From the Rolls-Royce experimental archive: a quarter of a million communications from Rolls-Royce, 1906 to 1960's. Documents from the Sir Henry Royce Memorial Foundation (SHRMF).
The principles of operation and features of the Cambridge exhaust-gas tester.
| Identifier | ExFiles\Box 102\1\ scan0150 | |
| Date | 18th January 1934 | |
| 2 Wheatstone bridge which satisfies the conditions. The two identical spirals of platinum wire, having resistances r1 and r2 respectively, are enclosed in two separate cells E1, E2 in a metal block. Each of the spirals forms one arm of the bridge circuit, the other two arms r3 and r4 being of manganin. An electric current (which is adjusted by means of a rheostat R to a definite value i, as measured on the ammeter A) is allowed to flow through the bridge, thereby causing the two spirals to become heated and to lose heat to the walls of the cells. If the spirals r1 and r2, and the gases surrounding them, are identical in every way, their temperatures will be equal. If, then, r3 and r4 are chosen to be equal, the bridge will be balanced and no current will flow through the galvanometer G.{Mr Griffiths - Chief Accountant / Mr Gnapp} If the two cells contain gases of different thermal conductivities the spirals will cool at different rates, and one spiral will be maintained at a higher temperature than the other. The consequent difference in the resistance of the spirals will cause a deflection of the galvanometer G, the extent of which will depend on the difference in the conductivity of the two gases. The construction is such that changes in the temperature of the gases affect both sides of the bridge equally. If, therefore, the cell E1 contains a standard gas—say, air saturated with water vapour, and the other the exhaust gas to be tested, the galvanometer can be calibrated to show directly the air-fuel ratio, since it has been demonstrated that a linear relation exists between the thermal conductivity and the air-fuel ratio. In practice it is found desirable to have four spirals, one in each arm of the bridge, and to dispense with the manganin resistances. Two spirals, in opposite arms of the bridge, are then exposed to the standard gas, and the other two to the gas under test. This arrangement enables more robust spirals to be used and a higher sensitivity to be attained than is possible with the two-spiral arrangement. The new Cambridge exhaust-gas tester, shown in Fig. 2, operates in accordance with this principle. It analyses the exhaust gas from internal-combustion engines, and indicates the result of this analysis on a scale calibrated in terms of air-fuel ratio over a range of 10 to 15. With the instrument it is possible to determine rapidly and with scientific accuracy the “leanness” or “richness” of a fuel mixture; information which not only enables the best carburettor setting to be obtained, but also denotes the completeness of combustion, and with proper interpretation indicates certain mechanical conditions of the engine. The instrument is supplied in an aluminium case with cover, and weighs 29 lb. Ten feet of rubber hose is supplied for conducting a sample of the gas from the exhaust pipe of the engine under test to the instrument, and a stainless steel fitting is provided for inserting into the exhaust. The four platinum spirals forming the arms of the Wheatstone bridge are contained within chambers inside the instrument, and two of these spirals are exposed to the gas under test, while the other two are exposed to moisture-saturated air contained in a sealed chamber. The spirals are heated a definite amount by current from two standard dry cells contained within the instrument case. The chambers containing the four spirals are enclosed within a heavy copper block for the purpose of equalising the temperature of the air or gas surrounding each spiral. After passing into the instrument the gas is filtered through a cartridge containing metallic wool before entering a chamber from which it diffuses into the test cell. It is then exhausted at the side of the instrument. A relief valve is provided in the under side to discharge any condensate that may accumulate and excess gas when the engine is operating at high speeds. A novel feature of the instrument is the moving system of the galvanometer, wherein the complete movement is supported magnetically and floats in the field of the magnet; this absorbs any shocks, takes the wear off the pivots, and results in a rugged instrument that will withstand hard usage. [Stamp Text] LIBRARY RECEIVED 18 JAN 1934 ROLLS-ROYCE LTD. Constructed by CAMBRIDGE INSTRUMENT COMPANY, LIMITED, 45, GROSVENOR PLACE, LONDON, S.W.1. Telegrams: “Unipivot, Knights, London.” Works: LONDON AND CAMBRIDGE. Telephone: Sloane 9146 (3 lines). Printed in Great Britain by The Cornwall Press Ltd., Paris Garden, London, S.E.1. | ||