My understanding is that in normal engine operation the predominant and desirable mode of lubrication is hydrodynamic. That is, a measurably thick film of oil is maintained between components and no contact occurs. In some proportion of normal operation, however, the hydrodynamic capabilities of any oil are exceeded, such as between camshafts and followers/rocker arms and between rings and cylinder walls near the top of the piston stroke, and under certain operating conditions such as start-up.
When the hydrodynamic capabilities of an oil are exceeded, metal-to-metal contact is avoided by (1) boundary lubrication and, finally, by (2) additive chemistries which sacrificially coat the metal surfaces.
I have come to believe that maximizing the oil's hydrodynamic effectiveness is a function of only two qualities: flow and HTHS viscosity. To a great degree those qualities are in opposition, such that increasing HTHS will tend to require greater kinematic viscosity which will tend to impede flow, and increasing flow will tend to result in lower HTHS viscosity.
My questions here have to do with what happens when hydrodynamic breakdown occurs. How exactly does boundary lubrication happen? One aspect of it seems to be related to polarity: that is, a polar oil will remain aggressively attached to metal surfaces, even in a layer that is only a few molecules thick, and will resist being scrubbed away.
I recall a recent post which quoted an industry expert noting that PAO oils are not polar and that, because of that, by themselves they were useless as boundary lubricants; and that consequently their abilities as boundary lubricants relied entirely on additives. If correct, what types of additives would those be, and how do they work?
Could it be that the non-polar nature of PAOs might help to explain why it is that conventional oils, despite many inferior laboratory qualities, are able to equal or exceed PAO oils' performance in terms of wear? In other words, that PAO oils are superior where cold performance and extreme heat are involved, and that they last longer, but that they are inferior in boundary lubrication, and that as a consequence their overall wear performance is only equal to conventionals, or even slightly inferior?
Could it be true, also, that Grp V oils, which are held to be highly polar in nature, can have superior properties when boundary lubrication is encountered, at least partly due to their polarity?
And how does the creation of sacrificial coatings, such as those formed by the breakdown of ZDDP, and perhaps moly for that matter, interact with the oil's boundary lubrication qualities? My current understanding is that boundary lubrication would tend to be breached first, then perhaps whatever molecular moly was coating the surface in question, and that the sulphur compounds formed by ZDDP breakdown actually react chemically with the metals and therefore become the outermost layer of the component, forming only when created by high heat from friction and acting as the final line of defense before metallic wear begins.
Finally, we have seen the replacement of ZDDP to some degree with other additives, and I am curious how these fit into the picture. Some show up in UOAs because they contain Boron, and I believe there may be others. How much is known about how these newer additive chemistries work, and how they interact with the older chemistries, which for now are still present in all motor oils?
It seems to me that the best wear protection would depend on optimizing all three qualities in a motor oil: hydrodynamic capabilities, to reduce the need for boundary lubrication; boundary lubrication itself; and AW/EP chemistries that act when boundary lubrication breaks down.
When the hydrodynamic capabilities of an oil are exceeded, metal-to-metal contact is avoided by (1) boundary lubrication and, finally, by (2) additive chemistries which sacrificially coat the metal surfaces.
I have come to believe that maximizing the oil's hydrodynamic effectiveness is a function of only two qualities: flow and HTHS viscosity. To a great degree those qualities are in opposition, such that increasing HTHS will tend to require greater kinematic viscosity which will tend to impede flow, and increasing flow will tend to result in lower HTHS viscosity.
My questions here have to do with what happens when hydrodynamic breakdown occurs. How exactly does boundary lubrication happen? One aspect of it seems to be related to polarity: that is, a polar oil will remain aggressively attached to metal surfaces, even in a layer that is only a few molecules thick, and will resist being scrubbed away.
I recall a recent post which quoted an industry expert noting that PAO oils are not polar and that, because of that, by themselves they were useless as boundary lubricants; and that consequently their abilities as boundary lubricants relied entirely on additives. If correct, what types of additives would those be, and how do they work?
Could it be that the non-polar nature of PAOs might help to explain why it is that conventional oils, despite many inferior laboratory qualities, are able to equal or exceed PAO oils' performance in terms of wear? In other words, that PAO oils are superior where cold performance and extreme heat are involved, and that they last longer, but that they are inferior in boundary lubrication, and that as a consequence their overall wear performance is only equal to conventionals, or even slightly inferior?
Could it be true, also, that Grp V oils, which are held to be highly polar in nature, can have superior properties when boundary lubrication is encountered, at least partly due to their polarity?
And how does the creation of sacrificial coatings, such as those formed by the breakdown of ZDDP, and perhaps moly for that matter, interact with the oil's boundary lubrication qualities? My current understanding is that boundary lubrication would tend to be breached first, then perhaps whatever molecular moly was coating the surface in question, and that the sulphur compounds formed by ZDDP breakdown actually react chemically with the metals and therefore become the outermost layer of the component, forming only when created by high heat from friction and acting as the final line of defense before metallic wear begins.
Finally, we have seen the replacement of ZDDP to some degree with other additives, and I am curious how these fit into the picture. Some show up in UOAs because they contain Boron, and I believe there may be others. How much is known about how these newer additive chemistries work, and how they interact with the older chemistries, which for now are still present in all motor oils?
It seems to me that the best wear protection would depend on optimizing all three qualities in a motor oil: hydrodynamic capabilities, to reduce the need for boundary lubrication; boundary lubrication itself; and AW/EP chemistries that act when boundary lubrication breaks down.