these components and a more crucial role for the topography and surface profile of the two surfaces in determining tribological performance. This paper will review the nature of the surfaces encountered in the piston assembly, valve train and journal bearings of the internal combustion engine and how mathematical models of engine tribology are endeavouring to cope with the extreme complexities the incorporation of surface topography potentially brings. Key areas for future research and the implications for design will be highlighted.

Predictive wear modelling of lubricated piston rings in a diesel engine

The tribological performance of piston rings in reciprocating internal combustion engines can only be fully understood when both lubrication and wear are considered in combination. To this end, a numerical model has been developed that predicts the dynamics, lubrication and wear of piston rings interactively for the first time. This paper reports the application of this new model to the piston ring pack of a diesel engine. With the overall aim of evaluating the correlation between theory and experiments, this analysis is divided into two discrete parts. First, the model is used to predict the lubrication performance of measured ring packs before and after periods of running, at constant speed and load, in a Caterpillar 1Y73 single-cylinder diesel engine: the objective being to establish the change in tribological behaviour with observed wear in the engine. Secondly, the model is used interactively to predict the lubrication and wear of the top compression ring from the same engine. This research advances the understanding of piston ring profile evolution with time and its dependence on complex interactions between lubrication and wear.

Theoretical modelling of cavitation in piston ring lubrication

Abstract Mathematical models of piston ring dynamics and lubrication are sensitive to the boundary conditions adopted to describe the cavitation occurring in the diverging outlet region of the lubricant film between the piston ring and cylinder wall. In this paper, such sensitivity is investigated by applying different models of gaseous cavitation, flow separation and fluid film reformation to the analysis of a single compression ring from a diesel engine. Significant differences are predicted in hydrodynamic pressure profiles, lubricant film boundaries, lubricant film thickness, oil flow and friction. Such indications of substantial differences in piston ring operating characteristics associated with the distinct cavitation boundary conditions considered highlights the need for further research in this field. However, the lack of detailed experimental data to validate the predictive models suggests that future progress must be based upon combined theoretical and experimental approaches to the problem. It is postulated that boundary conditions based upon Reynolds cavitation and fluid film reformation may be applicable at high loads, and fluid film separation of a form proposed by Coyne and Elrod at low loads.

Experimental Evaluation of Piston-Assembly Friction Under Motored and Fired Conditions in a Gasoline Engine

Piston-assembly friction measurement has been carried out on a single-cylinder gasoline engine using the IMEP (indicated mean effective pressure) method at realistic engine speeds and loads without any major engine modifications. Instantaneous and mean piston-assembly friction were measured under motored and fired conditions at different lubricant temperatures. The forces acting on the piston assembly were carefully determined by measuring the cylinder pressure, crankshaft angular velocity, and strain in the connecting rod. The difference between the resulting gas pressure, inertia, and connecting rod axial forces acting on the piston yields the piston-assembly friction. To achieve this with confidence, an advanced instrumentation, telemetry, and data acquisition system was designed and developed, giving special attention to the synchronization and simultaneous sampling of analog and digital channels. Experiments are reported for piston-assembly friction at a range of engine operating conditions with different lubricant formulations, with and without a friction modifier.

Tribological properties of ionic liquids as lubricants and additives. Part 1: Synergistic tribofilm formation between ionic liquids and tricresyl phosphate

The use of imidazolium tetrafluoroborate, IMM+BF4−, and hexafluorophosphate, IMM+PF6−, ionic liquids as lubricants was investigated at 25 and 100 °C to show lower friction coefficients but higher wear rates than a reference hydrocarbon lubricant. The ionic liquids readily form tribofilms at the lower temperature but have difficulty in forming partial films at the higher temperature. Wear tracks for a Plint TE77 reciprocating ball-on-plate test using ionic liquids show smoother surfaces, with small pits developing, compared to the reference hydrocarbon lubricant test result. Similar ionic liquids not containing fluorine were found to be less effective as lubricants. Addition of ionic liquids to a base grease and a formulated high temperature grease gave surprisingly large increases in the weld load for the Four Ball extreme pressure test. Little difference between base grease and base grease+5 per cent ionic liquid was observed for the TE77 test. There was no clear indication of the effect of alkyl substituent chain length on the imidazolium cation for the Four Ball test wear scar diameter. Addition of 1 per cent tricresyl phosphate (TCP) to ionic liquids rapidly establishes a tribofilm and reduces the wear volume by 64 per cent compared to the same test for the neat ionic liquid or neat TCP. Addition of 1 per cent TCP and 1 per cent ionic liquid to a Group III base oil also establishes a substantial tribofilm and reduces wear volumes compared to the base oil with 1 per cent TCP alone or the base oil with 1 per cent ionic liquid alone. Ionic liquids show promise as neat liquid lubricants by establishing a tribolayer chemically adsorbed to the steel surfaces. They are not as effective as a reference hydrocarbon lubricant in reducing wear of those surfaces by tribocorrosion. The fluorine-free ionic liquids investigated were not as effective as those containing fluorine. The addition of ionic liquids to grease, base or fully formulated, gave a substantial improvement in performance, which indicates a synergistic interaction with the additives present in the formulated grease. There is also clear evidence of a strong synergistic effect between ionic liquids and TCP, both for the neat ionic liquids and for 1 per cent dilution of each respectively in a Group III base oil to give a thick tribofilm and substantially reduced wear in the TE77 ball-on-plate test. The nature of the synergy between ionic liquids and TCP requires further investigation.

Experimental and Theoretical Study of Instantaneous Engine Valve Train Friction

A new method has been developed to directly measure valve train friction as a function of crank angle using specially designed timing belt pulley torque transducers fitted to the inlet and exhaust camshafts of a single-cylinder gasoline engine. Simultaneous and instantaneous friction torque of both the inlet and exhaust camshafts at any engine speed can be measured, with no apparent detrimental effect of timing belt loading on the output reading. Experiments are reported for valve train friction at a range of motored engine operating conditions with different lubricant formulations, with and without a friction modifier These are compared with the predictions of an existing valve train friction model based upon elastohydrodynamic lubrication theory. Measured friction decreased with increasing engine speed but increased with increasing oil temperature and the fuel economy benefit of friction modifiers was observed. The model yielded similar magnitudes of friction at medium engine speeds and above but predicted much lower friction with high oil temperatures at low speed. Comparison of theory and experiments also suggests that some oil may leak from hydraulic lash adjusters during the cam event with a consequent reduction in geometric torque.

Investigation of fundamental wear mechanisms at the piston ring and cylinder wall interface in internal combustion engines

Abstract This research examined the cylinder liner-piston ring system simultaneously from the metallurgical and metrological standpoints, using specimens cut from real engine components (rings and liners), in order to identify the mild and severe wear mechanisms. Work has been conducted using a Plint TE77 high frequency friction machine. Metrological analysis was performed using stylus contacting profilometer. Metallurgical analysis of the samples was carried out using a JOEL JSM-6400 scanning electron microscope. In addition to wear, the coefficient of friction (μ) was recorded for every piston ring - cylinder liner pair so as to observe the transitions between mixed and boundary lubrication. This paper presents the results obtained using flame sprayed Mo-coated spheroidal graphite cast iron, which is an old piston ring coating and is not available anymore, and relatively new Federal Mogul CKS-36™ top compression rings tested against a grey cast iron cylinder liner tested at two different bulk oil temperatures (90 °C and 140 °C), two different pressures (3.9 MPa and 6.5 MPa) and with two different lubricants (SAE 0W20 with a Friction Modifier and SAE 15W40).

Metal-metal friction characteristics and the transmission efficiency of a metal V-belt-type continuously variable transmission

Abstract The influence of metal-metal friction characteristics on the efficiency of a continuously variable transmission (CVT) of a metal pushing V-belt type was experimentally investigated using a commercial CVT unit of a metal belt assembly and pulley design. The experiments for transmission efficiency were carried out, varying the clamping force on the secondary pulley from 10.5 to 34.6 kN and the speed ratio from 2.36 (reduction ratio) to 0.44 (overdrive ratio). In order to analyse the metal-metal friction characteristics of each contact pair, a ring-on-disc tribometer was developed. Fluids giving a higher transmittable CVT torque capacity of CVT were found to have the potential for decreasing the maximum required pulley clamping force, resulting in the reduction of overall power loss in the CVT unit. The transmission efficiency of the CVT decreased under an overdrive speed ratio and lower load condition. The maximum difference in the efficiency between all the commercial automatic transmission and CVT fluids tested at the same pulley clamping force condition reached 3 per cent. This number depends on the friction losses caused by slipping behaviour between the belt segments and pulley, the segments and band, and between the bands. Furthermore, mathematical modelling of the friction loss in the belt was developed. Results calculated by this model were similar to those obtained experimentally.

Lubrication of an electroplated nickel matrix silicon carbide coated eutectic aluminium—silicon alloy automotive cylinder bore with an ionic liquid as a lubricant additive

Abstract For a chromium-nitrided steel piston ring section sliding against an electroplated nickel matrix silicon carbide coated eutectic aluminium—silicon alloy automotive cylinder bore in a reciprocating tribometer, the ionic liquid (IL) imidazolium tetrafluoroborate has an excellent tribological performance as a lubricant additive in the presence of tritolyl phosphate (TTP). A low friction coefficient of 0.015 and a substantial tribofilm was generated. The performance of this base oil/additive combination was superior to the base oil alone or the separate additives in the base oil. The substantial improvement in tribological performance given by addition of an IL and TTP, 1 per cent by volume of each, to a group III base oil indicates a synergistic interaction between the IL and TTP. The need to understand the chemical and physical nature of the tribofilm is emphasized.

Effect of Engine Operating Conditions and Lubricant Rheology on the Distribution of Losses in an Internal Combustion Engine

With new legislation coming into place for the reduction in tail-pipe emissions, the OEMs are in constant pressure to meet these demands and have invested heavily in the development of new technologies. OEMs have asked lubricant and additive companies to contribute in meeting these new challenges by developing new products to improve fuel economy and reduce emissions. Modern low viscosity lubricants with new chemistries have been developed to improve fuel consumption. However, more work is needed to formulate compatible lubricants for new materials and engine technologies. In the field of internal combustion engines, researchers and scientists are working constantly on new technologies such as downsized engines, homogeneous charge compression ignition, the use of biofuel, new engine component materials, etc., to improve vehicle performance and emissions. Mathematical models are widely used in the automotive and lubricants industry to understand and study the effect of different lubricants and engine component materials on engine performance. Engine tests are carried out to evaluate lubricants under realistic conditions but they are expensive and time consuming. Therefore, bench tests are used to screen potential lubricant formulations so that only the most promising formulations go forward for engine testing. This reduces the expense dramatically. Engine tests do give a better picture of the lubricants performance but it does lack detailed tribological understanding as crankcase oil has to lubricant all parts of the engines, which do operate under different tribological conditions. Oil in an engine experiences all modes of lubrication regimes from boundary to hydrodynamic. The three main tribological components responsible for the frictional losses in an engine are the piston assembly, valve train, and bearings. There are two main types of frictional losses associated with these parts: shear loss and metal to metal friction. Thick oil in an engine will reduce the boundary friction but will increase shear losses whereas thin oil will reduce shear friction but will increase boundary friction and wear. This paper describes how engine operating conditions affect the distribution of power loss at component level. This study was carried out under realistic fired conditions using a single cylinder Ricardo Hydra gasoline engine. Piston assembly friction was measured using indicated mean effective pressure method and the valve train friction was measured using specially designed camshaft pulleys. Total engine friction was measured using pressure-volume diagram and brake torque measurements, whereas engine bearing friction was measured indirectly by subtracting the components from total engine friction. The tests were carried out under fired conditions and have shown changes in the distribution of component frictional losses at various engine speeds, lubricant temperatures, and type of lubricants. It was revealed that under certain engine operating conditions the difference in total engine friction loss was found to be small but major changes in the contribution at component level were observed.