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).
Promotional leaflet for the Cambridge Exhaust Gas Tester, detailing its function and benefits for engine tuning.
| Identifier | ExFiles\Box 155\3\ scan0016 | |
| Date | 7th May 1935 guessed | |
| CAMBRIDGE EXHAUST GAS TESTER WHAT IT DOES IT SAVES FUEL IT SAVES TIME IT ENSURES EFFICIENCY THIS instrument is not a substitute for an able mechanic, but it gives him excellent data upon the combustion efficiency of an internal combustion engine, and enables him to determine precisely the effect of adjustments being made and accordingly to produce the best possible carburettor setting. A high air-fuel ratio indicates a weak mixture and greater combustion efficiency. Theoretically the engine should be operated on as weak a mixture as possible consistent with power requirements. On the other hand, too weak a mixture will reduce power, tend to overheat the engine, and consequently lead to increased consumption of oil, burning of valves, and other troubles. There is a best point in the carburettor adjustment and the Cambridge Exhaust Gas Tester shows when it is reached. It may be used on the test bench with cars under dynamometer loading or, better still, on the road under normal working conditions. Fig. 2. TRANSPORT COMPANIES, LARGE SERVICE GARAGES, MOTOR OMNIBUS CONCERNS, MOTOR MANUFACTURERS and LARGE USERS OF PETROL DRIVEN VEHICLES NEED THIS INSTRUMENT. SIMPLE TO USE RAPID IN OPERATION SUITABLE FOR ROAD OR BENCH TESTS CONVENIENT FOR DISTRIBUTION TESTS AIR-FUEL RATIO RICH 10 11 12 13 14 15 WEAK STD CAMBRIDGE INSTRUMENT CO. LTD. LONDON AND CAMBRIDGE. FIG. 3. AS motor spirit will ignite only when mixed with air, it is the function of an engine carburettor to mix them in the proper proportion for combustion. The ratio of pounds of air admitted to the engine to pounds of petrol to be mixed with it is known as the air-fuel ratio. With the usual motor spirit (85 per cent. carbon and 15 per cent. hydrogen) a mixture of 15 lbs. of air with 1 lb. of petrol ensures complete combustion. This ideal air-fuel ratio of 15 to 1 is not desired in general practice, even though it is economical in consumption, because the petrol engine does not develop maximum power at such ratio. Figure 4 indicates that maximum power is obtained in the average engine with an air-fuel ratio of 12'5 to 13'5. It is inconvenient to determine the quantities of petrol and air supplied to the carburettor, but the pre-combustion air-fuel ratio can be simply determined by analysis of the exhaust gas after combustion has taken place. The object desired is to determine the efficiency with which the fuel is being burned in the engine, or the percentage of complete combustion. In other words, you wish to know the miles per gallon on the road. Because no great loss of power results when the mixture is over-rich, there is a tendency, especially in passenger cars, to set the carburettor on the rich side and the owner pays for the wasted spirit. By using a Cambridge Exhaust Gas Tester calibrated in air-fuel ratio as a guide in setting, it is possible at the same time to obtain maximum power and maximum economy. Although combustion efficiency can be determined by chemical analysis of exhaust gases, this method is not only lengthy and complicated, but it does not provide a direct continuous indication. The Cambridge Exhaust Gas Tester is a practicable and robust instrument designed for continuous tests on cars, omnibuses or lorries operating over any sort of road. The standard instrument is calibrated for use on petrol driven vehicles but a correction scale can be applied when benzol spirit is used. FIG. 4. FIG. 5.—Instrument in use on a Fordson 2-ton truck. By courtesy of the Ford Motor Co., Ltd., Dagenham. THE INSTRUMENT THE principle of operation consists in measuring the thermal conductivity of the exhaust gas. A fixed relation exists between the thermal conductivity of the exhaust gas and the air-fuel ratio, so by measuring the former, one is able to determine the completeness of combustion taking place in the engine. The method, which is electrical and not chemical, is not new but has been employed for many years in measuring the gas components of flue gases and other industrial gas mixtures; instruments incorporating the principle are in use in many thousands of boiler houses and power stations. FIG. 6.—Instrument being used in dynamometer test. The complete instrument shown in Figure 1 is fitted in a cast aluminium case finished in red with chromium-plated fittings. It is waterproof and is not damaged by rain, snow or dust. It is fitted with a scale 5 inches long with clear bold figures as shown in Figure 3. Two dry cells which furnish the current necessary for operation are placed in a separate compartment in the instrument side and are readily accessible for renewal. A convenient carrying handle is provided and a floor stand for use in the garage is shown in Figure 2 and may be furnished if desired. Complete operating instructions are etched upon an aluminium plate attached to the instrument front. An important feature of the instrument is the unique method of suspending the moving system of the air-fuel indicator. The usual methods of mounting sensitive galvanometer systems are either to support the moving element with pivots, or to suspend it between fine tightly-stretched wires. Pivots may soon become blunt from vibrations and road shocks, and the movement of the coil becomes sluggish and its indications inaccurate. On the other hand, a suspended movement is susceptible to changes in level, and when used in a moving car the pointer may swing and jump so violently as to be unreadable. The Patented Cambridge Magnetically Cushioned Movement, shown in Figure 7, is so designed as to absorb the shocks of road testing without damage to the pivot point. The coil action is so controlled that the pointer remains steady and readable under all conditions encountered. The coil assembly, instead of being called upon to absorb shocks suddenly, floats in a magnetic field which cushions road shocks and protects the pivot from injury. This movement is very stable, rugged, and makes continuous exhaust testing practicable on the road. This instrument is provided with a sampling hose 10 feet long and a tail pipe with fittings that enable it to be held in position on the exhaust tube. A special chamber collects the condensate and thereby prevents water from entering the instrument. FIG. 7.—Cambridge Magnetically-Cushioned Movement. A, A1, Iron discs magnetically suspended. B, Iron core. C, Coil. E, Magnet. F, Zero adjustment post. G, G1, Control springs. H, H1, Pivots. | ||