Technical note by P. Magès on the influence of the filling ratio (Sheet No. 1, 1944)

In this document

This typewritten note, bearing the letterhead of the Société Anonyme André Citroën and signed "P. Magès, June-July 1944", constitutes "SHEET No. 1" of a reflection numbered in six points on the influence of the filling ratio on the thermal efficiency of the engine.

Magès' reasoning

The text progresses through successive deductions:

1. Thermal efficiency depends on the expansion ratio and the combustion speed p. 1.
2. For a given richness and end-of-compression temperature, the combustion speed depends on the pressure of the mixture at the end of compression — but this speed is limited by the phenomenon of "detonation", which bounds the usable compression pressure p. 1.
3. This compression pressure depends on the volumetric ratio (assimilated to the expansion ratio ρ) and the pressure at the end of intake p. 1.
4. Since one seeks to increase ρ to improve efficiency, it is therefore necessary to reduce the pressure at the end of intake, that is to say, decrease the filling ratio p. 1.
5. Magès draws the conclusion that an "under-fed engine" should have better thermal efficiency than a "supercharged engine", and announces his intention to verify if this gain is maintained once mechanical efficiency is taken into account p. 1.
6. Finally, he sets a reference "standard engine" with the values: filling ratio a' = 0.80, expansion ratio ρ = 6.2, indicated power Pi = 1, a'q = 1 p. 1.

A footnote specifies the definition of the filling ratio: ratio between the mass of mixture aspirated per cycle and the reference air mass (15°, 760 mm Hg) corresponding to the volume swept by the piston p. 1.

Context

This note is part of the research work conducted at Citroën during the Occupation, a period when engineers, constrained by fuel shortages, explored avenues for economy and improvement of the efficiency of petrol engines.

The notion of "detonation" mentioned here refers to the phenomenon of premature auto-ignition of the mixture, well known to engine designers of the time as a factor limiting compression ratios — an issue that would remain central in the subsequent developments of Citroën, particularly on the engines that would later equip the 2CV and the DS.

Archivist's perspective

This document has a methodological value: it exposes, in the form of a numbered technical syllogism, the intellectual path that leads Magès to formulate the hypothesis of an "under-fed engine" that is thermally more efficient than a supercharged engine p. 1.

It seems to be a first working sheet, probably followed by numerical and graphical verifications, as suggested by the announcement in point 5 p. 1. This hypothesis likely finds its extension in the graphical chart for determining the volumetric ratio and in the comparative table at equal displacement, which seem to numerically develop the avenues sketched out here.