Descriptive memorandum of the "Constant depression carburettor" by P. Magès (May 1935)

In this document

The defects of conventional carburettors

The note opens with the three conditions for "good efficiency": perfect atomisation, complete cylinder filling, maximum compression of the gaseous mixture p. 3. Magès explains that as the air intake nozzle of the carburettors of the time was fixed, these conditions could not be met at all speeds: atomisation was "very poor at low speeds", and excessive depression at high speed led to a "reduction in filling (12 to 15%)" and a proportional loss of compression p. 3.

The principles of the constant depression carburettor

The proposed system maintains constant depression, atomisation, filling and compression at all speeds, while the richness of the mixture and the ignition timing remain variable; fuel is cut off "when the engine is not needed (descents - slowing down)" p. 4.

Technically, a "regulator system" acts on the section of the diffuser, which remains "mathematically proportional to the volume of air sucked in", which controls, via variable-ratio levers, the lifting of a needle regulating the fuel supply p. 4. The fuel, distributed over the entire circumference of the diffuser (15 cm), arrives already mixed with a little air before final atomisation p. 4-5. The circuit cuts off during deceleration and is automatically re-established at a determined minimum speed p. 5.

Comparative table (filing no. 1124, 16 May 1935)

The application was filed "in Seine-et-Oise, on 16 May 1935, under no. 1124" p. 6.

The sheet of curves

Four handwritten graphs compare the "X..." carburettor and the constant depression carburettor: depression/filling/compression, power, acceleration, specific consumption — all dated "May 1935" and signed P. Magès p. 7.

Context

This memorandum is part of the research conducted by Paul Magès on carburetion in May 1935, outside of his employment at Citroën — which he joined in August 1925 at 17 and a half and where he would remain for 49 years, first as a self-taught technical writer and then as deputy manager of the Super-Control (autobiographical notes) —, several years before he entered the design office (1939-1940) and designed the hydropneumatic suspension for which he is best known. Variable depression carburettors, standard in the 1930s automobile, effectively suffered from the fixed adjustment limits mentioned here; research into constant depression heralded solutions that would become widespread later (notably SU-type constant depression carburettors).

Archivist's perspective

This document is probably one of the very first written testimonies of a personal patent filing by Magès, precisely dated by the mention of the filing on 16 May 1935 p. 6 — which allows the dating of the archive to be refined to the month, as already indicated.

The rigour of the argument, supported by a detailed comparative table and technical curves, suggests a methodical engineering approach, prior to his most famous works: it was only four to five years later, in 1939-1940, that he designed the braking limiter for the TUB van and entered the design office, before drawing the first plans for hydropneumatic suspension in February 1944 (plans of 17 February 1944). It seems that this file documents a stage in his reflection on the optimisation of the air-fuel mixture, a theme that joins, by analogy of method, his later search for constancy and regulation in hydraulic suspension.

The link with Patent no. 791.262, issued on 23 September 1935, is direct: it is very likely the official follow-up to this filing of 16 May 1935.