Centuries of vehicle development

At a very early stage in human history, it was realized by the more ingenious members of the species that the mobility to mankind provided by nature had some limitations. The human body was severely limited as to the loads it could carry, the distance it could carry them and the speed at which it could travel, even without a load. Furthermore, it is a safe guess that physical exertion was no more enjoyable then than it is today.

Major progress was achieved with the taming and training of suitable animals, which enabled heavier loads to be carried greater distances, often at greater speeds than the human was capable of attaining. There was, of course, the added advantage that most of the effort was provided by the animal, while the human being could travel at his ease, in comfort.

Heavy loads were dragged upon sledges until the next major development occurred, which was when an enterprising but unknown early engineer invented the wheel. The wheel made it possible to construct crude carts upon which even heavier loads could be carried more easily. However, the one drawback of wheeled vehicles was (and still is) the necessity of providing a reasonably smooth and hard surface upon which the wheels can run. The development of wheeled vehicles is therefore closely related to the development of roadways or dedicated tracks.

Wheels and tyres

Most light vehicles run on four wheels, which are usually made of steel or alloy. The wheels are fitted with hollow rubber tyres, which are filled with air under sufficient pressure to support the load they have to carry. The tyres provide contact with the road surface and therefore the grip to the road. The tyres also absorb small shocks caused by minor irregularities in the road surface. Larger shocks are taken by suspension springs and these allow the wheels to move vertically in relation to the rest of the vehicle.

Automatic transmission fluid

The fluid used in an automatic gearbox must be capable of performing many functions. It serves to operate the torque converter, actuate the clutches and brakes through hydraulic pressure, lubricate the various gears and bearing surfaces and act as a coolant to the internal components of an automatic gearbox. Each of these duties demands special properties, so it is essential that the fluid used contains additives, which will satisfy the recommended specification. In traffic conditions a torque converter gets very hot, and although an oil cooler is now common, the high temperature reached by the fluid must neither cause excessive oxidation nor vary the viscosity adversely to make the gear changes harsh or noisy. Additives, to resist wear of the rubbing surfaces and foaming of the fluid, are also essential because a long period between fluid changes is demanded, with some gearboxes now being sealed for life (i.e. no fluid changes). Another important consideration is the need to use a fluid that gives a stable friction value. A fluid of the incorrect type can cause the clutch and brake unit to engage either too quickly or too slowly. Automatic gearboxes traditionally used a fluid made to the General Motors specification DEXRON® II. There are, however, several different specifications of DEXRON® II and also DEXRON® III (including synthetic-based oils), therefore always refer to manufacturer’s literature before adding to, or replacing gearbox oil. It should be noted that the fluid used must be of a specification to suit the design of the box. Gearbox problems will occur if the wrong type of gearbox fluid is used. Older gearboxes often used an ATF (automatic transmission fluid) to the Ford standard F. Fluid level A dipstick is normally employed to indicate the correct volume of oil in the gearbox. The dipstick is usually accessible from the engine compartment. Alternatively the gearbox oil is checked by using a plug fitted to the sump of the gearbox casing. Refer to manufacturer’s literature before checking the level of the gearbox oil. If a dipstick is used, the dipstick is marked to show the correct level when the fluid is hot. Note that the volume of oil will vary depending on its temperature. Many of today’s automatic gearboxes require the level to be set at an exact temperature, which requires the use of specialised tools connected to the vehicle diagnostic system. The automatic gearbox oil level is normally checked with the engine running at idle speed. Since the reservoir level varies with the position of the selector, it is essential to set the lever as recommended by the manufacturer. Note that a number of faults may arise if the level is set either higher or lower than recommended.

Hydraulic system for a four- speed gearbox with torque converter lock-up

Development of the basic three-speed gearbox Developments in automatic gearboxes have resulted in gearboxes with four, five, six and even seven gears. However, these later gearbox developments are generally adaptations of the original three-speed designs, with additional epicyclic gear trains or overdrive units, which form more complex compound gear trains. In addition, the modern generation of automatic gearboxes make use of electronic control to replace or supplement some of the hydraulic valves and controls of the older designs. Within this book therefore, the basic understanding of the three-speed designs is sufficient to provide a general understanding of automatic gearbox operation. The following information provides a basic outline of the hydraulic system for a four-speed gearbox and section 3.14 does provide an initial introduction to electronic control of gearboxes. However, more detail of the electronic control systems and further studies of 4–6-speed gearboxes can be found in Fundamentals of Motor Vehicle Technology – Book 2. Four-speed hydraulic system Figure 3.91 shows a hydraulic valve block arrangement for a particular type of ZF transmission unit. This design has three forward speeds, an overdrive fourth gear and a lock-up converter clutch, as well as providing neutral and reverse gears. Figure 3.92 (on page 318) shows the hydraulic circuit along with identification of the valves and components connected to the circuit. Reference should be made to previous sections in this chapter relating to torque converter lock-up clutches (section 3.12.6) and overdrive units (section 3.13.3).

Hydraulic control of actuators

The clutches and brake bands of most automatic gearboxes are operated (actuated) by hydraulic means. An oil pump, driven at engine speed by the torque converter casing, generates the fluid pressure. A gear- type pump is typically used, similar to that used in an engine application. The pressure produced should be sufficiently high to lock the clutch plates or brake drum, but not so high as to waste energy. Distribution of fluid to the clutches and brakes is by means of a control valve, called a manual valve, linked to the driver’s selector lever. Although many valves are used in automatic gearboxes, the basic principle of operation of each valve is the same. The valves are contained within a chest, which is situated at the lowest point of the gearbox. In the following paragraphs, construction and operation of some of the valves is considered. Regulator valves (relief valves) The main purpose of this type of valve is to limit the hydraulic line pressure to a pre-determined maximum pressure. The three main types, shown in Figure 3.81, are as follows: Ball and piston-type relief valve Ball and piston-type valves are similar to the oil pressure relief valve used on engines. When the pressure acting on the valve produces a thrust greater than the force exerted by the spring, the valve lifts and so prevents any further increase in pressure. Operation is based on the relationship: Thrust = Pressure × Area Spool-type relief valve Simple spool valves are similar to the plunger-type except that they are waisted (reduced in area at the centre) to control the fluid flow. For the spool valve shown in Figure 3.81, the areas of A and B are the same, so the pressure in the waist region will exert an equal thrust upwards and downwards; as a result the pressure in this region will not affect the movement of the valve. When pressure is sufficient to lift the valve, a port is uncovered. Fluid then flows to the waisted part of the made by directing fluid from a single line to the appropriate clutch and also to the ‘release’ side of the brake servo. Besides reducing the number of hydraulic lines, this arrangement gives a quick down-change because as soon as the fluid is released from one side of the servo, the second gear immediately comes into operation. The majority of automatic gear changes are made while the drive is being transmitted through the gearbox. If a brake or clutch were released before the next gear units were partly engaged, the engine would suddenly speed up due to clutch (or brake band) slip. This condition, called flare-up, is prevented by overlapping the engagement of the new gear with the disengagement of the old gear. If the period of overlap is too great, then a harsh gear change condition, sometimes called ‘tie-up’, results. Tie up occurs because the two ‘gears’ oppose each other, i.e. each gear attempts to drive the output shaft at a speed dictated by its ratio, with the result that the gearbox partially locks up. Brake slip As with clutch slip (refer to 3.13.4), this fault produces similar symptoms to those given by slip of a main clutch in manual transmission. Again, as is the case with slip of the gearbox clutches, a stall test can be carried out. A test using each gear position may indicate which set of multi-plate clutches or which brake band is faulty. Manufacturer’s recommendations relating to the time taken for the test must be observed.

WCenturies of vehicle development

At a very early stage in human history, it was realized by the more ingenious members of the species that the mobility to mankind provided by nature had some limitations. The human body was severely limited as to the loads it could carry, the distance it could carry them and the speed at which it could travel, even without a load. Furthermore, it is a safe guess that physical exertion was no more enjoyable then than it is today. Major progress was achieved with the taming and training of suitable animals, which enabled heavier loads to be carried greater distances, often at greater speeds than the human was capable of attaining. There was, of course, the added advantage that most of the effort was provided by the animal, while the human being could travel at his ease, in comfort. Heavy loads were dragged upon sledges until the next major development occurred, which was when an enterprising but unknown early engineer invented the wheel. The wheel made it possible to construct crude carts upon which even heavier loads could be carried more easily. However, the one drawback of wheeled vehicles was (and still is) the necessity of providing a reasonably smooth and hard surface upon which the wheels can run. The development of wheeled vehicles is therefore closely related to the development of roadways or dedicated tracks. Over the centuries, there was not much in the way of major change other than some improvements in tools, some new materials and a better understanding of the technologies. Together, these changes combined to refine the wheeled vehicle and general mobility, but only within the limitations imposed by the source of power (i.e. the animals) and the basic materials available. Probably the biggest influence on the development of the wheeled vehicle occurred during the 1800s. During this period, the industrial revolution embraced major developments of known technologies but importantly, new technologies appeared and the understanding of the sciences accelerated at a rapid pace. VEHICLE EVOLUTION, LAYOUT AND STRUCTURE Chapter 1 1.1 VEHICLE EVOLUTION what is covered in this chapter . . . Vehicle evolution Vehicle layout Vehicle structure Routine maintenance Figure 1.1 An old car (Model T Ford) Figure 1.2 A modern car Although inventors and scientists did at that time understand the potential for producing ‘engines’, this potential could not be fully explored because of the materials available. However, production of more suitable materials along with an understanding of how to work and make use of those materials, enabled inventors and scientists to achieve some of their dreams, such as creating engines that used fossil fuels to produce power. As new materials and manufacturing methods were developed it became possible to make improvements to vehicles. Engines were of prime importance to the evolution of motor vehicles, but every aspect of the wheeled vehicle was improved and developed from the late 1800s through into the next century. 1.1.2 Choosing the right vehicle layout connected to the cart through shafts that were attached to a front axle, which was pivoted about its centre, thus providing a means of steering. Therefore, when it then came to replacing the horse with an engine, it was natural that front-wheel steering should be retained. The swivelling axle arrangement is not very satisfactory for powered vehicles. One reason is because a good deal of space must be left for the axle and wheels to swivel, and also because if one wheel strikes an obstruction (such as a large stone) it is extremely difficult to prevent the axle swivelling about its pivot. An alternative arrangement, where the wheels were carried on stub axles (which were free to pivot at the ends of the fixed axle) had already been used on some horse-drawn carriages, and this layout was soon adopted for motorized vehicles. One other alternative layout for steering the vehicle was tried, and that was to use the rear wheels, or axle, to steer the vehicle. It was soon found that rear-wheel steering had disadvantages which ruled it out for general use. For example, a vehicle steered by its rear wheels would steer to the right by deflecting the rear end to the left, making it impossible to drive away forwards from a position close to a wall or kerb, as illustrated in Figure 1.4.

Aurther

V.A.W. Hillier & Peter Coombes

Downlaod Book