Bustion Engines

In actual practice, therefore, transmissions are a compromise be tween the ideal and the limiting factors.

or a single spring multiplied by levers fulcrumed in the fly-wheel or pressure plate, and acting against the other members.

General Requirements for Transmission.

"Best car per t ormance" requires a power plant capable of driving the road wheels to the verge of slipping during acceleration and at maxi mum car speed. An internal combustion engine could be built pow erful enough to approach these conditions; such an engine would have a slipping clutch, a properly selected rear axle ratio, and it would drive the wheels directly. Considerations of weight, space limitations, cost, economy of operation, and sometimes excessive taxation, however, prohibit the use of such a large power plant, making it necessary to use a smaller engine.

An internal-combustion engine delivers its maximum power at a definite engine speed. Therefore, for best performance it is de sirable to maintain this definite engine speed constant regardless Gear-set, Gear-box, (Gear) Transmission.—The most com mon type of transmission is called gear-set, gear-box, or gear trans mission, and comprises a housing or case with shafts and bearings to carry the necessary gears, usually of the helical type. In the greater number of transmissions the gears are so arranged that by proper selection either three or four ratios of engine speed to car speed for forward driving, and one ratio for reverse driving, may be obtained. A typical three-speed forward and reverse transmission is shown in sectional view in fig. 17, in which a is the driving shaft and clutch member, h is the driven or spline shaft, i is the countershaft, b-e, c-f are constant mesh gears. The shaft i is stationary and gears e, f and g, fixed together, rotate on shaft i, whenever the clutch is engaged and the engine running. To engage the low gear, the shifter lever is moved so that gears g and d are brought into mesh. The torque is then trans mitted from gear b to gear e and through gears g and d to the spline shaft and universal joints to rear axles and road wheels. When in low

gear, the engine revolutions are from 9 to 16 times the revolutions of the road wheels, a condition which causes the vehicle to move forward slowly, but with considerable force, as is required when starting, climb ing a steep hill or pulling through sand or mud. To engage second, or intermediate gear, the gear shifter handle is moved so that gears c and f transmit power. When in second gear, the engine makes from 6 to 12 revolutions to i revolution of the road wheels, a condition which causes the vehicle to move faster than when using low gear, but with less force. To engage direct gear, the internal clutch teeth in c engage the external clutch teeth on b, and the torque is transmitted directly to the main spline shaft and on through to the road wheels. When in direct, or high, gear, the engine makes 3 to 6 revolutions to i revolu tion of the road wheels, a condition which gives high speed and best economy. To reverse the vehicle, sliding gear d is moved into en gagement with an idler gear. Torque is then transmitted from shaft i to the idler gear and from the idler gear to gear d, which drives shaft h in a reverse direction.

Other Types.—The planetary transmission, no longer used in Ameri can cars, employs a sun gear and a series of planet gears rotating about it. Sets of planetary gears are used widely in overdrives and automatic transmissions. Automatic or semi-automatic transmissions, in which the gears are shifted automatically or semi-automatically ac cording to speed, throttle position, and/or torque requirements of the vehicle, also are found. The fluid flywheel, used to supplement the conventional transmission or in conjunction with automatic geared transmissions, consists of two vaned rotors enclosed in a housing filled with oil or other liquid. To reduce engine speeds at high car speeds overdrives—giving a fourth car speed--are used widely. When in operation, the speed ratio of the engine crankshaft to propeller shaft is approximately 0.7 : I.