ropuhir Srirvrr Monihly 



263 



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- 



