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).