Condition monitoring through oil analysis tests

Abstract

The paper presents the tests in the oil analysis, used for the assessment of oil condition, as well as the requirements to be fulfilled by the tests, regarding the state of technical equipment, impurities and lubricants. Special attention should be paid to the occurrence of metal particles in oil and the tendency in the change of their number. The signs that point to changes in viscosity, oxidation increase and additive wear are also considered. The state of impurities in oil was discussed, with a particular focus on the number of particles, water content and metal impurities. It is inevitable to use an oil analysis programme in the case of motor oils, which provides several advantages: reduction of unscheduled vehicle downtime, improvement of vehicle reliability, help in organizing effectiveness of maintenance schedules, extension of engine life, optimization of oil change intervals and reduction of cost of vehicle maintenance. The paper also gives the results of the experimental research of physical- chemical characteristics of the motor oils sampled from the engines of Mercedes OM 345 vehicles in operational use. It was shown that there is a change in physical-chemical properties of lubrication oils. These changes are in a direct correlation with the state of all the elements of the tribomechanical motor system, i.e. with their functional characteristics. The conclusion based on the realized testing comes at the end of the paper. Introduction As a contact element of the tribomechanical system, lubricant is a carrier of information about the state of the whole system, from the aspect of tribological as well as other ageing processes. Therefore, an analysis of oils, based on a properly defined program, represents a very effective method for monitoring the state of technical systems, which ensures early warning signals of potential problems that could lead to failure and break down of technical systems. Using Oil Analysis programs for engine oils has several benefits: reduction of unscheduled vehicle downtime, improvement of vehicle reliability, help in organizing effectiveness of maintenance schedules, extension of engine life, optimization of oil change intervals and reduction of vehicle maintenance costs. Physico-chemical characteristics of lubricating oils Basic physico-chemical characteristics which determine the quality of oil are: a) physical characteristics: viscosity, density, flash point, cloud Point, pour point, volatillity, emulsibillity, deemulsibillity, foaming, air release, viscosity index, etc. b) chemical characteristics: neutralization number (TAN-total acid number), total base number (TBN), oxidation stabillity, chemical and thermal stabillity, corrodibillity, ash content and carbon residue, water content, compatibility, toxicity, etc. Diagnostics of the tribomechanical system of an internal combustion engine The diagnostics is based on the prediction (recognition) of damage and/or failure through characteristic diagnostic parameters. This allows prevention of delays and increases reliability, cost-effectiveness, and usage life. The diagnostics of the tribomechanical system can provide verification of the system condition, working capacity and functionality, and can point out the place, form and cause of a failure. The diagnostics is carried out through the detection of symptoms, determining the value of the characteristic parameters and their comparison with the limit values. If the engine assemblies are considered from the aspect of tribomechanical systems (e. g. piston-piston ring-cylinder, cam-valve lifter, bearing journal bearing) defined by tribological processes, it can be shown that the determination of the content of wear products, content of contaminants, state of lubricants and lubrication conditions have a significant influence on the implementation of maintenance of these systems. We should emphasize the importance of monitoring oil for lubrication of tribomechanical engine assemblies, which provides identification of potential causes and phenomena leading to damage and failure in the early stages of the functioning of the system. Prediction, i.e. detection of potential and/or current damage and failures in the system, checking the functionality of oil and determination of usage life are the main factors of the implementation of oil monitoring. Since mobile components of tribomechanical system engines are necessarily exposed to wear and contaminants and wear products deposit in the lubrication oil, it is necessary to monitor changes in fluid properties during exploitation, because the monitoring of lubricants is the key monitoring technique for maintenance as well as for achieving certain techno-economic effects. The analysis of the contents of different metals in lubricants is very important. Metal particles are abrasive and act as catalysts in the oxidation of oils. In motor oils, the origin of the particles may be from additives, wear, fuel, air and cooling liquid. Metals from additives can be Zn, Ca, Ba, or Mg and that indicates the change in additives. Metals originating from wear are: Fe, Pb, Cu, Cr, Al, Mn, Ag, Sn, and they point to increased wear in these systems. Elements originating from cooling liquids are Na and B, and their increased content indicates the penetration of the cooling liquid into the lubricant. The increased content of Si or Ca, originating from the air, points to the air filter malfunctioning. Wear of the parts is the main cause of malfunctioning in the process of exploitation of mechanical components of vehicles. Wear is characterized by the change in shapes and dimensions of working parts. Friction leads to surface wear which causes the increase of clearance between moving joined parts and the change in their mutual relations, thus resulting further in deviations from required specifications of assemblies and vehicles in general. Diagnostic procedure for the condition of lubricants 1) Selection of a type of vehicles and vehicle parts from which oil samples will be taken. 2) Consideration of the mode of the system (vehicle assembly) operation, lubrication systems, exploitation conditions and its purpose, and an overview of the basic technical characteristics. 3) Analysis of the tribological processes in components of motor vehicles. 4) Analysis of the failure of functional components of vehicles (as tribomechanical systems) 5) Information about the condition of a vehicle assembly and oil during the analysis and monitoring Results of motor oil investigation during exploitation In this part of the paper, the results of the experimental testing of motor oils in the Laboratory for fuels and lubricants, VTI Belgrade, are presented. The physico-chemical characteristics of oils were examined in accordance with standard methods, shown in Table 3. The analysis was done on fresh (new) oils and oils used in vehicle engine assembles. Testing of used samples was carried out in accordance with common criteria defined by the quality of used oil. The values of allowable deviation limits of individual characteristics of oil depend on the type of oil, working conditions and internal recommendations of the manufacturer of lubricants and users. The limited values of oil characteristics which condition the change of the oil charge from the engine are given in Table 4. They represent the criteria for the oil charge change. Deviation of only one characteristic results in the change of the oil charge, regardless of the characteristic itself. The research was carried out on three vehicles (MERCEDES O 345 buses) with the Mercedes-Benz engine, OM 447HLA type. This is a four-stroke engine with six in-line cylinders, turbo Diesel, liquid cooled and with combined lubrication, which meets Euro 2 emission standards related to exhaust gases. The research was conducted through periodic sampling of oil from the engines of the vehicles listed above. Apart from fresh oil ('zero' sample), samples were taken after 10.000 km, 20.000 km and 30.000 km. After 30.000 km oil charges were replaced in all three engines. During the oil sampling, the sampling sites were carefully chosen, taking care of actual oil usage, which enabled each sample to be a representative one. Each sample was taken from the living zone, i.e. zone closer to the elements in contact. The sampling of oil from the bottom of the motor housing (discharge outlet) was thus avoided, as the highest concentration of contaminants occurs there. This is achieved by simply modifying the outlet for oil, by extending it towards the active zone of oil within the housing with a tube of appropriate length. Special attention was also paid to the preservation of samples from contamination, both in the phase of sample taking and in the phase of manipulation, which is fully met by applying the prescribed procedures. A very high level of purity of all elements in the chain of the sampling systems was thus provided as well as the separation of samples in a way that does not perturb the integrity of their data on the state of the vehicle components from which the sampling was done. Conclusion On the basis of all the mentioned above, the following conclusions can be drawn: - motor oil VALVOLINE, API CF and ACEA E4, of SAE 10W-40 gradation, analyzed during exploitation, achieves its primary function and meets the prescribed replacement interval of 30.000 km in the EURO 2 engine category, which was found by the analysis of characteristic physico-chemical properties of oil and wear products (Fe and Cu) during exploitation; - the fall of viscosity is evident during the first 10.000 km, and after this period, viscosity remains approximately constant until the end of the interval changes of the oil charge. Maximum viscosity fall during the exploitation of the oil from all three engines is significantly below the allowed limit of 20 %; - after 30.000 km the TBN value has not exceeded the allowable limit for oil samples from all three engines; - the content of insoluble substances in the oil is negligible in comparison with the limit value, because there is no significant presence of oxidation products and mechanical impurities or insoluble substances such as coke, scale, dust, soot, particles originated from wear contact area of the engine tribomechanical systems and other mechanical impurities; - small decrease of the flash point values shows that there was no significant penetration of fuel into the system for lubrication; - content of iron and copper is significantly below the allowable limits for all three vehicles; - the appearance of water in the samples is not found, - after 30.000 km, oil is replaced, following exactly the manufacturer's recommendation about the oil charge change.