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move apart and upward and in so doing operate a valve which reduces the speed by partially shutting off the supply of steam from the cylinder. 239. Compound engines. In an engine which has but a single cylinder the full force of the steam has not been spent when the cylinder is opened to the exhaust. The waste of energy which this entails is obviated in the compound engine (see Fig. 311) by allowing the partially spent steam to pass into a second cylinder of larger area than the first. The most efficient of modern engines have three and sometimes four cylinders of this sort, and the engines are accordingly called triple or quadruple expansion engines. Fig. 185 shows the relation between any two successive cylinders of a cross-compound engine. By automatic devices not differing in principle from the eccentric, valves C1, D2, and E2 open simultaneously and thus permit steam from the boiler to enter the small cylinder A, while the partially spent steam in the other end of the same cylinder passes through D2 into B, and the more fully exhausted steam in the upper end of B passes out through E2. At the upper end of the stroke of the pistons P and P', C1, D2, and E2 automatically close, while C2, D1, and E1 simultaneously open and thus reverse the direction of motion of both pistons. These pistons are attached to the same shaft.
FIG. 185. Cross-compound engine cylinders
240. Efficiency of a steam engine. We have seen that it is possible to transform completely a given amount of mechanical energy into heat energy. This is done whenever a moving body is brought to rest by means of a frictional resistance. But the inverse operation, namely, that of transforming heat energy into mechanical energy, differs in this respect, that it is only a comparatively small fraction of the heat developed by combustion which can be transformed into work. For it is not difficult to see that in every steam engine at least a part of the heat must of necessity pass over with the exhaust steam into the condenser or out into the atmosphere. This loss is so
great that even in an ideal engine not more than about 23% of the heat of combustion could be transformed into work. In practice the very best condensing engines of the quadrupleexpansion type transform into mechanical work not more than 17% of the heat of combustion. Ordinary locomotives utilize at most not more than 8%. The efficiency of a heat engine is defined as the ratio between the heat utilized, or transformed into work, and the total heat expended. The efficiency of the best steam engines is therefore about 13, or 75%, of that of an 17 ideal heat engine, while that of the ordinary locomotive is only about, or 26%, of the ideal limit.
241. Principle of the internal-combustion engine. Let two iron or steel wires be pushed through a cork stopper and their ends s brought near together (1/32 inch will do) (Fig. 186). With an atomizer spray into the bottle a small amount of benzine or gasoline (the amount to use can be determined by trial), insert the stopper, and bring the tips of the heavily insulated wires leading from an induction coil to the underside of the wires a, b. A spark will pass at s; and, if the mixture is not too "rich" or too "lean," a violent explosion will occur, throwing the stopper as high as the ceiling. (A heavy round bottle must be used for safety. Wrap it well in wire gauze.)
FIG. 186. A mixture of gasoline vapor and air will explode
Within the last two decades gas engines have become quite as important a factor in modern life as steam engines. (See opposite pp. 190, 191, and 198.) Such engines are driven by properly timed explosions of a mixture of gas and air occurring within the cylinder.
Fig. 187 is a diagram illustrating the four stages into which it is convenient to divide the complete cycle of operations which goes on within such an engine. Suppose that the heavy flywheel W has already been set in motion. As the piston p moves down in the first stroke (see 1) the valve D
opens and an explosive mixture of gas and air is drawn into the cylinder through D. As the piston rises (see 2) valve D closes, and the mixture of gas and air is compressed into a small space in the upper end of the cylinder. An electric spark ignites the explosive mixture, and the force of the explosion drives the piston violently down (see 3). At the beginning of the return stroke (see 4) the exhaust valve E opens, and as the piston moves up, the spent gaseous products of the explosion are forced out of the cylinder. The initial condition is thus restored and the cycle begins over again.
Since it is only during the third stroke that the engine is receiving energy from the exploding gas, the flywheel is always made very heavy so that the energy stored up in it in the third stroke may keep the machine running with little loss of speed during the other three parts of the cycle.
FIG. 187. Principle of the gas engine
The efficiency of the gas engine is often as high as 25%, or nearly double that of the best steam engines. Furthermore, it is free from smoke, is very compact, and may be started at a moment's notice. On the other hand, the fuel (gas or gasoline) is comparatively expensive. Most automobiles are run by gasoline engines, chiefly because the lightness of the engine and of the fuel to be carried are here considerations of great importance.
It has been the development of the light and efficient gas engine which has made possible man's recent conquest of the air through the use of the airplane and airship.
242. The automobile. The plate opposite page 198 shows the principal mechanical features of the automobile in their relation to one another. It will be seen that the cylinders
of the engine are surrounded by water jackets which form part of a circulating system. The heat of the engine is carried by convection currents in this water to the radiator, where it is lost to the atmosphere through the air currents produced in part by a revolving fan (10). Unless some means were provided for cooling a gas engine, it would become so overheated that the pistons would stick fast. The power of the engine is transmitted to the rear axle through the clutch (11), the transmission (12), and the differential gearing.
First (Low Speed)
Second (Intermediate Speed)
243. The clutch and the transmission. Since a gas engine develops its power by a series of violent explosions within the cylinders, it is clear that it cannot start with a load as does the steam engine. In starting an automobile it is first necessary that the engine acquire a reasonable speed and that the power be applied gradually to the rear axle by the use of a friction clutch (11); otherwise the engine will stall. The shaft of the engine has upon its rear end a flywheel which, in the cone clutch, is turned to a conical shape inside. Close to this but attached to the transmission shaft is the clutch plate,
a heavy disk faced with leather, which FIG. 188. Automobile transmission
Third (High Speed)2
fits the inside of the flywheel and is pressed into it by a spring sufficiently strong to prevent any slipping when the clutch is engaged. The driver throws out the clutch by depressing a lever with his foot. In the disk clutch the bearing surfaces are two series of disks, one revolving with the engine shaft, the other with the transmission.
The amount of work done by a gas engine in a minute depends upon the work done by each explosion multiplied by the number of explosions per minute. Therefore it can develop its full power only while revolving rapidly. In hill climbing, for example, the speed of the engine must be great while that of the car is comparatively small. To meet this requirement a system of reduction gears called the transmission (12) is used to make the number of revolutions of the driving shaft less than that of the crank shaft (4) of the engine. In Fig. 188, (1), the gears are in neutral, gears 1 and 2 being always in mesh. By use of the gear-shift lever (14) gears 3 and 5 (Fig. 188) are made to slide upon a square shaft. Before shifting the gears the clutch is released to disconnect the power of the motor from the driving shaft; and, to avoid a clash when meshing the gears on the transmission shaft with those on the countershaft, care should be taken that they revolve at about the same speed. Fig. 188, (2), shows the low-speed connection. In shifting to second speed (Fig. 188, (3)) the clutch is released, gear 5 is thrown into neutral, and finally gear 3 is meshed with 4, after which the clutch is allowed to grip. In going to high speed (Fig. 188, (4)) gear 3 is shifted through neutral to engagement with gear 1. This connects the crank shaft of the engine directly to the driving shaft so that the two revolve at the same speed. For the reverse (Fig. 188, (5)) an eighth gear is thrown up from beneath so as simultaneously to engage 5 and 7. Such an interposition of a third gear wheel between 5 and 7 obviously reverses the direction of rotation of the driving shaft.
244. The differential. An automobile is driven by power applied to the rear axle. This requires the axle to be in two parts with a differential between, so that in wheel may revolve faster than the inner. large drawing opposite page 198, and from
Right Rear axle
FIG. 189. The differential
turning corners the outer It will be seen from the Fig. 189, that the pinion