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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.

245. The carburetor. The carburetor is a device for converting liquid gasoline, kerosene, etc. into vapor and mixing it with air in proper proportions for complete combustion. The simple principle of carburetion is shown in the upper diagram opposite page 199. 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 is drawn to the spray nozzle O. While the engine is running, the downward movement of the pistons in stroke 1 (Fig. 187) sucks air violently 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.

246. The ignition. The lower diagram opposite page 199 illustrates the principle of high-tension magneto ignition which is widely used on automobiles. A rolling contact R is mounted on the cam shaft, which revolves at half crank-shaft speed and is carried around the interior of the stationary fiber ring D. When the switch S is closed and the roller R passes across the metal segment G, a current of electricity passes from the magneto through the rolling contact to the central shaft C, and from there through the iron work of the car to the magneto by way of the primary coil of the induction coil. While the roller is in contact with the segment G the induction coil produces a shower of sparks between the points P of the spark plug, thus igniting the explosive mixture in the cylinder of 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 contact revolves but once. This, as shown in the diagram, is accomplished by having the crank shaft geared to the cam shaft in a velocity ratio of 2 to 1.

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1, radiator; 2, timing gears; 3, pistons; 4, crank shaft; 5, valve stems and push rods; 6, oil reservoir; 7, gasoline tank; 8, flywheel; 9, main rear bearing; 10, cooling fan; 11, clutch; 12, transmission; 13, universal joints; 14, gear-shift lever; 15, main driving gear and pinion; 16, electric control switch; 17, emergency brake lever; 18, service brake foot lever; 19, storage battery; 20, vacuum feed system; 21, muffler; 22, steering wheel

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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 advanced or retarded by a spark lever L which changes the position of the segment G by pulling around slightly the movable fiber ring to which it is attached.

The diagram applies to a one-cylinder engine. In case the engine has four cylinders, three additional segments must be added, as indicated by the clear spaces, together with three additional induction coils and spark plugs.*


247. The steam turbine. The steam turbine represents the latest development of the heat engine. In principle it is very much like the common windmill, the chief difference being that it is steam instead of air which is driven at a high velocity against a series of blades arranged radially about the circumference of the wheel that is set into rotation. The steam, however, unlike the wind, is always directed by nozzles at the angle of greatest efficiency against the blades (see Fig. 190). Furthermore, since the energy of the steam is far from spent after it has passed through one set of blades (such as that shown in Fig. 190), it is in practice always passed through a whole series of such sets (Fig. 191), every alternate row of which is rigidly attached to the rotating shaft, while the intermediate rows are fastened to the immovable outer jacket of the engine and only serve as guides to redirect the steam at the most favorable angle against the next row of movable blades. In this way the steam is kept alternately bounding from fixed to movable blades until its energy is expended. The number of rows of blades is often as high as sixteen.

FIG. 190. The principle of the steam turbine

*The pupil may well consult the more extended treatises for actual details of the many different systems of ignition used on automobile and airplane engines.

Turbines are at present coming rapidly into use, chiefly for largepower purposes. Their advantages over the reciprocating steam engine lie first in the fact that they run with almost no jarring, and therefore require much lighter and less expensive foundations, and second in the fact that they occupy less than one tenth the floor space of ordinary engines of the same capacity. Their efficiency is fully as high as that


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of the best reciprocating engines. The highest speeds attained by vessels at sea, namely, about 40 miles per hour, have been made with the aid of steam turbines. One of the largest vessels which have thus far been launched, the Berengaria, 919 feet long, 98 feet wide, 100 feet high (from the keel to the top of her ninth deck), having a total displacement of 57,000 tons and a speed of 221 knots, is driven by four steam turbines having a total horse power of 61,000. One of the immense rotors contains 50,000 blades and develops 22,000 horse power. The United States Shipping Board, on July 24, 1919, announced plans for

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