This invention relates to vehicle transmissions which include multi-ratio gearbox and in particular to transmissions which have an automatic ratio selection mode.

It is an object of the present invention to provide an improved form of vehicle transmission.

Thus according to the present invention there is provided a vehicle transmission comprising a multi-ratio gearbox, an electronic control means which receives inputs from a plurality of operating parameters of an associated vehicle and determines the appropriate gearbox ratio for the current vehicle operating conditions, and a ratio selection means which receives ratio selection signals from the electronic control means and selects the appropriate ratio, the transmission being characterised in that the electronic control means is arranged to signal a downshift in the operative ratio of the gearbox if any one of the following conditions occurs:-

a. a predetermined rate of depression of an associated vehicle accelerator is exceeded;

b. brakes of an associated vehicle are applied;

c. a zero depression condition of the accelerator has existed for a predetermined time period (typically 3 seconds) ;

d. an increase in the speed of the associated vehicle is detected when a zero depression of the accelerator exists, and

e. an increase in accelerator depression is detected combined with a decrease in vehicle speed,

Preferably no downshift will be made when any one of the above conditions occurs if this will result in an engine of the associated vehicle exceeding a preset speed.

This preset speed is chosen to prevent permanent damage to the engine and associated clutch and drive line components. For example, the electronic control means will prevent the engine exceeding its maximum rated speed and the clutch driven plate speed (when driven by the road wheels) exceeding its burst speed (typically 1.4 X max. engine speed).

The present invention will now be described by way of example only, with reference to the accompanying drawings in whic :-

Fig 1 shows in diagrammatic form the general layout of a vehicle transmission embodying the present invention;

Fig 2 shows in flow diagram form the criteria used in the downshifting feature, and

Fig 3 shows diagrammatically the generation of the clutch engagement control signal.

Referring to Fig 1 this shows an engine 10 with a starter and an associated starter circuit 10a which is coupled through a friction clutch 14 with a five speed synchronised lay-shaft type gearbox 12 via a gearbox input shaft 15. In the example described, fuel is supplied to the engine by a throttle 16 which includes a throttle valve 18 operated by accelerator pedal 19. The invention is equally applicable to electronic or mechanical fuel injected petrol or diesel engines.

The clutch 14 is actuated by a release fork 20 which is operated by a slave cylinder 22.

The selection of the operative ratio of the gearbox may be controlled manually or automatically.

A gear selector lever 24 operates in a gate 50 having two limbs 51 and 52 joined by a cross track 53. When the lever 24 is in limb 51 the transmission operates in manual mode and the lever 24 is biassed to return to a central position 51a when released. Each movement of lever 24 from central position 51a to a downchange position 51b selects a downchange of one ratio in the currently operative ratio of

the gearbox and each movement to an upchange position 51c selects an upchange of one ratio.

The position of lever 24 in limb 51 and 52 is sensed by a series of sensors (e.g. micro switches and optical sensors spaced around the gate) which are indicated collectively at 33 and whose signal outputs Vs are fed to electronic control unit 36. Control 36 in turn issues signals to a gear engaging mechanism 25 which includes, for example, hydraulic rams and solenoid control valves to move selector members to engage the desired gearbox ratio. One example of a suitable gear engaging mechanism is disclosed and claimed in the Applicants earlier co-pending UK Patent Application No. 952140.8.

The limb 52 of gate 50 has four positions; a 'P' position in which the lever 24 is placed to park the vehicle; an 'R' position in which the lever 24 is placed to select reverse; an ^, N' position in which the lever is placed to select neutral and a 'D' position in which the lever is placed when it is desired that the forward drive ratio of the gearbox should be selected automatically by control unit 36 in accordance with the current operating conditions of the vehicle.

When in the 'D' position the control unit 36 determines which ratio is most appropriate for the current vehicle operating conditions and issues appropriate ratio selection signals to gear engaging mechanism 25.

To determine the appropriate operating ratio control unit 36 receives signals from a plurality of vehicle operating parameter sensors such as signals Ve proportional to engine speed from engine speed sensor 26. Signals Vt are received from throttle valve position sensor 30 proportional to the current throttle opening and an accelerator pedal position sensor 19a is also provided to feed accelerator pedal position signals Va to control unit 36. In certain implementations of the system only one of the sensors 19a and 30 may be provided. Most commonly in such a situation it is sensor 19a which will be provided.

Control unit 36 also receives a gear signal Vg from gear position sensor 32 which corresponds to the gear ratio currently engaged, signals Vc from slave cylinder position sensor 34, which varying with the position of the slave cylinder 22, and signals Vdp proportional to clutch driven plate speed from speed sensor 42 which actually senses the speed of the gearbox shaft 15 (which is equal to that of a driven plate 40 of the clutch 14). A vehicle speed sensor 45, for example the variable reluctance type, is also provided to feed a vehicle speed signal Vv to control unit 36. Again in certain implementations driven plate speed sensor 42 may be omitted and the driven plate speed can be calculated by the control unit 36 from the vehicle speed and the gear ratio currently engaged.

A uzzer 50 is connected with control unit 36 to

warn/indicate to the vehicle operator when certain vehicle operating conditions occur. In addition to or in place of buzzer 50 a flashing warning light (not shown) may be used.

As indicated above control unit 36 also controls the engagement and disengagement of the clutch 14. This is done by the control unit 36 generating a reference signal Vr in a generator C (see Figure 3) which is representative of a desired engine speed. This reference signal is compared in comparator A with the actual engine speed signal Ve from engine speed sensor 26 to produce an error signal E which is compared in comparator B with the clutch actuator position signal Vc from sensor 34 to provide a clutch engagement control signal Vce which the control unit 36 outputs to the hydraulic control 38. Operation of a control unit 36 in this general manner is described in more detail, for example in the Applicants earlier European Patents 0038113 and 0043660.

In addition to controlling the engagement and disengagement of clutch 14 the control unit 36 also controls the throttle setting via a throttle control 37 when its control logic indicates that an override of the throttle opening set by the driver's operation of the accelerator 19 is desirable. In some applications the throttle control 37 may be omitted or torque reduction during gear changes can be achieved by varying engine timing and/or fuel supply (possibly using any existing electronic engine management system) .

A brake application sensor 46 is also provided (this may, for example, be a sensor operating on brake line pressure) which provides a signal Vb to control unit 36 when the brakes of the vehicle are applied. Alternatively, an accelerometer may replace sensor 46 or operation of the brake lights may be detected to indicate when the vehicle brakes are applied.

In accordance with the present invention the control unit 36 is arranged to command a downshift in the operative ratio of the gearbox if any one of the following conditions occurs:-

a) a predetermined rate of depression of an associated vehicle accelerator is exceeded;

b) brakes of an associated vehicle are applied;

c) a zero depression condition of the accelerator has existed for a predetermined time period (typically 3 seconds) ;

d) an increase in the speed of the associated vehicle is detected when a zero depression of the accelerator exists, and

e) an increase in accelerator depression is detected combined with a decrease in vehicle speed.

Condition a) above indicates a requirement for rapid

acceleration of the vehicle and by relying on a rate measurement (rather than a given angle of depression of the accelerator pedal as in a normal kickdown control) the downshift can be actioned earlier in the accelerator pedal movement thus significantly increasing the speed of the response of the vehicle. Typically the preset rate will be 2 per cent of the full accelerator travel per 5 milliseconds.

Conditions b) and c) above provide a degree of engine braking to assist in deceleration of the vehicle. These features may be standard in the transmission control or may only be invoked when the operator has selected a "sport" mode of operation of the transmission.

Condition d) above invokes increased engine braking when the vehicle is gathering speed going downhill. This is particularly significant when the vehicle is heavily laden or towing.

Condition e) invokes a downshift when the vehicle is climbing a hill in too high a gear. Further upshifts may be prevented in this condition until a decrease in accelerater depression is detected with a steady or increasing vehicle speed.

Figure 2 shows diagrammatically the tests applied by control unit 36 in deciding when a downshift should be made.

Boxes A & B determine when condition a) above exists, box C

determines when condition b) exists, box D determines when condition c) exists, boxes E & F determine when condition d) exists, and boxes A & G determine when condition e) exists.

Box H prevents downshifts into first gear and box I provides the overall check that the contemplated downshift which is to be actioned by box J will not cause the engine speed to exceed the preset level which will typically be of the order of 4000 r.p.m.

Although the invention has been described above in relation to a transmission with a synchronised layshaft type gearbox having both manual and automatic modes and which includes a main drive clutch whose engagement & disengagement is controlled by an electronic control unit which also selects the appropriate gearbox ratio, it will be appreciated that the invention is also equally applicable to a conventional epicyclic type automatic gearbox which operates via a torque convertor.