Note No. 560.108 — Reasoning on the use of the engine with a continuously variable transmission (1956)

In this document

This technical note by P. Magès, numbered 560.108 and dated 4 April 1956 p. 1, sets out a thirteen-point reasoning on the optimal use of the engine in the context of a continuously variable transmission, as opposed to a conventional gearbox.

The basic principle

Magès starts from a graphical observation: the engine power curve follows a general pattern noted A, while there exists, at each speed, a point of best efficiency whose geometric locus forms a curve B (p. 1, points 1-2). With a conventional gearbox, a need for low power (20 HP at 100 km/h in 3rd gear, 4000 rpm) forces this power to be drawn from a high specific consumption zone, point C on the graph (p. 1, point 3).

With a continuous and automatic variator, the engine could instead run at around 1800 rpm to reach point D, of minimum consumption, with "an appreciable gain in comfort" (p. 1, point 4). Magès draws a "GENERAL RULE" from this: only use the engine at the speed providing the required power on curve B (p. 1, point 5).

Envisaged practical consequences

He envisages that the throttle position be regulated by a cam linked to the rotation speed, corrected by the torque value, the latter being "practically a straight line on a 2 CV engine between 1000 and 4000" (p. 1, points 6-8).

On page 2, Magès pushes the reasoning further: with a continuous variator, curves A and B could become "very close if not identical" since the constraint of high power at low speeds disappears (p. 2, point 9). He proposes that the engine operate "always at full throttle", except in the transition zone near idle (p. 2, points 10-12), by resorting to "lean mixtures" associated with an automatic heating of the intake air (p. 2, point 11). The throttle would then disappear as a speed regulation component, replaced by a component automatically modifying the gear ratio according to the effort requested by the driver (p. 2, point 13).

Context

This reflection is part of the research conducted at Citroën in the 1950s around automatic transmissions and hydropneumatic suspension, a field where Paul Magès remained famous for his inventions on the DS. Continuously variable transmissions (today close to continuously variable transmissions, CVT) remained at the time a marginal field of experimentation compared to the dominant manual or semi-automatic gearboxes.

Archivist's view

This document is presented as a preparatory theoretical reflection, rather than a description of a finished mechanism: Magès thinks aloud, multiplying hypothetical formulations ("could one not", "it is not improbable") p. 2. It seems to be an upstream design stage, seeking to justify the theoretical interest of a continuous transmission before detailing its technical implementation.

Dated the same day as two other notes in the collection, it could constitute a milestone of reflection contemporary with more concrete work on the hydraulic transmission and its general diagram, even if the exact link between these documents is not specified in the text itself.