Note No. 560.122 by Paul Magès: principle of a hydraulic transmission (4 April 1956)

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Principle of the transmission

This note from 4 April 1956, numbered 560.122, sets out the principle of a hydraulic transmission: a generator G, driven by the engine, is hydraulically connected to one or more receivers R transmitting movement to the wheels (p. 161, §20). A blue diagram accompanies the text, representing the swash plate (angle α), the generator, the lines, the spring S and the receiver R connected to the wheel p. 161.

At a given speed corresponds a torque at the wheels, therefore a pressure P and a flow rate D, the latter being a function of the angle α of the generator plate (p. 161, §21).

Driver control

The driver acts on a pedal — which replaces the accelerator pedal — exerting a force K in opposition to the pressure P: they thus directly control the torque transmitted to the wheels (p. 161, §22-23). The carburettor remains at full load (§24), and Magès demonstrates how the engine speed automatically finds an equilibrium position according to variations in gradient, desired speed or available power (p. 161-162, §25-31).

Braking and cooling

In the event of a loss of engine power, the system can produce a "sudden braking jolt" because the receivers and generator reverse their roles (p. 162, §32). To regulate this, Magès provides for a second piston or a relief valve on the return circuit, the spring tension of which regulates deceleration and which is controlled by the brake pedal (p. 162, §32-33). This same pedal switches the fluid into a radiator to dissipate heat, and possibly towards a hydraulic motor operating a cooling fan: the braking energy is therefore transformed into heat and mechanical work (p. 162, §34). Finally, a device must gradually close the throttle valve to idle when K = 0 (p. 162, §35).

Context

Paul Magès is the Citroën engineer famous for the hydropneumatic suspension of the DS, presented in 1955. This 1956 note is part of a broader reflection by Magès on the uses of hydraulics in the automobile, beyond suspension: here, it is a question of imagining a transmission that would replace the classic mechanical gearbox with a generator-receiver pair controlled by pressure.

This type of research prefigures what Citroën would later develop under the name of hydrostatic transmission (T.H.), tested in particular on 2 CV and 3 CV models in the 1960s-1970s.

Archivist's perspective

Dated April 1956, this note is situated very early in Magès' research on hydraulic transmission — probably one of the very first written formulations of the principle, since it lays the theoretical foundations (generator/receiver, pressure regulation, braking and cooling management) without referring to a specific test vehicle.

It seems that this initial reflection nourished, nearly a decade later, the more concrete developments carried out at the B.E.A., as evidenced by the handwritten notes "Presentation T.H. 50 HP" of 1964 or the record of mileages of test vehicles. Magès' very pedagogical reasoning, constructed point by point (§20 to §35), reveals the engineer's working method: starting from a simple diagram to anticipate, from the design stage, limit cases (engine failure, sudden braking) — an approach that will be found in his later technical writings.