Magès note: replacing hydraulic regulation with a centrifugal governor (DS, 1959)

In this document

This technical note from the B.E.A. (André Citroën Design Office), dated and signed 24.12.59, proposes replacing the hydraulic regulation of the DS automatic clutch control with a centrifugal regulation (RC).

Findings on the existing hydraulic system

Magès lists the defects of the low-pressure pump system: "lack of precision in the automatic clutch and the evolution of settings according to temperature", an unpleasant "pumping" when starting in 1st and 2nd gear, as well as "sluggish disengagement when braking" p. 284.

Regarding endurance, he points out unresolved leaks at the low-pressure pump rotary seals, a piston "prone to seizing", and oil degradation due to high temperatures — "the main factor in the destruction of hydraulic components" p. 284. He links this phenomenon to oil circulation near the water pump (which can reach 100°) and provides a figure: separating the low-pressure pump from the water pump would save "10° at the reservoir and 16° at the gearbox block" p. 284.

He also details the effect of temperature on the internal clearances of the gearbox block, going "from 1 to 2 microns at 20°" to "5 or 7 microns at 90°" p. 284, creating a "vicious circle" of heating and loss of precision.

The principle of the centrifugal governor

The proposed solution eliminates the low-pressure pump, the "38 piston" and its spool valve in the hydraulic block, replaced by "a mechanical centrifugal governor acting on a hydraulic distributor", driven by a 3-groove belt on the high-pressure pump p. 283-284.

Magès expects a regulation "independent of temperature", an "instantaneous" response, and pumping "practically eliminated even in 2nd, 3rd and 4th gear" — to the point that he deems it necessary to "warn the customer not to abuse this advantage" p. 283.

He notes one drawback: the belt makes the disengagement travel adjustment inaccessible, which requires creating a new adjustment device, "easier to execute with precision because this adjustment is one of the most important on the DS" p. 283. A handwritten pencil annotation, partially uncertain, mentions an increase in the disengagement cylinder travel linked to disc wear p. 283.

Context

The DS, presented by Citroën on 6 October 1955, integrated an automatic clutch control coupled to its hydropneumatic suspension, a system whose principles date back to the plans drawn up by Magès as early as 17 February 1944 (Plans for hydropneumatic suspension with differential interaction) and the first tests on the 2 CV in 1946 (Notebook of the first tests). This 1959 note, written while Magès was at the Citroën design office — a position he would hold until his retirement in September 1974 (Photographs of the retirement) —, is part of the well-known limitations of the hydraulic technology of the time, before the subsequent developments of the model.

Archivist's perspective

This document illustrates the continuous work of Paul Magès to perfect a system already on the market, in response to concrete usage defects reported from the field ("main complaint..."). It appears to be a summary note intended to motivate a major technical evolution, probably for the attention of his management or a manufacturing team, given the care taken to justify each expected advantage p. 284 and to anticipate practical consequences p. 283.

This archive is part of the same file as the Summary of folder 41-59, dedicated to the developments of the DS and the hydropneumatic suspension, which confirms its place in an effort of continuous improvement of the vehicle.