J. Hondet note: compressed air as buffer energy for the 2 CV (May 1974)

In this document

This technical note from the D.E.A. Computing Centre, signed by J. Hondet, file 1.73, is dated May 1974 p. 5. It explicitly aims to "constitute a basis for the study of patents based on this principle" of energy storage via compressed air p. 5, with deliberately simplified calculations under the assumption of ideal gases.

Range and energy required

The author starts from the 2 CV 4: acceleration up to 60 km/h in 13 s with a 190 kg load requires 17,500 kg.m p. 5, and two kilometres of level driving at 60 km/h require an additional 52,000 kg.m, for a total of 69,500 kg.m p. 6.

Compression and expansion of air

Paragraphs II and III compare isothermal and adiabatic compression. For a pressure ratio p/p₀ = 200, the adiabatic compression energy reaches 76,200 kg.m/kg against 45,500 kg.m/kg in isothermal, with a final temperature of 1087 °C in the first case p. 8. The efficiencies calculated according to different expansion scenarios vary from 0.2 to 0.7 p. 10-11.

Weight of the casings

The note establishes that the weight of a cylindro-hemispherical casing is approximately 30% higher than that of an equivalent sphere p. 14. Based on 91,000 kg.m to be stored, the results give, in kg:

Comparison with the lead-acid battery

The author concludes that "the compressed air tank is therefore at least five times heavier than the lead-acid battery, for equal releasable energy" p. 16, a gap that worsens significantly if the range is multiplied by 20 (540 kg against 80 kg, p. 17).

Safety

The note mentions the legal rule of the product pressure×volume ≤ 80, compared to the "suspension spheres of the GX" (1.3 kg, 500 cm³) already known to the Service des Mines p. 17.

Context

The hydropneumatic system developed by Citroën, of which Paul Magès is the inventor, relies on spheres containing nitrogen under pressure separated from a liquid by a membrane. The reference to the "suspension spheres of our cars" and to the GX (Citroën model from the late 1970s) is part of this in-house engineering culture, where expertise on pressure vessels was already proven: Magès himself, who joined Citroën in August 1925 and was then deputy head of the Super-Control after a 49-year career (Draft letter from Paul Magès to Mr. Sarre and interview plan of 11 May 1969), had developed the plans for the hydropneumatic suspension as early as 1944 (Hydropneumatic suspension plans with differential interaction (17 February 1944)). In May 1974, the date of this note, he is still active, his retirement only occurring in September of that same year (Photographs of Paul Magès' retirement (1974)).

Compressed air as traction energy for vehicles has been the subject of competing research (pneumatic engines) at various times, but has structurally encountered the same weight and efficiency limits as those quantified here.

Archivist's perspective

This document does not appear to be signed by Magès himself, but likely stems from a study commissioned within his work environment, the D.E.A. Computing Centre being presumably linked to the design departments on which Magès depended, then deputy head of the Super-Control a few months before his retirement (Draft letter from Paul Magès to Mr. Sarre and interview plan of 11 May 1969). It is a piece of prospective engineering, intended — as the text explicitly states — to evaluate the solidity of envisaged patents p. 5, and therefore probably to decide, upstream, between several technological paths.

The interest of this note lies in its critical evaluation rigour: Hondet methodically demonstrates, with supporting figures, that compressed air is an energy dead end for automotive propulsion compared to the lead-acid battery p. 16-17, which likely contributed to steering Citroën's research towards other paths. The comparison with existing suspension spheres p. 17-18 also shows how the expertise acquired on the note of refusal to join the BHRA and hydropneumatic studies informed other internal technical reflections during the same period.