Magès patent project (1958): automatic regulation for continuously variable transmission

In this document

The principle: tuning the engine to the "economic curve"

Magès starts from an observation: with a conventional three or four-speed gearbox, the engine cannot be maintained at the operating point providing minimum consumption or maximum power p. 144. He illustrates this with the graph in figure 1, which compares the power required for level driving (curve A) and the power available at full throttle (curve B), with a network of specific consumption curves in grams per horsepower-hour p. 144.

Two numerical examples are given: at 75 km/h, a gear ratio of 25 km/h per 1000 rpm imposes 250 g/hp·h, whereas a ratio of 50 km/h per 1000 rpm would bring the engine to 1500 rpm, close to the minimum consumption ("point C") p. 145. Similarly, at 100 km/h, a ratio of 33.3 km/h per 1000 rpm would place the engine at the minimum consumption point for this power ("point D") p. 145. The objective is to have the engine operate permanently on this "economic curve" E, thanks to a continuously variable transmission coupled with automatic regulation p. 145.

The regulation device

The driver no longer acts directly on the throttle, but on a governor by simply compressing a spring; it is this governor that controls both the variator lever (for the gear ratio) and the opening of the carburetor throttle, in a coordinated manner p. 145-146. The text also mentions the possibility of varying the ignition timing point via a cam linked to the compression of the governor spring p. 146.

the document also describes the management of engine braking: releasing the pedal returns the engine to idle and the variator to the "overdrive" position, minimizing engine braking; a second pedal (or lever) allows, on the contrary, to increase the gear ratio, and therefore engine braking, without acting on the throttle p. 146-147, 151. A hydraulic braking system for the spring return (piston 22, calibrated orifice 27, reservoir 25) avoids overly abrupt gear changes during hesitant driving, for example in town p. 147, 149-151.

The components described (fig. 2 and 3)

The device includes an engine M driving the variator V, a centrifugal governor R with flyweights (28) and its hydraulic distributor D, powered by a pump P, acting on two receiver cylinders C1/C2 which move the control lever 2 of the variator p. 148-149. A cam (7) connected to the pedal 5 regulates the compression of the governor spring (21), while a second cam (8), on the same axis, controls the opening of the throttle 9 of the carburetor 10 p. 148-149.

Context

This patent project is part of Citroën's research into hydrostatic transmission, an alternative to the conventional gearbox that Paul Magès and his team explored for several decades (as evidenced by later archives in the collection, concerning the 2 CV, the GS, or the L vehicle).

At the end of the 1950s, the idea of a continuous variator associated with automatic regulation prefigures, in a general way and as a historical reference point external to the document, the principles that would later be developed in continuously variable transmissions (CVT) by various manufacturers. Nothing in the document, however, allows for a direct link to a specific Citroën production model to be asserted.

Archivist's perspective

Dated and signed "Magès 10.2.58" with countersignature "GUERARD" p. 144, this eight-page text, written on Citroën letterhead and then in an engineer's notebook, clearly has the form of a patent project written in Magès's own hand, dense with crossings-out and corrections — a sign of a text in the process of being drafted rather than a final filing.

It seems that this document lays, as early as 1958, the conceptual foundations for a coupling between a continuous variator and automatic regulation of the pedal/throttle/gear ratio torque, an issue that would return in Magès's later work on hydrostatic transmission. As such, it can be read upstream of later archives such as the Note by P. Magès on the adaptation of the engine for an automatic gear ratio variator or the handwritten notes "Presentation T.H. 50 CV", which extend the reflection on driving and engine behavior of vehicles with continuous transmission.

The precision of the dating ("10.2.58", p. 144) allows this document to be fixed as an early milestone, fifteen years before the more advanced developments preserved in the same collection.