Engine Lubrication System Research Articles

Theoretical background and results of a study on changes in oil alkalinity of diesel engines

Abstract Alkalinity is a primary indicator for neutralizing insoluble impurities in oil. Alkaline additives decrease as oils interact with fuel combustion products containing significant amounts of sulfur and nitrogen. Literature data suggests that adding 2-3% of an effective ashless dispersant of a specified type can be equivalent to increasing the alkaline number of the oil by approximately 10 mg KOH/g through the addition of a metal-alkalinity additive to the oil. Based on operational study results, it can be concluded that after the first 100 hours of operation, the alkalinity value ranges from 6-4.3 mg KOH/g, and after 300 hours of operation, the alkalinity value reaches its maximum value of 2 mg KOH/g. Oils with an alkalinity of less than 2 mg KOH/g already indicate an unsatisfactory condition of the oil when lubricating the engine. To maintain the alkalinity value at a given level, it is recommended to continuously introduce tribomechanical stabilizers into the engine lubrication system.

Features of the operation of the lubrication system for lubricating ships' low-speed engines

The article examines the optimization of high-alkaline cylinder oil consumption in lubrication systems of marine low-speed engines (MODs), as well as the peculiarities of operation of the cylinder lubrication system in view of modern trends in the development of low-speed marine engines. An overview of the lubricator lubrication systems of the cylinders of marine main engines was carried out and possible prospects for improving the lubricator lubrication systems of the cylinders of marine diesel engines were considered. Attention is paid to the causes of occurrence and methods of combating a whole range of problems, which represent the possibility of increased wear of cylinder sleeves, pistons, piston rings, this is related to the type and grade of fuel and its sulfur content. Sulfur in the fuel is neutralized with the help of cylinder oils with high alkalinity, problems arise when changing the grade of fuel, this requires changing the brand of cylinder oil (CLO BN). The possibility of using a cylinder oil mixing system (ACOM) to neutralize sulfur in the fuel and improve protection against low-temperature corrosion of MOD cylinder liners is proposed and considered. This system mixes and doses cylinder oil depending on the load of the ship's engine and the type of fuel. ACOM allows the dosing of oil on marine engines operating in dual fuel consumption (SDF) modes, where the ratio between pilot (ignition) and gas fuel is defined, the system mixes two different types of cylinder oil into one with the required BN base number, it can be determined as the oil's ability to neutralize acids produced during use. The results of optimal consumption of cylinder oil, which ensures minimal wear of cylinder liners of a diesel engine, are given. Features of lubrication regimes and control of the technical condition of the cylinder group of marine low-speed diesel engines during their operation at reduced rotation frequency are described

Performance Analysis of a Compact Offset Strip Fin Heat Exchanger for Lubrication System in Aero Engine

Abstract As a typical fuel-oil heat exchanger, an offset strip fin heat exchanger (OSFHX) can ensure the efficient and stable operation of the aero engine lubrication system. The paper establishes 20 kinds of numerical models of OSFHX that are verified by experiments. The effects of different structural parameters on the performance of lubricating oil in OSFHX were systematically studied. Based on dimensional analysis, the prediction model which can describe the overall performance of the OSFHX is obtained. The results show that with the decrease of fin distance (s), fin length (l), fin height (h), fin thickness (t), and fin shape angle (α), the heat transfer performance of the OSFHX is enhanced and the resistance is increased. With the decrease of s and h, and the increase of t and α, the comprehensive performance of the OSFHX is enhanced. The OSFHX exhibits the optimal performance when l = 4 mm. α has the most significant effect on the comprehensive performance of the OSFHX, successively followed by h, s, t, and l. The average error of Colburn factor (j) and friction factor (f) prediction models is 4.09% and 5.31%, respectively, which can realize the theoretical calculation of the performance of the OSFHX for aviation.

Transient Flow Characteristics of a Pressure Differential Valve with Different Valve Spool Damping Orifice Structures

Lubrication system failure is a significant cause of in-flight shutdown incidents in aviation engines. The pressure differential valve, an essential component of a certain type of aviation engine lubrication system, is responsible for controlling the flow rate and pressure of the lubricating oil. Comprehending the transient flow characteristics of the pressure differential valve is of paramount importance for the secure operation of lubrication systems. This paper establishes a transient flow model of a pressure differential valve based on a transient computational fluid dynamics (CFD) method, and the experimental validation demonstrates the effectiveness of the computational model. The internal flow characteristics of the differential pressure valve at different stages during the opening process were studied. Additionally, four transient lubricating oil flow models with different valve spool damping orifice were established to analyse the impact of damping orifice structure on valve spool movement characteristics, pressure control characteristics, and flow field distribution. The results indicate that when the diameter of the valve spool damping orifice increases from 0.3 mm to 1.0 mm, the valve spool displacement and fluid force increase by 88 % and 20 %, respectively. Meanwhile, the peak valve spool velocity, peak oil supply pressure, and steady-state value decreased by 15 %, 29 %, and 34 %, respectively. As the length of the valve spool damping orifice increases from 0.894 mm to 4.0 mm, the growth rate of valve spool displacement and fluid force gradually decreases, with the peak valve spool velocity decreasing by 7 %. This study has potential significance for the structural optimization and application of pressure differential valve in lubrication systems.

Enhancement of the lubrication system of an internal combustion engine

BACKGROUND: The modernized design of the lubrication system with an individual electric-driven oil pump ensures the oil supply to remote and loaded parts, regardless of the crankshaft speed, starting mode and engine warm-up. AIM: Study of the main directions for enhancement of the lubrication systems of piston engines in order to assess the option of using an autonomous electric drive. METHODS: The design of an adjustable inductor electric drive with microprocessor control is proposed as a drive. The ability to keep rotation speed of the pump gears accurately is ensured using a controlled inductor electric motor, depending on the pressure in the lubrication system. RESULTS: The design and technological implementation of the proposed method of driving an oil pump does not require major changes to the engine design. The layout of the oil pump at the engine is simplified. CONCLUSIONS: The proposed design of the lubrication system of an internal combustion engine ensures optimal rotation speed of the pump shaft regardless of the operating modes of the piston engine, resulting in an increase in engine operating life.

Повышение энергетической эффективности газопоршневой ТЭС за счет комплексного использования тепловых вторичных энергоресурсов

Recently, the advantages and prospects to use piston gas internal combustion engines for the combined generation of electrical and thermal energy have become increasingly obvious. The range of single capacities of gas piston units (GPU) ranges from 0,1 MW to tens of MW, which makes them more attractive when designing various power facilities. Most brands of gas piston units can operate in co-generation mode, that is, as a combined heat and power plant that simultaneously generates electrical and thermal energy. The purpose of this study is to substantiate the possibility of integrated use of heat of the cooling systems of the gas piston engine of a thermal power plant (TPP). The studies conducted are carried out using well-known methods of thermodynamic calculation of the internal combustion engine cycle, of determination of the components of its thermal balance and thermal calculation of the equipment for utilization of secondary thermal energy resources. The results of the analysis of the heat losses of the GPU drive engine have showed that the total losses during its operation are about 11544,5 kW. Average potential heat losses with flue gases are 45,87 %, and low-potential heat losses with water in cooling systems and oil in lubrication systems are 53,14 %. A schematic diagram is proposed for the integrated utilization of thermal renewable energy resources (RES) at a gas piston thermal power plant. The main difference between the proposed scheme and the existing technical solutions is the utilization of the heat of the low-temperature charge air cooling system after the second section of the compressor and the heat of the engine lubrication system. An analysis of the expenditure part of the energy balance of the drive engine in case of the integrated use of thermal RES in the GPU cycle, has showed that the implementation of energy-saving measures will make it possible to usefully use up to 93,05 % of the supplied energy in the generation of electrical and thermal power.

The structure of the polymodel complex for modeling the operation of the lubrication system of an internal combustion engineThe paper presents the structure of the polymodel complex for modeling the operation of the lubrication system of an internal combu

The paper presents the structure of the polymodel complex for modeling the operation of the lubrication system of an internal combustion engine with a pneumohydroaccumulator, outlines the general requirements for the composition of this model complex. The ways of solution for realization of component models and tasks of application of the given polymodel complex at estimation of efficiency of lubrication systems of internal combustion engines with pneumohydroaccumulator, for application at realization of urban passenger transportation are defined. Keywords internal combustion engine, pneumohydroaccumulator, lubrication system, polymodel complex, modeling

ИЗМЕНЕНИЕ ТЕХНИЧЕСКОГО СОСТОЯНИЯ ДВИГАТЕЛЕЙ ЛЕГКОВЫХ АВТОМОБИЛЕЙ В УСЛОВИЯХ ЭКСПЛУАТАЦИИ

This study presents the results of collecting and analyzing statistics on the wear of pistons of budget-class passenger car engines during active operation. The devices and methods of collecting experimental data on the wear of the cylinder-piston group of engines are described. The main ones were: measuring piston wear in four projections with a micrometer MK-100 GOST 6507-90 with a measurement error of ± 0.005 mm and measuring the compression properties of engines with a compressor, the latter showed low accuracy of displaying the technical condition according to the results of the study. The purpose of the study was to verify the effectiveness of common methods of diagnosing internal combustion engines, as well as to identify the dependence of wear on the frequency of oil change, the design features of the engine lubrication system (the presence of oil nozzles cooling pistons). As a result of the study, the inefficiency of the diagnostic method by measuring compression with various devices was revealed, since when the cylinder-piston group wears out with simultaneous wear of the rubber seals of the valves, the oil acts as an additional seal in the engine cylinder and "masks" the actual compression index of the internal combustion engine. In addition, the results of the study confirmed the effectiveness of the presence in the design of the lubrication system of piston cooling oil injectors designed to supply oil when a certain pressure in the lubrication system is reached to a more loaded cylinder wall and piston bottom – based on the revealed fact, it becomes possible to make recommendations on the refinement of the engine design during major repairs at automobile enterprises, service stations by introducing oil injectors into the engine block, which will lead to an increase in the life of the cylinder-piston group and longer trouble-free operation of the engines.

OPERATING TESTS OF THE DEVICE FOR ELECTROSTATIC ENGINE OIL TREATMENT

The article presents the results of preliminary studies aimed at establishing the main factors affecting the service life of sliding bearings. The researchers pay special attention to the dominant types of lubrication, the thickness of the lubricating film, the load-bearing capacity of the film, shear resistance during friction, as well as the aging of the oil and the loss of its properties over time. Taking into account the previous achievements, as well as the works of the authors in the direction of increasing the wear resistance and resource of sliding bearings, it is proposed to test the sliding bearings of the D-240 diesel engine on the SMC-2 friction machine. The tests were performed according to the "pad - roller" scheme. In this diagram, the roller simulates the neck of the crankshaft, it is made of HF cast iron. The pad presses against the roller a fragment of the liner, which was made from a real engine liner and is a bimetallic element with a steel base and an antifriction coating of AO-20-1 alloy. In the tests, the sliding friction of the liner fragment on the roller was modeled, and the load modes of the friction pair were set in such a way that mixed lubrication of the bearing or transitional lubrication from limit to hydrodynamic lubrication is achieved. The purpose of the tests was to establish the regularities of the impact of engine oil aging and its electrical processing on the wear resistance and service life of the bearing liners. For this, oil samples were taken from the engine lubrication system. Oil samples with 0, 150, 300 engine hours were tested. The complete experiment consisted of two series of trials. The first series of tests concerned the establishment of the effect of working on the wear resistance when lubricating the bearing with traditional oil taken from the engine. The second similar series of tests included the additional introduction of electrical treatment of the oil samples before it was fed to the bearing model. The values of weight wear of liner fragments were obtained experimentally. The wear resistance and service life of the liners were calculated according to the corresponding dependencies, which are also given in the article. As a result of measurements and calculations, the regularities of the effect of working on the conditional life of the liners were obtained, taking into account the effect of the electrical treatment of the oil.

Ambient temperature impact on the parameters of the electrical circuit of sensing element in cluster eddy current sensor of metal chips in the lubrication systems of gas turbine engines

The detection of the wear particles in the engines lubrication systems is one of the most effective ways to assess the state of the bearing assemblies of machines and mechanisms. The most promising existing systems for online monitoring of wear particles are based on the eddy current methods for monitored parameters conversion. The reason is that such systems have the ability not only to determine the particle size, but also to recognize its magnetic properties, which makes it possible (under certain conditions) to localize the place of the defect development. At the same time, it is known that the ambient temperature changes in the monitored zone are one of the main external factors that have a significant impact on the informative parameters of eddy current converters. This is of particular importance for debris monitoring systems in high-power plants lubrication systems (e.g. gas turbine engine), where the oil temperature can vary widely (-50...+160 C). The temperature impact on the parameters of the electrical circuit of sensing element in the cluster single-coil eddy current sensor of metal chips, which is a part of the debris continuous monitoring system prototype for high-thrust aviation gas turbine engine, is studied in the article. A typical design of the single-coil sensing element is considered. The study results of the temperature effect on the sensing element inductance, as well as on the parameters of the current lead that connects the sensing element to the matching transformer and to the measuring circuit are presented. Methods for reducing the impact of temperature changes in the monitored area on the parameters of the electrical circuit of the sensing element are proposed, too.

Influence of oil service life on selected performance parameters of an aircraft piston engine

This article presents an analysis of the influence of oil service life on the performance parameters of an aircraft piston engine lubrication system used in an ultralight aircraft. The ageing of oil between oil changes causes a change in its parameters (such as density, viscosity ...). These parameters have a strong influence on the level of protection of the lubricated components. Currently, in aircraft, oil changes are carried out according to a time schedule - oil is changed every fixed period (residual life) regardless of its actual condition. The task of this article is to test the possibility of indirect assessment of oil condition based on analysis of changes in selected parameters of engine lubrication system operation during normal operation. The oil warm-up speed during the pre-start procedure and the dependence of oil pressure on engine speed were assumed for the analysis. The study was conducted on an ultralight rotorcraft during normal operation. Selected first daily flights directly after oil change, and after 17, 32, 50, 66 hours of operation were analysed. It was shown that the warm-up rate changes in the samples analyzed, but that this change may also be due to factors other than oil operating time. In the case of the oil pressure vs. speed characteristics, different characteristics were shown for different operating time, but no specific dependencies were found.

Increasing the efficiency of the lubrication system of a modern internal combustion diesel engine

BACKGROUND: Leading manufacturers of engines for trucks are constantly competing and trying to create engines with the best consumer characteristics, which at the same time meet modern environmental standards. One of the most important characteristics for long-haul tractors is fuel efficiency. As a way to improve this parameter, it was chosen to reduce the loss of mechanical energy to drive the oil pump.
 AIMS: To determine the current level of mechanical losses in the drive of the oil pump of the KAMAZ 6ChN13/15 engine, to form a set of solutions to reduce losses.
 METHODS: The current pattern of mechanical losses distribution for the 6ChN13/15 engine was obtained by engine motoring at the engine bench with sequential removal of engine components. A comparative diagram of the overpressure at the outlet of the uncontrolled and controlled pumps was obtained empirically during tests at the engine bench. The evaluation of the positive effect from the use of a controlled oil pump was determined analytically.
 RESULTS: As a result, by using a controlled oil pump, it became possible to reduce the excess pressure at the outlet of the pump at nominal rotational speed by 25%, which theoretically should reduce the relative mechanical losses of the engine by 3%.
 CONCLUSIONS: The practical value of the study lies in determining effective ways to reduce mechanical energy losses and improve the consumer qualities of modern diesel engines.

Кинетика старения моторного масла в судовых дизелях при непрерывном и периодическом его доливе для компенсации угара

The article considers the balance of the engine oil aging components in the diesel engine lubrication system with the refinement of oil composition caused by the oil product loss. The motor oil aging process has been identified under continuous and periodic oil refill in order to replenish the lubrication system with fresh oil which would compensate the burnt off oil products. There were derived and solved the differential equations of the oil product balance for such aspects of aging as contamination by insoluble impurities and multifunctional additives impact, which helped understand the general regularities and kinetics of the studied process. Novelty of the approach to theoretical description of the process consisted in the fact of filtering the insoluble and colloidal components of mechanical impurities and additives by the purification and burnout means. Cyclic picture of aging allows to analyze the state of the oil aging before and after refill with its periodic refreshment. The dependence is transformed and presented in a form where it is possible to determine the state of the oil at each aging cycle, without examining the intermediate concentrations of impurities and additives. The advantage of the obtained kinetic integral expressions for oil aging is the ability to determine the conditions for a long-term or permanent mode of its use in the lubrication system of a diesel engine. There are given the formulas by which the intensity of filtration and the concentration of additives in fresh oil or the time of their impact are calculated at a given oil loss and provide a pre-rejection mode of using circulating luboil in a diesel engine.

ANALYSIS OF THE EFFICIENCY OF RAPID-TESTS
 TO DETERMINE THE TIME TO REPLACE USED ENGINE OIL

The article presents the results of the analysis of the effectiveness of express tests for monitoring the current state of engine oil during the period of its operation in an internal combustion engine and determining the limit state of used engine oil. The results of diagnostics by physicochemical, rheological and optical methods of changing the state of fresh motor oil from the lubrication system of a diesel internal combustion engine at regular mileage intervals are presented. A software product project is proposed in the form of a mobile application for diagnosing the state of engine oil in the lubrication system of an internal combustion engine based on paper chromatography using a drop test, which will allow individual motorists to determine the need to replace used engine oil and, as a result, increase the period services of tribocouplings of a motor vehicle and will increase their overhaul period.

МЕТОДОЛОГИЯ ОПРЕДЕЛЕНИЯ ОПТИМАЛЬНОЙ ПЕРИОДИЧНОСТИ ПРОФИЛАКТИКИ СМАЗОЧНОЙ СИСТЕМЫ АВТОТРАКТОРНОГО ДИЗЕЛЯ

The reliability of automotive engines is an important problem for the country's economy. Wear, and subsequently failures, often cause a rather long downtime of equipment, significant consumption of spare parts, complicate the operation of the machine and tractor fleet. The cost of overhaul of an automotive engine is more than half the amount of the cost of a new one, and the resource after repair is no more than half the resource of a new engine. Over the entire service life of automotive engines, 5-6 times more money is spent on ensuring their operability than on manufacturing. The main operations for the prevention of the lubrication system of a diesel engine are the replacement and refilling of engine oil, on which the engine resource largely depends. Therefore, the volume of refilling and the frequency of oil change determine the efficiency of the engine and are optimization parameters. The function of these factors is the total unit costs of prevention and repair. Minimizing these costs is the essence of the technical and economic method. The paper presents the results of operational studies related to the dependence of refilling parameters, engine oil aging and the failure rate of internal combustion engines. The data on the performance of a diesel engine with and without an oil level regulator are given. According to the results of the work, an adequate mathematical model was obtained with physical variables of the dependence of costs on the frequency of oil change and the volume of one-time topping. The substantiation of the kinetics of the pre-failure state of the main elements of a forced diesel engine during the aging of engine oil was made and the optimal values of preventive parameters were determined.

МЕТОДИКА ВЫБОРА ВАРИАНТА ТЮНИНГА СМАЗОЧНОЙ СИСТЕМЫ ДВИГАТЕЛЯ

Improving the reliability of automotive engines is an important problem for the country's economy. Engine failures, due to wear, cause significant consumption of spare parts, complicate the operation of machines. The cost of overhaul of automotive engines is 70-80% of their cost, and the secondary resource is 30-40%. Over the entire service life of automotive engines, 5-6 times more money is spent on ensuring their operability than on manufacturing. Therefore, increasing the resource of automotive engines by improving lubrication processes is an urgent problem. One of the effective methods of ensuring the operability of machines is functional tuning. The paper defines it and shows the place of tuning in the system of ensuring the reliability of machines. The method of choosing the option of tuning the engine lubrication system is substantiated. The technique has been tested on specific variants: visual indicator, electronic indicator, hydraulic accumulator, oil pumping system, autonomous lubrication of TKR, oil level regulator, limit automatic. Based on the cost indicators of these options and the values of the relative increase in engine life that they provide, a methodology for choosing the best option for tuning the lubrication system has been developed.

MEDIUM SIZE LNG TANKER LUBRICATING SYSTEM HEAT EXCHANGER (COOLER) ANALYSIS

As part of the project, the necessary steps are taken to carry out a calculation of a main engine lubrication system fitted on board an LNG tank vessel with a capacity of 45,000 dwt. The validation of the system is achieved by using a calculation program based on finite element theory (CFD). This is a new ship with a series of innovations regarding the construction of the cargo tanks, which was built in 2018. For sure, this engine is a dual fuel type, powered by marine fuel and natural gas. A general description of the naval lubrication system is made, then a specific calculation is performed for the essentials, such as: the main oil pump, the oil storage tanks, the oil separator, the oil filters and the oil cooler. Next, the focus is on the oil cooler, which is considered an essential part of the installation. In the first part of chapter five it is described in detail, from a constructive and functional point of view, then, using the Ansys program, the corresponding calculation is performed to verify it. A model is made according to the geometrical characteristics, then a series of specific simulations are performed. The main purpose of the simulation is to determine de operational behaviour of this heat exchanger from a temperature point of view and to establish its optimal operational status. This cand be done by establishing a proper simulation model and by conducting a precise simulation stage focused and key parameters. As noted in the final part of this study the temperature contour in the heat exchanger for cooled oil flows form minimum 291 0K to maximum 363 0K, this being its optima operational limit.

Maintenance Analysis Based on Reliability of Main Engine Lubrication System with Markov Method

Maintenance of the main engine lubrication system determines the engine’s performance and components based on the standard of Japan Institute of Plant Maintenance. The purpose of the system analysis is to determine the critical components and evaluate every lubrication system component as a base on maintenance planning as a preventive measure to avoid downtime during ship operations. Data needed are the ship’s motion, damage frequency, components’ downtime, and lubrication system diagram. Data was analyzed qualitatively with Failure Mode and Effect Analysis and Fault Tree Analysis as well as quantitatively with Overall Equipment Effectiveness, Markovian Decision Process, and damage distribution. Results show that LO filter crisis components with 120 RPN and LO Pump (standby) with 105 RPN. FTA analysis results there are 3 lost types cause happening failure system that is pressure oil low , overheating of the oil , and there is pollution in oil. At its steady-state conditions, have a probability of 0.45 to experience moderate damage and 0.55 to be severe damage. Therefore, it is recommended to carry out maintenance before passing the MTTF value of each component so that the system can work optimally.

The Initial Boiling Point of Lubricating Oil as an Indicator for the Assessment of the Possible Contamination of Lubricating Oil with Diesel Oil

This article provides a brief introduction to the indicators of the volatility and flammability of lubricating oils and fuels. It is proposed that the initial boiling point be used as an indicator of the contamination of lubricating oil with distillate fuel (i.e., diesel biofuel oil) in the context of the rapid detection of explosion risks in the crankcase. Detailed tests were carried out on lubricating oil samples (SAE 30 and SAE 40 grades, which are most commonly used in the lubrication systems of marine trunk engines) diluted with diesel oil at selected mass concentrations (0, 1, 2, 5, 10, 20, 50, and 100%). The oils were tested to determine their relevant properties: the flash point temperature and the initial boiling temperature. The flash point was determined in a closed crucible using the Pensky–Martens method, which is in accordance with PN-EN ISO 2719. The differences between the initial boiling point and the flash point of the tested lubricating oils were determined for different dilution levels of lubricating oil in diesel fuel. An approximate method for the calculation of the flash point of the oil based on the initial boiling point is proposed. The results of oil flash point measurements are compared with values calculated as a function of the boiling point for both lubricating oils tested. An evaluation of how well models fit the experimental results is reported. Conclusions are presented on the applicability of the proposed method during operational practice.

A combined risk-based and condition monitoring approach: developing a dynamic model for the case of marine engine lubrication

Abstract This paper focuses on the creation of a dynamic probabilistic model which simulates deterioration trends of a marine engine lubrication system. The approach is based on risk and the implementation is achieved through a dynamic Bayesian network (dBN). Risk can be useful for decision making, while dBNs are a powerful tool for risk modelling and prediction models. The model takes into account deterioration of engine components, oil degradation and the off-line condition monitoring technique of oil analysis, in the context of predictive maintenance. The paper aims to efficiently predict probability evolution for main engine lubrication failure and to decide upon the most beneficial schemes from a variety of lubrication oil analysis interval schemes by introducing monetary costs and producing the risk model. Real data and respective analysis, along with expert elicitation, are utilized for achieving model quantification, while the model is materialized through a code in the Matlab environment. Results from the probabilistic model show a realistic simulation for the system and indicate the obvious, that with more frequent oil analyses and respective maintenance or repairs, the probability of failure drops significantly. However, the results from the risk model highlight that the costs can redefine scheme suggestions, as they can correspond to low probabilities of failure but also to higher costs. A two-month interval scheme is suggested, in contrast to the most preferred practice among shipping companies of a three-month interval. The developed model is in general identified as a failure prediction tool focusing on marine engine lubrication failure.