Method and apparatus for measuring the strength of the air/fuel mixture supplied e.g. to an I.C. engine

An instrument for measuring the ratio of air to fuel of a combustible mixture being supplied to a combustion system, comprising:

• a cylindrical housing,
• a primary catalytic reactor in secure contact alignment within part of said housing comprising a matrix of annular cross section, said matrix having an open-ended central bore, said cylindrical housing having an end thereof closed except for an aperture therein being substantially coaxial with and of substantially the same diameter as said bore, said primary catalytic reactor being disposed a predetermined distance therefrom for forming an annular first mixing chamber,
• a central housing tube extending through said bore and said aperture,
• a secondary catalytic reactor in secure contact alignment with part of said tube, said tube having an end thereof substantially closed, said secondary catalytic reactor being disposed a predetermined distance therefrom for forming a cylindrical second mixing chamber,
• a monitoring cell, said secondary catalytic reactor being disposed a predetermined distance from the other end of said tube, said monitoring cell being securely mounted in a space thereby provided, and said primary catalytic reactor thereby surrounding said secondary catalytic reactor and an operative part of said cell, whereby the operating temperatures of said cell and of said secondary catalytic reactor are produced and maintained by a catalytic reaction at said primary catalytic reactor, and
• a pair of substantially identical lengths of partly coiled heat exchange tubes, one of said tubes receiving a portion of a flow of combustion products from said primary catalytic reactor and the other of said tubes receiving a portion of a flow of exhaust gas of said combustion system, each of said tubes having a known proportionate resistance to gas flow, being in a heat exchange relationship with the combustion products of said primary catalytic reactor, and providing respective predetermined portions of the flow of said exhaust gas and the flow of said combustion products to said second mixing chamber,
• wherein a controlled flow of a heating fuel and air is supplied to said first mixing chamber for providing a mixture thereof to said primary catalytic reactor.

A method of measuring the ratio of air to fuel of a combustible mixture being supplied to a combustion system operating on hydrocarbon fuel, for combustion therein, which method comprises the steps of:

• effecting the continuous complete combustion of a controlled supply of a hydrocarbon heating fuel with a controlled supply of an oxygen-containing gas to produce heat and a reference gas, said reference gas being a continuous supply of gaseous combustion products containing free oxygen in a known proportion,
• mixing a sample flow of exhaust gas from said combustion system with a sample flow of said reference gas in a controlled proportion,
• effecting a catalytic oxidation reaction of said mixture of said exhaust gas and said reference gas, to oxidise any unburnt components derived from the sample flow of said exhaust gas in the mixture,
• said reference gas ensuring the presence of a proportion of free oxygen in the gaseous combustion products of said catalytic oxidation reaction over the range of air/fuel ratios of said combustible mixture,
• utilising the heat of combustion of said mixture of said heating fuel and said air to maintain said catalytic oxidation reaction,
• using a quick-response oxygen-sensing cell of the solid-oxide electrolyte type to measure the proportion of free oxygen in the gaseous reaction products of said catalytic oxidation reaction, and
• deriving an air/fuel ratio of the combustible mixture being supplied to the combustion system in accordance with predetermined data and the results of said measuring step.