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Part I
Overture
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Chapter 1
Wear in the Heavy Duty Engine
1.1 Introduction
Stringent standards in different countries for both nitrogen oxides (NOx) and particulate emissions from diesel engines are becoming unified with national boundaries becoming blurred (Figure 13.1). Emissions increase with engine wear. The easily observable consequences of wear in a diesel engine are the increases in smoke and consumption of fuel and oil. An assured wear life of 8000 hours is expected for a typical off-road engine in the field. Similarly, the vehicles on the road are expected to have an assured wear life of a million kilometres or more. Running many engines in the laboratory or multiple vehicles in the field may help to establish the reliability of the engines and their life. However, it is too late to recognize a problem during these trials. To satisfy reliability, after every failure in the field the engineer is asked to run more vehicles for a geometrically increasing number of testing hours. To prove the engine reliability could take up to three years in the normal course; this is longer than the life of newer emission regimes, and hence it is costly to repeat reliability experiments. In addition, the estimation of life, even by modern reliability methods, is straddled with an error of 50%.
Modern design procedures are detailed and well established against gross failures like fatigue, breaking and loosening. Therefore, only the micron sized failures at the surfaces that result in wear affect the life of an engine. Apart from the basic design aspects that are grounds for ‘normal’ wear or aging, extreme loads, environments like a dusty atmosphere plus quality of maintenance, fuels and oils bring an earlier end to the life of an engine. If the engine that has lasted its wear life is studied, only less than 0.01% of the total mass of the engine has been wasted away. In other words, more than 99.9% of the mass has survived when the parts are replaced at the end of their wear life. If the normal and abnormal wear is understood early based on the application or design of an engine, it is possible to incorporate features and protections in the design so that the engine is able to complete its expected life with sufficient margin.
1.2 Engine Life
In a diesel engine, the critical parts that wear out are the piston and ring assembly, liners, valve train, valves and bearings. The minimum lives of these parts determine the life of an engine. In other words, the wear life of an engine can be described as the bottom overhaul interval when all the worn parts are changed. The overhaul lives of various engines of different sizes (Chapter 2 references) are plotted in Figure 1.1. The points in the figure are normalized for a common load factor of 0.8 (Chapter 2). Apparently, the life of an engine seems to be a direct function of the power produced by the engine per cylinder, which in turn depends on the size of the engine. The graph shows dependency of life of an engine on its bore.
1.3 Wear in Engines
1.3.1 Natural Aging
At a given load and speed, the pressure and the relative velocity of the pairs of wear parts in an engine vary cyclically undergoing varying degrees of lubrication. All the wear parts in the engine are separated by a thin film of oil. During most of the cycle, the hydrodynamic pressure generated at the wedge shaped interfaces (Figure 1.2) of the pairs of wear parts is sufficient to lift the surfaces beyond their micron sized roughness. However, within a cycle, the thickness of the oil film could drop so low that the pair may come in close contact in some zones. In addition, during starting and stopping in the normal life of an engine, the parts run at a relative velocity so small that the asperities of the two wear surfaces come in contact and very high contact pressures are generated at the roughness peaks. Since the peaks are random in occurrence and shape, an element of probability is introduced in their loading and in the direction of loading on the peaks, which is not always normal to the nominal surface (Figure 1.3). The pressure bends the roughness peaks in the local direction of application of load. Even if the local stress is less than the plastic limit of the material, the asperities break away in pieces after many cycles of load, due to fatigue. If there is a special affinity between the metals of the surfaces, it is possible that metal from one surface is transferred to the other surface causing adhesive wear. Such an eventuality is not unusual in the main and connecting rod bearings, which are highly loaded every engine cycle. Therefore, wear takes place when the surfaces separated by a thin oil film come in contact during operating conditions.
1.4 General Wear Model
The wear of a wide range of material combinations has been studied in unlubricated conditions in detail for different loads and relative speeds of wear surfaces (Archard 1956). As a broad classification, two contrasting mechanisms of wear have been observed. In nearly all experiments, and for all types of wear mechanism, once equilibrium conditions are established at the surfaces, the wear rate (kg s−1) is independent of the apparent area of contact. The wear rate is proportional to the load with the same surface conditions. In practice, this simple relation is modified because the surface conditions depend on the load. These rules of wear may be derived, on basic grounds, from the experimental results, or from more detailed theoretical calculations on the different wear parts of an engine.
1.5 Wear of Engine Bearings
The bearing overlay of about 20 microns thickness is an alloy of tin, antimony, copper and lead. This alloy has some important properties, namely, the ability to absorb lubricating oil on the surface and hence provide lubrication when the bearing is starved, the ability to absorb small particles of dirt (embeddability) without increasing the wear of the bearing or crankshaft and seizure resistance, so that it does not weld itself to the crankshaft material even under extreme loads or high speeds. Lastly, it must be able to be run slightly out of alignment without wearing out. Also, the material enables some kind of healing in the event of mild seizure and the material is transferred back to the parent bearing. The parts are heated by friction and also by heat transfer from the hot environment (e.g. hot combustion gases). The copious amount of oil flow between the surfaces enables washing away of both the debris and the heat. These wasted metal particles are found in the oil, sump and the oil filter.
1.6 Wear of Piston Rings and Liners
Usually, today there are three rings in engines that have a swept volume less than three litres per cylinder. Larger engines have four or five rings and they are always above the piston pin. Rings below the piston pin are out-dated, as the ...
