Louis Renault Patent No. 397,607: liquid speed change (1908)

In this document

This is the official booklet for patent of invention No. 397,607, filed by Louis Renault with the National Industrial Property Office. The cover page specifies: "Applied for on 18 December 1908. Issued on 27 February 1909. — Published on 13 May 1909" and notes that an equivalent application had been "filed in Germany on 21 December 1907" p. 132.

Object of the invention

The text presents "a clutch and progressive speed change mechanism" where the transmission between the engine shaft and the driven shaft "is done via two pump systems acting on a constant quantity of liquid" p. 132. A pump system, actuated by the engine shaft, delivers the liquid to a second pump system that drives the driven shaft.

Operating principle

The patent insists on a key characteristic: a portion of the power is always transmitted mechanically, while the rest passes through the "hydraulic transformer"; at "direct drive", the entire transmission becomes purely mechanical p. 132-133. The device also allows for complete disengagement — useful "in the case of an automobile" to let the engine run independently of the wheels when going downhill p. 132 — as well as reverse gear and engine braking.

Mechanical description

Pages 133-134 detail the architecture: a block of pistons (1, 2) keyed onto the engine shaft (3) via splines, ball-joint connecting rods, a tilting crown (6) adjusted via a lever (10) and a sliding piece (13), a distributor (15), and an analogous receiver system (17, 21-24) fixed to the chassis. A piston (25), balanced by a spring (26), automatically varies the inclination of the receiver crown (23) according to the liquid pressure — which regulates the speed as a function of the resistive torque, "regardless of the variations in the resistive torque" p. 134. Two safety valves (29) protect the device against overpressure in both directions of travel p. 134-135.

The final summary p. 135 reformulates the two main claims: the mechanical/hydraulic sharing of power, and the automatic control system via pressurized liquid piston. The document is signed "Louis RENAULT", by proxy of Émile Klotz p. 135. The single plate p. 136 illustrates the device in section (figs. 1 to 4).

Context

Louis Renault (1877-1944), founder of the Renault factories, filed this patent in 1908, at a time when the French automobile industry was seeking to simplify gear shifting, then provided by mechanical dog-clutch gearboxes that were often delicate to handle. The idea of a progressive hydraulic transmission, without jerks, prefigures principles that would be rediscovered and developed much later, notably in automatic and hydrostatic transmissions.

This document does not come from Paul Magès's own archives but seems to have been kept by him — the handwritten reference "Pb 4633 / 1959" p. 132 suggests a late classification, probably during his own research on hydrostatic transmission at Citroën in the 1960s-1970s.

Archivist's perspective

The presence of this old patent in the Magès collection is significant: it likely attests to a monitoring and prior research effort conducted by the engineer before developing his own hydraulic solutions for Citroën.

The handwritten annotations — underlining of the Renault name, circling of the filing date, reference "Pb 4633 / 1959" p. 132 — indicate that the document was read and referenced with attention, no doubt to establish the priority of ideas for transmission by pressurized liquid.

This patent can thus be read as a piece of historical context shedding light on Magès's later work on hydrostatic transmission, notably his notes on the automatic gear ratio variator. The 1908 Renault principle — hydraulic pumps progressively adjusting the speed ratio — announces, in its own way, certain issues that Magès would encounter fifty years later.