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Page:Popular Science Monthly Volume 89.djvu/277

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ropuhir Srirvrr Monihly

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��stand how important it is to consider the destructive effect of heat. When the spark ignites the fuel charge in the coniijusliun-cliamhcr the heat of the explosion is at least 2000° and even 3000°; the inner cylinder wall surfaces may be as hot as 350° anrl certainly no cooler than 180°; the ])iston heads may not be hotter than 300° but are more likely to be 1000°; the main shaft and crankpin-bearing oil varies in temperature from 140° to 250°; and the sump oil has a temperature of 90° to 150°.

What Happens When Oil Is Heated

Oil is a chemical compound of extraordinary complexity. The number of elements of which an oil is composed are few, but the number of ways in which these elements can be split up or combined is almost limitless. The properties of lubricating oil are dependent on the many chemical com- pounds present in the oil. De- stroy that stability (the ap- plication of heat is the surest way of accomplishing this), and the oil ceases in part to be what it was before heating, both chemically and physically. The change which takes place in boiling an egg is not so great as that which takes place in an oil exposed to higii temperatures.

After an oil has been used in a motor for a few hours it changes entirely in color. Yel- low originally, it becomes blue, and, after the motor has run for se\'eral da>-s, it turns black. What is more, a black deposit settles out— a deposit whicli consists of metal dust, rul)l)ed off the friction surfaces, carbon- aceous matter, and powder-like carbon. Of these the most destructive in their action — for oil is used over and over again in a motor car — are the metal dust and the carbon. They

���When the wrong oil is used the con- necting-rod bear- ings wear away

���Worn wrist-pins re- sult from bad lubri- cation

���Cylinder walls are scored when lubri- cation is poor

���Carbon deposits are formed when the oil works past the piston rings

���Too 1 i g h t an oil

causes leakage of

gases

��cut like sand, and their effect on the bearing surfaces can therefore be imagined. Since this powder-like carbon is de- posited as the result of heat, it is important that a heat- resisting oil be used. Unless an oil has the right physical and chemical properties it may do more harm than good in a motor. Oils are therefore tested in half a dozen different ways to ascertain their suitability for automobile lubrication. Some of these tests can be conducted only with scientific instruments; others can be made by anyone.

The Flash Test and What It Means

No oil can withstand the enormous temperature of 2000° and even 3000° to which it is subjected on the wall-surfaces of a motor during the brief moment of explosion when the piston is driven outward on its working stroke; fortunately the period of exposure is so short that only the outer lay- ers, so to speak, [of the oil are affected.

Above a certain temperature the vapors arising from an oil are inflammable, and the flash test consists simply in determining the temperature at which they will ignite with- out setting fire to the oil itself. A small test flame is brought quickly near the surface of the oil, contained in a little test cup, and as quickly removed after ignition of the vapors. The temperature is measured. If the flash point is much below 400° the oil is unsuitable for efficient lubrication, because it will obviously flash off the surfaces and also e\ai:)orate too quickly and will not last long.

A certain amount of gasoline always finds its way into the crankcase where it mixes with the lubricating oil. Sincegaso-

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