Man's physical universe

xanabras

348 FORMS OF ENERGY

lates dirt, in spite of air filters and oil filters. It should be changed as

soon as it becomes gritty, even though it still has good lubricating

properties. Good oil filters make oil last longer, but it may be that the

oil should be changed before it becomes gritty because it may become

diluted with unburned gasoline residues and it may become mixed

with water and corrosive sulfur compounds. A general rule is to change

the oil every 500 miles in the winter and every 1500 miles in the summer.

Crankcase dilution is likely to be much greater in the winter than

in the summer.

A heavy lubricating oil will last longer, but it will cut down on the

gasoline mileage and it will not give the best lubrication. A lubricating

oil is intended to lubricate, seal, cool, and scavenge rather than to last.

In order to adhere to metal surfaces, a lubricating oil must have a

low surface tension. On the other hand, an oil must have a boilingpoint

high enough to prevent it from vaporizing when in contact with

the hot metal surfaces. The higher the boiling-point is, the longer it

will last, but also the higher the surface tension will be.

Some of the

newer oils have substances added to them to lower their surface tension

and still keep the advantages of stability toward heat.

Lubricating oil must also seal the space between the piston and

the cylinder so that the oil will not slip through and be burned with

the gasoline — smoke issuing from the exhaust indicates that the

engine is burning oil — and so that gasoline will not leak into the

crankcase and dilute the oil. With the close-fitted parts of the modern

engines a light oil can be used, but as the engine becomes warm, a

heavier oil may be needed. A general rule is that heavier oil should

not be used unless the oil consumption at normal driving speeds is

increasing. One should use as light an oil as is economically possible.

A Few Lubricating Errors.

Buying an oil on the basis of its lasting qualities.

Not changing an oil often enough because the automobile has a

good oil filter.

Not renewing the oil

filter.

Changing the oil on the basis of mileage — a thousand-mile straight

run may leave a better oil than five hundred miles of slow driving,

starting, and stopping in winter.

Adding "dopes" to oil, thus upsetting their balance.

Testing an oil by its feel.

Selecting an oil on the basis of its color.

Using too heavy an oil.

Using too little oil in the crankcase.

Using too much oil in the crankcase. Keep the crankcase 3/4 full

rather than full.