INSTALLATIONS considerable misapprehension exists among oper ating engineers as to the oils that are suitable as fuel in a low pressure oil engine. It is unfortunate that many extravagant statements have been made concerning the results obtained with low-gravity oils. An operator should understand that an en gine's adaptability to burn the heavy oils is a matter that must be determined by a test on the particular engine in which he is interested. It may be safely stated that an expert can secure satisfactory operation regardless of the character of the fuel, providing the load be maintained at a constant value. The aver age operator, however, will find that the oils having a gravity lower than 24° Baume will give trouble. In the oil districts of the Southwest the low-pressure engine is handling pumping plants and is burning the heavy crude oil to the entire satisfaction of all concerned. These engines are operating on a constant load, and the smoky exhaust, that invariably accompanies the use of this heavy oil, is not objectionable in such isolated plants. In most installations, such as a light or an industrial plant, the load is far from being constant. On a varying load the ignition device will not maintain a temperature sufficiently high to burn all the heavy "untracked " particles of oil. The speed will be erratic, and the performance entirely unsatisfactory. Furthermore, such plants are usually wedged in among other factories or mercantile establishments where the disagreeable features of a smoky ex haust will not be tolerated.
Though the above statements concerning the operation on low gravity fuels may be accepted as facts, it is not actually the oil's gravity that makes it. objectionable, but rather it is its character istics that determine its usefulness. Regardless of its gravity, an oil must not contain much coke or dirt; neither must it hold any large degree of sulphur or water. Since practically all low gravity oils do contain a considerable percentage of coke and dirt, the gravity is usually taken as an indication of an oil's suitability. As can be readily seen, the dirt and coke gives off no heat and is useless in an engine cylinder. Both settle on the combustion chamber walls in a hard, thick scale, which is commonly called carbon. Filling up the hot bulb, it reduces the capacity of the bulb and cuts down the engine's output. Since it absorbs a large amount of heat, it remains incandescent and causes pre ignition by igniting the fuel charge early in the compression stroke. Frequently, settling on the cylinder walls, it reveals
itself in piston cutting and cylinder scoring. The sulphur unites with the water, and the resultant acid corrodes the cylinder. The water must be evaporated and raised to the temperature of the burning fuel charge. This requires heat; thus not only is the temperature of combustion lowered, but heat is abstracted. This results in an impaired efficiency. The water enters the combustion chamber along with the fuel particles; intermingling with the particles of oil, it chills the fuel charge with a consequent delay in the process of vaporization and combustion.
Since these objectionable characteristics are possessed by prac tically all oils lower than 24° Baume having an asphaltum base, the average plant will do well to avoid the use of oils heavier than this gravity. As a guide to purchasers of fuel oil for low pressure engines, the following specifications are worthy of attention.
Specifications. Fuel Oil for Oil .Engines.— The oil purchased is to be either crude oil, fuel oil or distillate oil, with a gravity not lower than 24° Baume, gravity to be tested at a temperature of 60° Fahrenheit. The oil shall not contain more than one-half of one (.5) per cent. of sulphur; not more than eight-tenths of one (.8) per cent. of water; and not to exceed six (6) per cent, of coke or dirt. Flash point below 275° Fahrenheit, open cup, and not lower than 160° Fahr enheit. When subjected to fractional distillation, at least fifty (50) per cent. shall distil over at a temperature below 680° Fahrenheit. The lawier heat value shall not be less than 18,000 B.t.u. per pound. For engines above 50 h.p. these specifications will secure a suitable fuel. For smaller powered engines opera ting less than twelve hours daily, the best fuel is a distillate from 32° Baume to 38° &tune. Fuels of this gravity are commonly marketed under refinery trade names, such as, Staroil; B. Solar Oil; Oriental Distillate, etc. These oils usually have a yellow greenish color, though some approach the color of kerosene. It is the best for medium-powered plants as it is heavy enough to be used without a great amount of preignition, and yet of high enough gravity to vaporize and ignite at a fairly low bulb temperature.