P. Magès note no. 570108: transmitting movement via hydraulic channels

In this document

This typewritten note by Paul Magès, referenced "570108" and titled Transmission of movement via hydraulic channels, sets out the "ENGINE-TRANSMISSION PROBLEM" p. 157.

The initial observation

Magès observes that on "today's congested roads", braking and acceleration count for more than top speed in achieving good average speeds safely p. 157.

Three compared solutions

To "have good acceleration", he considers three paths p. 157:

1. a large-displacement engine (few gears, but poorly used and fuel-consuming);
2. a small-displacement engine with numerous gears (manoeuvres deemed "unpleasant" for the customer) or an "admittedly complicated" automatic gearbox;
3. a small-displacement engine combined with a "continuous and automatic variator" covering the entire range, from infinity to a "6th gear".

This third solution "seems to him to be the most practical and economical" p. 157.

Why hydrostatic

Magès justifies the choice of the hydrostatic variator by its simplicity for the power to be transmitted, its use already proven "in general mechanics" and its adoption by aviation, which should be "transposed to the automotive level" p. 157. He acknowledges an efficiency perhaps lower than gears, but highlights the overall engine + transmission gain, comparing the idea to the variable-pitch propeller of aircraft p. 157-158.

Braking and auxiliary uses

He then describes the qualities expected of good braking — constant, cooled, progressive, with "maximum braking at the limit of adhesion without risk of locking" — obtained by throttling the transmission oil p. 158.

Finally, he lists the "OTHER ADVANTAGES" of pressurized fluid: powering the suspension (with automatic anti-roll considered), the dynamo at constant speed, ventilation (cooling, air conditioning, defrosting), and starting via the transmission pump used as a hydraulic starting motor, eliminating the electric starter p. 158.

Context

This reflection is part of the history of hydraulic engineering at Citroën, where Paul Magès would also develop the famous hydropneumatic suspension (DS, 1955). The principle of a pressurized fluid irrigating several vehicle components (suspension, steering, braking) would become a technical signature of the brand for several decades. The "hydrostatic transmission" project mentioned here refers to research that would continue at Citroën until the 1970s, without ever resulting in commercialization.

Archivist's perspective

This 1956 note seems to be a founding milestone: it formulates, from this date, a unified vision where the same hydraulic circuit would power transmission, suspension, braking, dynamo, and starting — a logic that will be found, developed and refined, in Magès' later work on hydrostatic transmission p. 157-158.

The file kept under the title "Hydrostatic transmission 59-74" attests that this idea was pursued for nearly two decades, until the 1973 notes which reiterate very similar arguments in favour of the hydrostatic variator. This 1956 note thus appears to be one of Magès' very first statements of principle on the subject.