Flash flash point of oil is merely that temperature at which the oil gives off a vapor that will ignite in the presence of an open flame. This point should not be below 125° Cen tigrade for two reasons. First, if the oil has a lower flash point than that given above, there is a fire risk due to oil vapors forming under atmospheric conditions. Second, if the flash point is low, the oil, as it rests in the engine fuel valve, may vaporize and ignite on account of the absorption of heat from the injection air and the cylinder head. If the flash point is exceedingly high, the ability of the oil to burn in the short time allowed during the admission period is lessened. In this the flash point is merely an indication of the vaporizing characteristics of the oil.
It should be added, at this point, that no person should be al lowed to approach an oil storage tank with an open light. Neither should an unprotected electric lamp be dropped into the tank. In cleaning out an empty tank, the cover should be re moved and the tank left open for at least forty-eight hours before a man is allowed to enter the tank. The oil vapors dispel very slowly; more than one death is traceable to carelessness in this matter.
Burning Point.—The burning point of an oil is that tem perature at which the oil ignites and burns in an open cup. By some it is claimed that the burning point in no way indicates an oil's usefulness as a Diesel fuel.
Nevertheless, if the burning point is high, with a corresponding high flash point, the oil will cause the engine exhaust to be smoky. This is attributable to the sluggish rate of combustion. If the burning point is fairly low, the fuel charge need not be so thor oughly atomized to obtain perfect combustion. As a matter of practical operation, a fuel oil should not have its burning point exceed 300° Fahrenheit or 150° Centigrade.
Sulphur.—The sulphur content should under no circumstance exceed 2 per cent. With this low value the action of the sulphur dioxide on the cast-iron parts is negligible. Even this small amount of sulphur will attack the exhaust pipe line if a water line runs into the exhaust header for cooling purposes. In the presence of the water a formation of sulphuric or sulphurous acid occurs with a consequent eating of the header. On certain early Diesels where water was introduced into the exhaust valve pots, the corrosion from the sulphur was very, evident. If the cooling of the cylinder head and exhaust header is carried at a temperature low enough to condense the resulting from the process of combustion, sulphuric acid may form in the exhaust. This is especially true
on low loads when the operator neglects to reduce the flow of cooling water.
The American Diesels had splash lubrication employing an emulsion of oil and water. The sulphur dioxide gas, blowing past the piston, came in contact with the emulsion, forming sulphuric acid. The acid attacked all the iron and steel parts of the crankcase and cylinder. The same effect may be observed in engines having a water-cooled piston. If the water connec tions leak, small portions of the joints will show the effects of acid. A corrosive action is at times observed in the cylinder immediately below the head joint. This is undoubtedly the result of water seeping past the flange gasket and uniting with the sulphur in the fuel, Water.—The presence of water in Diesel fuel oils is objection able from the viewpoint of both the purchasing agent and the plant operator. It requires no argument to prove that it is of no advantage to buy the water content of the oil. The water increases the net cost of the fuel while likewise increasing the freight charges. Ordinarily this is not of great moment since the percentage of water is never large.
The engineer is justified in protesting against an excessive amount of water since it increases his operating difficulties. In certain fields where the oil comes from flowing wells, instead of from "pumpers," the water and oil exist in an emulsion. It is practically impossible to separate the water and oil. The emul sion also entrains a large quantity of air globules. As this com bination of oil, water and air enters the fuel pump the air often breaks away from the oil and, collecting in the pump, disturbs the functioning of the valves. The air, water and oil mixture has a greater volume than a pure oil charge of equal heating value. This increased fuel volume demands an increased air-injection pressure to force it into the cylinder.
If the water is in a free state, it will settle at the bottom of the oil storage tank. If this is not drained at frequent intervals, the water level will reach the end of the fuel suction line, with the result that the fuel valve injects nothing but a water charge into the engine cylinder. The engine then drops its load and slows down. This constitutes one of the most serious nuisances that occur in a plant. When it does happen, the entire fuel line must be purged of the water and the storage tank drained. It is practically impossible to secure oil that is entirely free from water.