Prestrelle technical note: regulation of a hydrostatic converter (P.4331)

In this document

The general principle

This note from the B.E.A. Citroën, referenced P.4331 and signed "Prestrelle" (17-02-64), describes a device for the automatic regulation of the gear ratio of a hydrostatic torque converter, based on both the input power applied and the output resistive torque p. 72.

The text establishes the principle: when the input power is constant, "regulation is reduced to the equilibrium between 2 torques" represented mechanically by a beam whose arms balance according to the gear ratio p. 72.

First embodiment (Fig. I)

The transmission between the engine (1) and the wheel (2) passes through a generator (3) and one or more receivers (4), connected by supply and return lines p. 72. The beam (10), whose lengths vary thanks to rollers (11), receives on one side a force proportional to the resistive torque (piston 15) and on the other a constant force combining a spring (17) and a centrifugal governor (flyweights 22) linked to the accelerator pedal (28) via a cam (29) p. 73.

The document specifies that the pressure curve thus obtained (curve 1, Fig. III) remains below the maximum engine load (curve 2) and corresponds to its "best efficiency" p. 73. A distributor (32) also allows for the association of a variable-displacement receiver, the receiver displacement reduction cycle occurring after the increase cycle on the generator side p. 74. A clutch (49) device controlled by centrifugal pressure completes the assembly p. 74.

Variant (Fig. II)

In this second arrangement, it is the engine load that is set by the driver, with the engine speed becoming the result — the inverse of Fig. I p. 74. The centrifugal governor (100) produces a specific curve (Fig. IV) with two characteristic points A (maximum pressure clutch) and B (maximum power) p. 74-75. An additional device linked to the pedal depression speed (110 to 119) allows for rapid transient accelerations p. 75.

Summary of claims

The note concludes with five points summarizing the claimed principles: regulation by beam balancing pressure and displacement (1), distribution with hydraulic locking for variable receiver (2), setting of engine speed (3) or engine load (4) via the pedal, and control by the pedal depression speed (5) p. 75.

Pages 76 and 77 contain the complete diagrams (Fig. I and Fig. II) numbered up to 119, and page 78 the curves Fig. III and Fig. IV (engine torque / engine rpm).

Context

The hydrostatic converter mentioned here is part of the research conducted by Citroën, under the impetus of Paul Magès, to explore alternatives to the classic gearbox — in continuity with the work on the hydropneumatic suspension which had already demonstrated Citroën's mastery of hydraulics applied to the automobile.

These researches on the hydrostatic transmission (T.H.) continued for more than a decade at Citroën, with tests on 2 CV, 3 CV and then on the L vehicle, without leading to commercialization.

Archivist's perspective

This document is a dense and rigorous technical piece, likely intended to formalize an invention with a view to a patent filing, as suggested by the structure in "summary" of numbered claims p. 75.

It appears to be a contribution from the design office (signed Prestrelle) forming part of the hydrostatic transmission development program led by Magès. Dated February 1964, it precedes by several months the handwritten notes presenting the 50 HP T.H. (September 1964), and could have fueled the theoretical reflections that accompanied the actual tests conducted on vehicles, such as those recounted in the endurance monitoring of the 2 CV A12.

The dual approach described (setting engine speed or engine load via the pedal) clearly illustrates the exploratory approach of the B.E.A., which considered several regulation architectures before converging on solutions proven in the field.