Browse Topic: Lubricant viscosity
Hybrid electric vehicles (HEVs) with an increasing level of electrification, are becoming a major part of the global energy transition. To achieve lower engine tailpipe exhaust emissions and improve total fuel consumption, typically the HEV control system expertly and frequently switches between the internal combustion engine and electric motor drive, with multiple stops and restarts of the internal combustion engine (ICE). As a consequential result of this switching, are typically slower or even incomplete engine warm-up times, depending on the engine speed, load pattern and run time of the vehicle drive cycle. Along with the speed and load transient control, the engine stop and start processes are also challenging to control, with respect to cold start fuel and combustion by-products entering the oil. Consequently, contamination enters the engine oil but may not completely leave. These effects are highly transient over the drive cycle. Contaminants and in particular, fuel dilution, will affect the engine oil viscosity. To demonstrate this whilst yielding insights, a precisely controlled engine test cell, running the cold start Worldwide Harmonized Light Duty Transient Cycle (WLTC) for both, a non-hybridized ICE only vehicle and a HEV in charge sustaining mode operation is described. This also has on-line viscosity sensing and oil sampling. Typical data is shared along with engine oil comparisons. For complimentary insights, the impact of the fuel dilution on engine friction was investigated using a novel, precise, fully transient engine friction test rig, which measures gasoline direct injection high pressure fuel-pump friction and engine oil viscosity accurately. The cycle is based on measured data from vehicles tested on a chassis dynamometer. On-line friction data, with oil comparisons is used to show real-time data of the effect of fuel dilution on the frictional energy required, thus CO2 over the full WLTC.
The torque transfer response to rider throttle operation contributes to vehicle control in motorcycles equipped with a DCT (Dual Clutch Transmission). The clutch response is a key parameter to enhance torque transfer response. We have developed three new ECU (Electric Control Unit) control methods to enhance the clutch response on the DCT. The DCT clutch transfers torque by controlling the contact force between the clutch discs and the clutch plates. It is desirable to measure the hydraulic pressure value directly from the clutch piston chamber to control the contact force. However, since the clutch piston is a rotating body, it is impractical to place a hydraulic pressure sensor on it. Therefore, the hydraulic pressure sensor is placed along the clutch control oil line at the existing DCT system. Consequently, when oil flows in the oil line, pressure loss in the oil line causes a deviation between the hydraulic pressure sensor value and the clutch piston chamber pressure value, which limits the enhancement of clutch response. To enhance clutch response, we have studied the estimation of the hydraulic pressure value in the clutch piston chamber using the existing hydraulic pressure sensor value at the oil line. This estimation is based on the reaction force characteristics of the clutch piston and Bernoulli’s principle. By using the estimated hydraulic pressure, half-clutch control can be identified, which allows the application of higher feedback gain to enhance clutch response. We also implement correction of clutch control oil viscosity fluctuations based on the hydraulic pressure variations of the clutch control oil. With these technologies applied, the clutch response time is reduced 45% as reference compared to the existing DCT clutch control. This also reduces torque transfer response time, ultimately allowing for smoother vehicle control.
In pursuit of reducing carbon emissions and to fulfill the customers’ needs for fuel-saving and environmentally friendly cars, car manufacturers have been increasingly offering different choices of electrified cars to their customers. Among those different powertrain solutions, with a balance of energy source between on-board electricity and fossil fuels, plug-in hybrid electric vehicles (PHEV) are becoming a choice for more and more end users, particularly in regional car markets such as China in recent years. Owing to the diversified vehicle operating conditions, new challenges are brought to the engine oil to protect the hardware from issues such as piston deposit, water/oil emulsification, oil thinning caused by fuel dilution, stop-start bearing wear and corrosion. This technical paper seeks to understand the impact of different operating modes of PHEV on engine oil performance. One key finding is that extreme conditions were needed to accumulate water content in the oil. When the oil temperature is under 10°C and the coolant temperature is under 40°C, water content could be continuously built up in the oil. Conversely, when these conditions are not met, water in the oil can be easily vaporised and released from oil. On the other hand, when the ambient temperature is low (below -20°C), and under low-speed/frequent stop-start driving cycle, fuel can be entrained in oil much more easily, to as high as 20% (m/m). Fuel dilution can significantly reduce the oil viscosity. A SAE 0W-20 oil could effectively end up as a SAE 0W-8 oil under high fuel dilution rate, which promotes wear between friction pairs such as ring-liner and bearing-journal. Star Polymer (Hydrogenated Styrene-Diene) viscosity modifier demonstrated better viscometric control performance over OCP (Ethylene-Propylene Co-polymer). Finally, it has been noted that at equivalent vehicle running distances, wear metals like iron and aluminum are more prevalent in non-plug-in hybrid vehicles compared to PHEVs, with the lowest levels found in ICE vehicles. This trend is attributed to the decreasing severity of stop-start operations, ranked as follows: HEV > PHEV > ICE.
There is a lack of data to support the efficacy of traditional mileage and time-based criteria for oil changes in vehicles. In this study, used-oil samples from 63 vehicles were collected and analyzed. Besides dynamic viscosity, viscosity index and activation energy were evaluated as measures of thermal stability of viscosity. The results revealed that mileage and time of use are not significantly correlated with (p > 0.05) and are thus poor indicators of oil viscosity and viscosity thermal stability measures. These findings highlight the limitations of current criteria and underscore the need for new sensing and evaluation methods to reduce costs, waste, and environmental impact while ensuring vehicle performance.
Micro-dimple is one of the promising surface texturing technologies to reduce friction loss due to the generation of thicker oil film caused by the cavitation occurrence around the micro-dimples. In this study, the flow behavior of oil film around micro-dimples was directly observed by laser-induced fluorescence (LIF). LIF observation for the oil flow showed that micro- dimples induced the cavitation occurrence that contributed to increase the oil film thickness. This was in good agreement with the results of the friction test, and it was thus proved that the cavitation occurrence by micro-dimples is significantly effective for the friction reduction.
The engine power cylinder is comprised of the piston, piston rings, and cylinder. It accounts for a significant amount of total engine friction within reciprocating, internal combustion engines. Reducing power cylinder friction is key to the development of efficient internal combustion engines. However, isolating individual power cylinder tribocouples for detailed analysis can be challenging. In this work, a new reciprocating liner test rig is developed and introduced. The rig design is novel, using a stationary piston and a reciprocating cylinder liner. Friction is calculated from the force measured in the connecting rod which supports the piston. The rig allows for independent control of peak cylinder pressure, speed, and lubricant temperature. Using the newly developed test rig, several technologies for friction reduction are evaluated and compared. Friction reducing technologies include the use of a low-friction TiSiCN nanocomposite coating applied to the piston rings, a lubricant viscosity study with engine oils ranging between SAE 0W-16 to SAE 10W-40 viscosity grades, and the impact of a special organic friction modifying oil additive. Results indicate that significant reductions in friction may be obtained using specialty coatings and optimized lubricating oils. Finally, results from the new reciprocating liner test rig are compared to data generated in chassis dynamometer vehicle fuel economy testing, showing excellent agreement.
On urban and emission homologation cycles, engines operate predominantly at low speeds and part loads where engine friction losses represent around 10% of the consumed fuel energy but would account for 25% of the fuel consumption once combustion efficiency is taken into account. Under such mild conditions, engine and engine oil temperatures are also lower than ideal. The influence of oil viscosity on friction losses are significant. By reducing lubricant viscosity, engine friction, fuel consumption and emissions are reduced. Tribological and machine learning models were investigated to predict the effect of oil viscosity on fuel consumption during the FTP75 emission cycle with the use of detailed actual emission test measurements. Oil viscosity was calculated with the measured oil temperature. As the same vehicle transient is followed in the cold and hot phases, the models were evaluated by comparing their prediction of fuel consumption in the hot phase versus the measured value. The models were able to predict the fuel consumption in the hot phase within 1% tolerance for more recent vehicles equipped with GDI and more detailed test data. The proposed methodologies have the advantage of being able to have their reliability tested before application. One can test the model ability to predict the fuel consumption hot phase before applying them to predict fuel saving with lower viscosity oils than the reference one used in the actual emission test. The models were then used to predict fuel saving in the cold phase by change of the tested 5W30 oil for a 0W20 one.
One of the first tasks while designing pistons is to ensure the reliable engine operation with minimal friction losses. This is possible by ensuring the liquid friction in the piston-cylinder junction during the entire operating cycle. Therefore, it is important to assess the nature of friction in the piston-cylinder conjunction. This task can be broken down into a number of interrelated subtasks: determining the characteristics of the piston lateral movement, determining the piston deformations under thermal and mechanical loads, and calculating the hydrodynamic forces acting from the side of the oil layer in the conjunction. The use of software packages that solve these problems separately and their inclusion in the iterative process will lead to huge expenditures of computing time and is difficult to implement in carrying out design optimization problems. The authors have developed a mathematical model for the joint solution of the above problems, and carried out computational studies based on program codes developed by the authors. The main focus of this work is on solving the elastic-hydrodynamic problem. The solving of the Reynolds equation in a two-dimensional formulation, taking into account the change in the thickness of the oil layer from deformations caused by the effect of hydrodynamic pressures, after finite-difference approximation is reduced to solving a system of nonlinear equations by Newton's method. The effect of a change in the viscosity of the lubricant in the conjunction is taken into account. In the event of a contact between the piston and the cylinder, the contact area is excluded from consideration while determining the hydrodynamic pressure. In the contact area, a system of linear equations is solved that describes the elastic deformations of the piston. The influence of the finite-difference mesh size on the accuracy of the results is estimated. An important element in the effective implementation of the model was the specification of the compliance matrix only for the nodes of the friction surface of the skirt that coincide with the nodes of the finite-difference mesh, which can be obtained from the volumetric model of the piston structure, implemented using a software package. The developed mathematical model makes it possible to investigate the effect on the hydrodynamic friction characteristics and the dynamics of piston movement of the main parameters characterizing the design of the crank mechanism parts, the profile of the piston skirt, the mounting gap, the viscosity-temperature properties of the engine oil, and the engine operating modes. The results were obtained for a two-piece piston with a diameter of 130 mm. The study of the influence of the two-piece piston skirt profile on the hydrodynamic characteristics and lateral movement of the piston is carried out, the results for the investigated structure and recommendations for profiling are presented.
The piston assembly is the major source of tribological inefficiencies among the engine components and is responsible for about 50% of the total engine friction losses, making such a system the main target element for developing low-friction technologies. Being a reciprocating system, the piston assembly can operate in boundary, mixed and hydrodynamic lubrication regimes. Computer simulations were used to investigate the synergistic effect between low viscosity oils and cylinder bore finishes on friction reduction of passenger car internal combustion engines. First, the Reynolds equation and the Greenwood & Tripp model were used to investigating the hydrodynamic and asperity contact pressures in the top piston ring. The classical Reynolds works well for barrel-shaped profiles and relatively thick oil film thickness but has limitations for predicting the lubrication behavior of flat parallel surfaces, such as those of Oil Control Ring (OCR) outer lands. In these cases, a deterministic-based model was used to evaluate the role of surface roughness on the hydrodynamic pressure build-up and its impact on the lubrication performance of OCRs. Samples’ characteristics and results of piston ring-cylinder bore tribological tests were used as input data for the mathematical models and support the discussion of the simulated results. This paper also provides an overview and a conceptual examination of mixed lubrication models commonly used to simulate piston ring conjunctions. A companion paper, namely “Powertrain Friction Reduction by Synergistic Optimization of Cylinder Bore Surface and Lubricant - Part 2: Engine Tribology Simulations and Tests”, complements this work with additional simulation results and empirical data that advances the understanding of the interplay between lubricant viscosity and surface topography and its contribution on the performance of reciprocating engines.
This paper develops a lumped-parameter multi-plates wet clutch Offset Compound Gear (OCG) transmission dynamics and its thermal model for dual-speed rotorcraft applications with an active clutch slip-speed control. This model includes the Reynolds equation for the clutch oil film thickness, the clutch thermal model, the clutch transferred torques (viscous and asperity torque) and the clutch disengagement model. The wet clutch/OCG transmission system is implemented in Matlab® Simulink™ to manage the upshift clutch temperature rise, which is a main issue need to handle for a dual-speed helicopter transmission. Here, the clutch temperature rise is treated by injecting a certain amount of coolant during engagement so that the temperature rise for the wet clutch is much lower than that of an dry clutch. In order to transfer a required torque using the available power, the sizing of the wet clutch could be evaluated via the developed wet clutch/OCG transmission model. This study shows that the temperature rise drops as the wet clutch oil flow rate increases adding extra weights compared with the dry clutch. The simulation also captures a phenomenon that a larger clutch engagement pressure might be required for the wet clutch to transfer the same torque since the wet clutch oil viscosity drops as the oil temperature increases during the clutch engagement.
More stringent Federal emission regulations and fuel economy requirements have driven the automotive industry towards more sophisticated vehicle thermal management systems to best utilize the waste heat and improve driveline efficiency. The final drive unit in light and heavy duty trucks usually consists of geared transmission and differential housed in a lubricated axle. The automotive rear axle is one of the major sources of power loss in the driveline due to gear friction, churning and bearing loss affecting vehicle fuel economy. These losses vary significantly with lubricant viscosity. Also the temperatures of the lubricant are critical to the overall axle performance in terms of power losses, fatigue life and wear. In this paper, a methodology for modeling thermal behavior of automotive rear axle with heat exchanger is presented. The proposed model can be used to predict the axle lubricant temperature rise. It also can be used to study the effect of coolant temperature on the axle warm-up and efficiency for a typical EPA fuel economy driving cycle. Thermal axle consists of automotive rear axle with a heat exchanger added to the axle housing to control the axle lubricant temperature using energy from coolant. The proposed multi-mass model allows an assessment of the effect of operating parameters (heat exchanger effectiveness, coolant flow rate, coolant temperature, lubricant viscosity) on the axle performance and driveline efficiency. The model represents a useful tool to optimize coolant energy distribution between axle and other vehicle thermal system devices. The model can be used to develop more sophisticated thermal system control strategies with variable coolant flow control devices. The model will also be helpful to select axle lubricant for optimum axle performance. The model is validated by comparing its results with experimental data obtained from EPA fuel economy driving cycle vehicle tests.
There is still a need in the industry for engine oils that have low viscosities to improve vehicle fuel efficiency but also protect engines from wear. Viscosity modifiers (VMs) are chief additives responsible for adjusting the viscometric characteristics of automotive lubricants. Most notably, VMs have a significant impact on a lubricant's viscosity-temperature relationship as indicated by viscosity index (VI), cold cranking simulator (CCS) viscosity, and high temperature high shear (HTHS) viscosity of engine oils. Functional copolymers bearing branched, linear, or anti-wear functionalities have been synthesized and evaluated for viscometric and wear protection performance. The resulting polymers improved tribofilm formation, shear stability and CCS viscosities. Indirect benefits including Noack improvement and trim oil reduction were observed.
The aim of this study is to investigate how lubricants used for transaxles in hybrid electric vehicles (HEVs) and electric vehicles (EVs) give an impact on the cooling performance for electric motors. As a result, reducing lubricant viscosity improve heat transfer in both natural and forced convection conditions. Quantitative analysis could reveal that kinetic viscosity and heat conductivity of fluids are highly influential on the cooling performance. In addition, we investigated the effect of lubricant additive on fatigue life in bearing components by using a thrust needle roller bearing tester. Extreme pressure agent could control a morphology of the bearing raceway surface, playing a role in extending a fatigue life of the bearing.
Designing fuel economy lubricants is an art; finding the right balance between fuel economy and durability requirements is complex, with many trade-offs. To open new formulation spaces with ever increasing fuel economy, a deep understanding of how lubricating oils respond to different drive cycles, engine/transmission type and any coating properties, e.g. DLC, is required. In this paper, we describe how the implementation of WLTC requires lubricant optimization to deliver improved fuel economy under this test cycle and therefore, lubricant viscosity reduction becomes more important. We also illustrate optimization of the sludge system is key to reducing overall viscosity of lubricants for ultra low viscosity application, such as in SAE 0W- 8 viscosity grade oils. To meet the cleanliness challenges in an SAE 0W-8 environment, we describe a developmental sludge handling system with improved cleanliness at constant viscosity to conventional SAE 0W-8 lubricants. A SAE 0W-8 demonstration oil with the developmental sludge handling system at equivalent sludge handling to a conventional system showed lower viscosity properties and demonstrated improved fuel economy performance in a motored rig test over baseline oil with a conventional technology.
Applying friction modifier (FM) in low viscosity engine oil is one well known cost effective approach for improving a fuel economy of vehicles. At first, the characteristics and mechanisms of FMs on tribological phenomena were studied with surface analysis technics. The performance of FMs was also evaluated with engine component test and motored engine test to understand the friction property of FMs in engine application. Then the effect of driving cycle, lubricant viscosity and FMs in fuel economy performance under chassis dynamo were studied. Among tested FMs, molybdenum dialkyl dithiocarbamate (MoDTC) was the most effective at boundary lubrication, which is considered significantly important friction area for WLTP, latest procedure for fuel economy test, with low and ultra-low viscosity engine oil.
An instantaneous piston ring/liner friction model has been presented to estimate the minimum oil film thickness and power loss contributed by piston rings under hydrodynamic lubrication. The model is based on lubrication theory considering lubricant viscosity variation with respect to temperature. A numerical scheme is developed to solve Reynolds and load equilibrium equations simultaneously to obtain the cyclic variation of oil film thickness and power loss. The model considers the ring profile geometry, the ring mechanical properties and their effects on the tribological performance of piston ring. The relevant trends and relations between parameters are considered with relatively simple approach to compute the minimum oil film thickness and mechanical power loss. Besides, Design of Experiment (DOE) technique and ANOVA analysis are employed to determine the effect of influential parameters such as ring width, ring crown height, ring elastic properties and ring end gap on the average power loss and average minimum oil film thickness. The statistical observation revealed that the main effect of ring width, ring crown height, ring tension force have prime importance and affect the response factors.
It has been revealed by researches that lubricant properties have a great effect on the low-speed pre-ignition (LSPI) frequency in downsizing turbocharged direct-injection engines which are developed for better fuel economy. Droplets of lubricant or lubricant-gasoline mixture are considered to be the potential pre-ignition sources. Those droplets fly into the combustion chamber and ignite the gasoline-air mixture. To study lubricant droplets fundamentally, a novel set of droplet auto-ignition system is designed based on a Dibble Burner for this experiment. Influences of metallic additive contents, viscosities, lubricant diluted with gasoline and waste lubricant on the ignition delay of droplets are investigated by testing 12 groups of lubricants or lubricant-gasoline mixture. The equivalent diameter of each droplet generated by micro-syringes is around 2.1 mm. The co-flow temperature varies from 1123 K to 1223 K, and the experiments are carried out at atmospheric pressure. The auto-ignition process of each lubricant droplet in a dark background is captured and recorded by a high-speed camera. The results show that ignition delays of all groups significantly decrease with the increase of co-flow temperature. Besides, increasing calcium or decreasing zinc dialkyldithiophosphates (ZDDP) in lubricants obviously promotes the trend on auto-ignition of droplets. But magnesium content has little influence on the ignition delay of lubricant droplets. In addition, lubricant with a higher viscosity has a longer ignition delay. Compared with undiluted lubricant droplets, droplets of lubricant-gasoline mixture tend to possess a longer ignition delay at high temperature. Although the ambient pressure in this experiment is much lower than that in an automotive engine, the observed results of effects of lubricant properties on auto-ignition are consistent with the regularities summarized by some engine bench tests. The quantitative results of lubricant droplet auto-ignition may be used as a criterion to evaluate the pre-ignition performance related to lubricants in engines.
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