Riaz A. Mufti

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 performance of nanoparticles as lubricating oil additives

The prospect of modern tribology has been expanded with the advent of nanomaterial-based lubrication systems, whose development was facilitated by the nanotechnology in recent years. In literature, a variety of nanoparticles have been used as lubricant additives with potentially interesting friction and wear properties. To date, although there has been a great deal of experimental research on nanoparticles as lubricating oil additives, many aspects of their tribological behavior are yet to be fully understood. With growing number of possibilities, the key question is: what types of nanoparticles act as a better lubricating oil additive and why? To answer this question, this paper reviews main types of nanoparticles that have been used as lubricants additives and outlines the mechanisms by which they are currently believed to function. Significant aspects of their tribological behavior such as dispersion stability and morphology are also highlighted.

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.

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.

Analysis of piston assembly friction using the indicated mean effective pressure experimental method to validate mathematical models

Abstract The piston assembly friction is experimentally measured using the indicated mean effective pressure with detailed accuracy. The experimental results are compared with a relatively less complex piston assembly friction model typical of those used in industry, predicting the individual performance of compression rings, the oil control ring, and the piston skirt. The validation is carried out under fired conditions on a single-cylinder gasoline engine. The experimental results for an SAE 0W20 lubricant without a friction modifier were compared with the predictions. The predicted results correlate very well with the measurements, especially at higher lubricant inlet temperatures. The piston skirt friction was predicted using a simple concentric piston—cylinder model and a more realistic but computationally intensive method incorporating the pistons secondary motion. The results clearly indicate that a relatively less complex model can give realistic results under real engine conditions.

Effect of Lubricant Formulations on the Tribological Performance of Self-Mated Doped DLC Contacts: a review

Abstract Diamond-like carbon (DLC) coatings have emerged as one of the most promising surface coatings for applications involving boundary lubrication regime. Some of the characteristics that distinguish DLC coatings from other hard coatings include high hardness, low friction coefficient, wear resistance, and chemical inertness. Because of their low surface energies, these coatings cannot react effectively with different lubricant constituents. Doping of DLC coatings with metals and nonmetals, such as titanium, tungsten, silicon, chromium, fluorine, and nitrogen, can improve their interaction with lubricants to form tribologically beneficial films. Many experimental studies have been conducted on different types of DLC coatings using various lubricant formulations in the last few decades. The results obtained from these experimental studies are very scattered and contradictory, so the data should be consolidated in a more organized and apprehensible manner. By doing so, tribological behavior of various DLC–lubricant combinations can be understood in a better way, and a more logical continuation of research on DLC coatings can be carried out. In this review paper, most widely investigated metal- and nonmetal-doped DLC coatings, such as Ti-DLC, W-DLC, Si-DLC, Cr-DLC, WC-DLC, and multilayered a-C:H/W-DLC coatings, are considered for evaluation. Tribological performance of the aforementioned DLC coatings, in combination with various base oils and lubricant additives, is analyzed by comparing their average friction and wear coefficient values, which have been calculated from published experimental data. Only self-mated doped DLC contacts are considered in this study to eradicate the interference of ferrous and nonferrous counterbodies.

Theoretical and experimental evaluation of engine bearing performance

Abstract Measurements of total engine bearing friction have been carried out on a single-cylinder gasoline engine under fired conditions. The bearing friction was measured indirectly by measuring the total engine friction, piston assembly friction, and valve train friction. Total engine friction was measured via pressure-volume (PV) diagram, whereas piston assembly friction measurement was carried out using the indicated mean effective pressure method. Valve train friction was measured using instrumented camshaft drive pulleys. All the engine auxiliaries were independently driven electrically. Engine bearing friction was measured under different engine operating conditions. The hydrodynamic lubrication regime occurred under most of the engine operating conditions, although at lower engine speeds and higher lubricant temperatures a transition from hydrodynamic to boundary/mix lubrication regime occurred. The effect of two different lubricants SAE 0W20 and SAE 5W30 on engine bearing performance was investigated. The adverse effects of viscous oil at low lubricant temperature were notable, as were the benefits at higher lubricant temperature. Predicted results from a number of engine bearing friction mathematical models have been compared with the experimental results. The comparative study showed that the results generated through the short bearing mobility method under cavitation conditions were close to the experimental data. This experimental system can be used to study bearing design parameters, bearing materials, lubricant chemistry, and engine operating conditions on the bearing performance under realistic conditions in its original environment.

Technique of simultaneous synchronized evaluation of the tribological components of an engine under realistic conditions

Abstract One of the main drivers in developing advanced engines and formulating new lubricants is fuel economy. The direct relationship of frictional loss to fuel economy in an internal combustion engine has resulted in an increased interest in understanding engine performance at the component level. With the advances in data acquisition systems and sensor technology, extensive study of the main engine components, i.e. valve train, piston assembly, and engine bearings, have been possible, resulting in the development of a versatile engine friction measurement system. Total engine and component friction measurements were carried out on a single-cylinder Ricardo Hydra gasoline engine under fired conditions. For such measurements, the engine is fitted with more than 50 different sensors and to sample/log data from such a large number of transducers, an advanced high-speed synchronized data acquisition system was designed. This paper explains the experimental techniques used to measure simultaneously total and component friction under fired conditions. This system can be used as a powerful tool for screening lubricants, validating complex mathematical models at system level, and studying the effect of different additives and materials on the performance of each engine component under realistic engine operating conditions.

Effect of oil rheology and chemistry on journal-bearing friction and wear

Legislation and market pressures are calling for increased engine power, reduced engine size, and improved fuel consumption. The use of low-viscosity lubricants is considered as a means to enhance fuel economy by reducing viscous friction, particularly in engine bearings. Journal bearings mostly operate under hydrodynamic lubrication with a thin film of oil separating the journal and bearing shell. There are, however, certain conditions, especially under high load or low speed, when the film thickness will be low enough to allow boundary lubrication to occur. In this study, the effect of lubricants with different viscosities, different types of viscosity modifiers, different additives, different types of dispersants, and different lubricant formulations have been studied under hydrodynamic and boundary lubrication regimes. For hydrodynamic conditions, a high-temperature high-shear viscometer, meeting the requirements of ASTM D4741 was used to measure viscosity at 106 s−1. In addition, a new ultra high-shear viscometer, from PCS Instruments, was used to measure viscosity at shear rates near to 107 s−1. Bearing weight loss and load bearing capacity were measured on a rig developed in-house using a specially designed half-bearing shell loaded against a rotating journal. A PCS journal-bearing rig was used to measure the bearing friction under transient load.

Dispersion Stability and Tribological Characteristics of TiO2/SiO2 Nanocomposite-Enriched Biobased Lubricant

ABSTRACT The stable dispersion of nano-additives is highly desirable for the effective lubrication performance of nanolubricants. The compatibility of base oil with selected nano-additives is required for uniform and stable dispersion. This research evaluated the dispersion stability and tribological characteristics of nano-TiO2/SiO2 (average particle size 50 nm) as an additive in a biobased lubricant. The wear protection and friction reducing characteristics of the formulations were evaluated by four-ball extreme pressure tests and piston ring–cylinder liner sliding tests. Surface analysis tools, including scanning electron microscopy, energy-dispersive X-ray spectroscopy, and atomic force microscopy, were used to characterize the worn surfaces. Results showed that the nanolubricants demonstrated appreciable dispersion capability in the absence of a surfactant and an improvement in load-carrying capacity, antiwear behavior, and friction reduction capability.