Comparative curves manual gearbox / H.T. on DS 100 HP (Prestrelle, February 1962)

In this document

These four handwritten plates, signed "Prestrelle" and dated February 1962 (p. 80, 81, 83: "Trestrelle/Prestrelle 2-62"), compare on a 100 HP engine DS of 1398 kg the performance of the manual gearbox and the hydraulic gearbox (hydrostatic transmission).

Forces and efficiencies (p. 80-81)

Page 80 plots the "Maximum available force" of both gearboxes as a function of speed, with the manual gearbox ratios specified: 1st (1/12.8), 2nd (1/6.7), 3rd (1/4.48), for a maximum engine speed of 5,500 rpm on the mechanical side versus 5,000 rpm on the hydraulic side. An inset also compares the efficiencies of both gearboxes between 80 and 95% over the 20-180 km/h range.

Page 81 details the characteristics of the hydraulic receiver ("Variable displacement receiver", 37.5 cm³ generator, 82x2 cm³ receiver) and superimposes efficiency, power at the wheels, engine power (p. 85%) and compared fuel consumption of both gearboxes ("Manual gearbox consumption p.82%" / "Hydraulic gearbox consumption").

Accelerations and distances covered (p. 82-83)

Page 82 provides, for the manual gearbox, the speed/1000 engine rpm correspondences per gear, with associated coefficients:

For the hydraulic gearbox, the following are specified: 75 cm³ generator, 1/2 reduction, 2x625 cm³ variable displacement receivers. The graph of distances covered (0-2000 m) as a function of time (0-50 s) locates the "2-3" gear shifts around 12-13 s and "3-4" around 35 s.

Page 83 repeats these maximum accelerations on a finer scale (0-120 m, 0-10 s), showing the hydraulic gearbox (5,000 rpm max) slightly above the manual gearbox (5,500 rpm max), with the "1-2" shift marked around 4.5 s.

Context

The DS, presented by Citroën in 1955, is renowned for its innovation in hydropneumatic suspension, the work of Paul Magès. In parallel, Magès' teams explored from the 1950s-1960s a hydrostatic transmission — a continuous gearbox based on hydraulic pumps and variable displacement receivers — intended to replace the classic manual gearbox.

This type of experimental comparison (available force, efficiency, consumption, acceleration) is typical of the test campaigns conducted at the Automobile Design Office to evaluate the industrial viability of an innovation against the existing solution.

Archivist's perspective

These four plates form a coherent set of comparative measurements, probably intended to document, for Citroën's technical hierarchy, the interest (or limits) of the hydrostatic transmission compared to the manual gearbox on a high-end model powered at 100 HP p. 80-83.

The care taken in recording efficiency, consumption and acceleration suggests an important technical arbitration stage in the project's development, at a date (February 1962) prior to subsequent endurance tests on 2 CV and 3 CV referenced in the same collection, such as the Endurance test monitoring of the 2 CV A12 with hydrostatic transmission or the Handwritten notes "Presentation H.T. 50 HP" at the B.E.A..

It seems that this file belongs to the comparative development phase of the system, before its generalization to other test vehicles.