The PROBLEM of EFFICIENT CARBURATION.
By H. C. GAMBELL, (Director of The Memini Carburettor Company, Ltd.)
IT is a difficult matter enough to be confronted with -athe various practical problems of carburation, when one can worry out the solution by trying different methods until the desired result is gained, but these difficulties are increased enormously as soon as an attempt is made to commit one’s practical experience to paper. Nevertheless, it is possible that this effort to place a few of my own experiences on record may be of interest to some of the readers of MOTOR SPORT, for, being in constant touch with the carburettor problem, especially in connection with sporting cars, I have been able to devote a good deal of thought to their specific requirements.
If carburation only consisted in providing means for giving the highest possible speed to a racing engine, the apparatus would only need to consist of something capable of completely filling the cylinder with mixture at maximum revs., but even with racing cars the problem of carburation is not so simple, whilst for sporting cars used for road work, as well as for speed events, the whole subject is absolutely shrouded by complexities. Sometimes during my own researches into the field of carburettor improvement, I have visited the Patent Office, where literally thousands of applications are filed for alleged improvements in or relating to carburettors ; and while it is perfectly obvious’ that more time and money has been spent in endeavouring to improve the apparatus, it remains to-day more or less of a compromise incapable of giving the best results without the attentions of an expert, at any rate until it has been fitted, and the final adjustments have been made. As a carburettor manufacturer, I am fully aware that existing apparatus is far from perfect, not because the instruments cannot be turned out to give uniform results, but for the reason that so many different people have different ideas as to what constitutes perfection where
carburation is concerned. For example, one man may require the highest ultimate speed from his car, not caring very much about slow running on top gear, whilst another, with a car of identical make and type, may demand different running conditions altogether. Thus, there is a difficulty in arriving at any definite standard setting that will suit the demands of a large public, and each carburettor has to be tuned with infinite care, not only to suit the car, but also to suit the temperament of the owner. Indeed, I would go so far as to say that on many occasions there has been more trouble over the temperament of clients than in controlling the principles of carburation to satisfy their needs. Up to a point, it is possible to provide all-round efficiency in a carburettor which has been scientifically designed, but my experience forces me to the conclusion that no single instrument can be relied upon to give a super performance in each and every direction. In other words, the carburettor is—as was mentioned before—a compromise.
Apart from the problems inseparable from producing a good carburettor on a commercial basis, manufacturers of these appliances are often blamed for failures, which by right should be laid at the door of the engine designers. The most efficient carburettor made will not give good results on an engine which offers obstructions to the free passage of the gases, owing to the design of the inlet ports. I have in mind a very well-known high efficiency sporting car engine of which the sparking plugs invariably show that two of the cylinders are running on a weak mixture and the other two on a rich mixture. Various attempts have been made to overcome the difficulty by fitting modified forms of induction manifolds, but the trouble lies with the inlet passages of the cylinders, which happen to deflect the gases unevenly, so as to disturb the equal distribution of the gases.
In inch a case, perfect carburation can only be attained by streamlining certain bosses which cause the obstruction, thus deflecting the gases to the cylinders in the correct proportions.
As the process of carburation is not really completed by the carburettor, it follows that the design of the engine, especially with regard to the internal passages, must be such as to assist the process in every possible way.
Peculiarities of engine design are usually responsible for the difficulties experienced by manufacturers to find a carburettor that will effectively smother the engine designer’s faults, and a great deal of time is often taken up in selecting one that will give a reasonable all-round efficiency.
Basic Principles of Carburation.
Whilst these notes are not intended as a treatise on the principles of carburation, some reference appears to be necessary to fundamental facts, and it may be sately said that all standard carburettors at present
holding the market work on practically identical lines, namely the employment of a pilot jet for starting the engine, and for running it until the air velocity in the choke tube increases sufficiently to draw the petrol from the main jet.
In the early days of the jet carburettor, the main jet simply projected into the choke tube and supplied more and more gas as the air velocity increased ; but it was soon discovered that the ratio between the jet discharge and the air velocity did not remain constant, the first commercially produced carburettor to overcome the drawback being the Zenith, with its well-known compensator action.
Since the introduction of the latter, other carburettors have been produced, some with submerged jets, some with retarding suctions to oppose the main suctions, others with air leaks to relieve the suction on the main jet ; all aiming at, and to a certain extent achieving, the same object. When we come to the slow running jet, there is practically no difference between all the well-known makes on the market. The most common method is to provide a by-pass hole, which will be partly exposed on the upper side of the butterfly, or throttle, when the latter is nearly closed. In this position a suction of from 8 to ro lbs. per square inch is exerted on the pilot jet, thus providing a very rich mixture for starting purposes. As the throttle is opened, and the suction on the pilot jet decreases, the strength of the mixture falls rapidly
until the main jet comes into action, and at this point of minimum weakness, what is known as a “fiat spot” is liable to occur, more especially if a large choke tube is being used, as on engines where high maximum speeds are required. The elimination of the ” flat spot” is one of the most interesting operations connected with the tuning and adjtigting of carburettors, but it depends to a very large extent upon the design adopted. The most scientific solution of the problem is that of maintaining a constant suction in the carburettor, and to-day we have several good examples of the constant vacuum type. In the latter arrangement, the air intake to the carburettor
will only open in proportion to the suction exerted by the engine. In some of these carburettors, the opening of the air intake automatically increases the area of the main jet orifice and in others brings a series of jets into action progressively ; but, generally speaking, the results are achieved at the expense of mechanical complications, with corresponding risks of wear which are liable to render the carburettor inefficient with continued use. Springs, pistons, diaphragms and similar devices are capable of giving an extremely sensitive operation to a carburettor, but in my opinion, the greatest stability can only be obtained by eliminating moving parts wherever possible.
The Calibration of Jets.
Until the introduction of the Brown & Barlow Plowmeter, which is now used all over the world for the
calibration of carburettor jets, there was the greatest difficulty in creating any standard for jet sizes, for the most minute variations in the sizes of the orifices had the most disturbing effects when attempts were made at accurate tuning. The instrument is shown in one of the accompanying illustrations, and the jet to be calibrated is fitted to one of two adapters, situated at the lower end of one of two vertical glass tubes. One of these tubes is graduated from 20 to Ex) cubic centimetres and the other from zoo to 400 cubic centimetres, thus covering all sizes of jets likely to be required for any type of carburettor. The most common practice is to mark the jets with their drill sizes in millimetres. The standard flow is fixed for our drill size, and all jets are flowed before being passed into stock. A few makers stamp their jets in cubic centimetres of flow per minute, it is therefore advisable when tuning to find out what the number on the jet actually means. By the aid of this ingenious instrument it is possible to make the most accurate comparisons of different jets, and all carburettor manufacturers employ it to check the accuracy of the jets made on their machine tools. Another device, also shown in the illustrations, is a simple bench upon which the level of the carburettors
is checked, this being one of the tests imposed on all the instruments before they are passed out of the factory.
The photographs accompanying this article were taken at the London factory of the firm of which I am a director, and as many people have questioned me as to the origin of the Memini carburettor, this may be a fitting opportunity to mention that the appliance is the invention of Signor Memini, who was originally a mechanic in the Isotta Fraschini factory at Milan. He became absorbed in the fascinating subject of carburation and began to develop ideas of his own, making the first carburettor entirely by hand, and without any financial resources. Encouraged by successes in a small way, he decided to start manufacturing the ” Memini ” carburettor, and after several years hard work started a factory. The history of his racing successes is now well known to readers of this journal, the most recent being 1st and 2nd in the Grand Prix of Europe of 1925, and in the year 1923 Signor Ferdinand la Torre introduced the carburettor in this country, with the result that the British manufacturing rights were taken up, and production was started on a commercial basis.
In conclusion I should like to acknowledge the valuable assistance given to me in compiling this article by my co-director, Mr. S. Gresty, and to state that it is largely through his skill and knowledge that the Memini carburettor has had such a large measure of success in this country.