Chart No. 1: Compression ratio and filling to equalise compression pressures (1944)

In this document

This page is a graphic plate (Chart No. 1), drawn in purple ink on graph paper, signed "P. Magès" and dated "Juil. 44" p. 2.

Purpose of the chart

The title indicates that it concerns the "DETERMINATION of the COMPRESSION RATIO as a FUNCTION of the FILLING RATIO a' to OBTAIN EQUAL PRESSURES at the END of COMPRESSION" as well as the "CORRESPONDING THERMAL EFFICIENCY", the "INDICATED POWER at equal DISPLACEMENT" and the "CONSUMPTION at equal INDICATED POWER" p. 2. All comparisons are made in relation to a reference engine defined by: a' = 0.80, ρ (compression ratio) = 6.2, δq = 1, P = 1 p. 2.

The two juxtaposed graphs

On the left, a first system of axes shows on the ordinate the "Thermal Density" η_t (graduated 0.3 to 0.466) and on the abscissa a range from 0.7 to 1.3, illustrating "the variation of the indicated power or the effective displacement and the consumption" in relation to the reference engine p. 2. Two formulas frame this plot:

• at equal geometric displacement: P/P = δq₁·δ₁ / δq·0.466 ;
• at equal indicated power: δq₁/δq = 0.406/P₁ p. 2.

On the right, a second system of axes relates ρ ("Compression Ratio", graduated from 4 to 9) and a' ("filling", graduated from 0.4 to 1.2). The associated formula is: ρ = [0.8 × 6.2 / a']^1,3 p. 2.

Definition and complementary formula

A "Nota" box specifies: "we call filling a': Volume of air aspirated at 780 mm/15° / Volume generated per cylinder" p. 2. A second formula links thermal efficiency to the compression ratio: η_t = 1 − (1/ρ)^(γ−1), with γ = 1.384 p. 2.

Context

The compression ratio (ρ) and the filling rate are classic parameters for tuning internal combustion engines: their optimisation aims to improve thermal efficiency without degrading power or consumption.

In July 1944, Paul Magès had been at the Citroën design office since 1939-1940, where he worked on research into improving engine efficiency; this is the same period when he sketched, as early as 17 February 1944, his first hydropneumatic suspension plans, several years before the first tests on the 2 CV (May-July 1946) and the presentation of the DS on 6 October 1955. This plate is therefore part of a phase in his career where he was simultaneously conducting work on motorisation and the beginnings of his major invention.

Archivist's perspective

This plate bears witness to methodical graphic modelling work: Magès is clearly seeking to formalise, through charts, the trade-offs between filling, compression ratio, thermal efficiency and consumption — probably with a view to practical application on a "producer gas" or under-fed engine, as suggested by the file title p. 2.

The precision of the formulas and the explicit reference to a control engine (a' = 0.80, ρ = 6.2) suggest that this is a calculation tool intended to guide engineering choices rather than a simple theoretical illustration.

This document usefully complements the technical note on the influence of the filling ratio and the comparative table at equal displacement, both from the same file dated 1944.