A vehicle with rear-wheel drive has the components shown in Fig. 6.1. For purposes of discussion, we assume that the mechanical transmission has four gear meshes. (Functionally, a mechanical transmission can be thought of as a black box with gears inside. The meshing of these gears gives an output shaft speed lower than the input shaft speed. A specific gear mesh gives a specific ratio of input and output shaft speeds.) The engine can deliver torque over a given range; it will stall if the load requires torque above some maximum. A gear mesh is selected to match required output torque to available input torque. The output torque establishes the force the vehicle can exert as it moves. This force is often referred to a s drawbar pull, but it can also be a push as when a bulldozer is pushing material. Typical curves for the vehicle shown in Fig. 6.1 are given in Fig. 6.2. In gear one, tremendous force is developed, but forward speed is low. In fourth gear, the speed is high, but developed force is much reduced. Power is force times velocity. A transmission does not increase power; power is set by the engine.
The clutch disconnects the engine output shaft from the driveline so that the gears in the transmission can be shifted to a new mesh. With a manual clutch, the operator manually engages the clutch and manually shifts the transmission to a new gear mesh.
An automatic transmission has a series of fixed gear ratios. Depending on the design of transmission, these gear ratios each have their own unique clutch or combination of clutches. When a given clutch (or combination of clutches) is activated, the corresponding gear ratio is placed in the drive. The clutches (or combinations of clutches) are activated in appropriate sequence to move โupโ through the gears. As the next clutch (or combinations of clutches) is activated, the previous clutch is deactivated. The power is transmitted through as little as one mesh, but possibly through multiple meshes, at any given time.
As engine speed increases, the torque the engine can deliver decreases. A typical curve is shown in Fig. 6.3. Shifting from first gear to second gear can produce an output power curve as shown in Fig. 6.4. In this example, the shift is made when vehicle speed is 2.1 mph. At this shift point, output power is 72 hp. The potential power line shows that the potential output power is 89 hp. The shaded area in Fig. 6.4 is referred to as a power hole. If enough gears are provided, the power holes can be minimized, and the actual output power will closely approximate the potential output power line (Fig. 6.5).
The cost of an automatic transmission increases as the number of gear meshes and clutches increases. It is desirable to use as few shifts as possible to obtain a suitable output power curve. A torque converter is placed between the engine and the automatic transmission (Fig. 6.6) to reduce the number of transmission shifts required. Torque converters have been designed to transmit hundreds of horsepower.
