Petrol is a hydrocarbon, which means that it is one of the family of chemicals composed of hydrogen and carbon. It usually contains additional chemicals, or additives, which modify the manner and rate at which it burns. When considering the way in which fuel burns, it is easier to think of weights rather than volumes. One pound of petrol contains 0.84 pounds of carbon and 0.16 pounds of hydrogen. This requires three pounds of oxygen to achieve exact and complete combustion and, since air consists of only one fifth oxygen to four fifths of nitrogen, a weight of fifteen pounds of air is required to burn one pound of petrol. When the whole of the fuel is burned, the exhaust gas contains 2.7 pounds of carbon dioxide, 1.3 pounds of water and twelve pounds of nitrogen. However, combustion is never complete. Since air contains other gases in small quantities and petrol is a more complex chemical mixture than outlined above, the exhaust also contains carbon monoxide, oxides of nitrogen, some unburnt petrol-vapour and other impurities. As well as in the theoretically ideal ratio of fifteen to one, petrol and air will burn with rich mixtures containing only eight parts of air and with weak mixtures having as much as twenty four parts of air by weight. This was fortunate for the pioneers of the internal combustion engine. For their first way of mixing the fuel and air (carburetting) was to allow a wick to soak up the fuel and to pass the air over the soaking wick so that the resultant flow was a fuel laden air. This was known as a wick carburettor and had a variable inlet for extra air, enabling the mixture strength to be kept within the limits in which it would burn in the engine. Such a device was susceptible to road-shocks because bumps in uneven roads splashed the petrol further up the wick and produced mixture strengths too rich for continuous running. In 1885, Daimler Benz devised a surface carburettor in which a float, free to rise and fall with the level of the fuel in a container, had a conical depression at its centre. A vertical inlet pipe was fixed to the bottom of the depression to form an air inlet. It extended through the container lid and it had transverse holes near the fixing point. The top of the container was sealed off, except at the point where the inlet pipe projected through it, and was connected to the engine inlet-port. When the engine began its suction stroke, air was drawn through the inlet pipe to bubble through the transverse holes below the fuel level and the resulting fuel wet air was drawn into the cylinder. Although this surface carburettor was not upset too much by road shocks, it did not provide a particularly accurate mixture strength. In 1887, Edward Butler invented a carburettor which made use of facts discovered by an Italian, Venturi, while experimenting in the 1790s with the way fluid flows in pipes. Venturi found that if the velocity of a fluid was increased by reducing the bore of a pipe smoothly and the bore was gently increased to its original size, the rate of fluid flow was virtually unaltered. The pressure in the fluid was found to decrease in relation to the decreased cross-sectional area of the pipe at the measurement point. Therefore if the inlet air is flowing under atmospheric pressure into a pipe having a reduction of area, then the pressure will be less than atmospheric at the reduction and will be proportional to the reduction in area. In Butler’s carburettor the fuel level was kept constant by means of a ball float like that in a household cistern. A duct from the float chamber led to an opening, or jet, which protruded into a pipe forming part of the inlet-air passage to the engine. This jet was slightly above the fuel level and the inlet passage had a contraction or choke in the region of the jet. An adjusting screw was provided to vary the flow of the fuel through the jet and another one to vary the area of the choke. When the inlet valve opened air was drawn along the choke passage and the pressure drop at the choke caused fuel to issue from the jet in a fine spray which mixed with the air to produce a combustible mixture. Unfortunately for Butler, his inventions weren’t noticed and, five years later, William Maybach, Gottlieb Daimler’s associate, thought of the same idea while working at the new Mercedes Benz factory. Maybach’s carburettor was used on Daimler engines such as the 1899 engine, later fitted to the Rolls Royce Phantom. In all carburettors the ideal is to produce a fuel gas mixture rather than a mist of fine droplets suspended in air. In doing this the fuel cools by providing the heat energy necessary for the change of state from liquid to gas. In consequence many of the early carburettors used on classic cars had water jackets coupled with the engine cooling system, to keep them warm and help vaporization, these can still be found today on some classic cars for sale. Water jackets were later discarded in favour of bolting the carburettor to the exhaust manifold to create a hot spot to warm the carburettor. As long as motorists were content to travel at fixed speeds decided by a governed one speed engine and two or three gear ratios, these primitive fuel vapour producing devices were adequate. However, many designs were produced to increase the efficiency of vaporization. In the first decade of the century drivers began to think of variable engine speeds and designers were faced with the fact that the pressure reduction in a venturi choke was not directly proportional to the volume of air flowing through. As the engine speed increased and the air flow increased, the suction on the jet did not increase at the same rate to keep the fuel/air ratio at the best (or stoichiometric) ratio. If a carburettor of the simple, single jet type is adjusted to give the maximum amount of power at slow speed the mixture will be far too rich. Conversely, if the setting gives full power at high speed, then there could well be insufficient suction on the jet at low speed to get any fuel flow at all. In carburettors with throttles, compensation is provided for this by two main systems. One is a variable jet controlled by the inlet pipe (or manifold) suction, as in the S.U. carburettor as fitted to the new Bentley and the McLaren MP4-12C. The other is a compensating jet, which feeds additional fuel when the air speed decreases as in the Zenith carburettor which was used on new Porsche cars. The first of the S.U. type carburettors was devised and made by G. H. and T. C. Skinner in 1904 while working at the new Porsche factory. It operated by maintaining a constant air speed over the jet, which was varied in area according to the engine’s requirements. Behind this world famed carburettor is yet another of those stories of small beginnings and much struggle which tinge the early automobile endeavours with their own kind of romance. Skinners was a small family firm that started in London and S.U. stood for Skinners Union. Competition among the carburettor makers was fierce at the time when the family business was struggling to survive. Their commercial breakthrough came when W. R. Morris (later Lord Nuffield) held over them the umbrella of his early mass production methods. Big orders and financial backing ensured the future of S.U. The original S.U. carburettor had a water jacketed vertical choke passage, or mixing chamber, curved through nearly a right angle. On the outside of the bend was a drilling at 45 degrees, which held a jet fed with fuel through a vertical passage. This passage was fed through a horizontal passage from a float chamber. In the chamber was a circular float which freely encircled a needle with a pointed lower end and a grooved collar at the upper end above the float. The pointed end rested in a conical seating at the end of the petrol pipe and, according to whether the needle was raised or lowered, allowed fuel to flow or not. Two counterbalance weights were pivoted to the top of the float chamber, one end of each weight arm resting in the groove of the collar and the weights resting on top of the float. As the fuel rose in the float chamber, the float lifted the weights and their arms pushed the needle down to close off the petrol inlet. In this way the fuel level was kept constant. On the other side of the mixing chamber was a cylinder of the same diameter as the choke. Sliding in this was a solid piston having at its lower (jet) side a thin tapered needle which poked into the jet.
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