Engine oil - More Oils Aint Oils

You’ll Be Glad To Read This One. ..... Quiz follows

At the Club meeting of 6 December, Paul Glover, from Castrol, gave an interesting and informative talk about automotive lubricants (see Oils Aint Oils). I found his remarks on the respective function of detergents and dispersants in engine oils to be particularly interesting so I thought a few more words about them might be of value.

Detergents

Any detergent is, broadly speaking, a cleaning agent. In an engine oil, it releases 'dirt' by being preferentially taken up by surfaces, making them more easily 'wetted' by the oil. The particulate products of combustion (particularly carbon) become suspended, even when they have been deposited initially on a surface. Among other things, this prevents sticking of piston rings.

Dispersants

However, a dispersant acts dfferently, in that it stops sludge from being deposited. Sludge (comprising combustion products, unburnt fuel, carbon, water and lead anti-knock residues) can coat parts, block oil filters or worse. Stop-go, city driving conditions most favour its formation.

Boundary Lubrication - Engine Oil

To improve the condition of 'boundary lubrication' (particularly at the top of the cylinder and when starting cold), proprietary additives have been marketed for many years. First it was graphite (C), then molybdenum disulphide (MoS2) and, in recent years, polytetrafluoroethylene (P T F E), all in colloidal form. These are intended to deposit on rubbing surfaces, ie to 'plate out', so that, even if there is an instant of no oil film, the two metal surfaces still will not touch and therefore will not wear.

However, a dispersant is present specifically to attach to any particles that might 'Plate out', so therefore it can be expected to prevent these antifriction additives from doing just what they have been intended to do. Worse still, this may well use up all the dispersant in the oil so that none will be available to do the job the oil company put it there to do, i.e we then have a lose-lose situation! It is one for which I can work up no enthusiasm.

Boundary Lubrication - Gear Oil

On the other hand, there is no dispersant in gear oils, since they have no contact with the products of combustion. So, by implication; the addition of colloidal solid lubricant might be helpful in reducing the rate of wear of gears. Nevertheless, there is still a possible result of adding a colloid (no matter how chemically inert) to a gear lubricant containing extreme-pressure additives (which are designed to react with the metal of gears).

In a colloidal suspension, the particles are minute, approaching molecular size, and are electrically charged. They therefore will not settle out. If any do, it is because they are too large, ie larger than colloidal size and therefore originally in ordinary suspension. However, in service, extreme-pressure additives in gear oils can give rise to acidic by-products and these can modify the medium in which the colloid is held - oil in this case - possibly affecting the electrical charge. The suspended material might then deposit out (becoming unavailable for boundary condition lubrication) and may even build up in anti-friction bearings.

So, all of this means that, before adding colloidal C, MoS2 or P T F E to engine oil or extreme-pressure gear oils, one needs to be quite sure that there will be no undesirable results.

You Have Been Warned!

Ken Sandecoe SC316

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