A PRIMITIVE AND A MODERN AUTOMOBILE

The upper picture shows a typical automobile of 1899. It had a 1-cylinder engine of about 9 H.P. and a wheel base of 72 inches. The drive was through a belt, and changes in speed were accomplished by slipping the belt. The lower picture shows a modern 7-passenger, 8-cylinder sedan limousine. The engine develops 80 H.P. (Courtesy of the Packard Motor Car Company)

two full-Diesel engines, each developing 17,000 H.P. at 125 revolutions per minute. Her speed is 17 knots.

SUMMARY. An internal-combustion engine has the heat for running it developed inside the cylinders, not outside, as in the case of the steam engine. Its high efficiency is due to the very high temperature thus obtained in the cylinder.

In the four-stroke type of gas engine the cycle consists of intake, compression, power, exhaust.

The principal mechanical parts of an automobile are the engine (including carburetor, ignition system, and cooling system), the clutch, the transmission, and the differential.

QUESTIONS AND PROBLEMS*

1. Why is a gas engine called an internal-combustion engine? 2. Why do gas engines have flywheels? Why is a one-cylinder stationary gas engine of the four-stroke type (such as are commonly used for small-power purposes) especially in need of a flywheel?

3. What amount of useful work did a gasoline engine working at an efficiency of 25 per cent do in using 100 lb. of gasoline containing 18,000 B.T.U. per pound?

4. Why will an automobile go up a hill in low gear (crank shaft revolving rapidly) when it would stall in high gear (crank shaft revolving slowly)?

5. Suppose the rear wheels of an automobile were keyed fast to a continuous axle (no differential), what would be the effect on wear of rear tires in turning corners? Explain.

* Supplementary questions and problems for Chapter X are given in the Appendix.

CHAPTER XI

THE TRANSFERENCE OF HEAT

CONDUCTION

250. Conduction in solids. If one end of a short metal bar is held in the fire, the other end soon becomes too hot to hold; but if the metal rod is replaced by one of wood or glass, the end away from the flame is not appreciably heated.

This experiment and others like it show that nonmetallic substances possess much less ability to conduct heat than do metallic substances. But although all metals are good conductors as compared with nonmetals, they differ widely among themselves in their conducting powers.

FIG. 187. Differences in the heat conductivities of metals

Let copper, iron, and German-silver wires 50 cm. long and about 3 mm. in diameter be twisted together at one end as in Fig. 187, and let a Bunsen flame be applied to the twisted ends. Let a match be slid slowly from the cool end of each wire toward the hot end, until the heat from the wire ignites it. The copper will be found to be the best conductor and the German silver the poorest.

In the following table some common substances are arranged in the order of their heat conductivities. The measurements have been made by a method not differing in principle from that just described. For convenience, silver is taken as 100.

251. Conduction in liquids and gases. Let a small piece of ice be held by means of a glass rod in the bottom of a test tube full of

ice water. Let the upper part of

the tube be heated with a Bunsen burner as in Fig. 188. The upper part of the water may be boiled for some time without melting the ice. Water is evidently, then, a very poor conductor of heat. The same thing may be shown more strikingly as follows: The bulb of an air thermometer is placed only a few millimeters beneath the surface of water contained in a large funnel arranged as in

FIG. 188. Water a nonconductor

Fig. 189. If now a spoonful of ether is poured on the water and set on fire, the index of the air thermometer will show scarcely any change, in spite of the fact that the air thermometer is a very sensitive indicator of changes in temperature.

Careful measurements of the conductivity of water show that it is only about 1200 that of silver. The conductivity of gases is even less, not amounting on the average to more than that of water.

1

ether on the water does not affect the air thermometer

252. Conductivity and sensation. It is a fact of common observation that on a cold day in winter a piece of metal feels much colder to the hand than a piece of wood, notwithstanding the fact that the tempera- FIG. 189. Burning ture of the wood must be the same as that of the metal. On the other hand, if the same two bodies had been lying in the hot sun in midsummer, the wood might be handled without discomfort, but the metal would be uncomfortably hot. The explanation of these phenomena is found in the fact that the iron, being a much better conductor than the wood, removes heat from the hand much more rapidly in winter, and imparts heat to the

hand much more rapidly in summer, than does the wood. In general, the better a conductor the hotter it will feel to a hand colder than itself, and the colder to a hand hotter than itself. Thus, in a cold room oilcloth, a fairly good conductor, feels much colder to the touch than a carpet, a comparatively poor conductor. For the same reason

linen clothing feels cooler to the touch in winter than woolen goods.

253. The rôle of air in nonconductors. Feathers, fur, felt, etc. make very warm coverings, because they are very poor conductors of heat and thus prevent the escape of heat from the body. Their poor conductivity is due in large measure to the fact that they are full of minute spaces containing air, and gases are the best nonconductors of heat. It is for this reason that freshly fallen snow is such an efficient protection to vegetation. Farmers always fear for their fruit trees and vines when there is a severe cold snap in winter, unless there is a coating of snow on the ground to prevent a deep freezing. The cellular structure of steam-pipe covering (Fig. 190) utilizes the nonconducting nature of air. (See opposite p. 226.)

(1)

(2)

FIG. 191. A flame will not pass through wire gauze

254. The Davy safety lamp. Let a piece of copper-wire gauze be held above an open gas jet and a match applied above the gauze. The flame will be found to burn above the gauze as in Fig. 191 (1), but it will not pass through to the lower side. If it is ignited below the gauze, the flame will not pass through to the upper side but will burn as shown in Fig. 191 (2).

The explanation is found in the fact that the gauze conducts the heat away from the flame so rapidly that the gas on the other side is not raised to the temperature of ignition.