Page:Popular Science Monthly Volume 89.djvu/278

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��line is highly inflammable it follows that both the flash and fire points of used oil are lowered below the normal flash tem- perature.

The Fire Test

The temperature at which an oil will ignite from its vapors and continue to burn, called the "fire test," is not of much use in testing fresh oils, but it does reveal much about an oil which has been used in an automobile. Heat tends to decompose an oil chemically into its lighter and heavier constituents, and the crankcase of some motors is always hot enough to affect an oil. The lighter constituents will ignite at a much lower temperature than the original lubricat- ing oil. Hence, by applying the simple fire test, which consists in bringing a test flame quickly to the surface of the oil and allowing first the vapors to ignite and then the oil to catch fire from the vapors, it is possible to determine to what extent the oil resists heat. This applies chiefly to used oil. An oil that fails to meet the test satisfactorily will be used up very rapidly; it will be vaporized too easily.

Viscosity or "Body"

Of equal importance to the flash or fire test, is the determination of an oil's body — its viscosity. Water, which has very little body, is clearly less viscous than cane syrup. It is possible to meas- ure viscosity,' by measuring the rate of a liquid's flow. Sjjecial instruments have been invented to measure that flow in a given number of seconds under a given head or pressure and at a given tempera- ture. Viscosity is therefore usually ex- pressed in seconds at a given tempera- ture. If the oil is too light, has too little body, the rubbing surfaces will not be properly separated. Hence, an oil must be selected of such viscosit>' that it will reduce the fuel consumijtion for a gi\'en amount of power to a mininuim and yet prevent the pistons and bearings from "seizing." That selection results in a romi^romise between the attainment of proj)er lubrication and of the utmost fuel economy. The jioint of compromise lies somewhere between i8o and 800 seconds, depending on the conditions under which the motor is operated and the particular design of I lie motor.

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The Carhon Residue Test Another test consists in measuring the

��carbon residue after complete distillation in a small flask. Every oil will leave a carbon residue, as it must, because oil always contains a certain amount of "fixed" carbon. The amount and char- acter of the carbon left, however, is an indication of the grade of petroleum from which the lubricating oil was distilled and the care exercised in refining. All oils oxidize or polymerize when heated, forming sediment, the nature of which tells much about an oil. The heat of many explosions causes part of the film of the lubricating oil in the cylinder to flash oft and to escape with the exhaust gases. A residue, commonly called carbon deposit, is left behind, however, consisting of carbon, solid hydro-carbons, etc. Oil must be continuously fed in to renew the thin film. It is evident that by testing an oil by heating it in a tube over a Bunsen flame, we are subjecting it to a condition something like that which it must meet in an automobile motor and that decomposition must always result whenever oil is called on to en- dure heat. The duration of the heat- ing and the temperature, of course, af- fect the outcome of the experiments. Some manufacturers claim that they make "non-carbon" oils. An oil is com- jjosed of h\drogen and carbon in a chemical combination, just as water is a chemical combination of h\clrogen and oxygen. It is just as absurd to speak of "non-carbon" oil as it is to speak of "non-oxygen" water.

Tests for the Automobile Oiciicr

One of the easiest tests, which every automobile user can make for himself, is that which shows how the oil stands up under heat. A small (|uantit>' of the oil is heated (ner a Bunsen burner in a test tube until \i'lKnv \apors appear which will be in about fifteen minutes. If the oil turns black it is unsuitable for automobile lubrication; if it darkens but still remains clear, it is good.

Another test, easy to carry out, con- sists in shaking equal (|uantities of oil and water in a bottle for half an hour — the ennilsion test. A jioor oil (in part or wli<)li\) mixes [lermanently with the

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