Note No. 560.122 — Hydraulic transmission with generator and wheel receivers (1956)

In this document

General principle of the transmission

This note dated 4 April 1956, numbered 560.122, describes a hydraulic transmission system in which a generator G driven by the engine hydraulically powers "one or more receivers R" which transmit movement to the wheels (p. 3, §20). A technical diagram accompanies the text, showing the swashplate generator (angle α), the engine shaft, a valve/spring (S), and the receiver connected to the wheel p. 3.

Driver control

The flow D of the generator depends on the angle α of its plate; the pressure P thus produced acts on a piston that opposes a force K exerted by the driver on a pedal "replacing the so-called accelerator pedal" (p. 3, §21-23). The driver thus directly controls the torque transmitted to the wheels, rather than the engine speed.

Engine regulation and balance

Paragraphs 24 to 31 explain how, with the carburettor remaining at full load (reference to §12 of a previous note), the engine speed automatically finds an equilibrium position based on the gradient, the desired speed, and the force K applied by the driver p. 3-4.

Braking and energy recovery

In the event of a loss of engine power or braking action, "the receivers becoming generators and the generator becoming a receiver" (p. 4, §32), a relief valve is provided on the return circuit whose spring tension, adjusted by the brake pedal, determines the deceleration (p. 4, §33). The heated fluid can be directed towards a radiator, and possibly towards a "hydraulic motor" operating a cooling fan — the braking energy thus being converted "into heat and... into mechanical work" (p. 4, §34).

Finally, a device must gradually close the throttle valve when the force K decreases, down to idle when K = 0 (p. 4, §35).

Context

This note is part of the research conducted at Citroën in the mid-1950s on continuously variable transmission systems, in the wake of the work that would lead to the hydropneumatic suspension of the DS, presented in 1955. Here, Paul Magès explores, outside the strict field of suspension, an all-hydraulic transmission replacing the traditional gearbox and accelerator with a pressure-flow couple controlled by foot. This type of device was not adopted as such for the automotive production of the time, but testifies to the engineer's hydraulic inventiveness.

Archivist's perspective

The explicit reference to "§ 12" (p. 3, §24) indicates that this note extends previous reflection, probably that developed in Note No. 560.108, dated the same day and devoted to the same reasoning on a continuously variable transmission.

It seems that this piece constitutes a very detailed technical development stage, almost a step-by-step demonstration (paragraphs numbered 20 to 35), intended to validate the system's behaviour in steady state, acceleration, uphill, and braking.

The proximity of the date to Note No. 560.331, also from 4 April 1956 and devoted to the general diagram of the hydraulic transmission, suggests the same intense work file over a few days, where Magès successively refined the principle, controls, and general diagram.