For further guidance it may be pointed out that from past experience it has been found that the oils which have met with most success in Diesel engine air compressor lubrication, besides complying with the foregoing stipulations, have generally possessed properties lying within the following limits :- Specific gravity, .870 to -915 at 15 degs. Centigrade ; vicosity (Redwood), 400 to 1000 seconds at 70 degs. Fah. ; viscosity (Redwood), 75 to 125 seconds at 140 degs. Fag. ; closed flash point, not less than 400 degs. Fah. ; colour, red or yellow by transmitted light, but clear-not misty or smoky.

Both straight mineral oils and compounded oils containing small quantities of saponifiable oil have met with success on Diesel engine compressor service, but the majority of successful oils have been straight mineral oils.

(Marine Engineer Officers' Magazine.)

Oils for Lubricating Internal Combustion Engines. The following notes re lubricating oils by Messrs. Price's Patent Candle Co., Ltd.. will be found useful.

Choosing Oil

Generally speaking, the atmospheric temperatures met with nowadays in this country no longer touch the extremes of heat and cold. Moreover, present day engines, especially where thermostats or other devices for ensuring uniformity of working temperature are fitted, are less susceptible than earlier types to variations of atmospheric temperatures. On the other hand, whilst the water jacket keeps at a fairly even temperature, the crank case containing the oil supply varies materially, and we would not be surprised to see this problem tackled in the near future.

Fluid friction in the modern engine of small cubic capacity and high r.p.m. is a serious item. The closer the temperature of the oil be kept to a pre-determined most efficient figure, the better for all round results. Fuel is wasted, and there may be excessive carbon deposit whilst the temperature of the oil is too low ; whilst wastage likewise occurs and there is excessive wear, when the temperature of the oil is too high. This problem becomes acute as cubic capacity is reduced and r.p.m. increased- which is the modern tendency.

One of the first essentials in an oil for present day engines is a good ratio of viscosity ; in other words, the least possible increase or decrease in body as the lubricant is cooled or heated respectively. The most satisfactory results in this respect can only be achieved with rich compound lubricants.

Compound Versus Mineral Oils

For all internal combustion motors we recommend for preference compound lubricants, and a few remarks on this subject may be of interest.

At the outset we may instance Gas Engines and Oil Engines, for both of which types their makers, confirming our principles, also specify the use of compound lubricants. In the case of some makes of Oil Engines, the fuel used is liable to cause carbon and gummy deposits on the piston top and behind the piston rings, and with a pure mineral lubricant the engine will be useless from this trouble after a few hours' running. If, however, Engine Oils are used, which are very rich in suitable fatty matter, the engine may be run for weeks without any deposit accumulating.

The combustion of heavy fuel tends to result in the formation of a certain amount of carbonaceous and gummy residue, and the fatly oil alone is responsible for the fact that this harmful product does not accumulate. The fatty oil has a softening and solvent action on the gummy matter, with the result that a portion washes into the crank case, whilst the remainder is ejected with the exhaust. Motor engines on present-day fuels show the same symptoms, but, of course, to a less extent. It is logical to argue that the compound class of lubricants, which are practically speaking essential for satisfactory operation of oil engines are most efficient also for motor engines.

In the case of motor-cycles, troubles due to excessive deposit are more pronounced than with water-cooled car engines, and we find that the most successful lubricants are those richest in fatty matter.

With regard to the burning of the lubricant, the fatty oil leaves much less deposit than the hydrocarbon. The fatty oil contains oxygen in combination, and therefore burns | cleanly, whilst pure hydrocarbons give off volumes of sool.

As an instance, one may compare the burning of pure colza oil or of alcohol, with the burning of paraffin or of other mineral lamp oil. Colza oil and alcohol contain oxygen, and burn with a clear flame, requiring very little extra oxygen to give complete combustion. Hydro-car bons, on the other hand, " crack " and burn to soot and other carbonaceous matter, portions of which remain on the piston top and in the combustion chamber.

Oil Purification

By the term " oil purification " is included the cleaning or purifying of lubricating oil, which has been used, and which is full of particles of foreign matter, giving it a dirty appearance. It is now thoroughly established that if these particles be removed, and provided that the oil has not been diluted in some way, the purified oil is as good as ever it was. So far as removing the particles go this can be done perfectly easily and simply, provided that time taken is no object, by merely letting the oil rest quite undisturbed in a vessel, until such time as the effect of gravity has caused all the particles to settle to the bottom of the vessel. Also before this takes place the worst of the impurities can be got rid of by passing the oil through a more or less coarse strainer. To increase the speed of both processes the oil can be heated with advantage. Machines of small capacity have been used by causing a stream of water to mix with a small stream of the oil, both meeting and mixing in a funnel which enters a tank, in which the water is separated from the oil by reason of its difference in specific gravity. The oil is then allowed to run down an inclined grating so arranged that any particles left in it separate out by gravity, so that the oil reaching t he grating end is stated to be clean. Even in this type much of the work is really done by gravity, and therefore the capacity per hour is low. It is impossible by mechanical means to separate two liquids which are soluble one in the other, but it may be possible to separate them if their vaporising temperatures are different one from the other. So that in the case of a mixture of some oils with paraffin it will be possible by either passing steam through them, or by passing steam through pipes in the oil, to so raise the temperature that the paraffin passes off as vapour. If, however, a supply of " more or less " clean oil is required for such work as using as an anti-rust coating, oiling barrow wheel axles, etc., then an oil taken from crank cases of petrol engines, and allowed to clear itself by the action of gravity, is quite good enough. In such a case as deep a vessel as can be obtained should be used, as in time the dirt is separated by a greater distance from the cleaner oil at the top, than in a shallow vessel.