In this respect it is unlike the steam engine, which has a much wider range of effectiveness. Since all cars now use internal combustion motors, and throttling is unsatisfactory, as a means of controlling the engine, or changing the speed and power, so as to use it economically, a mechanical speed change system is essential.
This contains certain gears, which are designed to change the speed of the transmission shaft relative to the engine shaft.
Transmission Leverage.—It is simply using leverage in order to produce a more effective pull, or to attain greater speed, from a shaft which runs at a certain number of revolutions.
If we have a motor with a shaft speed of, say, 800 revolutions per minute, and an axle with a speed of 400 revolutions, the ratio would be 2 to 1. Now, to speed up the machine, so that the axle will turn 800 revolutions, would require an engine speed of 1600, which might be impossible.
Fig. 42. Progressive Transmission. Low.
Economy of Transmission Gearing.—From an economical standpoint, also, it would be undesirable, even though the engine should be able to make the speed.
Owing to the explosion impulses of the gasoline motor, a heavy fly wheel is necessary on the engine shaft, in order to store up power by momentum, and also to give a uniform speed.
In hill climbing, or in carrying heavy loads, the transmission shaft must have its speed cut down, while permitting the engine to run at full or normal speed.
