The gasoline engine must be disconnected from the driving wheels when it is started and when idling. This characteristic necessitates some type of unloading and engaging device to permit gradual application of load to the engine after it has been started. The torque, or turning effort, that the engine is capable of producing is low at low crankshaft speeds, increasing to a maximum at some fairly high speed representing the maximum, or rated, horsepower.

The efficiency of an automobile engine is highest when the load on the engine is high and the throttle is nearly wide open. At moderate speeds on level pavement, the power required to propel an automobile is only a fraction of this. Under normal driving conditions at constant moderate speed, the engine may operate at an uneconomically light load unless some means is provided to change its speed and power output.

The transmission is such a speed-changing device. Installed in the power train between the engine and the driving wheels, it permits the engine to operate at a higher speed when its full power is needed and to slow down to a more economical speed when less power is needed. Under some conditions, as in starting a stationary vehicle or in ascending steep grades, the torque of the engine is insufficient, and amplification is needed. Most devices employed to change the ratio of the speed of the engine to the speed of the driving wheels multiply the engine torque by the same factor by which the engine speed is increased.

The simplest automobile transmission is the sliding-spur gear type with three or more forward speeds and reverse. The desired gear ratio is selected by manipulating a shift lever that slides a spur gear into the proper position to engage the various gears. A clutch is required to engage and disengage gears during the selection process. The necessity of learning to operate a clutch is eliminated by an automatic transmission. Most automatic transmissions employ a hydraulic torque converter, a device for transmitting and amplifying the torque produced by the engine. Each type provides for manual selection of reverse and low ranges that either prevent automatic upshifts or employ lower gear ratios than are used in normal driving. Grade-retard provisions are also sometimes included to supply dynamic engine braking on hills. Automatic transmissions not only require little skill to operate but also make possible better performance than is obtainable with designs that require clutch actuation.

In hydraulic transmissions, shifting is done by a speed-sensitive governing device that changes the position of valves that control the flow of hydraulic fluid. The vehicle speeds at which shifts occur depend on the position of the accelerator pedal, and the driver can delay upshifts until higher speed is attained by depressing the accelerator pedal further. Control is by hydraulically engaged bands and multiple-disk clutches running in oil, either by the driver’s operation of the selector lever or by speed- and load-sensitive electronic control in the most recent designs. Compound planetary gear trains with multiple sun gears and planet pinions have been designed to provide a low forward speed, intermediate speeds, a reverse, and a means of locking into direct drive. This unit is used with various modifications in almost all hydraulic torque-converter transmissions. All transmission control units are interconnected with vehicle emission control systems that adjust engine timing and air-to-fuel ratios to reduce exhaust emissions.

More From Britannica

mass transit: The automobile and mass transportation

Oil in the housing is accelerated outward by rotating vanes in the pump impeller and, reacting against vanes in the turbine impeller, forces them to rotate, as shown schematically in the figure. The oil then passes into the stator vanes, which redirect it to the pump. The stator serves as a reaction member providing more torque to turn the turbine than was originally applied to the pump impeller by the engine. Thus, it acts to multiply engine torque by a factor of up to 2 1/2 to 1.

Blades in all three elements are specially contoured for their specific function and to achieve particular multiplication characteristics. Through a clutch linkage, the stator is allowed gradually to accelerate until it reaches the speed of the pump impeller. During this period torque multiplication gradually drops to approach 1 to 1.

The hydraulic elements are combined with two or more planetary gear sets, which provide further torque multiplication between the turbine and the output shaft.

Continuously (or infinitely) variable transmissions provide a very efficient means of transferring engine power and, at the same time, automatically changing the effective input-to-output ratio to optimize economy by keeping the engine running within its best power range. Most designs employ two variable-diameter pulleys connected by either a steel or high-strength rubber V-belt. The pulleys are split so that effective diameters may be changed by an electrohydraulic actuator to change the transmission ratio. This permits the electronic control unit to select the optimum ratio possible for maximum fuel economy and minimum emissions at all engine speeds and loads. Originally these units were limited to small cars, but belt improvements have made them suitable for larger cars.

Other mechanical subsystems