Transmission
The internal combustion engine used on a vehicle operates over a limited effective speed range of 1500-5000 rpm. At low engine speed, a reciprocating-piston engine does not develop sufficient turning-effort or torque to propel a vehicle forward from standstill. Even the greater torque produced at higher engine speed would be insufficient to accelerate the vehicle at a reasonable rate. The gearbox provides a way of varying the engine's output torque and speed to match the vehicle's speed and load. This chapter presents various transmission systems used in automobiles. Both manual and automatic transmission systems, and fluid couplings and torque converters are discussed. Latter part of the chapter includes electronically controlled semi-automatic and fully automatic transmissions. For better understanding some descriptions of mechanical components have been repeated in brief under electronically controlled automatic transmissions.25.1.
Need for a Gearing System and Determination of Gear Ratios
In order to achieve a high maximum vehicle speed, combined with good acceleration and economy over the whole speed range, a gearing system is required, which permits the engine to operate at the speeds corresponding to its best performance. Maximum engine power, torque and economy all occur at different engine speeds. As a result it becomes difficult to match the gear ratio for best performance, especially when variable operating conditions and driver demands are also to be considered. The engine requirement to suit a given operating condition is as follows.| Operating condition | Engine requirement |
| Maximum traction Maximum vehicle speed Maximum acceleration Maximum economy | Maximum engine torque Maximum engine power Maximum engine torque Engine at mid-range speed and under light load with a small throttle opening. |
Maximum Vehicle Speed.
Maximum vehicle speed is attained when the gear is set in top and the throttle is held fully open. A ratio of 1: 1 (direct drive) is chosen for top gear to keep the friction losses to minimum value.Consequently, the setting of top gear becomes the choice of a final drive ratio to suit the diameter of road wheel and engine characteristic.
Fig. 25.1. Power balance. A. Power required for driving the vehicle. B. Power available to drive the vehicle.
C. Balance between power available and power required.
Figure 25.1 illustrates the balance between the power required and the power available. Data for the power required are obtained from the brake power curve of the engine, and for the power available are based on the calculation of the power needed to overcome the tractive resistance of the vehicle when it is moving along a level road.
The tractive resistance, sometimes called total resistance, includes :
(a) Air resistance which is due to movement of the vehicle through the air.
(6) Rolling resistance which is due to friction between the tyre and road, and largely influenced by the type of road surface.
(c) Gradient resistance occurs when the weight of the vehicle acts against the vehicle motion during movement up a hill.
The power needed to propel a vehicle (Fig. 25.1A) increases with the cube of the speed. In this example, a power of 150 kW is needed to drive the vehicle at 200 km/h. The power output curve of the engine installed in this vehicle (Fig. 25.IB) indicates that the engine produces a peak brake power of 150 kW at 5000 rpm. To attain maximum road speed, the overall gear ratio {i.e. gear box ratio x final drive) of this vehicle must be set so that the peak of the power 150 kW occurs at a road speed of 200 km/h and an engine speed of 5000 rpm.
Once the relative positions of the two curves have been established, the vertical difference between the two curves gives the surplus power available for acceleration. This can be plotted as a separate curve to show the speed at which maximum acceleration is achieved.
If friction is neglected, the power output from a transmission system is similar to the engine brake power irrespective of the gear ratio. Therefore, a change in the gear ratio of the vehicle shown in Fig. 25.2 causes the peak power, P to move horizontally from the position if occupied in Fig. 25.1C. Lowering or raising the gear ratio moves the power available curve to the left (curve A) or to the right (curve C). These two conditions are called under-gear and over-gear respectively.
Fig. 25.2. Under-gear and over-gear.
In both of these gearing conditions the maximum possible speed is reduced. But, when compared with the optimum gearing needed to obtain the ideal maximum speed, the advantages of these two situations are as follows.
Undergear :
Since more power is available for acceleration in under-gearing, vehicle is livelier. Top gear performance being flexible, less gear changing is necessary when the vehicle encounters higher tractive resistances.Overgear:
Due to lower engine speed for a given road speed, better economy, lower engine noise level and less engine wear are achieved in over-gearing. A comparison of these two conditions indicates that under-gearing is more suitable for the average car, and hence under-gearing to the extent of about 10-20 percent is quite common. Therefore, the engine power peak occurs during 10-20 percent prior to the attainment of the maximum possible vehicle speeds.Maximum Traction.
Once the overall top gear ratio in set, the bottom gear (1st gear) is then decided. This gear is used when vehicle starts and is also needed when maximum tractive effort is required. Since tractive effort depends on the engine torque, the maximum tractive effort in a particular gear occurs when the engine delivers its maximum torque. The top gear performance, which was previously plotted as a difference in power in Fig. 25.3A, now indicates as a balance of forces. The driving force curve is similar in shape to the engine torque curve. The peak of the tractive effort curve occurs at a road speed controlled by the overall gear ratio and effective diameter of the road wheel. The difference between the effort and resistance curves represents the force available for acceleration.Figure 25.3B represents the effect of lowering the gear ratio on the tractive effort curve. A bottom gearbox ratio of 4:1 is used to produce sufficient tractive effort to meet the hill-climbing requirement. The gradual engagement of the clutch is necessary for sufficient building up of tractive effort. Once the clutch is fully engaged, and the engine is operating in the region of maximum torque, a small acceleration is possible provided the engine speed does not drop too low. The bottom gearbox ratio is obtained by the ratio of the maximum effort required and the maximum effort available in top gear. . vE
Fig. 25.3. Tractive effort curves.
Intermediate Gear(s)
Once the top and bottom gear ratios are set, the intermediate ratios are then determined to form geometric progression (GP). Therefore, all the individual ratios advance by common ratio. For example, if the top and bottom overall ratios are 4:1 and 16:1 respectively, then the sets of overall ratios for the 3 and 4 speed gearbox are 4, 8 and 16 (common ratio 2) and 4, 6.35, 10 and 16 (common ratio 1.59) respectively.For optimum speed and acceleration performance, the engine should be operated in the speed range between the limits of maximum torque and maximum power. The wider this operating range, the smaller is the number of gear ratios required. Most modern car engines have a narrow range, so gearboxes fitted in conjunction with these engines normally have at least four forward ratios.
Since most cars are under-geared, it is now common to use an extra gear, called a fifth gear, to offset some of the disadvantages associated with the under-gear condition. Normally, this gear is an overdrive, which is a ratio that drives the output shaft faster than the engine. Typical gear ratios for four and five speed gearboxes are as follows.
Fig. 25.4. Terminology of gear-tooth profile.
