Comparative table "For equal displacement": conventional engine vs. under-fed engine (1944)

In this document

This plate compares, "for equal displacement" p. 4, three engine configurations by means of cylinder-piston cross-section diagrams and numerical energy balances.

Conventional engine

Compression ratio of 6.2, absolute intake pressure at "full throttle" of 0.8. At full load: energy spent 246, energy on piston 100, losses 146, friction 15, useful energy 85, i.e., a consumption index of "246/85 = 2.9" p. 4.

In "reduced throttle" mode, this same engine shows: energy spent 182, energy on piston 74, losses 108, friction 15, useful energy 59, index "182/59 = 3.1" p. 4.

Under-fed engine

Absolute intake pressure at "full throttle" of 0.53, compression ratio 8.5. Balance: energy spent 162, energy on piston 74, losses 88 — "i.e., 18.5% less than Conv. Eng. at equal power" p. 4 —, friction 15, useful energy 59, index "162/59 = 2.75" p. 4.

Comparison

The document concludes: the two engines equipping identical cars, the first (conventional) "allows for greater 'acceleration' and 'speeds'", but at equal speed (therefore equal power), its consumption is "12.7% higher" than that of the under-fed engine, detailed calculation "(3.1-2.75)/2.75 x100 = 12.7" p. 4.

The page bears the handwritten signature "P.Magès", followed by an uncertain mention p. 4.

Context

At the time, this reflection was part of Paul Magès' career at Citroën: having joined in August 1925 as a self-taught individual, he became a technical writer and then deputy head of the Super-Control (autobiographical notes), and it was in 1939-1940, with the braking limiter for the TUB van, that he entered the design office — where he worked from 1944 on the hydropneumatic suspension (plans of 17 February 1944). This plate, dated the same year, testifies to parallel concerns regarding the thermal efficiency of petrol engines, playing on the compression ratio and the cylinder filling rate — a central concern for many automotive design offices, including Citroën's.

The principle of "under-feeding" (reduction of intake pressure compensated by a higher compression ratio) is part of general research into combustion optimization, in a context of fuel restrictions linked to the war.

Archivist's perspective

This plate synthesizes, in a very visual way, a numerical demonstration: it seems designed as a pedagogical argument or presentation aid, intended to convince that an under-fed engine, despite lower peak performance, offers real fuel savings at equal power p. 4.

It probably extends the more detailed calculations of the Technical note "Influence of the filling ratio on thermal efficiency" and relies on the values established in the graphic plate for determining the compression ratio, which suggests the same work file conducted by Magès during the summer of 1944 around this same issue of engine efficiency.

The presence of the signature reinforces the hypothesis of a summary document personally validated by the engineer, perhaps intended for an internal presentation.