Car manufacturers are facing increasingly strict regulations for harmful emissions from combustion engines. The development of internal combustion engines therefore focuses primarily on reducing fuel consumption and on their complete (ideal) combustion at every moment of engine operation. One of the most widely accepted technologies today that can reduce fuel consumption by up to 10%, especially when driving in urban environments, is start-stop technology. The start-stop system reduces fuel consumption by automatically switching off the combustion engine when waiting at traffic lights or in traffic jams, thus reducing engine idling times. On the other hand, the implementation of this technology places an additional load on other elements of the car's starting system, such as the battery, the starter and the spark or glow plugs, etc., as it significantly increases the number of engine starts.
In this study I concentrated on the development of glow plugs for diesel engines. The glow plugs are used to support the cold start of diesel engines. Their task is to preheat the combustion chamber of the diesel engine and thus facilitate the start. In addition, glow plugs continue to heat the combustion chamber for some time after the cold engine is started, thus contributing to more complete combustion of the diesel fuel and thus to lower emissions of particulate matter (soot) and harmful exhaust gases. Since the diesel engine must be ready to start in a relatively short time and generate as few harmful emissions as possible in cold operation, the glow plug must both heat up to a high temperature and work at a high temperature for a longer period of time. The manufacturers of diesel engines therefore demand ever higher operating temperatures for glow plugs as well as an extension of their service life.
The most sensitive element in the glow plug is a heating coil made of FeCrAl alloys. The FeCrAl alloys are known for their excellent oxidation resistance at elevated temperatures and were primarily used as heating elements. Their superior oxidation resistance is related to the formation of a thermodynamically stable and oxygen-impermeable protective layer of aluminium oxide (Al2O3) on the surface, which prevents further oxidation of the material. Despite the excellent anti-oxidation properties of these alloys, their oxidation resistance is limited when they are exposed to thermal cycles up to 1050 °C, such as in the case of glow plug operation when starting a cold diesel engine. The reason for this is the cracking of the alumina scale due to the difference in temperature expansion compared to the alloy matrix and the consumption of aluminium in the subsurface area of the alloy matrix due to the constant re-formation of the alumina scale. As soon as the aluminium content in the subsurface area is reduced beneath critical concentration (< 3 wt.%), the alloy can no longer reform the continuous alumina scale, which leads to catastrophic breakaway oxidation through the formation of rapidly growing iron and chromium-based oxides and aluminium and chromium nitrides (AlN, Cr2N).
To extend the service life of glow plugs, we have enriched the glow resistors made of a FeCrAl alloy with aluminium in the surface/subsurface layer, thus increasing the number of possible re-formation of the protective aluminium oxide layer. Various methods of aluminium deposition were used: PVD, powder pack aluminization with subsequent heat treatment and ALD process.
The aim of the research is to investigate physical and chemical processes that take place during the operation of a glow plug. Extensive microstructural investigations of cross-sections and longitudinal cross-sections of glow plugs were carried out after sustainability tests on laboratory equipment at the company Hidria d. o. o. and after operation in real engines at potential customers. The influence of the initial microstructures of various materials on the mechanism and kinetics of their oxidation under various conditions is explained and evaluated by research of basic physical and chemical phenomena during high-temperature oxidation. In the preliminary phase of the research, thermogravimetric analyzes were used to compare the oxidation properties of commercial materials based on FeCrAl alloys. The alloys studied differed in the production process, in the quantity and type of reactive elements, in the content of aluminium and chromium in the alloy, and in the microstructural and mechanical properties. In the next research step, an aluminum layer was applied to the heating resistors made of FeCrAl alloy by the PVD process and the influence of temperature and time on the diffusion processes in the alloy was determined by measuring the electrical resistance. The obtained results were then used to select the technological parameters of powder pack aluminization, which we developed to increase the amount of aluminum in the surface/subsurface layer, thus ensuring faster oxidation of the alloy in the initial heating phase and increasing the number of possible re-formations of protective alumina scale.
Due to the changed composition of the heating resistors, it was also necessary to adapt the method of joining them with other materials/parts of the heating element. The change of technological parameters of conventional arc welding, which has been used in production so far, does not allow successful joining, as the increased arc energy has a negative effect on other elements of the glow plug. We have therefore developed a laser welding technology to successfully join these elements.
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