Machinery Lubrication magazine published by Noria Corporation
Issue link: https://www.e-digitaleditions.com/i/490438
14 March - April 2015 | www.machinerylubrication.com and will be proportional to the liquid elec- trical conductivity. So as the temperature rises during a cold start, the filter charging will also increase with time during the warm-up period. Accordingly, as the temperature rises with time downstream of a charge source, there is a significant increase in the induced electrification of the liquid and a decrease in the effective length of the electrified oil. The combination of these two counter-effects will be a transient charging effect in the form of a voltage spike and an electrostatic charge surge downstream of the charge source where the oil flows. How low must the starting temperature be for this hazard to pose a practical problem? In general, the severity of this transient effect is influenced by a wide range of variables, such as the size and arrangements of the compartments in the circulation system, the base electrical conductivity of the circulating oil, the types of filters and pumps used in the system, the flow-volume rate, and the system's temperature profiles during the warm-up phase and at startup. Therefore, a complete system analysis is needed to answer the question. In one particular system that was recently analyzed, the starting temperature in the experimental setup was minus 41 degrees C, with the maximum voltage of 500 volts esti- mated at about minus 10 degrees C. For this system, any starting temperature below minus 10 degrees C could induce a severe spike. However, during experiments at higher temperatures, a similar but milder response was observed. Preheating the engine block is unlikely to mitigate the hazard of a voltage spike. While preheating might help the engine start, it may potentially amplify the voltage spike. Engine oil is often stored in an oil pan that is not in contact with the main engine block. So if the engine components are warm and the circulating oil is very cold, oil electrification will be enhanced. A system that can warm the engine oil and not the engine block would seem to offer a solution, and several such systems currently exist for specific engines. However, this solution is not practical for all power systems because oil in the pan may not be easily accessible. Another solution is to use a bypass system for certain components such as filters that can be triggered by a differential pressure across the component. While this is a promising technology and filter manu- facturers have begun to utilize this bypass system, there are still a few drawbacks. One is that the system is now more complex and more susceptible to failure. The other is that if new oil is used, the settings for the bypass condition should also be changed accordingly. Moreover, this technology can't be used for other components such as an oil pump, which can also induce charging in the oil. One might envision a change in the engine's arrangement with the oil storage unit placed within the engine block. This is analogous to systems in some hybrid-en- gine cars that store hot coolant inside the engine for better start-stop performance. Still, the best option would be electrical grounding of the engine compartments during early stages of a cold startup to prevent charge accumulation. While few if any studies have been conducted on these types of cold startup issues for automobiles, as advanced engines continue to include more electronics, this hazard could potentially pose a problem for them as well. This is both a practical and fundamental problem, and new research is needed to shed light on this phenomenon with respect to the tempera- ture effects and other transitory behavior of the system. 600 500 400 300 200 100 0 -100 0 100 200 300 400 500 600 700 Time ( in seconds) Voltage Output This chart shows the voltage output from a charge density probe over time. The solid line represents experimental measurements, while the dashed line is the theoretical prediction. (Ref. J. Electrostatics) COVER STORy