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.
THE AUTOMOBILE 349 Buying oil on the basis of the geographic location of its source. refiner can produce a satisfactory lubricating oil from any crude oil. Failing to use the scientific method in buying an oil. Slogans are effective in advertising, but they must not be confused with facts. Even with Good Brakes It Takes Time for an Automobile to Stop. People differ in their reaction time; but on the average, it requires a second between the time the mind decides to put on the brakes and the time that the muscles get into action. It is obvious that the number of feet driven during this "thinking time " r varies at different speeds. The distance traveled while using the brakes increases more rapidly than the increase in speed; for example, doubling the speed from 30 to 60 miles per hour increases the distance from 73 feet to 292 feet, or about four times. In general, the stopping-distance follows the square law; doubling the speed quadruples the distance required to stop. The Automobile Is Dangerous When Not Properly Controlled. In the United States 34,400 people were killed and 1,200,000 people were injured in 1940 by automobile drivers. The direct economic losses resulting from traffic accidents were estimated to be $1,600,- 000,000 in the same year. In England 41,900 civilians were killed by German bombs during the i(?-month period, January 1, 1940- July 1, 1941. England at war was a safer place than American highways. The automobile driver is the fifth most important death cause, exceeded only by heart disease, cancer, cerebral hemorrhage, and nephritis. o H P A