Note No. 560.331 — Hydraulic transmission: general diagram and controls

In this document

This technical note No. 560.331, signed by P. Magès and dated 4 April 1956, sets out the principle of a hydraulic transmission intended to replace a conventional gearbox.

General diagram

Two architectures are considered p. 5:

• a variable-displacement volumetric pump, driven by the engine, supplying a hydraulic motor coupled to each drive wheel, the operation resembling that of a mechanical differential (the maximum torque transmitted being limited by the most disadvantaged wheel);
• a variant with two generators placed at the ends of the crankshaft (to reduce torsion), connected by a pressure-balancing pipe when cornering, with the possibility of a valve to interrupt the passage of fluid "in difficult cases" p. 5.

A handwritten sketch illustrates the adjustment of the generator flow: an inclined plate between the engine shaft (X-X) and the circuit to the receivers, with forces F1 and F2 and a fluid return p. 5.

Adjustment and controls

The note then details, in eleven numbered points p. 6-7:

• at idle, the plate is perpendicular to the engine shaft p. 5;
• a spring tends to tilt the plate, while an automatic device creates a force F1 to straighten it if the speed drops too low, and a force F2 to tilt it further if the speed exceeds maximum power p. 6;
• the accelerator pedal first opens the carburettor throttle, then acts on the plate lever in the F1 direction p. 6;
• when releasing the accelerator, the hydraulic wheel motors act as pumps and brake the car via the engine ("engine braking"), the plate tilting to the maximum under the action of the spring p. 6;
• the brake pedal can, in its first travel, increase this engine braking, with a reaction piston restoring a feeling of effort to the driver p. 6;
• in its final travel, the pedal acts on a throttle sending the fluid to a cooling radiator p. 6;
• a portion of the pressurized fluid can supply an accumulator for hydraulic accessories p. 6;
• finally, braking energy can be partially recovered via a hydraulic motor driving a cooling fan, or even a flywheel storing kinetic energy subsequently returned to the fan p. 6-7.

The end of page 7 is illegible.

Context

This note is part of the research conducted at Citroën in the mid-1950s on continuously variable transmissions, in the wake of the work that would lead in particular to the hydropneumatic suspension of the DS, presented in 1955.

The idea of a continuously variable hydraulic transmission, without a conventional gearbox, would remain essentially experimental at the manufacturer, but testifies to the interest of the time in hydraulic solutions applied to the automobile.

Archivist's perspective

This note probably constitutes a direct development of the general reasoning set out on the same day in Note No. 560.108, and it is closely linked to Note No. 560.122, devoted to the same generator/receiver device.

It seems that Magès pushes the reflection here to the details of fine control (accelerator, brake, energy recovery), which suggests a stage already advanced in theoretical reflection, before any material realization attested by this document.

The attention paid to the recovery of braking energy (flywheel, fan) is notable and testifies to a concern for energy efficiency early in the engineer's work.