294 Alfa Romeo Selespeed transmission

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

The Magneti Merelli system has been designed for application to any conventional manual transmission. In this instance a five speed gearbox has been employed. The gears can be selected either manually, by actuating either a two-button control on the steering wheel or a conventional gear shift lever on the tunnel between the seats, or it can be set to operate automatically for operation in city traffic. City, or automatic mode, of operation can be selected at the flick of a console-mounted switch. The electronic control even takes into account driving style: if the engine speed is consistently rising above 5000 rev/min with the throttle depressed more than 60%, the clutch actuation cycle time is reduced to 0.4 sec, as compared with the 1.5 sec of less sporty driving.

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

As previously indicated, this transmission can be operated in either of two modes: city or semi-automatic. When the former is selected, using the switch, the panel in the engine speed indicator displays the word 'City', followed by the number of the gear that is currently engaged. In this mode of operation, the gear shifts are effected automatically, with gear changing patterns designed for driving when frequent gear shifting is needed in traffic. In the semiautomatic 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, but actuation of the clutch and the shifting of the gears are effected hydraulically. Four hydraulic actuators are employed: one is for the clutch, the second selects the gear by rotating the selector rods and the third, by moving them axially, engages the gears, Fig. 29.8. The fourth actuator is linked to the throttle control, for momentarily opening and closing it or double declutching during downshifts. As a result, all gear shifting can be effected without the driver having to take his hands off the steering wheel, except at very low speeds.

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

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

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

Gearbox Selespeed Sensors
Fig. 29.8 Top, clutch, and, bottom, gear selection and shift controls of the Alfa Romeo Selespeed transmission system
Selespeed Design
Fig. 29.9 Alfa Romeo Selespeed hydraulic system, based on a Magneti Marelli design: 1 Electrohydraulic unit; 2 Hydraulic pump; 3 Oil filter; 4 Oil inlet; 5 Oil outlet; 6 Hydraulic accumulator; 7 Hydraulic pressure sensor; 8 Bleed screw

required, reverse gear. When he takes his foot off the brake pedal, the vehicle moves away. Operation of the clutch and throttle is effected by the electronic control unit, on the basis of signals it receives from sensors detecting the throttle position, engine torque and which gear is selected. Once the speed rises above 10 km/h, the driver can use either the shift lever or the buttons on the steering wheel to change gear: he does not have to take his foot off the accelerator.

Even if full throttle is immediately applied, the system shifts up automatically through the gears, while inhibiting overspeeding of the engine. On the other hand, if overspeeding occurs on a steep down gradient, with the throttle closed, the system changes down to increase the effectiveness of engine braking. During downshifts, the system actuates the throttle, in effect double declutching, to synchronise the engine and transmission speeds.

If the accelerator pedal is released when approaching traffic lights or crossroads, the system automatically changes down and, as forward motion ceases, releases the clutch, again automatically, to avoid stalling the engine. Should the lights change from red to green as the driver approaches them, he will, of course, be in the appropriate gear for pulling away without actually stopping.

When the car is stationary with neither the brake pedal depressed nor the accelerator released, and the transmission in neutral, neither the drive nor his passengers 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 other with the gear shift lever, the latter takes priority. Any gear shift that would cause overspeeding of the engine is automatically blocked. Selection of neutral is inhibited at speeds of over 40 km/h: this prevents its inadvertent selection in critical situations, for instance during overtaking. Should, for example in a difficult parking manoeuvre, the driver be holding his door open, with neither the accelerator nor the brake pedal having been depressed for at least one second, neutral is selected automatically and the driver warned by the buzzer.

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Responses

  • RAHEL
    What should the engagement and clutch sensor be on alfa selespeed?
    9 months ago

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