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Cold engine startups at very low temperatures have been a problem for consumers, manufacturers of power systems and the petroleum industry. With a cold start, the flow of circulating oil (a dielectric liquid) in the system can induce voltage spikes in portions of the circulation manifold during the initial warm-up period. When exposed to this spike, sensitive components such as sensors and microprocessors may break down and ultimately shut down the engine if the component is critical to operation.
When a power system is cold, its circulating oil has a very high viscosity and very low electrical conductivity. The oil will warm as the engine heats up, but for a period after a cold start, there will be the danger of static electric buildup in the oil and of potentially damaging spontaneous discharge.
Flow electrification of liquids has been a source of numerous industrial hazards, primarily in the petroleum and power industries. This effect occurs in improperly grounded systems carrying fuels, lubricating oils and other hydrocarbon liquids. This is why some commercial gasoline fuel hoses in the United States have an attached ground wire to dissipate electric charge accumulation during fueling operations and why regulations exist to shut off the engine when pumping fuel into a vehicle.
Static electrification of a dielectric liquid is due to the presence of trace elements in the oil. Examples of substances that can carry electric charge in a non-conducting liquid include various oxidized oil components, contaminating agents, metal salts and other ionized additives. The concentration of any of these substances at which liquid electrification occurs can be as low as 1 part per billion. Because of this low concentration, it is impractical to remove these trace elements. If you could remove them successfully, subsequent handling could reintroduce the elements through recontamination.
Engine oils in power systems are electrically insulating liquids with electrical conductivities in the range of less than 1,000 picosiemens (pS) in normal ambient conditions. The value will depend on how pure the oil is and whether it has been altered with additive surfactants. For most liquids, the product of their viscosity and electrical conductivity is constant. As the temperature goes down, the oil’s viscosity increases exponentially, and its electrical conductivity decreases exponentially.
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.
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.
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 conductivity. 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 electrified 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 electrification 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.
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)
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 and will be proportional to the liquid electrical 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 estimated 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 manufacturers 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-engine 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 temperature effects and other transitory behavior of the system.