Oil aeration
Oil aeration can be a major problem in high-revving race engines. The presence of oil bubbles in significant quantities in an engine’s oil can lead to pressure loss and damaging cavitation in the oil pump. Controlling the issue is a multi-faceted task, requiring that all aspects of an engine’s oil system are optimised to combat it. The sump, oil pump, oil tank and even the oil itself can be engineered to minimise aeration, and thus improve both and engine’s performance and lubrication characteristics.
Before studying how aeration occurs, it is pertinent to take a quick look at the three forms of air in oil – dissolved air, air bubbles and foam. When present in oil in these forms the air is referred to as ‘bound’ air because it is bound, or connected, to the oil. Dissolved air is invisible and is harmless to oil function as a lubricant, but it can be released as bubbles and/or foam. Bubbles are small air pockets entrained and dispersed throughout the oil, while foam consists of pockets of air on the surface of the oil separated by thin liquid films. Air that is separated from the oil is referred to as ‘unbound’, or free air, and can become bound air through various mechanisms.
The level of aeration in the oil is determined by a range of factors, including engine speed, oil type and oil condition among others. It can be a particular problem in high-revving race engines – not only does the high rpm of components such as the crank churn the oil to a greater extent, the oil also flows through the lubrication system faster. This means it has less time to rest in the sump or the oil tank, where air bubbles would naturally separate out.
Another component that can significantly increase aeration in the oil is the oil pump itself, particularly in dry-sump lubrication systems. The aeration occurs on the scavenge stage of the pump, and the oil being sucked into the pump is more of an air-oil foam; as the oil is pulled between the pump’s gears it is further aerated. That is why oil tank design is of utmost importance for maintaining oil condition. As described in a number of previous RET-Monitor articles under the Oil System keyword, tanks are constructed so as to maximise the amount of air oil separation as the oil is fed down through the tank.
The problem of removing air from scavenged oil has also led to a number of devices designed to condition the oil before it reaches the tank. The most recent, and interesting, of these is an inline air-oil separator that uses centrifugal force to separate oil and air. Through the use of carefully designed channels, the air is removed through one outlet, with the de-aerated oil flowing through another.
While some oil is carried out with the air (which is fed back into the oil tank through a secondary inlet), most is returned through the main tank inlet in a far less aerated state than if it simply flowed from the pump. In terms of performance, the manufacturer claims a 30-70% reduction in aeration over running a direct scavenge feed to the tank, with the level of de-aeration dependent on rpm and other engine operating conditions.
On a final note, it is worth considering that oil pressure also has a direct relationship to its susceptibility to aeration. The greater the oil pressure, the more air it is able to absorb and, if the pressure drops, a portion of this dissolved air will form into bubbles. Therefore, if a high-pressure oil system suffers a pressure drop, more air bubbles will be introduced into the lubricant, potentially contributing to problems such as pump cavitation. Conversely, if a system is running at a lower pressure then there will be less air to release and, thanks to Boyle’s law, the size of these bubbles will also be larger, meaning they will separate from the oil more quickly.
Obviously the susceptibility of any particular lubrication system to aeration depends on a multitude of dynamic factors, but hopefully this provides an overview of the core issues.
Written by Lawrence Butcher