Machinery Lubrication magazine published by Noria Corporation
Issue link: https://www.e-digitaleditions.com/i/490438
12 March - April 2015 | www.machinerylubrication.com COVER STORy During the startup phase, the system normally has a warm-up period due to viscous heating and heat transfer from other engine sources. The oil temperature rises, decreasing its viscosity and increasing its electrical conductivity until a steady-state operating condition is reached. The variation of electrical conductivity with temperature is the principal cause of the electrostatic discharge during a cold start. The ability of a liquid to retain its electrical charge will depend on its electrical conductivity. In dielectric liquids, the time that an isolated liquid mass can remain electrified is known as its electrical relaxation time. It is inversely proportional to its electrical conduc- tivity. For different commercial oils, this time constant is in the range of 1 microsecond to 1,000 seconds for higher to lower conductivities. For any lubricating oil at very low temperatures during a cold start, the relaxation time of the liquid is closer to the upper limit, whereas under steady-state operation, it has values closer to the lower limit. Accordingly, during a cold start, the elec- trified oil will remain charged, and if moved, can give rise to charge accumulation in the circulating system. Once electrified, the distance that the oil can carry the charges depends on its electrical relaxation time as well as the bulk velocity of the flowing oil. In the warm-up phase of a power system, both the velocity and electrical conductivity of the circulating oil increase with time. At the start, the velocity and conductivity of the oil are low, and thus the electrification is limited to regions close to the charge source without electric charge buildup or any potential damage. On the other hand, with normal operations, any static electrifi- cation in the moving oil will travel very short distances. The oil will become neutralized, and the electrical charges will dissipate to the adjacent walls. However, as the engine warms up from a cold start, there can be a time interval in which the oil velocity is high enough and the conductivity is still low enough so that moving oil will give rise to charge accumulation with the potential to do damage. Yet another temperature effect involves the induced charge concentration behind a charge source such as a filter. In most cases, filter electric charging depends on a number of parameters related to filter geometry and flow conditions. For industrial filters used in power systems, the charging behind the filter is saturated This graphic illustrates the unsteady electrification of circulating oil during a cold startup. Oil temperature is depicted by the yellow/ orange bar, which darkens as temperature increases. The electrical charge concentration is shown in blue for the lowest concentration to purple for the densest. Oil Flow Distance from Charge Source Time 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.