The Motor Vehicle 2010 Part 15 ppt

1006 The Motor Vehicle relative to that for 2-wheel drive, the output from the automatic transmission is taken through a twin-planet gear set. To obviate vibration when the vehicle is cornering, the planetary gears run in precision needle roller bearings. The output from this gear set is transmitted to a short shaft to the rear end of which is connected the propeller shaft for the rear axle differential. A gear on this short shaft transmits the drive, through an idler, to a gear on the shaft connected to the propeller shaft extending forward alongside the engine to the front wheel drive gear set. The torque distribution, 65% rear and 35% front, is determined by the gear ratios of the front and rear differentials. To provide constant 4-wheel drive, mechanical or hydraulic locking devices in the differentials were ruled out. Instead, the electronic traction control system comes into operation as soon as one or more of the drive wheels starts to spin. This reduces the torque transmitted to the wheel, or wheels, to Fig. 38.26 Diagrammatic representation of the mechanical component layout for the Mercedes-Benz 4MATIC traction control system for 4-wheel drive Fig. 38.27 To avoid drive line vibration, the front axle gear set is bolted to one side of the engine oil sump and further supported by a bracket on the crankcase of this Mercedes V6 engine 1007Servo- and power-operated, and regenerative braking systems Fig. 38.28 From the engine crankshaft, the drive is taken through a planetary reduction gear directly to the rear propeller shaft and also through a second gear train, alongside the first, to the front axle gear set Fig. 38.29 Layout of the transfer gearbox with, above left, details of the twin planet train to a larger scale the road and redistributes it to the other wheels, until the spin speed falls below a predetermined value in relation to the speed of the vehicle. Additionally, an Electronic Stability Program (ESP), comes into action to apply the brakes on each side differentially when the vehicle is cornering. Adaptation of ESP to the 4MATIC has entailed introducing sensors to detect steering commands, lateral acceleration, yaw velocity, and brake pressure as 1008 The Motor Vehicle indications of the instantaneous dynamic status of the moving vehicle. Activation of the Brake Assist system is integrated into the 4MATIC control, so that the brake pressure can be built up rapidly for stabilising the vehicle. 38.16 Mercedes-Benz Brake Assist (BA) The Mercedes-Benz BA system was introduced at the end of 1996, to cater for the tendency of the majority of drivers to under-react to emergencies. Even if their cars are equipped with ABS, more than 90% of drivers tend to be either fearful of stamping on the brake pedal too hard lest they lose control, or they fail immediately to realise the seriousness of the situation and do not apply maximum braking soon enough. The first mentioned type of inadequate reaction can increase, by up to 45% the stopping distances from a speed of 100 km/h. In the event of excessively rapid depression of the brake pedal, indicating a panic stop, the full power of the booster is applied instantly by a solenoid- actuated valve housed within it, Fig. 38.30. As soon as the driver releases the brakes, the solenoid and, with it the booster, are deactivated. Since the system is used in conjunction with ABS, wheel lock is inhibited. If, in an emergency, a driver without BA were to fail to apply instantly maximum force to his brake pedal, the stopping distance of a car travelling at 100 km/h could be 73 metres but, with BA, the stopping distance would be only 40 metres. Even in the event of a hesitant reaction by the driver, BA can reduce that stopping distance by about 6 metres. Incidentally, hesitant is defined as an initial braking reaction producing a deceleration of 7 to 10 m/s, Fig. 38.30 If the electronic control of the Brake Assist system senses rapid depression of the brake pedal, indicating an emergency stop, it activates a solenoid valve in the brake servo unit to apply fully, instead of partially, atmospheric pressure to the right- hand side of the diaphragm. This provides maximum braking, although still modulated by the ABS system 1009Servo- and power-operated, and regenerative braking systems Fig. 38.31. Reactions producing deceleration values of less than 6 m/s or less are classified as inadequate. From Fig. 38.30, it can be seen that the brake actuation unit comprises a fairly conventional brake servo with the addition of a pedal travel sensor, a solenoid valve which in fact is the air valve, an electronic control unit, and a brake release switch. So long as the brakes are inactive, induction manifold depression acts equally on each side of the diaphragm. When the driver moves the brake pedal, the push rod opens the air valve, applying atmospheric pressure to the chamber on the right in the illustration. This moves the diaphragm to the left, until the air valve is closed. Thus, without BAS, the pressure in the hydraulic brake system is at all times proportional to the pedal travel. If the pedal travel sensor recognises a fear-induced excessively rapid movement of the pedal, the electronic control energises the solenoid in the centre of the brake servo unit, which opens the air valve fully, instead of partially: wheel lock is prevented by the ABS system. As soon as the driver releases the brake pedal, the release switch shown in the illustration breaks the circuit to the solenoid thus cutting out the boosting effect of the servo, Fig. 38.32. The speed of operation of the brake pedal is not, however, the only signal upon which the electronic control bases its decision to activate BA. Other 400 300 200 100 0 Pedal force, N 02 4 6 With BAS Without BAS Time, sec Pedal force Brake pressure With BAS Without BAS Time, sec 100 80 60 40 20 0 0246 Pressure, bar Fig. 38.32 As soon as the driver releases the brake pedal, a switch breaks the circuit to the solenoid to cut out the boosting effect of the servo Fig. 38.31 Comparisons of braking performances with and without Brake Assist 10 8 6 4 2 0 0 0.2 0.4 0.6 0.8 0. Adequate deceleration Hesitant driver reaction Inadequate driver reaction Time, sec Deceleration, m/sec 2 1010 The Motor Vehicle factors include the speed of the vehicle, the state of wear of the brakes, signals from the electronic control systems for the engine and transmission management and from other systems such as ABS and, in some installations, those controlling wheel-spin and vehicle stability. A major difficulty with such systems, however, is setting the threshold beyond which an emergency stop is automatically put into effect. This setting inevitably has to be a compromise that might not be appropriate for some drivers. For example, consider a nervous driver in an overtaking lane on a motorway, where traffic situations are liable to change with frightening rapidity. He might observe a change in the traffic movement ahead that does not call for emergency braking but, to be sure that he is ready to brake if the situation does become critical, he rapidly puts his foot on the brake, intending to apply relatively gentle braking yet, because of his nervousness, he moves exceptionally quickly. If the control interprets this as an emergency, he could find himself in a crash stop situation causing the driver of the car behind him to run into his back end. A similar situation could also arise at slower speeds in urban traffic, or if the driver suddenly realises that he is exceeding the speed limit in embarrassing circumstances! 38.17 Stability when steering and braking or accelerating (ESP) Modern micro-electronics is revolutionising vehicle control. The advances that have been made progressively by Mercedes-Benz exemplify the general trend. It started in 1978 when Automatic Braking Control (ABS) was introduced on their S-class W 127 Series. Clearly, the sensor that detects wheel lock can be used also for detecting wheel-spin. So a logical further development was an Acceleration Skid Control system (ASR), which was announced in 1987. With this system, if any of the wheels showed a tendency to spin, the engine torque was automatically reduced and the brake applied to the relevant driven wheel, or wheels, until stability had been assured. The aim, of course, was the enhancement of acceleration over the whole speed range. This facility is especially valuable when there are patches or streaks of ice, or perhaps mud, on the road. A further development was the introduction, in 1994, of what Mercedes- Benz terms Electronic Traction Support (ETS) on their six-cylinder S- and SL-class and, as an option instead of the Automatic Locking Differential (ASD), on the C-class. ETS simply brakes any driven wheel, or wheels, that show signs of inherent spin during acceleration from rest, and then releases the brake when the speed difference between the driven wheels is reduced to as small as practicable a level. The ultimate aim has been the development of an overall stability control system. This was achieved in 1995 with the introduction, by Mercedes-Benz, of their Electronic Stability Program (ESP), which is designed to enable the driver to maintain control in circumstances in which, without it, he would be unable to do so. Such circumstances might arise, for example, if the driver were cornering too fast, taking sudden evasive action or, for example, driving with the wheels on the near side on ice and those on the other side on dry tarmac. ESP is a combination of ABS and ASR, in that it stabilises the vehicle by 1011Servo- and power-operated, and regenerative braking systems braking intervention and torque reduction but, in contrast to these two systems which are activated only when required, ESP continuously monitors the situation and therefore is more effective. This greater effectiveness is attributable to two facts: first, it is coupled, through CAN data bus links, to the electronic controls for the engine and transmission, so that it can come instantly into operation in sudden emergencies; and second, its electronic control has many more inputs from sensors than did its predecessors. These inputs include not only throttle pedal position and individual wheel speeds, but also direct readings of transmission ratio and engine torque (instead of relying on pedal position only), as well as steering angle, yaw, lateral acceleration and brake pressure. The extra input enable ESP to actually anticipate loss of control, and to react virtually instantaneously by braking intervention on individual wheels. Incidentally, a CAN data bus is an electronic circuit that interlinks the various computer databases for controls such as engine and transmission management, ABS, and ETS, so that all are continuously updated with the information they need and therefore are ready instantly to perform their safety functions in an emergency. Sited under the back seats of the car are the lateral acceleration and yaw sensors, the latter being based on aerospace technology. Too high a rate of yaw warns that the vehicle is about to break away into a skid, while the lateral acceleration sensor provides information about any tendency to under- or oversteer. All the inputs are continuously compared with data pre-recorded on a map of limits of stability relative to steering wheel angles and vehicle speeds. Should the input values move into a critical region, indicating that breakaway is imminent, the ESP signals the hydraulic unit to apply the brake on the relevant wheel or wheels and, if necessary, reduce engine torque. The selective and precisely metered braking intervention takes place in a fraction of a second, and the driver is hardly aware of it. In the event of oversteer, the outer front wheel is braked, while if understeer is developing, the brakes and throttle control are applied, appropriately, to reduce the speed of the vehicle, with emphasis on braking on the inner rear wheel. The whole system is so sophisticated that it even takes into account how many people are in the car, how much luggage is carried and the depth of treads of the tyres. More information on this subject, and on a similar Toyota system, can be found in the chapter on Vehicle Safety, Section 36.16. 38.18 Regenerative braking systems A simple form of regenerative braking system is often employed on electric vehicles. It is necessary because the energy storage capacity of a battery of a weight and size practicable for installation in road vehicles is so small that one cannot expect to get more than 30 to 40 miles (38 to 64 km) out of it, even with regeneration of the energy that would otherwise be dissipated in braking. This type of vehicle has an electric motor and control system such that, when current is passed through it, it drives the vehicle, but generally when its control pedal is released, or more unusually during the initial movement of the brake pedal, the current supply to the motor is cut off and it actually generates current which is utilised to contribute to recharging the batteries. Thus, a braking torque is applied to the road wheels, by virtue of the fact that they are driving a generator. With the advent of electronically-controlled, constantly variable trans- 1012 The Motor Vehicle missions it has be come practicable to introduce regenerative braking for petrol and diesel vehicles. Leyland has been experimenting with such a system since before 1980, using its CVT with a flywheel for energy storage, while Volvo had a hydraulic accumulator regenerative system installed on acceptance trials in a London bus in 1985. Indeed, regenerative braking is particularly attractive for urban bus operation, since much of the power from the engine is used for acceleration from bus stops, soon after which it is dissipated again in braking for the next stop. Flywheel storage has some disadvantages. First, in the event of an accident in which excessive shock loading is transmitted to the flywheel, it might burst and cause casualties. Secondly, it adds significantly to the weight of the vehicle, thus offsetting some of the gains as regards fuel economy. Thirdly, it is bulky. Fourthly, it will run down overnight, so the engine has to be started electrically in the morning. Most of these disadvantages can, to a major extent, be designed out. For example, by using fibre-reinforced material for the flywheel it can be made so that it does not burst into large fragments when ruptured but rather tends to shear along the fibres and to be retained by them. The weight can be reduced by the use of a very dense material as a rim on a very light disc, so that its polar moment of inertia is high relative to its weight. Little can be done about its bulk, since it must have a flywheel of reasonably large diameter, though it can be installed with its axis of rotation vertical. To keep it spinning for a long time it could be housed in a vacuum, but this is hardly practicable; alternatively, the housing can be filled with a very light gas such as hydrogen or helium. Even so, to keep it freewheeling for, say, twelve hours is scarcely a reasonable demand. With a hydraulic accumulator, on the other hand, overnight storage presents no problem so that, in the event of, for example, a fire in a bus garage all the vehicles could be driven out instantly by drawing on the accumulator for energy, without having to wait for their engines to start and become warm enough to move off, and without generating any exhaust fumes. Disadvantages of high pressure hydraulic drive systems, however, include problems of leakage, and they are inherently noisy under certain conditions, owing to turbulence and very high local velocities of fluid flow. With low pressures and velocities, the system becomes unacceptably bulky. In trials of a Volvo bus in service in Stockholm the use of hydraulic regeneration has indicated average savings in fuel of between 28 and 30% in urban operation, though in an extreme case a saving of 35% was made. A significant proportion of this economy is attributable to the fact that, under load, the engine can be run virtually continuously at its most economical speed, the stored energy being used for acceleration and assistance in hill climbing. With suitable electronic control it might even be possible to stop the engine during braking and the initial stages of acceleration, though in the Volvo bus it is kept idling under these conditions. The layout of the control system of what Volvo call their Cumulo system can be seen in Fig. 38.33 and of the hydro-mechanical system in Fig. 38.34. Power is derived from a 180 kW diesel engine installed in conjunction with a fluid flywheel and four-speed gearbox, and with a final drive ratio of 4.87 : 1. A power take-off of the sort used for driving auxiliaries alternately drives and is driven by, according to the mode in which the vehicle is operating, a swashplate type hydraulic pump/motor with a 40° Z-shaft and spherical 1013Servo- and power-operated, and regenerative braking systems A B C D E F G K H J L Diesel engine Gear box Sensors Actuators A Throttle B Brake pedal C Drive/neutral sensor D Vehicle speed E Reservoir contents F Clutch engagement G Pump displacement H Shut-off valve J Pressure switch K Relief valve L Reservoir contents Fig. 38.33 Diagrammatic layout of the Volvo Cumulo control system, showing the sensors and actuators pistons. When this unit is operating as a pump the fluid is delivered from the reservoir into the hydraulic accumulator. During operation as a motor the flow of fluid is of course reversed. Pressure for regulation of the angle of the swashplate on the Z-shaft has to be obtained from a separate pump: if it were dependent on the pressure in the accumulator, control would be lost if that pressure became too low. The electronic control, with its 8-bit microprocessor, is programmed not only for normal conditions of operation but also for warming up the engine and charging the accumulators. It also monitors the system continuously at a frequency of 20 Hz to detect malfunction and check that the safety system is operational at all times. The inputs to microprocessor include a potentiometer coupled to the accelerator pedal for sensing the torque demanded by the driver and another connected to the brake pedal to sense the deceleration required. A position indicator senses whether the gearbox is in a drive ratio or neutral and a pulse pick-up senses the rotational speed of the propeller shaft and thus the vehicle speed. Another position indicator senses whether the power take-off clutch is engaged or disengaged. A potentiometer senses the displacement of the swashplate pump and a position indicator signals the volume of oil in the hydraulic reservoir, which determines when the engine should be brought into operation to take up the drive. Finally, there is a pressure sensor on the accumulator. When the accumulator is full, the incoming oil is diverted to the reservoir via a pressure relief valve. Energy stored in the accumulator is locked in overnight by the shut-off 1014 The Motor Vehicle A B A Power take-off gearing B Pump/motor unit C Hydraulic accumulator Fig. 38.34 Of the three interconnected cylinders in the Cumulo system, the two outer ones contain nitrogen gas, while the central one also contains gas but is separated by a free piston from the hydraulic fluid valve, which is actuated automatically by the electronic control system. Consequently, the vehicle can be driven out of the garage in the morning, using stored energy. When it is in the open air the diesel engine can be started and the bus driven away, still using the hydraulic energy. As the speed rises to 22 mph (35 km/h), or if the hydraulic pressure drops below a predetermined level, the engine is automatically accelerated from idling up to the same speed as the propeller shaft, at which point the engine and gearbox take over from the hydraulic drive. When the brakes are applied, the engine reverts to idling and the hydraulic motor to a pumping mode, to charge up the accumulator. The friction brakes come into operation only if the control pedal is depressed beyond a spring-loaded detent. Fully charged, the accumulator stores 0.22 kWh energy, which is adequate for running a half-laden vehicle at a constant slow speed for about three-quarters of a mile (1.2 km) or accelerating it, at a constant rate of 1.8 m/s 2 , up to the cut-in speed of the engine. The accumulator in Fig. 38.34 comprises three interconnected cylinders, the outer two containing only compressed nitrogen and the inner one both gas and hydraulic fluid separated by a free piston fitted with Teflon seals. When fully discharged, the gas pressure is about 200 bar and, fully charged, about 350 bar. The system is designed to bring a half-laden city bus to rest from 31 mph (50 km/h), the difference between this figure and that of 22 mph (35 km/h) for acceleration from rest being accounted for by the rolling and aerodynamic resistances. While the overall efficiency of the transmission is about 80 to 85%, that of the hydraulic system is 90 to 96%. C 1015 Chapter 39 Anti-lock brakes and traction control For the sake of simplicity a single wheel will be assumed in the following description of the fundamental principles of anti-lock systems. Basically, an anti-lock system comprises a sensor to detect incipient wheel-locking, together with a system for relieving momentarily the hydraulic pressure to the brakes, to prevent locking before it actually occurs. As explained in Section 38.11, when the wheel is slipping only to a small extent, the deceleration will be low in comparison with the value appertaining to the approach of sliding and so, when the deceleration exceeds a certain value, the control releases the brake, the deceleration of which will then fall to a low value and so the brakes will be reapplied. This release and reapplication of the brakes must take place in an extremely short time if the system is to work satisfactorily and, in practice, the cycle will occur up to as high as 15 times per second. In the early systems the wheel deceleration was measured by purely mechanical means but in present-day systems electronic circuits are used because they give much quicker responses and can be controlled more easily. These circuits are beyond the scope of this book and only an outline of their action can be attempted. The deceleration sensor usually consists of a toothed disc attached to the hub of the wheel, and a pick-up placed near to the periphery of the disc. The pick-up is essentially a horseshoe magnet with a winding, and the projections of the disc act as a succession of keepers which bridge the poles of the magnet thus momentarily causing an increase in the magnetic flux through the winding and setting up a current in it. The frequency of this current will depend on the speed of the disc and the rate at which that frequency changes will be proportional to the deceleration of the disc. This rate can be measured relatively simply electronically and can then be used to supply a signal for the control of the brakes. The remainder of the system therefore consists of valves actuated by the signal and which control the actuation of the brakes. A sensor may be provided for each wheel to be controlled but sometimes it is practicable to control the wheels in groups. A common system is to have a sensor for each of the front wheels and a single sensor with its disc on the propeller shaft for the two rear wheels together. It will be appreciated that the incorporation of these anti-lock systems is facilitated by the use of power [...]... that of the left-hand side the piston will move to the left to close the valve Because of the decrease in the volume of the stem of the valve that projects into the chamber B, the pressure in the brake cylinder will drop and the brake will be released When the signal to the control valve ceases the right-hand side of the actuator piston will again be opened to the atmosphere in the reservoir and the valve... lifted so as to open the port Z to the atmosphere via the gap opened at C Thus the valve equalises the pressures at Y and Z and the brake actuating pressure will at all times be equal to the pressure determined by the brake pedal valve As soon as the anti-skid pressure on the right-hand side of the brake actuators is released by the cessation of the signal from the electronic module the brakes will be... will also depress the piston assembly of the sensitivity valve, Fig 39.2(c), and this will restrict the passage of air from the brake pedal valve to the brakes The pressure that acts on the lefthand side of the brake actuator diaphragms also acts on the underside of the balanced exhaust valve and if it exceeds the pressure acting on the upperside from the port Y the central portion of the diaphragm will... cylinder Ideally, the modulator is installed in the engine compartment, to keep the pipe lines to both the master cylinder and the wheel-brake cylinders as short as possible Four-channel versions of the modulator are available for vehicles in which 1030 The Motor Vehicle the brake pressure to each of the four wheels is regulated by a separate solenoid valve For vehicles in which the brakes on two front... flows through the solenoids From the illustration, it can be seen that the pressure generated by the master cylinder opens the uppermost valve, against the force exerted on its return spring At the same time, the lower valve is held closed by the lower return spring Consequently, the fluid from the master cylinder flows through the open valve, down through the duct on the right-hand side of the armature,... by the falling pressure in the return circuit from the wheel cylinder With the lower valve open, the fluid in the wheel cylinder passes back through the solenoid-bypass circuit, past the hydraulic accumulator and through the first check valve and the return pump cylinder, and then on through the second check valve to the master cylinder As the fluid release phase ends, this flow is expedited by the. .. system, which counters the tendency for the vehicle to yaw when the wheels on one side are on a firm surface and those on the other on ice PH is the brake pressure applied to the wheels on the firm ground and PL is that applied to those on the ice Anti-lock brakes and traction control 1035 As soon as the pressure is released for the first time on the nearside brakes, that on the brakes of the offside wheels... that chamber is that of the control system, as dictated by the force applied by the driver to his pedal Then, when the solenoid was energised and the pressure below the solenoid valve dropped to atmospheric, the valve in the base of the memory chamber dropped on to its seat, so the pressure in that chamber could fall only at the slow rate dictated by the size of the orifice on the right, just below that... diaphragms Hence, when the brake pedal is depressed, air will pass freely from the service (lower) reservoir to the port Y of the balanced exhaust valve, Fig 39.2(b) This pressure will deflect the outer portion of the lower diaphragm against the force of the spring so that air will pass to the port Z and thence to the left-hand sides of the brake actuator diaphragms, thereby applying the brakes Under poor... releases pressure from the brake actuation cylinder into the pressure accumulator, the pison of which moves to the right At the same time, the motor rotates the eccentric, pushing the piston of the return pump to the left, forcing fluid back past the now open ball valve to the brake master cylinder Anti-lock brakes and traction control 1031 cylinder during ABS operation Interposed between the pressure release . The Motor Vehicle factors include the speed of the vehicle, the state of wear of the brakes, signals from the electronic control systems for the engine and transmission management and from other. left-hand side the piston will move to the left to close the valve. Because of the decrease in the volume of the stem of the valve that projects into the chamber B, the pressure in the brake cylinder. but if the wheel on the other side then spins, the electronic control closes the throttle or reduces the Fig. 39.10 Layout of the anti-lock brake system of the Ford Sierra with (above left) the