operates in the usual way. The water circulation passes through the jacket by way of the pipes J and K. When the engine is running at heavy loads with full charges of oil delivered by the oil pump through the sprayer G, a second pump is caused to come into action, which discharges a very small quantity of water through the water sprayer valve F. This water passes into the vaporizer and combustion chamber, together with a little air, which enters by the automatic inlet valve, which serves as sprayer. This contrivance is found useful to prevent the vaporizer from overheating at heavy loads.
Fig. 15.—Crossley Oil Engine.
The principal difference between this engine and the Hornsby engine already described lies in the use of the separate ignition tube H and in the water sprayer F, which acts as a snifting valve, taking in a little air and water when the engine becomes hot. Messrs Crossley inform the writer that the consumption of either crude or refined oil is about ·63 of a pint per horse-power on full load. They also give a test of a small engine developing 7 B.H.P., which consumed ·601 pint per B.H.P. per hour of Rock Light refined lamp oil and only ·603 pint per B.H.P. per hour of crude Borneo petroleum oil.
Engines in which the oil is vaporized in a device external to the cylinder have almost disappeared, because of the great success of the Hornsby-Ackroyd type, where oil is injected into, and vaporized within, the cylinder. It has been found, however, that many petrol engines having jet carburettors will operate with the heavier oils if the jet carburettor is suitably heated by means of the exhaust gases. In some engines it is customary to start with petrol, and then when the parts have become sufficiently heated to substitute paraffin or heavy petroleum oil, putting the heavy oil through the same spraying process as the petrol and evaporating the spray by hot walls before entering the cylinder.
Mr Diesel has produced a very interesting engine which departs considerably from other types. In it air alone is drawn into the cylinder on the charging stroke; the air is compressed on the return stroke to a very high pressure generally to over 400 ℔ per sq. in. This compression raises the air to incandescence, and then heavy oil is injected into the incandescent air by a small portion of air compressed to a still higher point. The oil ignites at once as it enters the combustion space, and so a power impulse is obtained, but without explosion. The pressure does not rise above the pressure of air and oil injection. The Diesel engine thus embodies two very original features; it operates at compression pressures very much higher than those used in any other internal combustion engines, and it dispenses with the usual igniting devices by rendering the air charge incandescent by compression. The engine operates generally on the Otto cycle, but it is also built giving an impulse at every revolution. Mr Diesel has shown great determination and perseverance, and the engine has now attained a position of considerable commercial importance. It is made on the continent, in England and in America in sizes up to 1000 H.P., and it has been applied to many purposes on land and also to the propulsion of small vessels. The engine gives a very high thermal efficiency. The present writer has calculated the following values from a test of a 500 B.H.P. Diesel oil engine made by Mr Michael Longridge, M.Inst.C.E. The engine had three cylinders, each of 22·05 in. diameter and stroke 29·52 in., each cylinder operating on the “Otto” cycle. The main results were as follows:—
| Indicated power | 595 horse |
| Brake power | 459 ,, |
| Mechanical efficiency | 77% |
| Indicated thermal efficiency | 41% |
| Brake thermal efficiency | 31·7% |
(D. C.)
OILLETS (from an O. Fr. diminutive of (œil, eye, in Mod. Fr. (œillet; other English variants are oylets, eyelets, or eyelet-holes), the architectural term given to the arrow slits in the walls of medieval fortifications, but more strictly applied to the round hole or circle with which the openings terminate. The same term is applied to the small circles inserted in the tracery-head of the windows of the Decorated and Perpendicular periods, sometimes varied with trefoils and quatrefoils.
OILS (adopted from the Fr. oile, mod. huile, Lat. oleum, olive oil), the generic expression for substances belonging to extensive series of bodies of diverse chemical character, all of which have the common physical property of being fluid either at the ordinary temperature or at temperatures below the boiling-point of water. Formerly, when substances were principally classified by obvious characteristics, the word included such a body as “oil of vitriol” (sulphuric acid), which has of course nothing in common with what is now understood under the term oils. In its most comprehensive ordinary acceptation the word embraces at present the fluid fixed oils or fatty oils (e.g. olive oil), the soft fats which may be fluid in their country of origin (e.g. coco-nut oil, palm oil), the hard fats (e.g. tallow), the still harder vegetable and animal waxes (e.g. carnaüba wax, beeswax), the odoriferous ethereal (essential) oils, and the fluid and solid volatile hydrocarbons—mineral hydrocarbons—found in nature or obtained from natural products by destructive distillation.
The common characteristic of all these substances is that they consist principally, in some cases exclusively, of carbon and hydrogen. They are all readily inflammable and are practically insoluble in water. The mineral hydrocarbons found in nature or obtained by destructive distillation do not come within the range of this article (see Naphtha, Paraffin, Petroleum), which is restricted to the following two large groups of bodies, formed naturally within the vegetable and animal organisms, viz. (1) Fixed oils, fats and waxes, and (2) Essential, ethereal or volatile oils.
1. Fixed Oils, Fats and Waxes.
The substances to be considered under this head divide themselves naturally into two large classes, viz. fatty (fixed) oils and fats on the one hand, and waxes on the other, the distinction between the two classes being based on a most important chemical difference. The fixed oils and fats consist essentially of glycerides, i.e. esters formed by the union of three molecules of fatty acids with one molecule of the trihydric alcohol glycerin (q.v.), whereas the waxes consist of esters formed by the union of one molecule of fatty acid with one molecule of a monohydric alcohol, such as cetyl alcohol, cholesterol, &c. Only in the case of the wax coccerin two molecules of fatty acids are combined with one molecule of a dihydric (bivalent) alcohol. It must be pointed out that in common parlance this distinction does not find its ready expression. Thus Japan wax is a glyceride and should be more correctly termed Japan tallow, whereas sperm oil is, chemically speaking, a wax. Although these two classes of substances have a number of physical properties in common, they must be considered under separate heads. The true chemical constitution of oils and fats was first expounded by the classical researches of Chevreul, embodied in his work, Recherches sur les corps gras d’origine animale (1823, reprinted 1889).
(a) Fatty (fixed) Oils and Fats.—The fatty (fixed) oils and fats form a well-defined and homogeneous group of substances, passing through all gradations of consistency, from oils which are fluid even below the freezing-point of water, up to the hardest fats which melt at about 50° C. Therefore, no sharp distinction can be made between fatty oils and fats. Nevertheless, it is convenient to apply the term “oil” to those glycerides which are fluid below about 20° C., and the term “fat” to those which are solid above this temperature.
Chemical Composition.—No oil or fat is found in nature consisting of a single chemical individual, i.e. a fat consisting of the glyceride of one fatty acid only, such as stearin or tristearin, C3H5(O·C18H35O)3, the glycerin ester of stearic acid, C17H35·CO2H. The natural oils and fats are mixtures of at least two or three different triglycerides, the most important of which are tristearin, tripalmitin, C3H5(O·C16H31O)3 and triolein, C3H5 (O·C18H33O)3. These three glycerides have been usually considered the chief
