This article explains how electrostatic discharge from oil can destroy sensitive and crucial engine components. All thermomechanical power systems contain a dielectric fluid – namely the circulating lubricant oil – where its circulation can create friction and cause a static electric charge to build up. The charge can induce voltage spikes in portions of the circulation manifold during the initial warm-up period. The spike can destroy a sensitive component such as a sensor or microprocessor, and if that component is critical to operation, the engine will shut down. 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. That’s why some commercial gasoline fuel hoses in the United States have an attached ground wire to dissipate electric charge accumulation during fueling operations and there exist regulations to shut off the engine when pumping fuel into a vehicle.
We have all sat in an airport waiting for a plane that fails to arrive or take off on time. sometimes the flight is delayed; sometimes it is canceled. there can be many reasons for that, including one that has only recently become clearly understood.
In the past two decades, the U.S. Federal Aviation Administration has compiled numerous reports on jet engine shutdowns during cold start-ups. The cause of these accidents has been attributed to burnt-out electronic components inside the engine due to electrical discharge. The connection between the start-up at low temperatures and the electronic-component failure has only recently come to light.
All thermomechanical power systems contain a dielectric fluid—namely the circulating lubricant oil—where its circulation can create friction and cause a static electric charge to build up. The charge can induce voltage spikes in portions of the circulation manifold during the initial warm-up period. The spike can destroy a sensitive component such as a sensor or microprocessor, and if that component is critical to operation, the engine will shut down.
When a power system is cold (lower than -10 ̊C), 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 a hazard of static electric build-up 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. That's why some commercial gasoline fuel hoses in the United States have an attached ground wire to dissipate electric charge accumulation during fueling operations and there exist regulations to shut off the engine when pumping fuel into a vehicle.
Static electrification of a dielectric liquid is due to the presence of some trace elements in the oil products. Examples of substances that can carry electric charge in a non-conducting liquid are various oxidized oil components (as a result of processing), contaminating agents (acquired during processing and handling), 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 such low concentration, it is impractical to remove these trace elements and even if one does so successfully, subsequent handling can reintroduce the elements through recontamination.
Even with the presence of trace elements, stagnant undisturbed oil is uncharged in the bulk and charged only very close to the solid surfaces with which it is in contact. The liquid motion and the convection of the trapped charges in the liquid give rise to a convective electric current often referred to as streaming current.
The ability of a liquid to retain its electrical charge depends 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, the velocity and the electrical conductivity of the circulating oil both increase with time. At the start, the velocity and conductivity of the oil are low and therefore the electrification is limited to regions close to charge source without electric charge build-up or any potential damage.
On the other hand, with normal operations, any static electrification in the moving oil can travel very short distances (less than 1 mm). The oil will get neutralized, and the electrical charges 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 such that moving oil will give rise to charge accumulation with the potential to do damage.
Yet another temperature effect has to do with the induced charge concentration behind a charge source such as a filter. In the most general case, filter electric charging depends on a number of parameters that have to do with filter geometry and flow conditions. These and other parameters were discussed in a 1977 paper by Peter Huber and Ain Sonin in the Journal of Colloid and Interface Science.
Preheating the engine block is unlikely to mitigate the hazard. A system to warm the oil and not the block would be a solution.
For industrial filters used in power systems, the charging behind the filter is often saturated and 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 an exponential increase in the induced electrification of the liquid and a decrease in the convective 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 in the 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 temperature profiles of the system during the warm-up phase as well as the starting temperature. Therefore, a complete system analysis is needed to answer the question.
In the system that my colleagues and I have analyzed, the starting temperature in the experimental setup was -41 ̊C and the estimated maximum voltage of 500 V was estimated at about -10 ̊C. For this particular system, any starting temperature below -10 ̊C can induce a severe spike. However, during experiments at higher temperatures, we observed a similar but milder response. For example, after starting the system at 0 ̊C, we measured a maximum voltage of 150 V.
Preheating the engine block is unlikely to mitigate the hazard of a voltage spike. While preheating might help the engine to start, it may, in fact, 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.
Experimental data on the use of single-block heating for different engines for an extended period of time, detailed initially by E.W. Wiens in a 1972 paper in Canadian Agricultural Engineering, show that the engine oil may rise only between 3 and 6 ̊C while the temperatures of the engine blocks and the coolant change 30 to 60 ̊C.
A system that can warm up the engine oil and not the engine block would be a solution, and there exist several proposed systems for particular engines, but this solution can’t be practical for all power systems because oil in the pan may not be easily accessible.
Another solution is to use a by-pass system for some of the components such as filters that can be triggered by a differential pressure across the component.
While this can be a promising technology and filter manufacturers have begun to utilize this by-pass 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 by-pass 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 to the arrangement of an engine to put the oil storage unit within the engine block. This is somewhat analogous to systems that exist in some hybrid-engine automobiles that store the hot coolant inside the engine for better start-stop performance.
The most robust option will be well-electrical grounding of the engine compartments during the early stages of a cold startup to prevent charge accumulation.
To assess this potential hazard for a given power system, the charging sources in the oil circulation system such as the filter and the pump need to be characterized in terms of the magnitude and the polarity of the charge they induce in the liquid. Once such data is available, one can utilize recently developed analysis on unsteady charging in circulation systems to predict the severity of charging during a cold start. The analysis was included in a paper written by David S. Behling and this author, “Transient electric charging of dielectric liquids in recirculation systems,” published in October 2013 by the Journal of Electrostatics.
Flow-induced electrification of dielectric liquids has been studied extensively since World War II. By and large, all these studies, theoretical and experimental, have been restricted to steady charging under ambient temperatures. The electrostatics of a startup circulating oil is both unsteady and critically dependent on temperature.
While, we are not aware of any open report on cold start-up problem of this sort for automobiles, as advanced engines continue to include more electronics, this hazard could potentially pose a problem for them, too.
This is a practical problem and new studies are needed to shed light on this phenomenon with respect to temperature effects and other transitory behavior of a system.