OILS FOR POWER PURPOSES preparation and classification of oils for power purposes and chiefly for use in the internal combustion engine have not received the consideration and study they deserve, and which is essential if the supply of oils for this work is to be extended. Indeed, we have, at the present time, a spirit of almost resentment on the part of engineers that our chemists do not adapt the large variety of oils obtainable to the Diesel engine, and a similar attitude on the part of the latter, that the engine is not adapted to the use of any and every description of heavy oil. The responsibility rests, of course, on neither, but there can be no doubt that each side of the argument can contribute something towards the solution of the problem. The results would certainly be profitable to both and the respective industries. At the present time, the range of oils suitable for burning in the internal combustion is not large, and there are oils of certain grades and characteristics which are excluded by reason of some deficiency or of possessing some ingredient which the existing types of engines refuse to work on. This widening of the scope of suit able Diesel oils not only would relieve the drain on certain classes of oils, but would, at the same time, reduce their cost on account of eliminating the necessity of special distillation, storage and transport, which has now to be carried out.
The characteristics of an oil suitable for the Diesel type of engine have been classified by the majority of manufacturers of these engines, though these do not altogether indicate very clearly the precise description of oil, or assist in their selection. The specific gravity of an oil is not by itself any criterion of its suitability in this connection, and, though the calorific value is an important point, there does not appear to be any relation between this factor and engine consumption. The one point on which there appears to be general agreement is flash point, for in all specifications it is fixed as having to exceed F. Viscosity is a vital point where fineness of atomization is essential, and very viscous fuels require heating before introduction into the engine. a point which must be taken into account in selecting an oil, as son?e descriptions become sufficiently mobile once they are introduced into the piping system on the engine, but under normal atmospheric conditions they are of too high a viscosity to be handled in their cold state. An important consideration also is that of ash
content which is regarded by some authorities as the most important factor in the selection of a Diesel engine oil, that is if freedom of trouble with the exhaust value and unconsumed residue in the combustion chamber is to be secured. The ash content should not exceed .06 per cent. The petroleums of the world may be roughly divided into two distinct categories, namely, those having an asphalt base, and those having a paraffin base. The most suitable for the purpose under con sideration are obviously those having a paraffin base, the largest supply of which is found in America, chiefly in Pennsylvania. The fields producing these are slowly becoming exhausted, and that is one reason why the Diesel engine should be so constructed as to employ any description of oil.
Petroleums possessing an asphalt base are generally disliked for use in this type of engine, as this constituent will not distil, and, when heated, forms coke. On a long sea run this disposition has been the cause of serious trouble, while it detracts from the economic running of the engine. The undecided position of this aspect of the subject is indicated by the fact that in some quarters a proportion of 5 per cent asphalt was regarded as the maximum an oil should contain, while in other directions 15 per cent or even higher was stated as allowable.
Gas and tar oils are also in use on some engines, though the sources from which these are obtainable are not sufficiently extensive to enable a large supply to be relied on. The opinion of a high German authority on the latter of these descriptions was that, even if they were obtainable in sufficiently large quantities, they were not sufficiently consistent in quantity, and were the cause of too frequent break-downs during running. Tar oils and gas oils mixed in various proportions may be used, but this should not contain more than 25 per cent of the former.