Carburetion in theory and practice

The subject of carburetion is one which is vital to the automobile movement and has, therefore, received more scientific attention during the past few years. This has been necessary on account of the demands of the public for wide ranges of engine speed, controllability, and quietness.

The author has been unable to discover much scientific book work dealing with this subject, with the exception of that very useful book of Sorel's upon the subject of alcohol motors. There is, undoubtedly, a considerable interest taken, both by automobile engineers and owners, in the carburetor question, and this has been much greater recently on account of the increased price of fuel.

It is the intention of the author that this book should provide in convenient form information upon the properties of various fuels, how the said fuels require treatment for use in a motor car engine, and what has been done in the past in order to obtain the necessary data upon which to base the theory.

The user will find much information which will give him a clearer understanding of the principles of carburetion and enable him to effect economies in working, and the designer should be saved many hours of labour by the use of the data contained herein.

The closing chapters consist of descriptions of some of the best known carburetors, with criticisms thereon, but the number of carburetors which has been produced is so large that it is impossible to include all.

A certain amount of the information contained herein has been embodied in lectures and papers and press contributions, and is reproduced in a slightly different form by kind permission of the editor of the Automobile Engineer, now Internal Combustion Engineering, and the editor of the Automobile of America.

By the use of the word "carburetion" it must be understood that this word will designate the art of mechanically mixing or blending a liquid fuel with a certain amount of air, and that whether this art is carried out to the limits of perfection or not, is an indication of whether the carburetion is good or bad. Carburetion will be considered to be more or less complete by reason of the manner in which the air is mixed with the molecules of the liquid fuel, or whether the fuel is divided into its finest possible particles in such a way that every particle of fuel is surrounded by a certain quantity of air to the limit of homogeneity of the mixture.

Homogeneity signifies "having the same properties or character in every direction." A homogeneous fuel is one of uniform composition throughout, so that samples taken, however large or small, from any part of the bulk of the fuel are exactly alike in composition. As far as the explosive mixture supplied to an engine is concerned, if two gases are the active agents, such as coal gas and air or oxygen, the intimate mixing of these in the inlet arrangements, combined with the turbulence in the cylinder during the compression stroke, result in a fairly homogeneous mixture at the moment of ignition. There are special cases, however, in which such is not the case, and where stratification is aimed at. This result can be obtained to some extent when special provision is made.

The motor car engine using liquid fuel does not, however, require a stratified mixture, but a homogeneous one, as will be seen later on, and in order to obtain this the carburetor should be designed for that end.

Another term which will frequently be used is that of "depression at the orifice," or " head over the orifice," expressed in inches of water. This means that the difference of pressure between that of the atmosphere and that adjacent to the fuel orifice is sufficient to support a column of water the number of inches in height which is expressed. Saturation of air by a vapor occurs when the air is unable to support any more vapor in that form, so that the addition of vapor causes precipitation of the liquid from which the vapor has arisen.

Raising the temperature of the mixture will, however, allow the air to retain a greater percentage of vapor, as will also a reduction of pressure.

Fuel is discussed as containing a certain number of "fractions" or constituents which distil off as the temperature of the fuel is raised. Petroleum spirit consists of many different hydrocarbons, and in specifying any spirit, its final boiling or distillation point is one of the most important factors, together with the percentage of the total quantity of the fuel distilling off at different temperatures.

The "lighter fractions" designate the more volatile benzenes of the hexane series when referred to motor spirit, but with reference to crude oil may imply all those constituents distilling below 150 C., which usually comprise commercial motor spirit.

Fuel may be mixed with air in several ways. The first and the oldest form of carburetion is by passing the air through a volume of liquid fuel. On the other hand, the volume of air can be treated by spraying into it a certain quantity of fuel in a more or less finely divided state. There is another form of carburetion, which is virtually distillation or evaporation by means of applied heat, and it is quite conceivable that if a volume of air is passed over a liquid, and a higher temperature than the normal is applied to this liquid, the evaporation of the liquid will be accelerated above what it is under ordinary atmospheric conditions. Assuming that the rate of evaporation of the fuel is in proportion to the amount of air passing, and that the air is brought sufficiently near to the surface of the fuel, a satisfactory form of carburetion will follow.

It is naturally somewhat difficult, when dealing either with air or with fuel in quantities, to obtain a homogeneous result in the mixture. For this reason it is preferable to treat small quantities as desired ; furthermore, when small quantities of air and fuel are dealt with, there is not so much risk of accident from any involuntary ignition of the explosive mixture in the generating chamber, as is the case where a larger volume is dealt with in a chamber of considerable capacity.

An engine such as is used in the modern motor vehicle is not running under constant demand, and it is therefore preferable to create an explosive mixture in accordance with the demands of the engine, rather than to store up any quantity of explosive mixture to meet any sudden demand which may come upon the engine. In this practice we are more nearly approaching the modern trend in stationary gas engine practice, where a suction producer is fitted, and the suction producer in that case corresponds to the carburetor of an engine, rather than to the gas holder which was previously used when coal gas was employed. We find in a gas set, where the engine sucks directly upon the carburetor, the amount of carbureted air which is drawn in is in direct response to the demands of the engine.

There is probably no part of a modern motor car which has undergone more useful development in recent years than the carburetor. The improvements which have taken place have made it possible to obtain that great range of speeds of engine rotation with which we are all now familiar. Furthermore, these results have been accompanied by other advantages, such as the reduction of petrol consumption, more perfect combustion, the prevention of overheating, and ease of starting.

When we come to investigate how these ends have been obtained, we find that there is no one method or principle which stands out with prominence beyond several others. This fact cannot be disputed, as the result of numerous severe competitive tests show. It may be that, as a result of a series of trials undertaken by one firm of motor car manufacturers, a particular carburetor suits a particular engine somewhat better than other competing carburetors, on all-round results. In another instance we may find again that a different carburetor, working upon an entirely different principle, is more suitable to another class of engine of approximately the same size. It is a particularly interesting fact that such good results as have been obtained recently should have been possible with different instruments.

Let us revert for a moment to the early types of jet carburetors in which the fuel supply to the engine was more or less controlled by the size or number of the jet orifices. This type of instrument, it will be remembered, was fitted with a choke tube, either of one constant diameter located round the jet orifice, or with a conoidal tube, the position of which could be varied with regard to the jet. Taking the first one as exemplified by the old Longuemare, this tube was surrounded by an annulus through which supplementary air was admitted by a hand- controlled device. The arrangement was crude, as it was only capable of giving a correct mixture automatically for the one speed for which the choke tube was suited. As the speed increased so did the suction, and this latter had to be counteracted by the admission of air from an external source. It will be remembered that in these early devices extra air valves working against springs were often provided to reduce the amount of hand manipulation necessary with such an instrument.

The early Krebs sought to combine the extra air valve with the carburetor by means of air pressure actuating a diaphragm against the resistance of a spring, and in such an arrangement it is possible to design the air ports so that the jet is surrounded by a constant pressure difference with regard to the external atmosphere. A further development of this principle was claimed in the Gillet-Lehmann device, in which a direct connection was made by means of a small pipe between the float chamber and the induction pipe at one or more points. Assuming that the restricting screw or screws were set properly, with a device of this nature it was possible to regulate the pressure difference under which the instrument worked with some degree of nicety.
Looking at the matter from the point of view not usually apparent, we may consider that all devices of this nature were forerunners of what are now known as constant vacuum carburetors, and these in detail will be dealt with later.

Again going back to the early days, we can call to mind another line of development, which aimed at restricting the efflux of the liquid fuel as the engine suction increased. Such devices took the form of spirals or bears' poles of metal in the jet orifice. Obviously arrangements of this sort could scarcely be predetermined with regard to their detail so as to give any great accuracy in working, and their effect as regards uniform carburetion at all speeds could only be arrived at by means of trial and error. These devices were undoubtedly the forerunners of some of the instruments of to-day, in which the main feature is the variation of jet orifice in accordance with the demands of the engine, The modern form of such an instrument is designed so that the orifice consists of at least two parts, which rotate relatively to each other, and in which the holes or orifices are circular, segmental, triangular, or any suitable shape, and which give an orifice opening in proportion to the air and throttle opening.

Instruments of this type can be designed previously with a great degree of accuracy, and require very little final adjustment. There are, however, further developments of these instruments working in conjunction with additional air devices, the latter controlled either pneumatically or hydraulically, which in modern designs can be arranged to give excellent results. In such a combination, however, more than one type of adjustment is required, and the instrument immediately becomes liable to derangement and erratic working in the hands of the inexperienced user. Furthermore, the air-controlling arrangement is liable to suffer as the operating mechanism wears, the spring control loses its original liveliness, or the moving parts stick or become loosened.

Now it is very obvious from general principles obtaining in nature, where a body is turned from one state into another, i. e., either from a solid to a liquid state or from a liquid to a gaseous state, a certain amount of interchange of heat must take place in order to effect this change of state, and the amount of heat absorbed is, of' course, in proportion to the latent heat of the body. In the case of a liquid such as petroleum spirit, which is of a complex nature, one cannot exactly state what its latent heat of evaporation is, but it is of the order of 160 calories per kilogram, equal to 288 B.Th.U. per pound of fuel evaporated; that means to say, that every pound of petroleum spirit which is passed through the carburetor requires an addition of heat equal to 288 British thermal units in order to evaporate it so that the resulting mixture shall remain at the same temperature as the incoming air. This heat can be applied in several ways, either by raising the temperature of the incoming air by drawing that air over, say, the exhaust pipe, or by heating the induction pipe between the mixing chamber of the carburetor and the engine valves. Heat can also be added to the liquid fuel itself before mixing with the air, but such heating has its limitations by reason of the volatility of the fuel and the low boiling point of some of its fractions. It does not really signify how the heat is added as long as the temperature of the resulting mixture remains what is desired. By this latter expression, of course, a great deal depends upon the locality, and the duty which the car has to perform, and theoretically it is more suitable for the temperature of the incoming mixture to be as low as possible consistent with the liquid remaining in the evaporated or suspended state without precipitation.

There is one point in connection with carburetion which is very frequently referred to, but about which very little useful data is obtainable. This point is the effect of the inertia of the liquid in the jet and passage leading thereto. In those types of carburetors fitted with a modulating pin, the inertia of the liquid is negligible, as there are more important details than the flow of the fuel which are affected by inertia. Take, for instance, the Stewart instrument: there is bound to be a certain amount of lag in its action as the moving part has considerable mass, and, when the throttle is opened, this mass must respond both against the action of gravity and its own dashpot. When the throttle is closed there is again a certain lag, but owing to the fact that the valve is off its seat there is an area for air-flow considerably greater than the normal. The result is, that owing to the decreased suction very little petrol will be drawn up the centre tube, and the presence of the pin will further baffle the flow of liquid to the centre tube. Conversely, when the throttle is opened rapidly a desirable state of affairs is reached, namely, a large suction is produced at the jet during the time the valve is rising to its normal position, and a correspondingly increased richness of mixture follows. This richness enables the engine to pick up quickly, owing to the known fact that a rich mixture, within certain limits, produces a more powerful explosion.

So-called constant suction carburetors are not really by any means operating under the conditions indicated by their title, as the suction is continually varying, but not in the manner that it does in the ordinary jet and choke tube instruments. For instance, one may calculate out and find in practice that a certain carburetor will operate normally under a certain depression, and this depression will usually occur when the throttle is almost closed and the engine running slowly. However, if the throttle be open the value of the depression immediately decreases until a condition of stability is reached when the engine is running at a speed corresponding with its throttle opening. It will be found that in practice the depression is not the same as before, and, as the engine speed increases, the difference in pressure inside and outside the carburetor has undergone further slight but perceptible changes, which vary in accordance with the design of the particular carburetor. The changes of depression are most important, as, if the value reaches too low a point, it is practically impossible for the carburetor to pass sufficient petrol through its jet properly to carbureted the amount of air passing.

One must bear in mind that although high efficiency is aimed at in the design of a carburetor working under a small depression, there is a certain limiting size of orifice beyond which it is inexpedient to go, on account of the difficulty of getting sufficient petrol through it when the engine demand is high. The author has had several difficulties of this nature quite recently, and it would appear that in practice the experimental feature is borne out with regard to the falling off in the ratio of petrol flow to area of orifice as the area of the orifice increases above the maximum desirable size before referred to.

In the types of carburetors using a modulating pin, one is able to take advantage of two fairly easily controllable factors, namely, the positively varying jet orifice and the possibility of suiting the air opening to any particular type or size of engine. This latter feature is a most important one when what is known as "high efficiency" work is concerned. The details necessary for the fundamental principles of design of the air aperture will vary with each particular job, and depend, of course, upon valve areas, compression spaces, and so forth, which have a bearing upon the particular results aimed at.

A desirable feature in modern carburetors is that of easy starting, and this is frequently attained by the use of a starting well. This well may consist of a certain volume of liquid, which at other times may be used as a dashpot for the moving element of the carburetor, or, on the other hand, may be a volume of petrol standing in a tube above, or adjacent to, the actual jet orifice.

A good deal is often made of the necessity for the ability to vary a carburetor to suit climatic conditions or those of temperature, and details as to why this becomes necessary are given in a later chapter. The automatic carburetor of the varying jet type is the easiest to alter to suit special conditions, and, in addition to the modulating pin scheme, there is that in which a number of similar jets are uncovered by a predetermined plunger method. Such an instrument is at once one that adapts itself automatically to a wide range of demand, and embodies the necessity for a large area for high speed work, with the concentration of air-flow necessary for starting and slow running. Furthermore, this instrument embodies a combination adjustment which acts throughout its entire range of working in the same proportion, and when the flow of fuel is set for one speed or working position, it remains correct for all other positions.