“Lean mixtures are clue to cleaning up motor exhausts” — The Engineer, 30 March 1972

In this document

The lean mixture, Shell's preferred path

The article, signed by David Millichamp for The Engineer (30 March 1972), reports on the work of Shell's Thornton Research Centre, near Chester, presented by Alun Thomas, head of the light fuels division. According to him, fuel composition (excluding lead content) has little effect on pollutants, and “the ball lies in the engineers' court to find a solution” p. 55.

The flagship method is the lean mixture: burning air-fuel at a ratio of approximately 22:1 instead of the usual 14:1, thanks to a special “mixture generator” p. 55. The results demonstrated by Shell are given in a list:

The device vaporises the fuel in an electric heater then mixes it with high-pressure air in a mixing chamber; the flow is constant and the excess must be “burned off” p. 55, which makes it inapplicable as it stands on a vehicle. Thomas also mentions the “lean burn” system developed by General Motors in the United States p. 55.

Catalytic reactors

In parallel, Shell developed with British Leyland a single-box catalytic reactor, tested on a Triumph GT6 equipped with the “slant-four” engine p. 55. Conventional systems operate in two stages (reduction of NOx, then oxidation of CO and hydrocarbons); the durability required by the projected American legislation is 50,000 miles p. 55-56. The single-box reactor, which is less expensive, still raises doubts regarding its durability p. 56, and Thomas points out a risk of over-reduction producing ammonia p. 56.

Crankcase, evaporation and lead

The article specifies that only 60% of hydrocarbon emissions come from the exhaust pipe, 20% from the crankcase (blow-by) and 20% from tank/carburettor evaporation p. 56. The removal of lead is presented as complex: loss of power and consumption estimated at 10% for an engine moving from 99 to 91 octane, cost of 200-250 million pounds over nine years for the British oil industry p. 56. Lead also “poisons” catalysts p. 56.

Context

The early 1970s marked the entry into force of the first American automotive anti-pollution standards, which strongly influenced Europe, which was still devoid of its own legislation — something the article explicitly highlights. Magès' hydropneumatic suspension and his work on carburetion/injection are part of this same decade where the industry was looking, in parallel with mechanical innovations, for answers to emerging environmental constraints.

Archivist's perspective

This clipping probably belongs to a technical monitoring file compiled by Magès or his team on carburetion and anti-pollution, as suggested by its proximity to other translations and press clippings devoted to injection and combustion chambers, for example Chapoux (UTAC) or the Zenith-Stromberg documentation.

The text clearly illustrates the technical debates of the time between “upstream” solutions (lean mixture) and “downstream” solutions (catalysis), a tension that probably nourished the reflection of Citroën engineers on future fuel supply systems.