Transmissions and Driveline • The most powerfull engine in the world is of little use unless the power from the engine can be safely and effectively transmitted to the ground. This is the primary function of the transmission and driveline. In addition to being able to transmit the torque and power from the engine, the transmission and driveline also must allow the vehicle to operate over a wide range of speeds-from a stationary to the maximum speed of the vehicle. This implies that the system must have some method of disconnecting the engine from the remainder of the driveline to allow the vehicle to remain stationary. Furthermore, the transmission also must be designed to satisfy the conflicting requirements of quick acceleration, high speed and adequate fuel economy. • Figure 6.1 illustrates the need for a transmission. This figure shows the tractive force at the wheels plotted against vehicle speed. • As shown in Fig. 6.1, a high torque multiplication is desired to accelerate the vehicle from a standstill. However, if first gear were all that were available, the maximum speed of the vehicle would be limited to 100 km/h. Although this might be an acceptable top speed for some, the penalty in fuel economy would be excessive. As shown in Fig. 6.1, the car is capable of exceeding 100 km/h by a large margin, but only by having more gear ratios available. At the other extreme, if the car had only sixth gear, any attempt to accelerate the car from a standstill inevitably would damage the engine or, at best, result in extreme clutch wear. Friction Clutches • The friction clutch is the link between the engine and the transmission, and it exists to provide the operator with the ability to engage and disengage the engine from the transmission. The clutch consists of a cover, a pressure plate, and a disc with friction facings. The cover is bolted to the engine flywheel; thus, it rotates with the engine at all times. Inside the cover is a pressure plate, which also rotates with the cover and the flywheel. Sandwiched between the pressure plate and the flywheel is the friction disc. This disc is connected to the transmission input shaft by means of splines. • Torque is transmitted to the transmission when the pressure plate is forced against the friction disc, thus squeezing the friction disc between the pressure plate and the flywheel. The axial force required to squeeze the disc is supplied by a series of springs arranged circumferentially around the pressure plate. • When the driver wishes to disengage the clutch, he or she does so by depressing the clutch pedal. Through a series of either mechanical linkages or hydraulics, this action causes the clutch fork to move against a release, or thrust, bearing. This bearing allows the axial force to be transmitted from the clutch fork to the diaphragm spring, while minimizing the wear that would exist if a stationary, solid piece of steel were used to contact the rotating spring. This action causes the diaphragm spring to pull the pressure plate away from the friction disc. At this point, no torque can be transmitted to the transmission shaft, and the clutch is disengaged. Engagement of the clutch is produced when the driver releases the clutch pedal. The diaphragm spring now returns to its unloaded position, thus applying the necessary axial force to the friction disc and allowing torque to be transmitted. • The clutch designer must consider activating force, torque delivery, energy loss, temperature rise, and wear. Thus, the friction material must be selected to provide all of the following: • A uniform coefficient of friction over the surface • A coefficient of friction that remains stable with temperature changes • Good thermal conductivity • Resistance to wear • Resistance to thermal fatigue • Good high-temperature strength Gear Theory • A gear is a rotating machine part having cut teeth, which mesh with another toothed part in order to transmit torque. Two or more gears working in tandem are called a transmission and can produce a mechanical advantage through a gear ratio and thus may be considered a simple machine. Geared devices can change the speed, torque, and direction of a power source. The most common situation is for a gear to mesh with another gear, however a gear can also mesh a non- rotating toothed part, called a rack, thereby producing translation instead of rotation. • The gears in a transmission are analogous to the wheels in a pulley. An advantage of gears is that the teeth of a gear prevent slipping. • The torque required at the driving road wheels of a vehicle is larger than the torque available at the engine flywheel. For example, the engine may develop a torque of 100N m and require a torque of 1500N m at the driving wheels. This would require a torque multiplication of 15 times the engine torque. In order to operate the vehicle it is necessary to provide some means of multiplying engine torque. Use of the gears is the most commonly used method of torque multiplication on vehicles. • Figure shows a pair of gears and their action may be compared to the action of two simple levers. The radius of each gear is related to the number of teeth on the gear. In this example the radius of the large gear may be taken as 40mm and that of the small gear as 10 mm. The gear ratio = revolutions of input gear/revolutions of output gear. In this case the small input gear must rotate four times to produce one revolution of the large output gear. The gear ratio in this case is 4:1. If the small gear is the input gear that carries a torque of 50N×10mm = 500N mm; the torque on the large gear = 50N×40mm = 2000N mm. This simple pair of gears provides a torque multiplication of 4. Straight-Tooth Spur Gears • Straight-tooth spur gears have straight teeth parallel to the axis of rotation. When the teeth engage, they do so instantaneously along the tooth face. This sudden meshing results in high impact stresses and noise. Thus, these gears have been replaced with helical gears in most transmissions. However, these gears do not generate axial (or thrust) loads along the shaft axis. Furthermore, they are easier to manufacture and can transmit high torque loads. For these reasons, many transmissions use spur gears for first and reverse gears. This accounts for the distinctive "whine" when a car is reversed rapidly. Helical Spur Gears • Helical gears have teeth that are cut in the form of a helix on a cylindrical surface. As the teeth begin to mesh, contact begins at the leading edge of the tooth and progresses across the tooth face. Although this greatly reduces the impact load and noise, it generates a thrust load that must be absorbed at the end of the shaft by a suitable bearing. Straight-Tooth Bevel Gears • These gears, have straight teeth cut on a conical surface. They are used to transmit power between shafts that intersect but are not parallel. They are used in differentials. Similar to straight-tooth spur gears, they will be noisy. However, in the differential, they rotate only when the axles are rotating at different speeds. Spiral Bevel Gears • These gears have teeth cut in the shape of a helix on a conical surface. They can be used for final drives to connect intersecting shafts Hypoid Gears • These gears have helical teeth cut on a hyperbolic surface (Fig. 6.13). They are used in final drives to connect shafts that are neither parallel nor intersecting. These gears have high tooth loads and must be lubricated with special heavy-duty hypoid gear oil because greater sliding occurs between the teeth. The sliding increases with the amount of offset between the shaft axes. Manual Transmissions • The earliest transmissions were sliding gear types, in which the gears were splined to the appropriate shafts and were engaged and disengaged by the driver. They were used universally through the late 1920s but have since been replaced in passenger cars by synchronized, constant-mesh transmissions. Figure 6.17 shows a photograph of a four forward-speed plus reverse transmission, with a schematic of the transmission. • The transmission has three shafts: the input shaft, the countershaft, and the output shaft (or mainshaft). The clutch gear (1) is an integral part of the transmission input shaft and always rotates with that shaft. The countershaft gears normally are machined from a single piece of steel and sometimes are referred to as the cluster gears. These are mounted to the countershaft on roller bearings, and the countershaft is fixed in place so it does not rotate. The gears on the output shaft, called speed gears, also are mounted on roller bearings. They are always meshed with the cluster gears and continuously rotate around the main shaft. The speed gears are locked onto the main shaft by the action of the synchronizers and, when locked, transmit torque to the output shaft. Automatic Transmissions • Just like that of a manual transmission, the automatic transmission's primary job is to allow the engine to operate in its narrow range of speeds while providing a wide range of output speeds. • The key difference between a manual and an automatic transmission is that the manual transmission locks and unlocks different sets of gears to the output shaft to achieve the various gear ratios, while in an automatic transmission, the same set of gears produces all of the different gear ratios. The planetary gearset is the device that makes this possible in an automatic transmission. The Planetary Gearset • Any planetary gearset has three main components: • The sun gear • The planet gears and the planet gears' carrier • The ring gear • Each of these three components can be the input, the output or can be held stationary. Choosing which piece plays which role determines the gear ratio for the gearset. Planetary gear sets have several advantages over compound gear trains, as follows: 1. They are strong and compact. Because the load is distributed over many teeth, individua tooth loading is less than that experienced by a conventional gear train. 2. The gears are in constant mesh, which eliminates the risk of damage due to engagemen or disengagement. 3. All elements rotate around the same central axis. This provides advantages in packaging choice of output element, lubrication, and control. 4. Gear ratios can be changed with no interruption in torque transfer. Torque Converter • Automatic transmission cars use a torque converter. • A torque converter is a type of fluid coupling, which allows the engine to spin somewhat independently of the transmission. If the engine is turning slowly, such as when the car is idling at a stoplight, the amount of torque passed through the torque converter is very small, so keeping the car still requires only a light pressure on the brake pedal. • In a torque converter there are at least three rotating elements: the impeller, which is mechanically driven by the prime mover; the turbine, which drives the load; and the stator, which is interposed between the impeller and turbine so that it can alter oil flow returning from the turbine to the impeller. • Torque converter has three stages of operation: • Stall. The prime mover is applying power to the impeller but the turbine cannot rotate. For example, in an automobile, this stage of operation would occur when the driver has placed the transmission in gear but is preventing the vehicle from moving by continuing to apply the brakes. At stall, the torque converter can produce maximum torque multiplication if sufficient input power is applied (the resulting multiplication is called the stall ratio). The stall phase actually lasts for a brief period when the load (e.g., vehicle) initially starts to move, as there will be a very large difference between pump and turbine speed. • Acceleration. The load is accelerating but there still is a relatively large difference between impeller and turbine speed. Under this condition, the converter will produce torque multiplication that is less than what could be achieved under stall conditions. The amount of multiplication will depend upon the actual difference between pump and turbine speed, as well as various other design factors. • Coupling. The turbine has reached approximately 90 percent of the speed of the impeller. Torque multiplication has essentially ceased and the torque converter is behaving in a manner similar to a simple fluid coupling. In modern automotive applications, it is usually at this stage of operation where the lock-up clutch is applied, a procedure that tends to improve fuel efficiency. Continuously variable transmission • A continuously variable transmission (CVT) is a transmission that can change steplessly through an infinite number of effective gear ratios between maximum and minimum values. This contrasts with other mechanical transmissions that offer a fixed number of gear ratios. The flexibility of a CVT allows the driving shaft to maintain a constant angular velocity over a range of output velocities. This can provide better fuel economy than other transmissions by enabling the engine to run at its most efficient revolutions per minute (RPM) for a range of vehicle speeds. Direct-Shift Gearbox • The Direct-Shift Gearbox is an electronically controlled dual clutchmultiple-shaft manual gearbox, in a transaxle design - without a conventional clutch pedal, and with full automatic, or semi-manual control. • The internal combustion engine drives two clutch packs.The outer clutch pack drives gears 1, 3, 5 (and 7 when fitted), and reverse— the outer clutch pack has a larger diameter compared to the inner clutch, and can therefore handle greater torque loadings. The inner clutch pack drives gears 2, 4, and 6.Instead of a standard large dry single-plate clutch, each clutch pack for the six-speed DSG is a collection of four small wet interleaved clutch plates (similar to a motorcycle wet multi-plate clutch). Due to space constraints, the two clutch assemblies are concentric, and the shafts within the gearbox are hollow and also concentric.Because the alternate clutch pack's gear-sets can be pre-selected(predictive shifts enabled via the 'unused' section of the gearbox), un-powered time while shifting is avoided because the transmission of torque is simply switched from one clutch-pack to the other.T his means that the DSG takes only about 8 milliseconds to upshift. In comparison, the sequential manual transmission (SMT) in the Ferrari F430 Scuderia takes 60 milliseconds to shift, or 150 milliseconds in the Ferrari Enzo.
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