plied 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 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 form most commonly used) 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 and Fig. 185) 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. 185 (1) the gears are in neu

tral, gears 1 and 2 being always

in mesh. By use of the gear-shift FIG. 185. Automobile transmission

lever (14) gears 3 and 5 (Fig. 185) 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. 185 (2) shows the low-speed connection. In shifting to second speed (Fig. 185 (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. 185 (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. 185 (5)) an eighth gear simultaneously engages 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.

Left rear axle

Driving shaft
from engine

-Drive pinion

B

Right rear axle

246. 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 turning corners the outer wheel may revolve faster than the inner. It will be seen from the large drawing opposite page 210, and from Fig. 186, that the pinion attached to the driving shaft rotates the main bevel gear B, to which are rigidly attached the differential gears 1 and 2. The left axle is directly connected to gear 3, and only indirectly connected to the main bevel gear B through gears 1 and 2. In running straight both rear wheels revolve at the same rate; therefore, while gears 3 and 4 and the main bevel gear are revolving at the same speed they carry around with them pinions 1 and 2, which are now, however, not revolving on their bearings. When the car is turning a corner, gears 3 and 4 are turning at different rates; hence pinions 1 and 2 are not only carried around by the main bevel gear but at the same time are revolved in opposite directions on their bearings.

To 3 wheel

Tó wheel

FIG. 186. The differential

247. The carburetor. The carburetor is a device for converting liquid gasoline, kerosene, etc. into spray and mixing it with air in proper proportions for complete combustion. The simple principle of carburetion is shown in the upper diagram opposite page 211. Liquid gasoline comes through the supply pipe and enters the float chamber through the valve V. By acting on the levers L the float closes the valve V when the gasoline reaches a certain level. From the float chamber the gasoline passes to the spray nozzle O. While the engine is running, the downward movement of the pistons in stroke (1) (Fig. 184) causes air to move swiftly past the spray nozzle into the region called the venturi, where the jet of gasoline is emerging from O. The spray of fuel thus formed intermingles with air in the mixing chamber and passes by the throttle to the engine as a highly explosive mixture.

248. The ignition. The lower diagram opposite page 211 illustrates the principle of battery ignition which is in extensive use on automobiles. A low-tension current, usually of from 6 to 8 volts, passes from a storage battery through the primary coil of an induction coil, through a moving contact, and thence through the framework of the car to the battery. While the engine is running a rotor (see diagram) makes successive contacts with the 4 or more terminals connected to the 4 or more spark plugs. The mechanism is so timed, or adjusted, that at the right instant for the explosive mixture to be ignited, and while the rotor is touching the proper rotor contact, the cam separates the grounded moving contact from the stationary insulated contact, thus breaking the primary circuit to produce a momentary high-tension current in the secondary of the induction coil. In this way a spark is produced at the terminals of each spark plug as shown in Fig. 184 (3). As many rotor contacts and spark plugs are employed as there are cylinders in the engine.

Since the power stroke of the piston occurs but once in two revolutions of the crank shaft, it is necessary that the crank shaft revolve twice while the cam shaft revolves but once. This, as shown in 2 of the diagram opposite page 210, is accomplished by having the crank shaft geared to the cam shaft in a velocity ratio of 2 to 1.

The explosive mixture requires a very short but measurable time for combustion; hence the full force of the explosion occurs a short time after the spark ignites the mixture. Therefore, at high speed the spark should occur a little earlier with reference to

the position of the piston than at low speed. The spark is usually advanced or retarded by slightly rotating the plate to which the moving and stationary contacts are attached.

249. The Diesel and the semi-Diesel engine. The Diesel engine is a form of internal-combustion engine which depends for ignition of the fuel upon the heat developed by very high compression of air within its cylinder. Into this highly compressed and very hot air the oil spray is injected as a stream under still higher pressure during the first part of the power stroke and burns non-explosively, maintaining during this part of the stroke a pressure which is practically of constant value. The pressure then falls off during the remainder of the stroke.

In the semi-Diesel engine (shown on opposite page) the air is not compressed to as many hundred pounds to the square inch as in the full-Diesel type and the oil burns much more rapidly, taking on to some extent the characteristics of an explosion. In the diagram two valves (the air valve and the exhaust valve) are shown immediately above the small compartment called the vaporizer. During the "suction" stroke to the right a cylinderful of air is driven in through the air valve by the outside pressure and then compressed into the vaporizer during the return stroke. Near the instant of maximum compression atomized oil is forced into the intensely hot air, where it ignites and causes the power stroke. Semi-Diesel engines are widely used because they are reliable, simple in construction, and combine many of the best features of both the Diesel engine and the explosive gas engine.

The full-Diesel engine is more complex, but is also reliable, economical, and adapted to a wide range of fuels from kerosene through heavy oils to tar spray. They are used on submarines and for a great variety of purposes on land, and are increasingly coming into use on large merchant ships. They may, to a large extent, replace steam locomotives, especially for short hauls and for use in arid regions. The Gripsholm, a 23,500-ton ship of the Swedish-American Line is driven by