Semi-Automatic Transmission

Over the years, Citroen have found several different ways to spare the clutch foot yet leave the driver in control of which gear is actually selected.

Sensaud de Lavaud

Little is known about the Sensaud de Lavaud transmission which was intended for the Traction Avant, beyond it having played a substantial role in the Traction's development bankrupting Citroen. It is believed that it may have been an early example of what is now a reasonably conventional torque-convertor automatic gearbox, but it is also understood that Adolphe Kegresse (also behind the half-tracks based on RWD Citroens, as used to explore much of Africa and Asia in the '20s) also designed a transmission for the Traction. An ingenious design, based around dual clutches and shafts, the concept was not developed due to the arrival of WW2 - but appears very similar to the dual-shaft design best known through VW's DSG (developed and manufactured by Borg-Warner) and Porsche's PDK (from ZF). Porsche also used a double-clutch transmission for racing in the mid '80s.

It is not known how Kegresse planned to control the design, without the complicated electronics and hydraulics used by the modern systems.

Embrayage Centrifuge

Falling a little short, perhaps, of qualifying as a fully-fledged semi-automatic gearbox, the centrifugal clutch fitted to many 2cvs, Dyanes and Amis cannot go unmentioned here.

In addition to the normal clutch, the flywheel contained a secondary clutch not dissimilar to a very large diameter but thin drum brake. When engaged, the friction material gripped the clutch drum, transmitting drive. However, when revs fell to idle, the clutch was automatically disengaged, separating drive.

As a result, cars equipped with this clutch design were virtually impossible to stall - ideal for a car that was intended to be the first car owned by most buyers. Equally importantly for a car with so little performance, there was no loss of efficiency or hit on performance or fuel economy. Normal use of the clutch was still required to change gear whilst underway, however.

The biggest drawbacks to the system - apart from a small extra maintenance penalty when the clutch needed replacing - were that the car could not be bump-started (but, since it had a starting handle, that wasn't a problem), and that parking on a hill relied on the handbrake alone. With earlier A-series, fitted with front drum brakes, this was similarly not an issue, but the much less efficient handbrakes on later cars with front disk brakes marked the end of the concept, with some early '70s Amis being the only cars to mix disk & Trafficlutch.

BVH

The DS brought perhaps the best known semi-automatic gearbox, with the BVH - Boite de Vitesses Hydraulique. Apart from the control of the gears and clutch, the BVH was exactly the same transmission as the "normal" manual gearbox fitted to the lower-spec ID.

Control of gear selection was achieved through a system of hydraulic valves and servos built into the lid of the gearbox, in place of the conventional linkage, and operated by the fingertips on a lever on the steering column. To ensure that the car was not started in gear, the lever doubled as control of the starter motor.

Any movement of the lever also controlled the clutch. A set process of events followed selection of a different gear - clutch disengagement, gear deselection, gear selection, re-engagement of the clutch. A number of adjustments could be made to the speed at which these were carried out, so whilst it was possible to have a D which changed gear imperceptibly, it was equally possible for a badly maintained car to lurch and judder.

The clutch control was also linked to the brake pedal, with the idle speed reduced when the brake was pressed. Properly set up, the clutch would be engaged at the higher idle speed, but disenaged at the lower, so that "creep" could be easily achieved by simply removing the driver's foot from the brake.

By the end of DS production, a conventional Borg Warner automatic box was also available, although very few were built.

C-Matic

For the GS and, later, the CX, Citroen headed off down yet another tack with the C-Matic. The concept of "automating" a manual 'box was continued from the DS, but simplified slightly. Rather than use a conventional clutch, with the sophisticated control required to move away from rest smoothly, a torque convertor was used, as on normal automatic gearboxes. However, that didn't help change between gears, so an electronic microswitch was used to detect movement of the gearlever, which then, via . An electrovalve then cut fluid pressure to the torque convertor, smoothing the change. Whilst the 'box was based on the four-speed CX & GS manual transmissions, both lost a gear to become three-speed (in line with then-common automatic boxes) in C-Matic form. Both GS & CX C-Matics require a particular fluid - sold by Total as Fluide T. This is no longer available generally, as Total donated their entire UK stock some years ago to the club, who are now the UK's only source.

C-Matic was discontinued in both CX & GSA in the early 1980s. For the CX, it was replaced by a conventional ZF 3-speed automatic box, but the G became available only with a clutch pedal.

Sensodrive

In 2003, the C3 became available with a semi-auto termed Sensodrive. In an echo of the DS's BVH, it comprised a conventional manual 'box and clutch, and traded both clutch pedal and conventional gear selector for a set of "F1-style" paddles and a simple up-down manual selector. Computer controlled, the biggest step forward of Sensodrive over the BVH was that it didn't need you to change gear. If you couldn't be bothered, it'd do it for you.

EGS

In a further refinement of the concept, the C4 saw EGS take over from the previous Sensodrive system. Conceptually very similar, with a six-speed manual controlled by computer, improvements in computer technology mean that the EGS box is smoother and more refined than it's immediate predecessor.

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