264 



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. 



Popular Science Monthly 



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 



