Aeration: the process by which air is circulated through, mixed with or dissolved in a liquid or substance.
The primary purpose of oil, in the context of the powertrain, is to lubricate the mechanical components. As such, the lubrication system is designed with jets, galleries and returns, which are filled by pumps through filters. In other words, oil must travel through a complex network of passages.
The agitation of the fluid through these passages is the leading factor in aerating oil. Air becomes bound to the oil as it splashes its way through the lubrication system, similar to how bubbles are formed when you splash water in a bathtub. This phenomenon worsens as the velocity of the fluid increases (i.e. increased engine speed and/or reduced viscosity with temperature or additives).
Once the oil is aerated, it will remain so until the air can escape. This is largely influenced by the residence time of the fluid in the sump or oil pan. The shorter the residence time, the more aerated the oil will be. Higher engine speeds reduce residence time and the advent of downsized engines with smaller sumps are exasperating the problem.
Types of aeration
Within a hydraulic system, air can be trapped or otherwise exist in pockets of free or unbound air. In this context, aeration is referring to the existence of air that has become bound to the oil (through agitation or other means).
Once air is bound to oil, it can propagate as bubbles suspended in the liquid. This is known as entrained air. The level of entrained air in an oil is the balance between the rate of air incorporation and the rate of air release.
Since entrained air is of a lower density than the oil, it will eventually rise to the surface in an attempt to escape. Once at the surface, the air will form thin liquid films (known as lamellae). Depending on the strength of the lamellae and the rate of air added, a network of separate air chambers may form, otherwise known as foam. The viscosity and surface tension effects influence the amount of foam that can form on the surface of the oil.
At any pressure, there is a possibility of air dissolved in oil. Even a beaker of oil sitting on a table at atmospheric pressure may have some air dissolved, complete invisible to the naked eye.
In hydraulic systems, oil may be pressurized at some or all areas of the circuit. Any entrained air (air bubbles) in the oil could become dissolved as the pressure increases. If the pressure decreases again, the dissolved air may return as air bubbles.
The concept is akin to a bottle of your favorite soda. Carbon dioxide is dissolved under pressure, invisible to the naked eye, until you open the cap. Once opened, the mixture rapidly depressurizes, causing the carbon dioxide to precipitate out of solution as the gaseous bubbles we know well.
For every gas and liquid mixture, there is an absorption coefficient of the gas in a solution at a given pressure. Known as the Bunsen coefficient, it is defined as the volume of gas in litres, reduced to 273.15°K and 1 atmosphere. Thus, at a given pressure there is a theoretical saturation limit of dissolved air within a given oil.
As a result, dissolved air is particularly problematic for hydraulic actuation systems (i.e. cam phasers, lifters/ tappets, dampers). Air precipitating from a pressure drop can cause unexpected adverse effects in hydraulic performance. Even when air is fully dissolved in solution, the compressibility characteristics of the oil change. This leads to a “spongy” behavior, as the oil essentially loses its hydraulic efficiency (outlined in the next section).