The use of Direct Method Surface Layer Activated to Measure Wear in Diesel Engine 931627
As a technique for monitoring engine wear, direct measurement surface layer activation (SLA) offers some important advantages compared to either SLA by the tracer technique or traditional engine wear tests conducted in a test cell or in the field. With SLA, wear can be monitored real time as the engine is running, and under realistic operating conditions. By measuring the loss of activity due to wear in the component or components of interest directly, wear rates in terms of dimensional changes can be calculated. Using the direct measurement SLA method, the wear rate for a given duty cycle, oil condition and level of filtration can be established in 24 hours of engine operation. The dependence of top piston ring face wear rates on oil contamination level resulting from different filtration levels was measured in a heavy duty diesel engine. By transforming the wear rate into a component life prediction, the effect of filtration efficiency on component life is shown quantitatively. The wear calculated by SLA was compared to actual wear as measured by profile traces with excellent agreement. In a medium duty engine, three components are being monitored simultaneously, the top ring, the corresponding cylinder liner, and a cam follower, by irradiating each part to produce a different radioactive isotope. Issues involving the selection of suitable isotopes including the material to be irradiated, isotope half life, gamma ray energy, and interferences of peaks in the gamma ray spectrum are discussed. Wear data is shown for engine break-in at steady state full load and speed.
THE USEFUL life of a diesel engine is determined, as a practical matter, by oil consumption. As oil consumption starts to increase markedly, the engine is considered to be due for rebuilding. The increase in oil consumption usually occurs for two reasons. First, crown land deposits can prevent the piston rings from fully sealing allowing blowby to occur, and second, ring and/or liner wear is excessive. Newer engine designs are moving the top piston ring much closer to the combustion chamber. This prevents deposits from forming and reduces the oil contribution to emissions since the oil film thickness at this level approaches zero. While deposit formation is reduced, ring and liner wear becomes a more important component in defining engine life.
Other wear systems in the engine also determine useful engine life but are more difficult to perceive. Overhead wear can change engine timing and subsequently fuel economy and performance. However, wear in other components such as bearings may only be easily observed through oil analysis.
It is generally thought that, as the oil degrades, wear throughout the engine increases. In most cases, wear is defined by the concentration of iron in the oil as determined by spectroscopic analysis. While this is a simple test to carry out, it provides no indication as to the wear rates of the key individual components. Determining those individual wear rates is usually a tedious and expensive proposition.
Conventional engine wear testing is carried out either in a test cell or in the field. In the cell, a 300 - 500 hour engine test is required, followed by a tear down and inspection of the key components. This inspection can be simply visual, or include weight loss or dimensional measurements. The engine is often run in an over stressed condition, either in terms of engine speed and load or high levels of contamination added to the oil. The wear rates determined in this type of test are unrealistically high so that a test may be run in a practical length of time, and bear no relationship to actual engine use. Also, a separate engine build is required for each major change in variable, such as engine configuration, oil formulation and filtration level. It is very expensive to do systematic testing to investigate the relative importance of each of the major variables on engine life. The results also contain a high degree of uncertainty since it is impossible to rebuild an engine exactly as it was before. There is even less control over the operating conditions in a field test so the effect of major variables cannot be easily studied systematically. Field testing is usually reserved for the final confirmation that the specific combination of engine hardware, oil and filtration is robust enough for actual operation.
Oil degradation occurs by a combination of the following factors:
Consumption of the additive package resulting in loss of antiwear films and oxidation resistance
Contamination by engine fluids such as fuel and coolant
Contamination by decomposition products such as combustion gases, moisture, oxidation products and fuel soot which can form acidic products
Contamination by wear inducing particulate from external sources such as the intake air and internal sources such as wear debris and core sand from casting.
It is to be expected that each component and system has a different sensitivity to the relative contributions to wear from oil chemistry and oil cleanliness. While the oil additive package can determine the robustness, to some extent, of the oil chemistry, filtration is the primary method to control oil cleanliness. It is the objective of our ongoing research to determine the relative sensitivities for all the major wear systems to chemistry and cleanliness so that filtration specifications can be established for a given engine application and usage.