The Motor Vehicle 2010 Part 15 ppt

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 fallsbel

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 Theoutput from this gear set is transmitted to a short shaft to the rear end ofwhich is connected the propeller shaft for the rear axle differential A gear onthis short shaft transmits the drive, through an idler, to a gear on the shaftconnected to the propeller shaft extending forward alongside the engine tothe 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 controlsystem comes into operation as soon as one or more of the drive wheelsstarts 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

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 fallsbelow a predetermined value in relation to the speed of the vehicle.Additionally, an Electronic Stability Program (ESP), comes into action toapply the brakes on each side differentially when the vehicle is cornering.Adaptation of ESP to the 4MATIC has entailed introducing sensors to detectsteering 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 caterfor 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 losecontrol, or they fail immediately to realise the seriousness of the situationand do not apply maximum braking soon enough The first mentioned type

of inadequate reaction can increase, by up to 45% the stopping distancesfrom 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 actuated valve housed within it, Fig 38.30 As soon as the driver releases thebrakes, the solenoid and, with it the booster, are deactivated Since the system

solenoid-is used in conjunction with ABS, wheel lock solenoid-is inhibited

If, in an emergency, a driver without BA were to fail to apply instantlymaximum 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, BAcan reduce that stopping distance by about 6 metres Incidentally, hesitant isdefined 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

Fig 38.31 Reactions producing deceleration values of less than 6 m/s or lessare classified as inadequate.

From Fig 38.30, it can be seen that the brake actuation unit comprises afairly conventional brake servo with the addition of a pedal travel sensor, asolenoid 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 manifolddepression acts equally on each side of the diaphragm When the drivermoves the brake pedal, the push rod opens the air valve, applying atmosphericpressure to the chamber on the right in the illustration This moves thediaphragm to the left, until the air valve is closed Thus, without BAS, thepressure in the hydraulic brake system is at all times proportional to thepedal travel

If the pedal travel sensor recognises a fear-induced excessively rapidmovement of the pedal, the electronic control energises the solenoid in thecentre of the brake servo unit, which opens the air valve fully, instead ofpartially: wheel lock is prevented by the ABS system As soon as the driverreleases the brake pedal, the release switch shown in the illustration breaksthe 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 signalupon which the electronic control bases its decision to activate BA Other

Without BAS

Time, sec

100 80 60 40 20 0

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

Hesitant driver reaction

Inadequate driver reaction

Time, sec

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 transmissionmanagement 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 thresholdbeyond which an emergency stop is automatically put into effect This settinginevitably has to be a compromise that might not be appropriate for somedrivers For example, consider a nervous driver in an overtaking lane on amotorway, where traffic situations are liable to change with frightening rapidity

He might observe a change in the traffic movement ahead that does not callfor emergency braking but, to be sure that he is ready to brake if the situationdoes become critical, he rapidly puts his foot on the brake, intending to applyrelatively gentle braking yet, because of his nervousness, he movesexceptionally quickly If the control interprets this as an emergency, he couldfind 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 thespeed limit in embarrassing circumstances!

38.17 Stability when steering and braking or

accelerating (ESP)

Modern micro-electronics is revolutionising vehicle control The advancesthat have been made progressively by Mercedes-Benz exemplify the generaltrend 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 enginetorque was automatically reduced and the brake applied to the relevant drivenwheel, or wheels, until stability had been assured The aim, of course, wasthe enhancement of acceleration over the whole speed range This facility isespecially 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 Benz terms Electronic Traction Support (ETS) on their six-cylinder S- andSL-class and, as an option instead of the Automatic Locking Differential(ASD), on the C-class ETS simply brakes any driven wheel, or wheels, thatshow signs of inherent spin during acceleration from rest, and then releasesthe brake when the speed difference between the driven wheels is reduced to

Mercedes-as small Mercedes-as practicable a level

The ultimate aim has been the development of an overall stability controlsystem This was achieved in 1995 with the introduction, by Mercedes-Benz,

of their Electronic Stability Program (ESP), which is designed to enable thedriver to maintain control in circumstances in which, without it, he would beunable to do so Such circumstances might arise, for example, if the driverwere cornering too fast, taking sudden evasive action or, for example, drivingwith the wheels on the near side on ice and those on the other side on drytarmac

ESP is a combination of ABS and ASR, in that it stabilises the vehicle by

braking intervention and torque reduction but, in contrast to these two systemswhich are activated only when required, ESP continuously monitors thesituation and therefore is more effective This greater effectiveness is attributable

to two facts: first, it is coupled, through CAN data bus links, to the electroniccontrols for the engine and transmission, so that it can come instantly intooperation in sudden emergencies; and second, its electronic control has manymore inputs from sensors than did its predecessors These inputs include notonly throttle pedal position and individual wheel speeds, but also directreadings of transmission ratio and engine torque (instead of relying on pedalposition only), as well as steering angle, yaw, lateral acceleration and brakepressure The extra input enable ESP to actually anticipate loss of control,and to react virtually instantaneously by braking intervention on individualwheels Incidentally, a CAN data bus is an electronic circuit that interlinksthe various computer databases for controls such as engine and transmissionmanagement, ABS, and ETS, so that all are continuously updated with theinformation they need and therefore are ready instantly to perform theirsafety functions in an emergency

Sited under the back seats of the car are the lateral acceleration and yawsensors, the latter being based on aerospace technology Too high a rate ofyaw warns that the vehicle is about to break away into a skid, while thelateral 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 vehiclespeeds Should the input values move into a critical region, indicating thatbreakaway is imminent, the ESP signals the hydraulic unit to apply the brake

on the relevant wheel or wheels and, if necessary, reduce engine torque Theselective 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, toreduce the speed of the vehicle, with emphasis on braking on the inner rearwheel The whole system is so sophisticated that it even takes into accounthow 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 similarToyota 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 electricvehicles 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 thatone 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 inbraking This type of vehicle has an electric motor and control system suchthat, when current is passed through it, it drives the vehicle, but generallywhen 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 actuallygenerates 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 thatthey 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 forpetrol and diesel vehicles Leyland has been experimenting with such asystem since before 1980, using its CVT with a flywheel for energy storage,while Volvo had a hydraulic accumulator regenerative system installed onacceptance trials in a London bus in 1985 Indeed, regenerative braking isparticularly attractive for urban bus operation, since much of the power fromthe engine is used for acceleration from bus stops, soon after which it isdissipated 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 mightburst and cause casualties Secondly, it adds significantly to the weight of thevehicle, 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 bestarted electrically in the morning

Most of these disadvantages can, to a major extent, be designed out Forexample, 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 bereduced by the use of a very dense material as a rim on a very light disc, sothat its polar moment of inertia is high relative to its weight Little can bedone 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 spinningfor 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 thevehicles could be driven out instantly by drawing on the accumulator forenergy, without having to wait for their engines to start and become warmenough to move off, and without generating any exhaust fumes Disadvantages

of high pressure hydraulic drive systems, however, include problems ofleakage, and they are inherently noisy under certain conditions, owing toturbulence and very high local velocities of fluid flow With low pressuresand velocities, the system becomes unacceptably bulky

In trials of a Volvo bus in service in Stockholm the use of hydraulicregeneration has indicated average savings in fuel of between 28 and 30% inurban operation, though in an extreme case a saving of 35% was made Asignificant proportion of this economy is attributable to the fact that, underload, the engine can be run virtually continuously at its most economicalspeed, the stored energy being used for acceleration and assistance in hillclimbing With suitable electronic control it might even be possible to stopthe engine during braking and the initial stages of acceleration, though in theVolvo bus it is kept idling under these conditions

The layout of the control system of what Volvo call their Cumulo systemcan 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 of4.87 : 1 A power take-off of the sort used for driving auxiliaries alternatelydrives 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

H J

The electronic control, with its 8-bit microprocessor, is programmed notonly for normal conditions of operation but also for warming up the engineand 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 theaccelerator pedal for sensing the torque demanded by the driver and anotherconnected to the brake pedal to sense the deceleration required A positionindicator senses whether the gearbox is in a drive ratio or neutral and a pulsepick-up senses the rotational speed of the propeller shaft and thus the vehiclespeed Another position indicator senses whether the power take-off clutch

is engaged or disengaged A potentiometer senses the displacement of theswashplate pump and a position indicator signals the volume of oil in thehydraulic reservoir, which determines when the engine should be broughtinto operation to take up the drive Finally, there is a pressure sensor on theaccumulator When the accumulator is full, the incoming oil is diverted tothe 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

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 bestarted and the bus driven away, still using the hydraulic energy As the speedrises to 22 mph (35 km/h), or if the hydraulic pressure drops below apredetermined level, the engine is automatically accelerated from idling up

to the same speed as the propeller shaft, at which point the engine andgearbox take over from the hydraulic drive When the brakes are applied, theengine reverts to idling and the hydraulic motor to a pumping mode, tocharge up the accumulator The friction brakes come into operation only ifthe control pedal is depressed beyond a spring-loaded detent Fully charged,the accumulator stores 0.22 kWh energy, which is adequate for running ahalf-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/s2, up to the cut-inspeed of the engine

The accumulator in Fig 38.34 comprises three interconnected cylinders,the outer two containing only compressed nitrogen and the inner one bothgas 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 restfrom 31 mph (50 km/h), the difference between this figure and that of 22mph (35 km/h) for acceleration from rest being accounted for by the rollingand aerodynamic resistances While the overall efficiency of the transmission

is about 80 to 85%, that of the hydraulic system is 90 to 96%

C

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 below in comparison with the value appertaining to the approach of sliding and

so, when the deceleration exceeds a certain value, the control releases thebrake, the deceleration of which will then fall to a low value and so thebrakes will be reapplied This release and reapplication of the brakes musttake place in an extremely short time if the system is to work satisfactorilyand, 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 purelymechanical means but in present-day systems electronic circuits are usedbecause 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 theiraction can be attempted

The deceleration sensor usually consists of a toothed disc attached to thehub of the wheel, and a pick-up placed near to the periphery of the disc Thepick-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 themagnet thus momentarily causing an increase in the magnetic flux throughthe winding and setting up a current in it The frequency of this current willdepend on the speed of the disc and the rate at which that frequency changeswill be proportional to the deceleration of the disc This rate can be measuredrelatively simply electronically and can then be used to supply a signal forthe control of the brakes The remainder of the system therefore consists ofvalves 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 thepropeller shaft for the two rear wheels together It will be appreciated thatthe incorporation of these anti-lock systems is facilitated by the use of power

1016 The Motor Vehicle

operated brakes, also that they are somewhat expensive and are used thereforeonly on the more expensive cars and on certain classes of commercial vehicle

39.1 Dunlop-Maxaret system

The Dunlop-Maxaret system was developed for application to the drivingwheels of the tractor unit of articulated vehicles in order to prevent thevehicles from jack-knifing The system has been most successful in doingthis

The general layout of the system is shown in Fig 39.1 and details of thevalves appear in Fig 39.2 When there is no incipient wheel slide there will

be no signal current from the electronic module to the control valve and so

the port A, Fig 39.2(a), will be open to atmosphere through the gap at C and

there will be atmospheric pressure on the right-hand sides of the brake actuator

B A

Fig 39.1 Dunlop-Maxaret anti-slide system

diaphragms Hence, when the brake pedal is depressed, air will pass freelyfrom 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 Zand thence to the left-hand sides of the brake actuator diaphragms, therebyapplying the brakes Under poor road conditions depression of the brakepedal will again pass air to the brake actuators to apply the brakes but ifwheel slide becomes imminent the electric modules will pass current to thesolenoid of the control valve, the gap C will be closed and air will pass fromthe anti-skid (upper) reservoir to the right-hand side of the brake actuatordiaphragms and release the brakes

The pressure air from the control valve 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 hand side of the brake actuator diaphragms also acts on the underside of thebalanced exhaust valve and if it exceeds the pressure acting on the uppersidefrom the port Y the central portion of the diaphragm will be lifted so as toopen the port Z to the atmosphere via the gap opened at C Thus the valveequalises the pressures at Y and Z and the brake actuating pressure will at alltimes be equal to the pressure determined by the brake pedal valve

left-As soon as the anti-skid pressure on the right-hand side of the brakeactuators is released by the cessation of the signal from the electronic modulethe brakes will be reapplied This action will be repeated with a frequency ofseveral cycles per second as long as the wheel-slide condition continues

39.2 Lucas-Girling WSP system

The general layout of the Lucas-Girling system, as applied (for the sake ofsimplicity) to a single wheel, is shown in Fig 39.3 It is designed for usewith brakes employing fluid application Under normal conditions the brake

is applied by the master cylinder in the usual way because the valve A of theactuator unit of the system is open as shown

The valve is held open by its spring and by fluid pressure on the left-handside of its piston, this pressure being maintained at a constant value by apump that is driven by the engine of the vehicle When a signal is passed bythe electronic module to the solenoid of the control valve, oil from the pumpwill pass to the right-hand side of the actuator piston and as the effective area

of this side is greater than that of the left-hand side the piston will move tothe 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 brakecylinder will drop and the brake will be released When the signal to thecontrol valve ceases the right-hand side of the actuator piston will again beopened to the atmosphere in the reservoir and the valve will open The actionwill be repeated with a frequency up to some 15 Hz, this frequency beingmodified to some extent by auxiliary circuits in the electronic module Whenseveral wheels are to be controlled each must have its sensor, electronicmodule circuit, control valve and actuator but the pump will be common toall On the other hand, to reduce cost, some vehicle manufacturers elect tosense the occurrence of wheelspin per axle instead of per wheel

1018 The Motor Vehicle

Electronic module Reservoir

Brake

cylinder

Master cylinder Actuator

Fig 39.3 Lucas-Girling WSP system

39.3 Ford Escort and Orion anti-lock systems

For the Ford front-wheel-drive Escort and Orion, the Lucas-Girling, low

cost system is used It has sensors for detecting wheel-lock on only the frontwheels, locking of the rear wheels being initially inhibited by a pressure-limiting valve These models have an X-split brake control system, as described

in Section 38.14 so, when operating, the anti-lock system alternatively relievesand reapplies the pressure to the brake not only on the front wheel that isabout to lock but also on the diagonally opposite rear wheel, Fig 39.4.Having two instead of three or four wheel-lock sensors of course is aneconomy, but other measures, including the substitution of a mechanicalinstead of an electronic sensing and control system and the avoidance of anyneed for separate, electric or engine-driven hydraulic pump to supply thebraking pressure also make major contributions to the overall cost reduction

A flywheel incorporating an overrun device serves as the mechanical sensor.This is driven from the front wheel by a toothed belt which gears it up to 2.8times the driveshaft speed The flywheel, a modulator valve unit and a cam-actuated reciprocating pump are all in a common housing, Fig 39.5

In normal conditions the flywheel accelerates and decelerates with theroad wheel, and the hydraulic modulator valve is functioning as shown inFig 39.6, in which the black areas are those in which the hydraulic pressurerises as the brakes are applied In this condition the pump plunger (11) isheld clear of the cam (10) by the plunger spring (12)

If, however, the angular deceleration of the wheel attains a value equivalent

to a 1.2g deceleration of the vehicle, wheel lock is likely to occur and so the

overrun torque generated by the flywheel, due to its inertia, rotates it a fewdegrees relative to the hub This rotation occurs within a ball-and-rampmechanism (4), which causes the axial displacement shown in Fig 39.7 Theconsequent axial movement displaces the dump-valve lever (9) about itspivot, thus opening the dump valve (7)

Fig 39.4 Ford Escort and Orion anti-lock system Note that, as compared

with Fig 39.5 the control units are upside down

The opening of the dump valve releases the pressure above the de-boostpiston (15) and consequently also relieves that in the pipeline to the brakes.Since the downwards pressure on the pump plunger (11) has also been released

by the opening of the dump valve, the master cylinder pressure acting on thepiston forces it into contact with the cam (10) Even so, the consequentreciprocation of the pump plunger cannot generate any hydraulic pressure solong as the dump valve remains open

Simultaneously, the de-boost piston, under the influence of the hydraulicpressure below it, rises to allow the cut-off valve (13) to close, as in Fig.39.8, thus cutting off the input from the brake master cylinder and relievingthe pressure in the pipelines to the brakes Therefore, the road wheels accelerate

to the speed of the still decelerating flywheel At this point the flywheel,moving back and contracting its ball-and-ramp mechanism, is accelerated at

a rate controlled by the clutch that can be seen in Fig 39.5 As the lever (9)

is released, the dump valve closes

This allows the reciprocating pump to increase the pressure above the boost piston and in the pipeline to the brakes If it again causes the roadwheel to lock, the cycle of events is repeated but if, without wheel lockoccurring, it rises to equal the pressure applied by the driver’s pedal to themaster cylinder the cut-off valve (13) is opened again, the pump disengagesand the master cylinder is reconnected The effects of the whole sequence ofoperations on the input to the brakes and on the wheel spin is illustrated inFig 39.9

de-1020 The Motor Vehicle

39.4 Ford Granada, Sierra and Scorpio anti-lock systems

The ABS (Anti-lock Braking System) on the rear-wheel-drive Granada, Sierra and Scorpio, each of which have a Y-split (Section 37.14) brake

system, is the outcome of co-operation between Ford and the German brakemanufacturers ATE It has an electronic control, with electro-magnetic sensors

on all four wheels, Fig 39.10, and an electrically driven pump and hydraulicaccumulator for maintaining sufficient reserve pressure to enable the anti-lock system to release and reapply the brakes repeatedly at rates of up totwelve times per second The electronic control module has two microprocessorswhich not only duplicate the processing of the incoming signals but alsomonitor each other continuously to check that both are functioning properly

In the event of a total system failure the brake control reverts to conventionaloperation without anti-lock control and an indicator on the dash is illuminated

to warn the driver

Fig 39.5 Sectioned control unit for the Ford Escort and Orion

Frequency signals from the four wheel sensors are translated by the electronicmodule first into wheel-speed and acceleration values and then into vehiclespeed and wheelslip When the slip becomes so great that wheel-lock isimminent, the control alternatively energises and de-energises the appropriatehydraulic inlet and outlet solenoid valves in the ABS electro-hydraulic unit,Fig 39.11, to relieve and reinstate the pressures in the lines to the brakes.There are three hydraulic circuits, one for each front wheel and the third for

4

2 6

Fig 39.6 Positions of the valves in the normal brake operating condition

4

17

9 7 14 15 Brake Master

4 Ball and ramp

5 Pump outlet valve

6 Flywheel spring

7 Dump valve

8 Pump inlet valve

9 Dump valve lever

16 Cut-off valve spring

17 Dump valve lever pivot Common key to Figs 39.6–39.8

1022 The Motor Vehicle

2

4

7 Brake

13 16

Flywheel lost motion

Dump valve opens

Vehicle speed

Dump valve reduces brake pressure Pump re-applies brakes

Accelerating shaft picks up flywheel

Flywheel accelerated

by clutch

Flywheel over runs

on clutch 1

2 3

Fig 39.9 Brake pressure and wheel speed plotted against time throughout the

sequence of operations of the Lucas Girling anti-lock brake system

reduction of oversteer by reducing the brake torque on the most heavilyloaded rear wheel At the same time, the reduction in the overall braking ofthe vehicle is minimal because of the effect of the apportioning valve inlimiting the contribution by the rear wheels to only a fraction of the total.During operation without anti-lock the front brakes are actuated by themaster cylinder with assistance from its integral hydraulic servo, while therear ones are actuated by pressure from the hydraulic accumulator Thisaccumulator is maintained at 140 to 180 bar by the electric pump Thecontrol valve linked to the piston rod of the tandem master cylinder, Fig.39.11, maintains a constant relationship between the hydraulic output pressurefrom the servo and the input force applied by the brake pedal to the mastercylinder.

39.5 Traction control

For the similar Ford models that have also four-wheel-drive based on the use

of viscous couplings, Section 31.10, a further development of the ABS electroniccontrol system takes into account also the interactions between the fourwheels, through the viscous couplings, under varying engine torques In

other words, what is termed traction control is incorporated This entails

automatic alternate application and release of the brake on either drivenwheel as soon as the microprocessors detect that it is about to spin Obviously,therefore, the electronic control module has to differentiate between wheel-lock and wheelspin With full traction control, as soon as the driving wheel

on one side spins the brake is applied on that side, but if the wheel on theother 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 electro-magnetic pick-up to a larger scale

A K

G

A Hydraulic accumulator

B Control valve

C Hydraulic booster

D ABS master cylinder

E High pressure pump

F Electric moter

G ABS valve block with six solenoid valves

H Pressure warning switch

J Main valve

K Hydraulic fluid reservoir

Fig 39.11 Two views of the Teves combined master cylinder and ABS unit The control valve equalises booster and master cylinder

output pressures

rate of fuel injection to reduce the torque output from the engine All thesecontrol operations are effected within milliseconds.

For starting from rest the traction control system must be much moresensitive to drive slip than for either simple acceleration from one speed toanother or deceleration in anti-lock systems Additionally, it must alsodifferentiate between wheel-speed differential due to the vehicle’s beingsimultaneously driven round a corner The electronic control unit is virtuallyidentical to that for the Mk IV system, Fig 39.14, which is described in thelast two paragraphs of Section 39.6

39.6 Teves Mk IV ABS and traction control

In Fig 39.12 the Teves Mk II system, for the Ford two- and four-wheel-drivecars, Sections 39.4 and 39.5, is compared diagrammatically with the Mk IVsystem Cost reduction, to render the equipment suitable also for less upmarketcars, was the primary incentive for the Mk IV development The principaleconomies were the substitution of a vacuum for a hydraulic servo, or booster,and the use of a pump of higher output, at extra cost, to obviate the need for

an accumulator With the abandonment of the hydraulic booster J containingthe control valve, equalisation of the output pressure from the ABS pump tothe brakes with that from the master cylinder has had to be effected by means

of valve A incorporated in the centre of the piston of the master cylinder.The Mk IV system, too, has acceleration/deceleration sensors on eachrear wheel, and the brake pressure to both rear wheels is reduced or increased,

as appropriate, to the level necessary to control the locking, or spinning,wheel All-wheel control gives the sensitivity needed for traction control,and greatly reduces the possibility of rear-end instability in all modes The

A Master cylinder

K

E

Fig 39.12 Comparison between the Teves Mk II (right) and Mk IV (left) anti-lock

brake systems The hydraulic circuit to only one brake is shown since the others are identical to it

1026 The Motor Vehicle

electric pump is switched on by the electronic control system only when it

is required to build up the brake pressure in the anti-lock or anti-spin modes(ABS or traction control operation), so little energy is consumed

From Fig 39.13 it can be seen that the general arrangement of the Mk IVsystem is similar to that of the Mk II except that, in the Mk IV, the mastercylinder and reservoir together have become a separate unit Also, the systemillustrated is designed for an X-split braking system, as compared with the Y-split of the Ford two-wheel drive layout

If a wheel tends to lock, the electronic controller closes the actuated hydaulic inlet valve to its brake and opens an outlet valve in the lineback to the reservoir Simultaneously, it switches on the electric motor Sincethe inlet valve to the brake circuit is closed, the fluid delivered from thepump can only force the piston in the master cylinder back until the valve inthe centre of its piston, Fig 39.12, opens to release all fluid in excess of thatrequired for ABS operation back to the reservoir This ensures that the pressuregenerated by the pump cannot exceed that induced by the driver through hisbrake control pedal, and that the driver does not lose the feedback from (thefeel of) his brake control

solenoid-As the unlocked wheel accelerates back to the appropriate speed, theoutlet valve D2 in its brake circuit closes and the inlet valve D1 opens, so thepump brings the brake application pressure back up to the level dictated bythe force exerted on the pedal If the wheel again starts to lock, the sequence

is repeated Incidentally, the non-return valves shown in Fig 39.12 preventfluid from flowing back to the reservoir under pressure exerted by either thepump, hydraulic accumulator or master cylinder when the brakes are applied.For safety, the electronic control circuit, Fig 39.14, is duplicated Thereare two identical microprocessors, each with its own comparator The

Fig 39.13 Schematic layout of Teves ABS IV anti-lock brake system

comparators check both the internal and external signals from the speed sensors and to the valves respectively If they do not correspond thedefective circuit is switched off and a warning lamp illuminated on the dash.There is also a continuous monitoring system for checking the performance

wheel-of the sensors, connections, solenoid valves and hydraulics In the event wheel-of

a failure, the brake system reverts to operation without ABS, and again thedriver is warned by a lamp on the dash

This system can be expanded to include traction control The extra cost issmall because the same sensors and valves are used, though some extravalves do have to be introduced Some expansion of the hardware and software

in the electronic controller is necessary, too, since a traction control systemmay have to apply, instead of release, a brake to prevent wheelspin and,moreover, it has to prevent the wheels from spinning at any vehicle speed It

is also required to intervene in the engine control, as described in Section39.5 An advantage of the X-split hydraulic system shown in Fig 39.13 isthat the driven wheels can be controlled individually to provide optimumtraction, instead of perhaps having to reduce the traction on both wheels tothat obtainable from the more lightly loaded wheel when cornering

39.7 Advanced anti-lock braking systems

At this point it is necessary to examine, in greater detail than earlier in thischapter, all the phenomena associated with braking, skidding and steering.Indeed, the design of an automatic braking system (ABS) entails much morethan simply alternately cutting off and re-establishing the hydraulic pressuresupply to the brakes For instance, the system must not operate at a frequencythat could cause resonant vibrations in the drive line For the same reason,precipitous pressure drops and rises have to be avoided

As indicated previously in this chapter, the peripheral accelerations of the

Logic-block

Internal signals

Sensors

Comparator 1

Comparator 2 Internal

Fig 39.14 The Teves MK IV electronic control system is duplicated

1028 The Motor Vehicle

wheels serve as indicators of impending onset of wheel locking In thiscontext, there is a difference between the dynamic characteristics of drivenand undriven wheels For instance, if a gear is engaged, especially 1st or 2nd,while the vehicle is being braked, the effective mass moment of inertia of therotating driven wheels will be perhaps as much as four times that of theundriven wheels This affects the rate of response of the wheels to braketorque variations during ABS operation Indeed, if this were not taken intoconsideration, the deceleration of the driven wheels could rise well into what

is termed the unstable braking range before the ABS system intervenes.

39.8 Braking force coefficient and slip factor

If the brakes are fully applied, the retarding force rises rapidly to a maximumand then, if the wheels lock and the vehicle therefore slides, falls off initiallyprogressively but almost immediately followed by a rapid drop to a low levelalthough not to zero During sliding, the coefficient of friction between thetyre and road is therefore significantly lower than that when the wheel isrolling Where a film of water covers the road, aquaplaning can occur, Section36.5, resulting in loss of all braking and steering control

The braking force coefficient is defined as:

µhf = Fn/Frwhere Fn = the normal, or vertical, and Fr is the horizontal friction forcebetween road and tyre The latter coefficient ranges from about 0.05–0.1 onice, to 0.2–0.65 in wet conditions, to 0.8–1.0 on dry road surfaces

As the hydraulic pressure applied to a brake increases, the braking torqueand therefore the drag force at the periphery of the tyre rises at a steady rate

So long as the brakes are on, however, there is always a degree of slipbetween the tyre and the road, owing to distortion of the rubber in theregions through which the braking forces are transmitted to the road It, ofcourse, increases with increasing brake pressure This state of affairs exists

throughout what is termed the stable range of braking.

When the drag force exceeds the limit set by the coefficient of friction

between the tyre and road, wheel lock will occur, so the wheel will be sliding

instead of rolling along the road Also, the braking force coefficient will fallrapidly, by at least 10–20% and then remain constant regardless of any

increase in brake pressure This is termed the unstable range.

Without some wheel slip, neither braking nor acceleration can occur Asimilar situation arises with steering: without a steering slip angle, there can

be no side force and therefore no steering control Consequently, if thewheels lock, steering control will be largely lost because the rubber hasdeformed to, or close to, its limit under the longitudinal braking load, leavinglittle or no spare capacity for lateral deformation If the brakes are appliedwhile the vehicle is cornering, the total friction and rubber deformationavailable has to be apportioned between the braking and steering forces: thecurves in Fig 39.15 give some idea of the proportions Clearly, the predominantforce is that due to the inertia of the vehicle, which tends to cause it tocontinue in a straight line, in conformity with Newton’s first law

The braking slip factor is defined as:

λ = (V – V )/V

where Vf = vehicle speed and Vu = the velocity at the periphery of the tyre.From this equation, it can be seen that slip occurs as soon as the wheel speedfalls below that which corresponds to that of the vehicle, in other wordswhen λ = 1 Both slip (or incipient wheel lock) and actual wheel lock (sliding)are detectable by wheel-speed sensors.

39.9 Bosch anti-lock (ABS) systems

Bosch produce a range of anti-lock brake systems including ABS 2S, ABS5.0, ABS 5.3 and the ABS/Automatic Brake force Differential lock, ABS/ABD 5 These are described and illustrated in detail in the Bosch PublicationNo.1 987 722 193 ‘Brake Systems’, 2nd edition, June 1995 and also, although

in lesser detail, in the Bosch Automotive Handbook, 2nd edition, which is

available in the English language What follows here is largely a summary ofthe basic principles, and some details of the ABS 2S system

The heart of this system is the hydraulic modulator which, interposedbetween the brake master cylinder and the wheel-brake cylinders, implementsthe commands from the ECU These commands are executed by means ofsolenoid-actuated valves which regulate the pressures in the wheel-brakecylinders The hydraulic modulator contains, in addition to the solenoid-actuated valves, an electric motor-driven fluid-return pump, and one hydraulicaccumulator chamber for each wheel-brake cylinder Ideally, the modulator

is installed in the engine compartment, to keep the pipe lines to both themaster cylinder and the wheel-brake cylinders as short as possible

Four-channel versions of the modulator are available for vehicles in which

Lateral force coefficient

Fig 39.15 Note that the braking force coefficient remains high as brake, or wheel, slip increases, while the lateral force coefficient falls off rapidly

1030 The Motor Vehicle

the brake pressure to each of the four wheels is regulated by a separatesolenoid valve For vehicles in which the brakes on two front wheels areregulated individually and those on the back axle by a single solenoid valvethere are 3-channel versions As can be seen from Fig 39.16, each valve hastwo ports and can be moved by the solenoid to any of three positions, according

to whether the pressure to the wheel cylinders is to be increased, held orreduced The cam-actuated plunger type return pump transmits back to thebrake master cylinder the fluid released cyclically from the brake actuation

Wheel

speed

sensor

ECU

Wheel brake cylinder

Fluid return pump

Brake master cylinder

Fig 39.16 The three phases of brake pressure modulation Top, pressure build-up:

solenoid armature in its lowest position, opening the upper and closing the lower

solenoid valve Middle, pressure hold: armature in its mid-position, closing both solenoid valves Bottom, pressure reduction: solenoid armature in its uppermost

position, opening the lower solenoid valve This 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.

cylinder during ABS operation Interposed between the pressure release valveand the pump is a hydraulic accumulator, the function of which is to absorbthe surges that otherwise would occur as a result of the sudden opening ofthe pressure release valve.

Hydraulic restrictors downstream of the outlet valve and upstream of theinlet valve suppress noise and prevent pressure oscillations from being reflectedback to the brake master cylinder and pedal The movement of the armature

is approximately 0.25 mm, switching times are only a few millisec, and themagnetic forces must be consistent throughout its stroke Moreover, pressures

of up to about 350 bar have to be contained Clearly, therefore, all the nents have to be produced to close tolerances and the assembly operationsstrictly controlled Filters in the inlet and outlet ports prevent entry of debrisinto the valve

compo-39.10 How the system functions

On the basis of data transmitted to it from wheel speed and other sensors, theECU issues commands to the hydraulic modulator The latter modulates thepressure generated by the master cylinder, Fig 39.17, before it is transmitted

to the wheel-brake actuation cylinders To understand this process, it is necessaryfirst to know how the solenoid valves function

When the driver actuates the brake pedal, the first pressure build-up phasebegins During this phase, no current flows through the solenoids From theillustration, it can be seen that the pressure generated by the master cylinderopens the uppermost valve, against the force exerted on its return spring Atthe same time, the lower valve is held closed by the lower return spring.Consequently, the fluid from the master cylinder flows through the openvalve, down through the duct on the right-hand side of the armature, and out

Vehicle speed Reference speed

Fig 39.17 Braking control characteristics for dry road conditions, where λ1 is the slip

switching threshold, + a1 and + a are the thresholds of peripheral acceleration, and –a

is the threshold of peripheral deceleration of the wheel

1032 The Motor Vehicle

to the wheel cylinders The rising pressure in this fluid supplements the forceexerted by the return spring holding the lower valve on it seat

As soon as a wheel or wheels lock, the ABS system comes into operation

and the ECU calculates the first of a series of reference speeds, Vref At thesame time, the solenoid valve is fully energised to lift the lower valve off itsseat The upper valve is held on its seat by its return spring supplemented bythe falling pressure in the return circuit from the wheel cylinder With thelower valve open, the fluid in the wheel cylinder passes back through thesolenoid-bypass circuit, past the hydraulic accumulator and through the firstcheck valve and the return pump cylinder, and then on through the secondcheck valve to the master cylinder As the fluid release phase ends, this flow

is expedited by the eccentric-actuated fluid return pump, which is driven by

a tiny electric motor controlled by the ECU

As soon as the wheel unlocks, the fluid release phase ends and the solenoidvalve returns to either its pressure build-up or its pressure hold position,

depending upon the rate at which the wheel decelerated towards the Vref

position To move the valve to its pressure hold position, only half the maximumvoltage is applied to the solenoid Consequently, the armature is lifted onlyhalf way up its travel, which is far enough to lift the upper valve until it isseated by its return spring, but not enough to open the lower valve Thismovement increases the compression in the return spring holding the uppervalve on it seated against the force exerted by the hydraulic pressure generated

in the master cylinder With both valves now seated, the circuit to the wheelcylinder is closed and the pressure in it therefore retained at the level atwhich the wheel unlocked

The rate of repetition of these control cycles varies, according to thecondition of the road, from 4 to 10 cycles per second Similarly, the ECUregulates, again according to the road condition, the rises and falls in pressure

39.11 The reference speeds

On the basis of the signals it receives from wheel-speed sensors mounted onboth the driven and undriven wheels, the electronic control unit calculates a

series of reference speeds, Vref each of which is the datum on which the nextstage in the succession of pressure holds, releases and re-applications isbased On the assumption that the clutch is disengaged when the brakes areapplied, this reference speed is based, at moderate speeds, on the signalsfrom the fastest rotating wheel When, however, ABS is in operation duringemergency stops at higher speeds, the reference speeds are extrapolateddirectly from that at the beginning of the ABS cycle

The acceleration and deceleration rates at the peripheries of the wheelsare supplemented by data on the actual deceleration of the vehicle This isbecause, once the vehicle enters the unstable braking range, the signals fromthe undriven wheel-speed sensors are no longer reliable as control variables:

in this range the slightest increase in pedal pressure may be enough instantly

to induce wheel lock

The threshold defining the peripheral deceleration of the wheel beyondwhich ABS becomes active should not significantly exceed the maximumpotential deceleration of the vehicle If it were to do so, the wheels wouldenter well into the unstable range before ABS came into operation On theother hand, the control system should not respond too early, otherwise valuable

braking distance might be lost Therefore the electronic control continually

calculates different reference speeds, Vref, as the basis for determination ofthe threshold, λ1, at which the solenoid is switched into its brake pressurerelease phase, Fig 39.17

From this illustration, it can be seen that if the curve of peripheral

deceleration of the wheel falls below a predetermined rate –a, the solenoid

is switched briefly into its brake pressure-hold mode and the reference speed

Vref is lowered to V1 If, at the end of this brief hold phase, the peripheral

speed continues to fall faster than that of the vehicle, it follows that V1 iscoincident with λ1, so the solenoid is switched to its pressure release mode

When the peripheral deceleration rises above the –a level, indicating that the

wheel has entered the stable braking range and is slightly underbraked, thesolenoid switches to pressure hold It then stays on hold longer than the first

time, until the wheel has accelerated to a level +a and is continuing to +a1,

at which point the pressure is allowed to progress upwards again in a series

of brief pressure hold and pressure increase stages, as also illustrated in Fig

39.17 When the acceleration curve again drops below the –a level, the cycle

is repeated, unprompted by a λ1 signal

If the ECU recognises slippery road conditions, it switches to a different

pattern of responses to wheel lock, Fig 39.18 Here, the level –a is, of

course, much closer to the zero, and the whole sequence is effected moreprogressively Apart from these changes, phases 3 and 4 become 2, 3 and 4

In phase 3 which, in Fig 39.17 is phase 4, the hold period is shorter but isfollowed by a pressure release period, prior to a longer period of pressurehold Then come a series of brief pressure rises and holds up to the end ofphase 5, when the pressure is again released, followed by a protracted holdperiod and then a stepped increase Both sequences are described and illustrated

in much more detail in the Bosch publication quoted at the beginning ofSection 39.9

λ 1

Peripheral speed Switching signals

1034 The Motor Vehicle

39.12 Wheels on one side on ice and on the other on tarmac

Another special feature of the Bosch systems is compensation for the yawingeffect that occurs if, for example, the two nearside wheels are on mud or ice

and the two offside on tarmac, or vice versa This is a yawing moment delay

system, abbreviated by Bosch to GMA When the brakes are applied, thebraking effect on the two wheels on the tarmac is, of course, considerablygreater than that of the two on the slippery side, and therefore causes apronounced tendency to yaw The effect is more pronounced with smallvehicles than larger and heavier ones having longer wheelbases This isprimarily because the latter have greater inertia about the vertical axis, so therate of yaw is slower, allowing the driver more time in which to applysteering correction to counter the tendency

For large cars, with relatively uncritical response characteristics, Boschapply their system GMA1 A comparison between the rates of pressure build-

up with and without yawing moment correction are illustrated in Fig 39.19

On each side, the braking force is regulated by the ABS system to providethe maximum attainable overall braking effect In the following we assumethat it is the nearside wheels that are on the slippery surface

Master cylinder pressure

Brake pressure P H without GMA

Steering angle with GMA

Time

Fig 39.19 GMA is the Bosch abbreviation for their yawing moment delay 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

Brake pressure P L

As soon as the pressure is released for the first time on the nearsidebrakes, that on the brakes of the offside wheels continues to increaseprogressively, in short rise-hold stages When the pressure on the offsidebrakes attains the lock level, continuing signals from the nearside wheelsbecome irrelevant, so the pressure on the offside is then controlledindependently, to obtain the optimum overall braking performance There is

a delay in attaining maximum pressure on the offside wheels but it is only ofthe order of 750 msec, so the effect on braking distance is very small.System GMA2 is suitable for vehicles with more critical responsecharacteristics On release of the pressure to the nearside brakes, that to theoffside brakes begins a series of hold-reduce phases, which are more protractedthan those of system GMA1 When the pressure next increases on the nearside,

so also does that on the offside where, however, the pressure accumulationtimes are longer At the same time, those on the nearside increase progressively.This program of events continues so long as the brakes are applied.Because the effects of yaw on steering response become increasinglycritical as speed increases, system GM2 divides vehicle speed into fourcategories or ranges, each being allocated a different yawing moment build-

up delay While the pressure build-up periods remain constant for the nearsidewheels, those for the offside wheels are reduced stage by stage with increasingspeed The aim is at reducing the yawing moment build-up with increasingspeed Whereas GMA2 is implemented by two supplementary microprocessorsoperating in parallel, with mutual monitoring, GMA1 is effected by large-scale integrated circuitry in the ECU

Yet further refinement of the system is needed to cope with the situationarising if the nearside wheels are on, for example, ice and the offside wheelsare on tarmac during cornering With GMA in operation, the effect is toincrease dynamic loading on the front suspension and decrease it on the rear.The result is a corresponding increase in the lateral forces on the front and

a decrease on the rear This tends to cause the vehicle to slide towards thenearside In this situation, steering correction is difficult to apply To counterthis effect, the GMA system is deactivated when one of the sensors serving

it signals that lateral acceleration rates have risen to 0.4g In these circumstances,

the higher level of braking on the offside front wheel tends to generate, aboutthe vertical axis of the vehicle, a torque tending to rotate it outwards relative

to the curve, thus compensating for the inwardly directed torque The outcome

is a mild tendency to understeer, which is relatively easy for the driver tocorrect

39.13 ABS for cars with 4-wheel drive

Various 4-wheel drive layouts have been outlined in Section 23.4 In allsystems, it is necessary to reduce the engine overrun torque if the ABS is to

be effective on low friction surfaces This can be done either by increasingthe idling speed of the engine permanently, or by an electronic control thatdoes so automatically Another problem is that engine rotational inertia canreduce the sensitivity of the ABS system to variations in friction coefficients,especially on slippery surfaces To solve this problem, supplementary signalprocessing and logic functions must be incorporated in the electronic controlsystem The requirements for these functions differ for each type of 4-wheeldrive layout

1036 The Motor Vehicle

Consider the type of layout in which the rear axle and inter-axle differentiallocks are either equipped with viscous couplings or are locked manually Ifonly the rear axle differential lock is engaged, the two wheels rotate together

so some of the overall braking and traction effects of the back axle may belost on slippery surfaces It follows that, if the wheel on one side is on a highand the other on a low friction surface, braking and traction on the higherfriction surface will be the overriding force until the wheel on that side locks

or spins Consequently, in the Bosch systems when the rear axle differential

is locked, the system is automatically switched into a mode in which only theresponses of the wheel on the high friction side are taken into account

If, for example, the nearside wheel is on ice, that wheel will slide and theyawing torque due to the greater drag force at the offside wheel will have astrong tendency to cause the vehicle to yaw towards the offside Shouldmaximum braking force then be applied suddenly to the front wheels, thevehicle would slide uncontrollably To counter this possibility, GMA is applied

to the front wheels, so that stable conditions and steering control can bemaintained

To reduce the effect, under slippery conditions, of the dynamic couplingbetween all four wheels and the engine and transmission inertias, Boschincorporate a linear-deceleration switch This switches the response threshold

–a to half its normal value when the coefficient of friction falls below 0.3, so

that incipient wheel lock is detected earlier At the same time the referencespeed rises at a lower rate and is restricted to lower levels Consequently,incipient wheel lock is recognised earlier and the control system becomesmore sensitive

Rapid application of the brakes on slippery surfaces could cause all thewheels to lock simultaneously The Bosch system avoids this because itselectronic control responds by setting the reference speed based on the slippingwheels so that it does not exceed that appropriate for maximum potentialdeceleration of the vehicle Additionally, ABS pressure release is triggered

by a critical difference in wheel speed and a peripheral acceleration threshold

of –a.

In 4-wheel drive systems incorporating a freewheel with the inter-axleviscous coupling, rapid application of the throttle can trigger wheel spin onall four wheels To counter this, the supplementary system just describedmust again be incorporated in the electronic control However, since thefreewheel automatically uncouples the rear wheels, ABS operation remainssatisfactory under all conditions of braking Nevertheless, ABS operationcan be improved by the incorporation of automatic engine drag torque control

In installations in which differential locks that are actuated automatically

by the electronic control, they need to be automatically released when thebrakes are applied Consequently, although the special signal processing andlogic features already described are necessary to cater for all four wheelsslipping, additional supplementary devices are not required

39.14 Traction control in general

Wheel spin and sliding can be a result of not only an increase in braketorque, but also an increase in drive line torque, or traction, beyond thatwhich can be transmitted by friction between the tyre and road Therefore, asystem controlling traction automatically, as a supplement to ABS, is desirable

as a means of retaining stability and accurate control over the vehicle, especially

on slippery surfaces Modern systems of this kind generally control not onlythe drive torque but also the drag torque generated when the throttle issuddenly closed, or a shift is made into a lower gear This is done on the basis

of a comparison of the rates of acceleration, or deceleration, of the drivingwheels with those on the undriven wheels, and appropriately adjusting thethrottle control Thus, engine braking is retained and the wheels preventedfrom sliding

The requirements for such a system are that it must prevent wheel spinunder the following conditions:

1 On slippery road surfaces

2 When the surface is slippery under the wheels on only one side of thecar

3 Under conditions of alternating patches of slippery and dry surfaces

4 During acceleration while cornering

5 When pulling away from a stand-still on a steep slope

Traction control offers the following benefits as regards improved stability:

1 By preventing wheel spin while cornering, it increases the lateral forceavailable, at the contact patch between the road and tyres, for steeringthe vehicle

2 By obviating sudden transitions between wheel spin and grip, it reducesthe wear and tear on the drive line

3 It prevents the tyres from dragging on corners, as they can do withdevices such as differential locks and viscous couplings

These systems have to override driver control of the throttle pedal Therefore there can be no mechanical linkage between the pedal and the throttle or, for diesel engine

driven cars, the governor control lever Instead, a drive-by-wire system isemployed, in which a pedal position sensor translates throttle pedal angleinto an electric signal This, together with other signals from, for exampleengine temperature and engine speed sensors, is transmitted to the electroniccontrol unit These signals are then translated into a control voltage for anelectric servo-motor that actuates the throttle valve or governor control level

on the engine Wheel spin is monitored on the driven wheel on each side ofthe vehicle If one or both wheels spin, combinations of brake, throttle andignition controls can be employed In some simpler torque control systems,either the brakes or throttle valve control are used alone to stop wheel spinbut response times can be so long as to cause unacceptable delays

39.15 Bosch ASR2-DKB traction control system

Robert Bosch GmbH produce a range of automatic torque control systemseach based on a different method of control They are designated ASR systems,the letters being an abbreviation for Antriebsschlupfregelung, which is threewords in one, roughly meaning drive slip regulation One of these is theirASR2-DKB system, in which the letters DKB stand for Drosselklappe andBremsen (throttle valve and brakes) In fact, it is a combination of their ABS2S anti-lock brake system, Section 39.9, with their ASR, or torque controlsystem

1038 The Motor Vehicle

When wheel lock occurs, the ABS system comes into operation On theother hand, when wheel spin occurs, a pilot valve is automatically switched

to introduce ASR This activates hydraulic accumulators for supplying pressure

to the brakes on the driven wheels without application of the brake pedal.The accumulators are charged by electrically driven hydraulic pumps Whenwheel spin is detected, pressure modulation is applied to the wheel-brakecylinders of the driven wheels by the solenoid valves of the ABS system,which the ASR system utilises to control wheel spin in the same manner asfor ABS: that is, by alternating between the pressure build-up, hold andrelease Return pumps help to discharge the fluid from the wheel-brakecylinders but, with ASR, it passes into the accumulators instead of, withABS, to the master cylinder

By applying the brake on the driven wheel that is spinning on, for example,

a patch of low friction surface, this system in effect serves as a differentiallock By preventing the wheel on the low friction side from spinning, itenables maximum torque to be delivered through to that on the other side.During this process, ASR may also be applied, if necessary, to the wheel onthe high friction side

39.16 Bosch ASR2-DKZ/MSR system

This system comprises the ASR 2s ABS with throttle control (Drosselklappe)and ignition (Zündung) control for ASR, combined with injection control toreduce the delay before engine overrun torque, MSR, control becomes effective

To prevent wheel slip, braking control is not utilised

Initially, the ignition is retarded to inhibit wheel spin If this fails to havethe desired effect, the electronic control suppresses some of the ignition andinjection pulses simultaneously To ensure smooth operation of the enginewhen normal operation is resumed, the ignition initially remains retardedand, after a brief pause, progressively returns to normal The relativeeffectiveness of the different methods of intervention is illustrated in Fig.39.20

Throttle valve + wheel brake

Throttle valve

only

Throttle valve + ignition

ASR response time Fig 39.20 The relative effectiveness of the methods of intervention for application of traction control

Bosch also have other ASR systems, including the ASR5 with engine andbraking intervention and ASR5 with engine intervention only Details ofthese can be obtained from the Bosch publication quoted at the beginning ofSection 39.9.

39.17 Lucas-Girling Skidchek GX

Following some years’ experience with their original Skidchek system forheavy commercial vehicles, Lucas-Girling introduced the GX version Thiswas virtually a redesign based on an analysis in detail of the performance ofthe original system in service The fundamental considerations, they found,were as follows

To minimise the delay between the driver’s observing a need to brake andthe actual application, a rapid rise in air pressure is essential It is this that isliable to cause a skid, because of the tendency for the pressure to overshootthe target pressure level and apply the brakes too hard Unfortunately, however,

it is not practicable to reduce the rate of pressure rise for the initial applicationwithout unacceptably increasing the stopping distance, as measured from theinstant the driver depresses the foot valve

The only possibility for reduction of this tendency to overshoot, therefore,

is early detection of wheel-lock One requirement is to have a sensor on eachwheel and to trigger the air pressure release sequence following detection ofincipient wheel-lock on the slowest wheel of the pair on an axle, or of twopairs on a bogie Lucas-Girling considered that to economise by using onlyone sensor and placing it on the propeller shaft could increase the reactiontime This is because vibrations in the drive line can generate spurious signalsand the electronic detection system then has to wait a few milliseconds toestablish whether these signals are real before it can initiate an anti-locksequence Obviously, earlier detection of incipient wheel-lock might also bemade by designing the electronic control system to respond to factors otherthan just wheel acceleration – for instance, to rate of change of acceleration,possibly in relation to wheel and vehicle speeds

Once the initial brake application has been completed and the anti-lockcycles begin, however, it is possible to modify the rate of pressure build-upfor each successive reapplication This is done in the GX system by the use

of a modified relay valve, which memorises the pressure at which the wheels

previously locked The new valve, called the memory-controlled relay (MCR)

valve, is illustrated in Fig 39.21 It is similar to a conventional relay valveexcept that a solenoid, a latch valve and the memory chamber have beenadded

When the driver depresses the brake pedal, air from the foot valve enters

at the control port The solenoid is not energised, so this air lifts the latchvalve and flows rapidly down, past the solenoid valve – seated on its exhaustport – into the control chamber, pushing the control piston down This causesthe piston to seat on the end of the tube below, which is the exhaust port forthe brake application system, and to push this tube downwards against itsreturn spring The latter action opens the modulation tube valve, by lowering

it from its seating, thus allowing air to flow rapidly from the reservoir port,past this valve to the brakes

When the electronic control unit detects a wheel-lock, it energises thesolenoid, lifting the solenoid valve off its exhaust seat and closing it on to the

1040 The Motor Vehicle

control pressure supply port This allows the pressure in the control chamber

to exhaust past the solenoid to atmosphere, so the control piston rises again,closing the modulation tube valve and allowing the pressure in the brake line

to exhaust rapidly to atmosphere through that tube valve

In the meantime, while the brakes were on, two things have happened:first, air from the control port has leaked through the restricted orifice in thecentre of the latch valve and equalised the pressure above and below it;secondly, the non-return valve in the base of the memory chamber has lifted,

so the pressure in that chamber is that of the control system, as dictated bythe force applied by the driver to his pedal

Then, when the solenoid was energised and the pressure below the solenoidvalve dropped to atmospheric, the valve in the base of the memory chamberdropped on to its seat, so the pressure in that chamber could fall only at theslow rate dictated by the size of the orifice on the right, just below that seat.Consequently, when – in response to a signal from the sensor indicating thatthe road wheel has regained its appropriate operating speed – the electroniccontrol de-energises the solenoid, the pressure remaining in the memorychamber is dependent on both the original pressure that initiated the wheel-lock and the time that the wheel has taken to recover to its normal speed Thelatter time of course is a function of the grip – or lack of it – of the wheel onthe road surface

This de-energisation of the solenoid – closing its exhaust valve and openingthe control valve supply port – releases the control pressure into the controlchamber The resultant drop in control pressure pulls the latch valve down on

to its seat, leaving only a small passage open past the seat to the control

Fig 39.21 MCR valve operation for rapid reapplication of brakes

Memory chamber

Control port Latch valve