25.26.
Electronically Controlled Semi-automatic Transmission
The objective behind the development of electronically controlled semi-automatic transmissions is to produce a gearbox with the efficiency and controllability of a manual transmission, but the ease of driving offered by a clutchless system. In all designs, semi-automatic transmission is concentrated around layshaft or epicyclic-type gearboxes, with automated actuation of clutch engagement and disengagement.
Early designs of the 1960s and 1970s, such as the Manumatic gearbox (developed by AP and subsequently installed in some Hillman cars) and the system developed by NSU for their Wankel-engined Ro80, used a micro-switch under the gear level knob to detect the driver’s hand-pressure and hence his intention to change gear. When the switch was operated, a solenoid valve was energized to allow either vacuum or hydraulic pressure to actuate the clutch mechanism. Since clutch control was of a rather on-off nature, these transmissions were not particularly smooth and therefore fluid couplings were sometimes used to cushion the drive. Deficiencies in the control electronics still remained. These systems could make little impact on the market.
During the late 1980s using the readily available cheap and powerful microprocessors, low-cost semi-automatic gearboxes with good performance was developed. In Europe, the Valeo Company in France, Fichtel and Sachs in Germany, Magnetic Marelli in Italy and AP Borg and Beck in Britain have all developed fully commercialized systems with semi-automatic operation of a conventional lay-shaft gearbox. The heart of all of these systems is a microcomputer-based electronic control unit, which monitors engine and transmission speeds, as well as detects the positions of the accelerator pedal and gear lever. Commands issued by the control unit then energize an electric, vacuum or hydraulic actuator to engage or disengage a conventional dry-plate clutch. This provides the driver all the advantages of a manual gearbox without the fatigue of manipulating the clutch. This renewed interest has led to the development of a variety of modified semi-automatic control strategies, presented below.
25.26.1.
Some Semi-automatic Control Systems Electronic Clutch Control.
This is the basic semi-automatic system described above. With the vehicle at rest, the driver engages a gear and depresses the accelerator pedal to move away. Once the vehicle is moving, gear changes are effected by shifting the gear lever, causing very fast gear changes, Clutch operation is fully automated, but during the gear-change interval the driver must still control engine speed with the accelerator pedal. When the vehicle is slowed, the clutch is not disengaged until the vehicle is almost at a standstill. This maximizes engine braking but avoids a stall. Some of the first vehicles having electronic clutch control include the Ferrari Modial (introduced in 1993 with the Valeo TEE2000 system), Renault Twingo Easy (introduced into Europe during
1994 with an AP Borg and Beck system) and Volkswagen’s Golf diesel Ecomatic (also introduced in 1994).
Electronic Clutch and Engine Speed Control.
This system is the outcome of further enhancement of the system described above. The accelerator cable is replaced by an electrical connection between a position sensor mounted on the accelerator pedal and a servomotor operating the engine’s throttle disc. Normally, the throttle disc is actuated directly at the command of the driver, but during gear changing, the control unit intervenes and takes command of engine speed to synchronize it with transmission speed for the newly selected gear. This action ensures smooth and comfortable clutch engagement and minimizes clutch wear.
Remote Gear Selection.
In this system the features described above are further enhanced by the incorporation of a remote gear selector. The usual gear selection linkage is replaced by an electrical connection between the gear change switch (usually a small lever or a push-button) and a transmission-mounted electro-hydraulic actuator. The small lever is flicked forwards or backwards to shift down or up a gear. The gear lever being an electrical switch requires only small operating pressure and it can be fitted at a place best suited to the driver.
Automatically Shifted Manual Transmission.
A transmission with electronically controlled clutch and remotely selected gears as above can be further enhanced for complete automation of the gear-selection process. As a result a small, light and highly efficient automatic transmission, well suited to small-capacity cars is developed.
25.26.2.
AP Borg and Beck ACTS Semi-automatic Transmission
Figure 25.29 illustrates the Automatic Clutch and Throttle System (ACTS) developed by AP Borg and Beck. ACTS permits the driver to change gear using a conventional “H”-gate lever, but with automated control of clutch and engine-speed during the gear-changing process. Since ACTS uses a conventional manual transmission, it is comparatively simple and low-cost. However it has additional sophisticated electronic control unit and a variety of sensors and actuators. These include
(i) An accelerator pedal position sensor.
(ii) A throttle disc actuator with a built-in position sensor.
(Hi) A gear lever load-switch to detect the driver’s hand pressure on the knob.
(iv) A gear position sensor.
(v) A magnetic pickup to sense engine speed.
(vi) A magnetic pickup to sense gearbox input shaft speed.
(vii) A controllable source of hydraulic energy consisting of a fluid reservoir, electrically driven pump, pressure accumulator and electro-hydraulic control valve.
(viii) A clutch release cylinder with a built-in position sensor.
The hydraulic system operates clutch release cylinder, which engages and disengages the circuit through a conventional release lever. Pressure to the reverse cylinder is controlled by a solenoid valve mounted on the hydraulic power unit. This valve is operated by the controller in response to signals from the sensors. A servo loop based on the release lever transducer and the
engine and transmission speed sensor provides fine control for the clutch friction plate speed gradient, ensuring very smooth engagement.
Fig. 25.59. Automatic clutch and throttle system (ACTS) incorporated in semi-automatic transmission (AP Borg & Beck).
The throttle motor is equipped with a feedback potentiometer to report throttle opening to the control unit. During normal driving the motor’s position is controlled by the driver through another potentiometer, mounted on the accelerator pedal. During gear changing, however, the microcomputer intervenes to temporarily modify the engine speed. A switch in the gear lever detect the driver’s hand pressure to change the gear and so signals the controller to disengage the clutch. Once a gear change has been made the gear position sensor sends a digital code to the controller to report the gear selected, and so a decision is made to re-engage the clutch.
Operation of ACTS.
Before starting a vehicle fitted with ACTS the transmission must be placed in neutral to close the inhibitor switch for operation of the starter motor. When the engine starts, the clutch cylinder position sensor locates the current release lever position (thus compensating for clutch wear) and the controller establishes an engine idle speed reference.
With the vehicle still at rest, the driver moves the gear lever to engage first gear, so causing the controller to disengage the clutch. But it remains disengaged until the driver depresses the accelerator, causing the controller to re-engage the clutch. During reengagement the controller continuously modifies the throttle disc and release lever positions to ensure a smooth make-up but simultaneously maintaining the engine speed desired by the driver. With this sequence, it is possible to start the vehicle with a high engine speed (for hill-starts or rapid acceleration) or with a low engine speed (for leisurely acceleration). The microcomputer detects if the driver initially selects an inappropriate gear and issues a warning to the driver (by a buzzer). In this case the clutch engagement takes place only when a suitable gear is engaged.
With the vehicle in motion, the driver performs gearshifts by moving the lever to the desired gear position. The microcomputer then momentarily takes command of clutch position and engine speed. Engine speed is modified depending on an upward or a downward gearshift as detected by the gear position sensor.
When an upshift is detected, and the engine speed is greater than that of the transmission input shaft, the microcomputer engages the clutch immediately and engine-speed control remains with the driver. When downshift is detected, the engine speed is usually below that of the transmission input shaft then the microcomputer delays clutch re-engagement and commands the throttle motor to momentarily increase engine speed to match that of the transmission input shaft. Once the clutch has-been re-engaged engine-speed control reverts to the driver.
If the vehicle is operated down to a very low speed, the engine revolutions fall and the microcomputer disengages the clutch to prevent a stall. The clutch then remains disengaged until a suitable gear is selected and the accelerator depressed. As a means of reducing drive-line shunt in stop-go driving it is possible for ACTS to allow an amount of controlled clutch slip. Any large discrepancy between the actual engine speed and the driver-demanded engine speed is detected by the microcomputer and a limited amount of slip is introduced accordingly.
25.26.3.
Valeo Electronic Clutch Transmission TEE 2000
Valeo’s TEE 2000 system (Fig. 25.60) has many components in common with AP Borg and Beck ACTS. The system is to some extent less complex as it does not intervene to adjust engine
speed during gear changes like ACTS and therefore a little more skill from the driver is necessary. It uses an electromechanical (rather than electro-hydraulic) actuator system for clutch operation. The actuator is consisted of an electric motor, which drives the release arm through a worm reduction gear. To reduce input power to the motor the actuator uses a strong spring, working against the clutch diaphragm spring, to counterbalance the disengagement load. Even with a relatively small motor, clutch actuation takes less than one-fifth of a second.
Fig. 25.60. Valeo electronic clutch transmission.
25.26.4.
Volkswagen Ecomatic System
The Ecomatic system has been commercialised by Volkswagen for use on certain diesel-en-gine cars. In this system the clutch is operated by a vacuum servo unit controlled by the Ecomatic ECU through a solenoid valve. Ecomatic is a semi-automatic transmission, which improves fuel economy and reduces exhaust emissions by disengaging the engine from the transmission when the driver reduces pressure on the accelerator to slow down the vehicle. In case the driver totally releases the accelerator pedal for more than about two seconds, for example in a traffic queue, the engine is switched off and the clutch is disengaged. When the engine is not running a 50% oversize 92 Ah battery, charged by a 90A alternator, is used to provide power for normal electrical loads plus the electrically driven vacuum, coolant and power steering pumps.
The engine is again started by simply engaging first gear and depressing the accelerator, which automatically operates the starter motor and engages the clutch to allow the vehicle to move off.
Diesel engines are specifically suited to this type of operation. It is because, when warm, these engines generally start on the first turn of the crankshaft whereas, petrol engines normally require several turns.
The Ecomatic adds about 40 kg to the weight of a standard vehicle. With a 1.9 litre diesel engine it offers 22% lower urban fuel consumption, 36% reduction in CO emissions, 22% reduction in CO2, and 25% in HC and NO.
