The Motor Vehicle, Page 831 Chapter - 29 Semi- Automatic Gearboxes and Continuously Variable...

841

Chapter 29

Semi-automatic gearboxes andcontinuously variabletransmissions

Automatic transmissions embodying torque converters suffer two majorshortcomings. One is inevitable loss of energy in the converter and the otheris the complexity and weight of the epicyclic gearing and clutches and brakesrequired if ratio changes are to be made without interrupting the power flowto the road wheels. Moreover, the smaller the engine the greater is thesignificance of the power lost relative to the total power available.

Over many years, therefore, a great deal of effort has been devoted to thedevelopment of automatic and semi-automatic transmissions, to provide twopedal control without the use of a hydraulic torque converter. Some introduceda fluid coupling to replace the clutch. Others were based on a magneticclutch, notably the Ferlec unit. Then there was the Smiths Industries Easidrivetransmission, which was based on a magnetic powder clutch, The MotorVehicle, 7th edition. In all these, the aim has been to achieve higher mechanicalefficiencies than are possible with torque converter transmissions. Until recently,none achieved widespread acceptance but, now that electronic controls areavailable, the prospects are good. Indeed, several automotive manufacturershave gone into production with semi-automatic transmissions.

29.1 AP semi-automatic gearbox

This is a four-speed and reverse gearbox in which bevel gears are used asordinary gearing for some of the ratios and as epicyclic trains for the otherratios. It incorporates a single-stage torque converter whose reaction memberis held by a one-way clutch up to the change-over point. The automaticchanges are controlled by the speed of the vehicle through a governor drivenby the output shaft of the box and by the position of the accelerator pedal.The selector lever has seven positions: reverse, neutral, 1, 2, 3, 4 and D2, thislast one giving full automaticity. The selector lever can be used to givemanual control at all times, the gear selected being held until the leverposition is changed. In automatic operation, kickdown changes are available.

842 The Motor Vehicle

The layout of the gearbox is shown in Fig. 29.1; there are four bevel gearsS1, S2, S3 and S4, the latter being always the driven gear while either S1 or S2can be the input or driving gear according to whether the clutch A or theclutch B is engaged. The clutches are actually multi-plate ones and incorporatea piston to enable oil under pressure to engage them. The plant carrier hastwo compound planet pinions, only one of which is shown in the diagram.The ratios are obtained as follows—

1st Clutch A is engaged so that S1 becomes the driving gear, the spragF prevents backward motion of the planet carrier which thereforeremains stationary because the reaction torque that acts on it isbackwards. The drive is from S1 to P1 and thence from P2 to S4, theplanet pinion rotating on its fixed pin.

2nd Clutch A remains engaged and the brake D is applied to fix the sunS3. Rotation of the sun S1 then causes the planet carrier to rotateforwards and so the planet P2 drives the sun S4 forwards.

3rd Clutch A remains engaged but the brake C is applied to fix the sunS2. Rotation of S1 again causes the planet carrier to rotate forwardsbut gives a higher ratio because S2 is equal in size to S1 whereas insecond gear S3 was larger.

4th Both clutches A and B are engaged so that the suns S1 and S2 arelocked together and the gear has to rotate solid thus giving a 1 : 1ratio.

Reverse The clutch B is engaged and the brake E is applied to hold theplanet carrier stationary. The drive is from S2 to P1 and thence fromP2 to S4.

The oil pressure to apply the brakes and engage the clutches comes, whenthe engine is running, from a pump that is driven off the cover of the torqueconverter. To enable a towed start to be obtained a second pump driven offthe output shaft is provided. The change-over from one pump to the other ismade by the flow-control and tow-start valve seen at the bottom left-hand

3rd gear brake C

Input

S2 S1Fixed casing

Output

FS4

Planetcarrier

Reverse brake E2nd gear brake D

S3

P2 P1

Forwardclutch

A

BReverse clutch

Fig. 29.1 AP automatic gearbox running gear

843Semi-automatic gearboxes and continuously variable transmissions

corner in Fig. 29.2, where its position corresponds to that assumed when theengine is running and oil is passing from the main pump through the flow-control valve spool to the control system. In this condition the tow-startvalve is held in the position shown so that the auxiliary pump can dischargefreely back to the sump. When the engine is not running the flow-controlvalve is moved to the left by its spring and blocks the pipe coming from themain pump, the tow-start valve spool is also moved to the left thereby closingthe passage to the sump and connecting the auxiliary pump to the controlsystem so that if the vehicle is towed the auxiliary pump can supply oil to thecontrol system.

The pressure of the oil supply is maintained at the designed value by theregulator valve which is the lower spool in the kickdown and regulator valveassembly. The oil from the pump passes through the diagonal hole drilled inthe spool and thus sets up a pressure that moves the spool upwards andrestricts the entry of oil so that a balance of forces is reached between theupwards oil pressure and the downwards spring force.

The control system. The box can be used either as a manually-controlledbox, the gear changes being produced by moving the selector lever to therequired position, or the changes, except into reverse, can be obtainedautomatically by putting the selector lever into the D position. Consideringthe manual operation Fig. 29.2 shows the selector valve in the positioncorresponding to N, or neutral, all the brakes being off and the clutchesdisengaged. If the valve is now moved upwards to reverse then oil will passfrom pipe a to pipe b and so to the reverse brake; it will also pass to the top

Valve To throttlelinkage

To 3rd gear brake

Kick-down & regulator Selector valve 2nd & top valve 3rd gear valve

To kick-downservo

To reversebrake

To forwardclutch

To t

orqu

e co

nver

ter

Sump

Sump

Sump

Flow control Tow start

SumpGovernor valve

Sump

From auxiliary pumpFrom main pump

a b

cp

s

j

d

l

f

j

k

e

h

k k

ml

To reverseclutch

g

To 2

nd g

ear

brak

e

Sump

Fig. 29.2 AP Automatic gearbox control system


of the second and top gears valve and will move the spool downwards toopen pipe c and so pass oil to the reverse clutch which, on engagement givesthe reverse gear. When 1 is selected the valve is moved downwards and oilpasses from a to d and engages the forward clutch to give first gear; no brakehas to be applied because the planet carrier is held by the sprag. Pipe b isopened to the sump to release the reverse brake and the spool of the secondand top gears valve moves back to its top position thus opening pipe c to thesump via pipe p. The supply to the forward clutch remains open throughoutall the subsequent changes so that the forward clutch remains engaged. Bymoving the selector lever 2 oil is passed from pipe e to f; it enters the secondand top gears valve and raises the middle spool so that oil flows via pipe gto the second gear brake, giving the gear. When 3 is selected the oil flowsfrom pipe h to j and so to the third gear valve where it depresses the lowerspool so that oil flows to the third gear brake via pipe s; at the same time thepipe f is opened to the sump so that the second gear brake is released. When4 is selected oil flows from pipe h to k and, if the engine speed is not low,from k to p and thence via c to the reverse clutch, thus locking S1 and S2together and giving the 1 : 1 ratio.

When the selector is put in the D position the selector valve reaches itslowest position, pipe a remains connected to pipe d so that the forwardclutch remains engaged while pipe h is connected to pipe k to give a supplyof oil to the governor valve. As this valve moves to the left because of theincreasing speed of the car it first connects k to l and thence to the secondgear brake via the second and top gears valve and pipe g, thus giving secondgear. Further movement causes the pipe 1 to be connected to the sump, thusreleasing the second gear brake and at the same time k is coupled to m andhence to the third gear brake; at the same time the bottom spool in the secondand top gears valve is moved upwards to give a free flow of oil from thesecond gear brake to the sump and take that brake off quickly. When thegovernor valve moves to its extreme left position the pipe k is connected topipe p and hence to pipe c so that the reverse clutch is engaged as well asthe forward clutch and gives the 1 : 1 ratio.

The kickdown changes are produced by the operation of the cam on thespool of the kickdown valve which results in oil passing from pipe k to t andso to the kickdown servo. This is a small ram which bears on an arm of thegovernor linkage and forces the governor valve spool to the right therebyproducing the required downwards change.

29.2 AP hot-shift automatic gearboxWith fuel prices increasing, it became obvious long ago that the trend for thefuture must be towards smaller engines driving lightweight cars. This stimulatedAutomotive Products to re-examine the prospects for developing a simplerand more efficient automatic transmission based on the traditional, highlyefficient, manual gearbox. A major advantage was that both manual andautomatic could, if necessary, be produced on the same production line, or atleast a minimum of investment would be required in new manufacturingfacilities and equipment for production of the automatic transmission.

A prime requirement was the development of a hot-shift system – changinggear without interrupting the power flow to the wheels. Closing the throttleand re-opening it again for each gearshift is not only a tricky operation, but


also it can even be dangerous – for example, potentially initiating skids onice– and can render emission control difficult. Consequently, a new system,based on the use of two clutches and transferring the drive from one to theother for each shift, was developed.

To understand how it has evolved, look at Fig. 29.3, ignoring the peripheralequipment, including the clutches, and concentrating attention on the mainshaftand the layshaft, top and bottom respectively. The main difference betweenthis and the equivalent manual gearbox is that the gear pairs for second andthird speeds have been interchanged – the order, left to right, in the conventionalbox would be 4, 3, 2, 1. Additionally the mainshaft has been divided, leavingfirst, third and reverse gears driven conventionally through the main clutchA, on the engine flywheel, while second and fourth gears are integral with asleeve rotating freely on the mainshaft and driven through a second, smallerclutch B, mounted on the rear end of the gearbox. This second clutch isdriven by a quill shaft extending from the engine crankshaft right through thegearbox.

The main clutch is released by the hydraulic actuator, top right in theillustration, and engaged by its diaphragm spring, in the usual way. Since thesecondary clutch is not used for driving away from rest, but only for gearchanging, it is not subject to much slipping and therefore can be smaller andis of the wet type. It is engaged by hydraulic pressure acting on the rear face

Fig. 29.3

Pump and controlvalves

Hydraulic cylinder

A

B

C

Wetclutch

Actuators4 2 3 R 1

Standard clutch


of the pressure plate C. Hydraulic power is provided by the pump, top left,driven by the continuously rotating portion of the second clutch, and thegears are shifted by hydraulic rams, represented by the two rectangles at thebottom of the illustration.

Overall control is exercised by a computer, preferably working in associationwith the engine management computer. So far as the transmission is concerned,the inputs to this control come from a governor, and sensors for indicatingwhich gear is engaged, throttle position and engine and gearbox outputspeeds – the throttle-position sensor of course is, in effect, a torque-demandsensor. The outputs go to control the valves for the hydraulic actuators of theclutches and gear shift rams. All gears are held in engagement by hydraulicpressure and are sprung out into neutral.

The sequence of operations is as follows: with the engine idling andgearbox in neutral, selection of drive, using the manual control, first causesdisengagement of the primary clutch and then a shift into first gear. Depressionof the accelerator initiates automatically the gradual engagement of the primaryclutch, thus setting the vehicle in motion.

At an appropriate speed, the governor signals a gear change. Since thesecondary clutch is already disengaged, second gear can be selected by itsactuator operating through its synchroniser. Then, the rear clutch engageswhile the front one simultaneously disengages, to perform the hot-shift.Finally, the hydraulic pressure to the first gear is released, so it springs outof engagement, and the primary clutch is re-engaged, to keep all parts rotatingat engine speed and thus keeping the demands on the synchroniser for thenext gear shift to a minimum. Subsequent gear shifts are made similarly, bytransferring the drive from one clutch to the other, which is why the secondand third gears in the box had to be interchanged for this design.

The claims made for this fully automatic transmission are as follows: itsefficiency is as high as that for a manual box except in that a little power isabsorbed by the hydraulic pump. Both the weight and cost are certainly nomore, and are expected ultimately to be less, than for a conventional automaticbox. Other advantages include ease of servicing, minimum of investmentrequired in new tooling and the ease with which the control can be integratedwith an electronic engine-management system.

For front-wheel-drive cars, a compact four-speed design is attainable,using a three-shaft layout, Fig. 29.4. Similarly, a six-speed version can bedesigned on a three-speed basis, Fig. 29.5, and with so many gears it ispossible to approximate fairly closely to the characteristics of an idealcontinuously-variable-ratio gearbox, Fig. 29.6. The outcome, AP claim, wouldbe a potential saving of 25% in fuel consumption.

29.3 Ricardo ALT automatic transmissionLate in 1987, the Ricardo Automatic Layshaft Transmission (ALT) unit, Fig.29.7, was announced. It was designed by Peter Windsor-Smith and developedby Ricardo Consulting Engineers. Peter Windsor-Smith also played a majorpart in the design and development of the Maxwell transmission for buses,which is described in detail in Section 29.7. Although the ALT transmissionis being developed, in the first instance, for cars, it has potential also forheavy duty applications.


0 10 20 30 40 50 60 70 80 90 100 mph

60

50

40

30

20

10

Fue

lco

nsum

ptio

n

Speed

4th

5th

3rd2nd

1st

6th‘Ideal’ gearbox

6-speed automatic transmissioncomputed fuel consumptions for each gear on the levelm

pg

Fig. 29.6 Six-speed automatic transmission; computed fuel consumptions for eachgear on the level

Basically, the gearbox is identical to a conventional five-speed synchromeshlayshaft type unit but multi-plate wet-clutch units replace the synchronisers.These clutches are hydraulically actuated, automatic control over the hydraulicsystem being effected electronically. A signal from the automatic controlinitiates engagement of the clutch in the constant-mesh gear pair appropriateto the prevailing engine torque and vehicle speed. As the conditions change,the clutch serving a gear pair that has subsequently become more appropriateautomatically takes over from the first. If all clutches are disengaged, nodrive can be transmitted. The clutch actuators do not rotate, so there is

A

R 1 3

B

2

A

2 4 6

B

1 R 3 5

Fig. 29.4 Fig. 29.5

4


virtually no leakage past their seals. Two-pedal control is practicable, sincethe conventional single-dry-plate clutch can be dispensed with, and startingthe vehicle from rest is effected by the highly developed multi-plate clutchthat engages first gear.

29.4 Alfa Romeo Selespeed transmissionAmong the recently introduced semi-automatic gearboxes is the Selespeedtransmission control system, designed by Magneti Marelli and installed inthe Alfa Romeo 156. Its electronic control works in association with that forthe Bosch Motronic fuel injection system installed on that vehicle. By virtueof the resultant integration of the controls over transmission and engine,accurately regulated re-establishment of torque progressively to the wheelsis assured, following its interruption for gear shifts. This is of particularbenefit on slippery surfaces, and significantly enhances occupant comfort.

The Magneti Merelli system has been designed for application to anyconventional manual transmission. In this instance a five speed gearbox hasbeen employed. The gears can be selected either manually, by actuatingeither a two-button control on the steering wheel or a conventional gear shiftlever on the tunnel between the seats, or it can be set to operate automaticallyfor operation in city traffic. City, or automatic mode, of operation can beselected at the flick of a console-mounted switch. The electronic control

Fig. 29.7 The Ricardo automatic layshaft transmission (ALT), as originally developedfor a 1.1-litre car, has multi-plate wet clutches where the synchronisers wouldotherwise be


even takes into account driving style: if the engine speed is consistentlyrising above 5000 rev/min with the throttle depressed more than 60%, theclutch actuation cycle time is reduced to 0.4 sec, as compared with the 1.5sec of less sporty driving.

Either the button or the gear shift lever can be used to select either of twopositions: change up, or change down. The buttons, however, automaticallycut out of operation as the speed falls below 10 km/h. At higher speeds, thesteering wheel mounted control is perhaps most appropriate because thedriver is unlikely to want to change gear while the wheel is turned throughanything other than relatively small angles. Below 10 km/h, the lever on thetunnel must be used for changing gear. This largely obviates the possibilityof inappropriate gear shifts being inadvertently called for during, for example,parking manoeuvres. The gear shift lever has two additional positions: bypushing it to the right before moving it forwards or backwards, it can be usedto select either neutral or reverse respectively.

As previously indicated, this transmission can be operated in either of twomodes: city or semi-automatic. When the former is selected, using the switch,the panel in the engine speed indicator displays the word ‘City’, followed bythe number of the gear that is currently engaged. In this mode of operation,the gear shifts are effected automatically, with gear changing patterns designedfor driving when frequent gear shifting is needed in traffic. In the semi-automatic mode the gear shifts are selected manually by the gear shift lever,but nevertheless with two-pedal control.

Clutch movement and gear selection are controlled electronically, butactuation of the clutch and the shifting of the gears are effected hydraulically.Four hydraulic actuators are employed: one is for the clutch, the secondselects the gear by rotating the selector rods and the third, by moving themaxially, engages the gears, Fig. 29.8. The fourth actuator is linked to thethrottle control, for momentarily opening and closing it or double declutchingduring downshifts. As a result, all gear shifting can be effected without thedriver having to take his hands off the steering wheel, except at very lowspeeds.

The hydraulic system, Fig. 29.9, is powered by an electric motor because,as compared with engine driven hydraulics, it is more readily controlled bythe electronic system to conserve energy. To ensure that there is enoughenergy in the system to select the gears for moving off, the electric pump isswitched on by the opening of the driver’s door. While he is entering the carand preparing to move off, it charges a hydraulic accumulator. When theaccumulator is fully charged, the motor is automatically switched off to saveenergy, and comes on again only when needed.

LEDs light up in a window in the dial of the engine speed indicator, todisplay at all times which gear is currently engaged. If the ignition is switchedoff with the transmission in neutral, a warning buzzer sounds: a gear must beengaged, otherwise the key cannot be withdrawn from the ignition switch.Two seconds after the engine is stopped with the car stationary, the electronicsystem deactivates itself. To restart the engine, the brake pedal must bedepressed and the accelerator released, at which point the control systemautomatically selects neutral.

To move off when the vehicle is stationary, the driver pushes the brakepedal down and then, using the gear shift lever, can select first, second or, if


Fig. 29.8 Top, clutch, and, bottom, gear selection and shift controls of the Alfa RomeoSelespeed transmission system

Fig. 29.9 Alfa Romeo Selespeed hydraulic system, based on a Magneti Marellidesign: 1 Electrohydraulic unit; 2 Hydraulic pump; 3 Oil filter; 4 Oil inlet; 5 Oiloutlet; 6 Hydraulic accumulator; 7 Hydraulic pressure sensor; 8 Bleed screw

required, reverse gear. When he takes his foot off the brake pedal, the vehiclemoves away. Operation of the clutch and throttle is effected by the electroniccontrol unit, on the basis of signals it receives from sensors detecting thethrottle position, engine torque and which gear is selected. Once the speed

5 R

3 4

1 2

2/4/RM

1/3/5

5/RM3/41/2

8

6

1

7

54

3

2


rises above 10 km/h, the driver can use either the shift lever or the buttons onthe steering wheel to change gear: he does not have to take his foot off theaccelerator.

Even if full throttle is immediately applied, the system shifts up automaticallythrough the gears, while inhibiting overspeeding of the engine. On the otherhand, if overspeeding occurs on a steep down gradient, with the throttleclosed, the system changes down to increase the effectiveness of enginebraking. During downshifts, the system actuates the throttle, in effect doubledeclutching, to synchronise the engine and transmission speeds.

If the accelerator pedal is released when approaching traffic lights orcrossroads, the system automatically changes down and, as forward motionceases, releases the clutch, again automatically, to avoid stalling the engine.Should the lights change from red to green as the driver approaches them, hewill, of course, be in the appropriate gear for pulling away without actuallystopping.

When the car is stationary with neither the brake pedal depressed nor theaccelerator released, and the transmission in neutral, neither the drive nor hispassengers can inadvertently select a gear. Additional safety features include,for example, selection of reverse gear is inhibited if the car is moving forward.Should two different gears be selected, one with the buttons and the otherwith the gear shift lever, the latter takes priority. Any gear shift that wouldcause overspeeding of the engine is automatically blocked. Selection ofneutral is inhibited at speeds of over 40 km/h: this prevents its inadvertentselection in critical situations, for instance during overtaking. Should, forexample in a difficult parking manoeuvre, the driver be holding his dooropen, with neither the accelerator nor the brake pedal having been depressedfor at least one second, neutral is selected automatically and the driver warnedby the buzzer.

29.5 Van Doorne Variomatic and Transmatic transmissionsThe twin-belt Variomatic automatic transmission developed by Van Doornewas first used in 1955 on Daf cars – more recently taken over by Volvo. By1980, steel belt derivatives were at an advanced stage of development byFiat, Borg-Warner and others.

The Variomatic transmission system, as originally produced, Fig. 29.10comprises six main sub-assemblies: the propeller shaft, power divider, twobelt-drive units, and two final-drive reduction gear units. Of these, the propellershaft, with flexible couplings at each end, instead of universal joints, transmitsthe drive from a centrifugal clutch to the power divider, which is mounted ona sub-frame beneath the rear of the vehicle. The input shaft to the powerdivider has a bevel gear on its rear end, which meshes with two bevel pinions,one on each side, to turn the drive through 90°.

These pinions, therefore, are driven in opposite directions, but they rotateindependently of their transverse shaft the outer ends of which carry the twodriving, or primary, pulleys. Splined on to the centre of this shaft, and floatingaxially between the two pinions, is a coupling sleeve with dogs on its ends.When the coupling is in its mid-position, neither of the pinions is engaged,so the transmission is in neutral. If it is moved in one direction along thesplines, its dogs engage with slots in the forward-rotating pinion and, if it isslid in the other direction, it engages the pinion rotating in the reverse sense.


Fig

. 29

.10

Van

Doo

rne

Var

iom

atic

tra

nsm

issi

on


One of the flanges of each driving pulley is fixed on the shaft, while theother can be moved axially on splines either to squeeze the belt outwards, ifthe two are closed together, or to allow it to sink deeper into the groove ofthe pulley, if they are moved apart. Similarly arranged flanges on the drivenpulley move in the opposite sense to those on the driving pulleys, and thusvary the ratio of the drive over a range of slightly more than 4 : 1. The ratioof the bevel drive in the power divider is rather less than 2 : 1 and that of thefinal drive reduction is about 4.75 : 1. In practice, the maximum overall ratiois of the order of 4 : 1.

The flanges in the driving pulley are moved apart by a combination ofmanifold vacuum and the centrifugal force on bob-weights, and they areclosed together by a diaphragm spring. This spring also transmits the torquefrom the shaft to the moveable pulley flange, thus obviating the need for asliding splined joint and consequent problems due to its sticking. A controlvalve cuts the vacuum assistance out of operation below about quarter throttleand above about three-quarters throttle. In each case, this increases the ratio– for engine braking with the throttle closed, and for maximum accelerationwith it wide open.

Unlike the driving pulleys, the driven ones are moved only by springs, butin this case a combination of a diaphragm and coil spring is used, the twotogether giving an almost constant closing load characteristic. Ratio adjustmentis therefore controlled solely on the driving pulleys, the driven ones beingautomatically self-adjusting to accommodate to them.

Because of the inherent load-sensitivity of the drive, and the use of aseparate pair of pulleys for each wheel, the system has a differential drivecapability, which can be explained as follows: when the vehicle turns acorner, the acceleration of the outer wheel occasioned by the external forces– i.e. applied to it at its contact with the road – causes the torque transmittedby the belt to be reduced and thus lessens the tension in it. Consequently, thebelt is squeezed radially outwards and the effective diameter of the drivingpulley automatically increased. The converse effect of course is obtainedwith the deceleration of the inner wheel, which pulls the belt down deeperinto its groove.

Since both the driven pulleys and the spur-type final-drive reduction gearsare carried by the swinging arm of the rear suspension, and the axis aboutwhich it swings intersects the neutral axis of the tension-loaded belt at thepoint where it leaves the driving pulley, wheel displacement neither causesmisalignment of the pulleys nor varies the tension in the belts. In overrunconditions, when the opposite run of the belt is loaded in tension, the loadingis so much lighter that the variations in geometry are of no consequence.

A later version of the same transmission, Fig. 29.11, has a single belt,with a differential gear and final drive reduction gear together forming atransmission unit, which is carried on two transverse members secured byrubber mounting beneath the vehicle structure. A de Dion axle with single-leaf springs is employed.

As before, the drive line from the propeller shaft is taken to the twofloating bevel gears, with a forward-or-reverse selector coupling sliding betweenthem. Thence, however, it is taken to a single centrifugal- and vacuum-actuated drive-pulley assembly, and through the belt to the spring-loadeddriven pulley, which is splined on the end of the final drive reduction pinion


Fig

. 29

.11

Var

iom

atic

tra

nsm

issi

on w

ith

a si

ngle

bel

t


spindle. The reduction gear, of the helical spur type, drives a ring gear boltedto the differential cage, the differential gears being connected by constantvelocity joints to the swinging driveshafts out to the wheels.

29.6 Van Doorne Transmissive BV steel CVTThe Van Doorne Transmatic continuously variable transmission (CVT), furtherdeveloped by Fiat and Ford and adapted for the Uno and Fiesta models in1987, is similar in principle to the single-belt Variomatic but is much morecompact and has a single segmented metal drive-belt, Figs 29.12 and 29.13.Again the pulley ratio spread is about 4 : 1 but the maximum overall ratio isonly about 25 : 1. This is for an experimental installation in the Fiat Strada.

From the illustration it can be seen that it is particularly suitable for front-wheel-drive cars. A centrifugal clutch, top left, transmits the drive to theprimary pulley and, through a quill drive, to a hydraulic pump, top right, the

Fig. 29.12 Fig. 29.13


adjustable flanges of both the primary and secondary – driving and driven –pulleys slide axially on linear ball bearing splines, that of the primary onebeing controlled by hydraulic pressure in the cylinder to the left of it, whilethe secondary one is closed by the coil spring plus hydraulic pressure in thesmaller-diameter cylinder on its right. A splined muff coupling between thetwo gears to the left of the secondary pulley is disengaged in its centralposition, engages forward drive if moved to the left, and reverse – throughthe medium of an idler gear – if moved to the right.

Of especial interest is the segmented steel belt. It comprises a set of platesabout 2 mm thick by 25 mm wide by about 12 mm deep, with slots in eachside to receive the two high-tensile steel bands which hold them together,rather like two strings in a necklace. The sides of the plates slope to matchthe V-angle of the grooves of the pulleys in which they seat. Unlike aconventional V-belt, however, this one transmits the drive by compression,instead of tension, in the run on one side between the pulleys. The run on theother side of course is unladen and runs free. Buckling of the strand undercompression is prevented by tension in the high-tensile steel bands. Theprincipal advantage of this type of transmission is that its torque ratio issteplessly variable. Transmission efficiencies of between 90% at a 1 : 1 ratioand 86% at the extremes of the ratio range are claimed. The whole unit istotally enclosed in a cast housing and the belt-pulley interface is lubricatedby oil passing from the primary pulley actuation cylinder through ducts intothe base of the V-groove.

29.7 The Maxwell automatic transmissionAutomatic transmissions based on hydraulic torque converters have someserious disadvantages: they are bulky, heavy, complex, costly, their servicingrequires a high degree of skill, and losses in the converter entail a fuelconsumption penalty. The Maxwell automatic transmission was designed toovercome all of these disadvantages, and is especially useful for rear-enginebuses in which the engine is installed transversely. Most of its servicing canbe done without removing the engine-transmission unit from the vehicle:with a conventional unit, removal and replacement of the engine andtransmission, for replacement of a small component such as a brake bandvalued at only a few pence, may cost as much as about £1000.

At the front of the Maxwell transmission is a fluid coupling. Splined intothe rear end of this and coaxial with it is a quillshaft, which extends rearwardsthrough a short hollow mainshaft and integral gear cluster. On the rear endof the quillshaft is the primary gear.

Arranged round the mainshaft are four short layshafts each carrying apinion meshing with a layshaft gear, and having, at its rear end, anotherpinion which meshes with the primary gear. Secured to the rear face of eachlayshaft pinion is a pneumatically actuated multi-plate wet clutch. Engagementand disengagement of this clutch couples and uncouples the pinion with theprimary gear. All the gears and pinions are in constant mesh.

The whole assembly is enclosed in a housing of rectangular shape, thelayshafts being positioned one in each of its four corners. Access for attentionto the clutches in service is gained by removing four bolted-on circular coverplates from apertures in the end of the housing.

In Fig. 29.14 are two diagrams, one showing the first gear layshaft, the


Input First

Output

Input

Output

Fourth

Fig. 29.14 Showing the transmission paths when the clutches for the first and fourthspeeds (left and right respectively) are engaged in the Maxwell transmission

mainshaft and output gear assemblies, and the other showing the correspondingarrangement for the fourth gear layshaft. Note that the spiral bevel outputgears are in a bolted-on housing near the front end of the box, so that theycan be easily removed in service and, important to the vehicle designer, thedriveshaft can be taken almost straight back to the back axle. This is why thistransmission is so suitable for rear-engine buses: with a conventional trans-mission, having its output shaft at the rear end, the drive has to be takenthrough an acute angle for connection to the final drive unit in the back axle.This acute angle drive also of course adds to the overall length.

The clutches are actuated by an electro-magnetic valve, which is controlledautomatically by an electronic system. Prior to a gear’s being selected forstarting from rest, all the clutches are disengaged. To select first gear, theclutch on the first gear layshaft is engaged; then, to change upwards again,this clutch is progressively disengaged while that for the next gear is beingengaged. For the selection of reverse, the first speed layshaft gear is slid bya pneumatic actuator, against the resistance of a return spring, out of meshwith its mainshaft gear and into mesh with an idler gear in constant meshwith the first gear on the mainshaft, and then the first gear clutch is engaged.

A useful feature of this box is the fact that the first and second speedclutches can be held in partial engagement, to act as retarders when either thethird or fourth speeds are engaged. While the retarder is in operation, oilfrom a pump in the base of the box is delivered at a rate of up to 68 litre/minthrough ducts in the casing into the hollow layshafts for first and secondspeeds and then radially out between their clutch plates, to dissipate the heatgenerated. It is then collected and returned through a separately mounted oil-cooler to the sump. The retarder is brought into operation by the initialmovement of the brake pedal, during which it takes up some lost motionbefore the service brakes are brought into operation.

29.8 Leyland continuously variable transmissionWith a transmission the ratio of which can be varied progressively fromreverse through zero to the maximum needed it becomes possible to operate


the engine virtually continuously at its most economical speed, Fig. 29.15,regardless of changes in load and vehicle speed. Furthermore, the jerk-freeratio changes, Fig. 29.16, and total absence of shock loading due to clumsygear shifting means a minimum of wear and tear on the transmission line.Consequently, a continuously variable transmission (CVT) has been the aimof automotive designers since the invention of the road vehicle. A majorobstacle to such a development has been the fact that the requirements forcontrol over such a transmission are too complex to be exercised satisfactoryby a human being. Now that microprocessors can be used for this control,however, not only has this obstacle been removed but also prospects havebeen opened up for its use in conjunction with regenerative braking, seeSection 38.18.

Leyland’s CVT is developed from an idea originally patented by W.D.Hoffman in 1899. Further development was begun in 1928 in the USA byF.A. Hayes, and his transmission was installed in about 600 cars in thenineteen-thirties. Yet more work was done in the UK after the Second WorldWar by Forbes Perry, for some of the time with BL. The outcome was firstthe installation of an experimental transmission of this type in a Triumph

150

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90

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800 1000 1200 1400 1600 1800 2000 2200 2400 2600 2800 3000

0.36

0.380.39

0.37

0.40

0.41

0.42

0.49

Engine speed (rev/min)

A B SFC at A = 0.368 at 1040 rev/minSFC at B = 0.476 at 2240 rev/mini.e. 22% improvement at A

0.35

Fig. 29.15 Fuel consumption and power map of the Leyland Terrier diesel engine. Bycontrolling it to operate mostly between about 1400 and 1600 rev/min, optimum fuelconsumption can be obtained. The line drawn from 1000 rev/min represents the poweroutput of the engine when operated with the CVT

0.430.44

0.45

0.460.470.48


Dolomite and then, by 1983, the announcement of a similar, but much larger,transmission for installation in the Leyland Terrier truck.

In essence, the Leyland CVT, Fig. 29.17, comprises what is termed avariator and a planetary gearset. The variator is interposed between theprimary gears and planetary gearset, as shown diagrammatically in Fig.29.18. In a common housing, both are driven in parallel by the engine, via afluid coupling. Each is coupled by a clutch, one connecting the sun gear, theother the annulus to the output. It should be noted that, for simplicity, theidler gear has been omitted from between the two primary gears in Fig.29.18, so the direction of rotation of the variator is not the same as in Fig.29.17. Control is exercised by a microprocessor.

The torque input to the variator passes from the primary gears to a layshafton which is mounted a pair of discs, with a single disc output memberinterposed between them. Torque is transmitted from the input to the outputdiscs by what are termed rollers, but which look more like wheels, rather likeplanetary gears but with friction instead of teeth transmitting the drive. Itfollowed that the output disc rotates in a direction opposite to that of the twininput discs. From the output disc, the torque is transmitted through a largediameter cup-shape component to the sun gear of the gearset. The planetcarrier is driven by the mainshaft and primary gears, while the annulus gearis the output from the gearset.

In principle, the variator is simple: the axes about which the three rollersrotate are simultaneously tilted through equal angles so that their peripheries

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Trac

tive

effo

rt (

lbf)

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Tractive effort

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Road speed (mile/h)

Fig. 29.16 A particularly useful characteristic of the Leyland CVT is its jerk-free ratiochanges as compared with, for instance, the five-speed manual shift transmission, asshown here


Fig. 29.17 In the Leyland CVT the idler between the primary, or input, gear pinionsets the layshaft rotating in the same direction as the mainshaft. Immediately behindthe primary gears, on the layshaft, is the variator. The mainshaft passes above it,straight through this section of the box to the rear section, where it carries a gearmeshing with a pinion that drives the planet carrier. The sun gear is coaxial with anddriven directly by the variator

roll along a track of different diameter on the input disc to that on the outputdisc. In Fig. 29.18, the track on the input disc is of smaller diameter than thaton the output disc, so the transmission is in a geared-down ratio. If the rollersare tilted so that both tracks are of equal diameter, the drive ratio is 1 : 1,while a further swing to make the track on the input larger than that on theoutput disc takes it into overdrive.

The tracks are of a toroidal section, such that the rollers are always incontact, regardless of the angle through which they are tilted. Moreover, thedisc and roller assembly is axially preloaded hydraulically, at loads varyingup to about 15 tonne, according to the torque being transmitted. Even so,there is no metal-to-metal contact between the peripheries of the rollers andthe discs, since a thin elasto-hydrodynamic film of oil is drawn in betweenthem, as between the rollers and races of bearings. The torque therefore istransmitted by shear in this film so the rate of wear of the rollers and tracksis extremely low.

In what follows, it is necessary always to bear in mind that the directionsof rotation of the input and output from the variators illustrated in Figs.29.17 and 29.18 are different. Reverse, forward and neutral gears are obtainedby using the variator to increase or decrease the speed of the sun gear relativeto that of the planet carrier which, as previously mentioned, is directly drivenfrom the engine. The overall ratio, Fig. 29.19, is a function of the ratios ofthe variator and the gearing from the primary gear to the planet carrier.


OutputInput

Fig. 29.18 Diagrammatic representation of the variator and planetary gearset, butwithout the idler between the primary gear pair. The two clutches, in turn, couple anduncouple the sun gear and annulus gear to the output pinion

In this illustration the change from low into high regime occurs at thepoint when the overall ratio is 0.43 : 1 which, in the conventional transmissionof the Terrier, is approximately equivalent to second gear. For operation inthe low regime, the annulus gear clutch is engaged and the sun gear clutchdisengaged. At the change-over point both the sun gear and annulus arerotating at the same speed and in the same direction, so no load is transmittedthrough their clutches. This is termed the synchronous speed, or synchronousratio, of the transmission.

To progress from low to high regime, the sun gear clutch is engaged andthe annulus clutch immediately disengaged. In the high regime all the torqueis taken through the variator, while the planet carrier and annulus rotateunladen. However, should there be a requirement for holding low regime, forinstance for engine braking, the electronic control can be called upon toactuate the clutches appropriately.

The really clever part of the whole transmission is the system for controllingthe ratio of the variator. Central to this control system is the microprocessor,a significant feature of the demand on which is that it is continuous. Incontrast, a microprocessor in a conventional automatic transmission springsinto action only when a gear shift is called for. Consequently, the CVT hasa 16-bit microprocessor with 8 kbyte of software. It actuates an electro-hydraulic valve controlling a low pressure system for loading the variatoraxially in proportion to the torque transmitted, as well as for steering therollers and actuating the clutches. The three main inputs to the electroniccontrol unit are driver demand and the speeds of the engine and of thevehicle.

Ideally, the engine would operate continuously at its most economicalspeed, while the speed of the vehicle is regulated by the transmission. However,since constant speed implies constant power output, the only way to obtainextra power for acceleration or climbing a hill is to relieve the enginemomentarily of the transmission load, so that the power thus made availablecan be utilised for accelerating the engine to a speed appropriate for itsrequired operation at a higher power output. Then the engine and transmissionhave to be coupled together again to re-establish a steady-stage level ofoperation at that output.


Changing the ratio of the variator is effected by steering the rollers ontheir tracks. Signals from the microprocessor regulate the movements of thehydraulic valves which, in turn, control the hydraulic actuators of the steeringmechanism. From Fig. 29.20, it can be seen that the rollers are carried inroller bearings in carriages, vaguely resembling pulley sheaves, which tiltabout trunnions at their ends. From the cross-section of the tilting mechanism,Fig. 29.21, it can be seen that the trunnions pivot on the ends of Y-shaped

Fig. 29.20 Each roller of the variator rotates in a carriage pivoted about a trunnion ateach end. These trunnions are carried on the ends of Y-shaped centrally pivoted levers(out of sight here) the legs of which project radially inwards, as shown in Fig. 29.21

Output torque

Low High

Rev Forward

CVT speed ratio (Nout/Nin)

0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8

Fig. 29.19 The modes of operation of the Leyland CVT, from reverse, throughforward in low, and on up into high regime

Overdrive


rockers, the legs of which extend radially inwards to register in slots in acontrol sleeve. This sleeve can be rotated a few degrees in either direction toalter the angle of tilt of the rollers by steering them inwards or outwards untilthey run along tracks of diameters appropriate to the speed called for by thedriver.

Equalising the share of the load between the rollers in each set of three isalso an important requirement. Failure to do so greatly reduces the potentialrating of the transmission. So long as the reactions at the ends of the arms ofthe rockers remain equal, the sleeve remains coaxial within the shaft. If,however, the reactions at the end of one arm or the other increases or decreases,the resultant displacement of the leg of the Y causes the sleeve to deflect toone side or the other, within the clearance between the sleeve and the shaft.This steers the roller, causing it to tilt so that it downshifts or upshiftsappropriately until the reactions are again equal. During this process thesleeve progressively returns to the position in which the reactions are againall in equilibrium.

Since the input and output discs rotate in opposite directions, steering therollers in one direction causes their peripheries to move simultaneously inwardson one and outwards on the other disc. The pivot geometry is such as to givethe rollers a castoring tendency, so the unit can run in only one direction,which is why the epicyclic gear has to be utilised for obtaining reverse, aspreviously described. Such a transmission could lead to fuel savings of ashigh as about 22% and is intended to replace the conventional transmissionin vehicles, such as buses and delivery vans, used for stop–start operation.

Fig. 29.21 Cross-section of the rollers and their tilting and steering mechanisms

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