Turbine oils for Automotive Vehicles

Since  gas  turbine  engines  for  automobiles  have  a  potential  which  no  doubt will  soon  be  realized,  mention will  be  made  of the  lubricant  requirements  for  the  drives. The oil will be used to lubricate both bearings and gearing. The  bulk oil  temperature will  probably  be at least  300 degree F during  operation, but  the oil will  not  be  subjected   to hot combustion gases  and thus should remain    comparatively   clean.

Whether   the oils  used will preferably  be  petroleum or  synthetic or  blends  of  the  two  is a  question. Whatever type, they will   no doubt contain various additives, such as   antifoam    agents, oxidation inhibitors, metal deactivators, and antiwear agents.

Speculation as  to  consumption  and  renewal  of  such  oils   indicates that  the oil  reservoir  will  have  a capacity  of about  3 quarts  and that  the   consumption will be  almost  nil  but  that  an  oil  change  yearly  can be  expected.

Dispersion of Silicone Antifoam Agents

Most often  the  additives  employed  in   compounding  gear  oils  are  readily  soluble  in the  base  oil; therefore , simple  mixing, as described  above, is  sufficient  to obtain a  satisfactory  blend. On the other hand,  since  silicone fluids which are  frequently  used  as  antifoam  agents have  limited  solubility in oils, they  must  be  thoroughly  dispersed, or they  will settle  out. In view  of this  Beerbower  and  Barnum  made  a study  of  dispersions  of  silicone  defoamers  in oils  using  radioactive tracers  to  determine  dispersion  and  separation of these  additives. They  concluded  that  to  form  stable  dispersions  of  silicones  in oil  the  droplet  size  of the  additive  should  be  in the  10  micron  range. This  could  be arrived at by  heat  and the use  of  high  intensity  mixing  approaching colloid  milling. In laboratory mixers, a speed of 4800 rpm and a temperature of 200 degree F “gave blends in which the silicone was uniformly dispersed after 70 days of storage”.

Foam inhibition in gear oils

The rapid movements of gears tend to aerate oils and produce foam. This tendency may be aggravated by the presence of some additives, such as EP agents. Also, the higher the viscosity of the oil the more permanent the foam as a rule. Cases have been noted where foam became so great that it filled the gear case to overflow and long before this the gear teeth failed to obtain sufficient lubricant. While  foaming of gear oils might  result  from vaporization of entrained  water or driving of air  out  of solution, the general  cause in gear sets is churning  of air  into  the oil  by  agitation.

Little trouble is experienced from foaming of gear oils in service because the use of foam inhibitors in such lubricants is almost universal. It has been suggested that there is a difference between a foam inhibitor and a foam depressant, the latter being an insoluble material. Robinson and Woods^44 use the term “antifoaming agent” to embrace all aspects of the destruction, elimination, or prevention of foams. These investigators state that a foam inhibitor may act:

“ (1) by causing  coalescence of smaller bubbles into large bubbles at or below the surface, (2) by causing the rupture  of  individual bubbles at the surface, (3) by destroying the inherent stability of the liquid  films, or (4) by causing any or all of  these actions simultaneously.”

McBain et al.^39 found that the most complete defoamers for oils are generally, but not always, insoluble. This is true of silicone fluids which have wide usage for this purpose. There is an optimum amount of antifoam agent required which is quite low. Consequently most suppliers furnish defoamers as dispersions or solutions so that low dosages will be more accurate. While Woods and Robinson^50, in testing varying proportions of  DC 200 fluid in two oils, found that 0.01 per cent gave  the greatest  foam  inhibition, actual  usage  in most  gear oils is only a fraction  of this proportion. Thus, Klaus  and Fenske^34, using two oils which foamed badly with no additive, found  that both responded to silicone  antifoam additives at concentrations as low as 0.00001 weight per cent and that maximum effectiveness  was achieved with 0.00005 per cent  or greater.