Aircraft technical Basics: Aircraft Engines - RAF Flying Training Manual - Chapter VII.- Engines: Carburation
CARBURATION
Liquid Fuel
31. The carburettor and induction system supply the combustible mixture to the cylinders, but to understand how they work it is necessary first to know something of the nature of the fuel used. If the fuel were a gas most of the difficulties of carburation would disappear, but the fuel used in all the engines we are discussing is a liquid at ordinary temperatures and pressures. It is a mixture of several substances known chemically as " hydrocarbons " (i.e. compounds of carbon and hydrogen) and these, though they all bum, differ considerably in their chemical and physical properties, notably volatility (readiness to turn into vapour) and resistance to detonation. The evaporation can be accelerated by warming the fuel or by reducing the pressure on it, very small drops will evaporate more quickly than a large volume of the liquid. The temperature must be raised or the pressure reduced before the fuel arrives at the cylinder, because in order to burn properly it must be either vapour or very fine drops like a mist, and it must be intimately mixed with the air. Large drops like rain are apt to pass right through the cylinder and out through the exhaust port and so are wasted. However the vapour is produced, heat will be absorbed in the process and unless heat is supplied from outside, the fuel and the air mixed with it will get cold. This is the reason for the familiar chilling sensation when petrol is spilt on the hand, it is even possible to produce ice by this means. Aero engine fuel, must therefore be readily vaporised, reasonably low in specific gravity (weight for bulk), and free from injurious chemical impurities. It must also resist detonation, for fuels differ considerably in this important respect. This latter property is expressed as an " Octane number," which represents the proportion of two pure hydrocarbons in a mixture which has the same anti-detonating quality as the fuel being considered. These constituents are iso-octane (which is a very good anti-knock fuel) and normal heptane (which is a very bad one) and by mixing the two almost any degree of knock resistance can be produced. A fuel with an octane number, of, say, 77 (the fuel known in the Service as D.T.D. 224 is 77) is equivalent when tested under certain standard conditions to a mixture of 77 per cent. octane with 23 per cent. heptane "Straight run" petrols, i.e. those obtained by simple distillation of crude oil, cannot be produced commercially with octane values over about 72, and as this is not enough for modem engines the knock-resistance is increased by adding benzole or a " dope " known as tetra-ethyl-lead (T.E.L.) or both. T.E.L. is not in itself a fuel, but has the property, when added to petrol in very small quantities, of greatly increasing the anti-knock quality of the petrol. Too much of it is, however, harmful and even small quantities, if used continually, may damage an engine not designed to take it. Iso-octane itself, formerly prohibitively expensive, can now be produced synthetically and it is used to give a very high anti-knock value to certain fuels. The use of the correct fuel for any particular engine is important, not so much to produce the power required as to prevent damage. The amount of energy obtainable from the fuel is known as the " calorific value " and is measured in British Thermal Units per pound of fuel ; this figure, contrary to widespread belief, is almost the same for all ordinary fuels, apart from special ones such as alcohol.
The induction system
32. The fuel passes from the carburettor through the induction system to the cylinders, but because the carburettor is much influenced by induction system, the characteristics of the latter will be described before those of the carburettor. It must be realised that although the carburettor may produce a correct mixture of fuel and air, what matters is the mixture which arrives at the cylinders, and this is not by any means always the same thing. It is impossible to fix a carburettor direct to each cylinder in a multi-cylinder engine. There must be a pipe or passage, the length and shape of which will vary for different cylinders. If the fuel were completely vaporised this would not matter much, but the liquid part of the charge, whether in the form of mist or larger drops, tends to deposit itself on the walls of the pipe especially at corners and branches where its inertia causes it to fly to the outside of the bend rather than follow the air with which it is supposed to be mixed. This tendency is accentuated by the fact that the flow is not steady but pulsates in accordance with the induction strokes of the cylinders concerned. Moreover, the amount of fuel deposited on the inlet pipe will depend on the temperature of the wall and the pressure inside, and as this is constantly varying, not only with the pulsations just mentioned but with throttle opening and local changes in velocity, it follows that the amount of liquid fuel still suspended in the air which eventually reaches any individual cylinder is a very uncertain quantity
33. This variation in mixture affects power output and wastes fuel, therefore in order to reduce it as far as possible, it is usual to have not more than three cylinders supplied from one carburettor (except in supercharged engines) and to warm the induction pipes either by oil or by the engine coolant circulating round them. When the induction system is quite cold, as when first starting, excessive condensation takes place inside and rough running under these conditions is the well-known result. The exact amount of air theoretically required to burn completely any given weight of fuel is fixed by the chemical composition of the latter, and such a mixture—about 14 : 1—is known as the " chemically correct," but, as it is not possible in practice to burn quite all the fuel that enters a cylinder, a slight excess is needed to use up all the air and so to develop maximum power. This applies to individual cylinders, but the mixture supplied by the carburettor must be a little richer. This is because less power is lost by using a slightly rich mixture than a slightly weak one, and as the cylinders cannot all have the same mixture any deviation from the desired mixture must be on the rich side. The average therefore must be a little richer than the weakest mixture for maximum power for a single cylinder. When full power is being developed a still richer mixture is used. This is partly because it is less prone to detonation, which of course is a danger at full power, and partly to cool the hot parts of the cylinder, especially the exhaust valves which tend to get very hot under these conditions. This cooling effect is due to the evaporation of the excess of liquid fuel which the rich mixture contains. When running at less than about 9/10 full power, however, this cooling and anti-detonating effect is not needed, so the mixture can be weakened, and under cruising conditions, when perhaps only 50 per cent. of the available power may he needed, a mixture weaker even than the chemically correct can be used. As the loss of power is more than compensated by the lower consumption of fuel, a mixture about 10 per cent. weak can and should be used for economical running under cruising conditions, but it must not be used when anything like the full power is being taken from the engine. For the economical use of weak mixture, a good distribution is particularly important, this in turn requires proper temperature control which is largely in the hands of the pilot, and the use of volatile fuel.
34. So long as the engine is running at a steady speed and throttle opening, although the mixture may vary slightly between individual cylinders it is on the average the same as that which leaves the carburettor. This does not apply during acceleration or deceleration, however, for then it becomes temporarily weakened or enriched unless something is done to correct it. The reason is simply that the amount of fuel which is always present on the walls of the induction system varies according to the pressure inside. During slow running, when this pressure is low, very little fuel will adhere to the surface but when the throttle is opened the pressure is increased and so some of the fuel passing through will be deposited on the way, leaving a weakened mixture to arrive at the cylinder. This condition only lasts for a few seconds and, once the induction pipe is thoroughly "wetted," the cylinders receive their normal mixture once more. This weakening of mixture during acceleration is a characteristic of the induction system and is compensated by providing a specially rich mixture from the carburettor at the right moment. The reverse happens, of course, when the engine is suddenly throttled back, but the momentary enrichment, even if it does cause a slight loss of power, does not matter and no special steps are taken to correct it. It will be seen from the foregoing that the carburettor is called upon to provide widely varying mixtures at different times.
The carburettor—elementary form
35. We have seen that the carburettor has two duties ; first to reduce the fuel to the right form, that is vapour or mist, and secondly to mix it with the air in the right proportion. First let us consider the elementary carburettor shewn diagrammatically in Fig. 109. On its way to the engine, the air passes through a part of the carburettor, known as a choke tube (A). This by virtue of its shape increases the velocity of the air with as little resistance to flow as possible, just as the upper surface of an aerofoil (which a section of the choke tube closely resembles) increases the local velocity of the air stream over it and thereby contributes to the lift. On account of this increase in velocity the pressure is reduced, this depression causes the fuel to issue from an outlet, (B) under the atmospheric pressure behind it. In order to control the flow of fuel, it is made to pass through a calibrated orifice, or jet, (C) the rate of flow being dependent on the size of the hole ; the depression above it ; and the " head " of fuel behind it. This head is kept constant by a float mechanism in a float chamber, (D) which works in precisely the same way as the ball cock in the domestic water cistern, its action will be clear from the diagram. It may be mentioned in passing that the needle is never on its seat while the engine is running, but allows just enough fuel to pass to keep the level correct in the float chamber. This simple carburettor is completed by a throttle valve, (F) which controls the pressure in the induction pipe and so the power of the engine.
The Diffuser
36. Unfortunately such a simple instrument is quite incapable of satisfying the widely varying needs of an aero engine, its most glaring fault is that as the air speed through the choke increases so the mixture gets richer. Various means have been adopted to avoid this, the commonest is an assembly known as a "diffuser" which not only provides the necessary "correction" of the mixture but helps to break up the fuel into the spray required. Fig. 110 shows the diffuser. It will be seen that fuel still flows through a jet on its way from the float chamber, but instead of the depression being equal to that in the choke tube it is modified by admitting air at points just above the jet. This is done by surrounding the passage leading from the jet to the choke by another open to the air at the top and communicating with the inner tube by a series of holes drilled in the latter. When there is no depression in the choke, the level of fuel in both the inner and outer passages is the same as that in the float chamber, but as the air velocity and hence the depression increases, the level in the outer tube will fall, just as a liquid will fall in one side of a U-tube when the pressure is reduced in the other. The greater the depression, the lower will this level be ; more air will get in through the greater number of holes exposed and less fuel will issue from the jet. By suitably arranging the number and size of the holes any desired correction may be obtained. The amount required is dependent on a variety of factors such as induction pipe characteristics, valve timing, firing intervals, etc., it can only be found by experiment. In addition to reducing the depression on the jet, the air entering through the holes in the diffuser mixes with the fuel and helps to break it up into small drops, which are further broken up when they emerge into the high-speed air stream in the choke tube.
The Jet System
37. A representative type of aero-engine carburettor is shown in diagrammatic section in fig. 111. The parts already described are plainly visible, and the main jet and diffuser are adjusted to give normally a slightly rich mixture for maximum cruising conditions, as previously explained. In addition, another fuel outlet into the choke tube will be noticed ; this is used for three purposes. First, as an outlet for the "power jet" which, as shown in the picture, is an extra jet, not leading to the diffuser, brought into action by a valve operated by a cam connected to the throttle so that when the throttle is wide open extra fuel is provided for the full-power enrichment. (See para. 33.) Secondly, there is the accelerator pump which is also connected to the throttle ; this pumps in an extra supply of fuel just as the throttle is opened. The enrichment is only given during acceleration and, by a delay device not shown, for a few seconds afterwards the pump is inactive while the engine is running steadily. The third use for the extra fuel outlet is for "take-off enrichment", the need for which may not be understood until supercharging has been described. Certain engines are capable of giving, for take-off only, a power output greater than the normal maximum, and just as the power jet gives a rich mixture for ordinary full-power running so, for the same reasons, this extra jet gives an even richer mixture (about 40 per cent. rich) for taking off. This jet is brought into effect by whatever device is used to bring the full take-off power into action, and, since it uses a great deal of fuel (as well as for other reasons), take-off power must not be used for longer than is necessary.
(a larger copy of the diagram can be found here.)
Slow Running
38. When the throttle is nearly closed and the engine is running slowly, very little air is being used and its velocity through the choke tube is not enough to bring the diffuser system into action. A separate slow running system is therefore added. This, in fig. 111, is shown in the small passage at the right of the throttle, and is in effect a small separate carburettor which delivers a somewhat rich mixture to the edge of the almost closed throttle valve. Here it mixes with the air which at this point has a high velocity and therefore produces a strong depression, although its speed through the choke is low. The double outlet illustrated and the passage through the throttle itself are designed to ensure a smooth changeover from the slow running system to the main diffuser when the throttle is opened enough to bring the latter into action. A cut-off valve is provided to stop the engine quickly when required.
Mixture Control
39. So far, with the exception of take-off enrichment, we have been dealing with carburation conditions which apply generally, and some modern car carburettors are, in fact, very similar to the one just described in outline, but aero engine carburettors have to deal with one condition which is almost unknown in motor car practice—change of altitude. As the aeroplane goes up the air gets less dense and, if no correction were applied, the mixture would become unduly rich. To overcome this a separate control is provided to weaken the mixture at high altitudes, and this can also be used when a weaker mixture than normal is permissible under cruising conditions low down. Several forms of mixture control have been used, but the one shown in fig. 111 is satisfactory and is widely used. It consists of a valve which when opened connects the mixture passage just above the diffuser to the air intake, thus breaking down the depression on the jet and reducing the flow of fuel through it. It should be noted that it is not merely an extra air inlet, though some air does flow through it. The various jet systems described are automatically brought into action by the throttle, but the mixture control has to be operated separately by the pilot. On some engines he has to adjust it accurately for every change in altitude, speed or throttle opening, and this requires a degree of attention which the pilot may not be able to spare, so on other engines an automatic mixture control is fitted. With this the pilot has only to set the control at "weak" or "rich" according to whether the engine is running at cruising or nearly full power. In either case, the correction for altitude is made automatically, and, in the "weak" position, the mixture is automatically that which gives the greatest practicable economy. The pilot has to set it at weak or rich according to the operating conditions, and it is important that he should not fail to do so if excessive fuel consumption and damage to the engine are to be avoided. The automatic mixture control is achieved by means of a special linkage, controlled by an aneroid capsule through an oil-operated relay system. Details of this mechanism may be found in the handbooks of the engines to which it is fitted.
