A TECHNICAL TALK—
ON THE SUBJECT OF SUPERCHARGERS.
Their Development, Principles and Design. HEN one considers what a very short time it is since critics were asserting that supercharging could never be made of any use to the ordinary motorist,
and would remain a freak device for getting very high speeds for racing purposes only, it is interesting to trace its development, and note the somewhat unexpected results that have been achieved.
In common with practically every feature of the modern car, it owes its success entirely to racing. In the perpetual search for greater power from a given weight and size of mechanism, it has played a bigger part than any other single innovation in the history of the internal-combustion engine. Also, as we shall see, it has enabled many characteristics, more usually regarded as touring features, to be developed beyond their original limit.
In the days when the greater part of the sporting motorist’s season was occupied with hillclimbs and speed trials, the art of ” super-tuning ” was practised to a greater extent than is now the case. By this, one does not mean that engines were better or more highly developed than they are today. Such a contention would be obviously absurd. However, for very short events of this sort the question of long distance reliability was a very minor consideration. There was none of the feeling of “Well, my motor is not as fast as his but then he’ll probably blow up.” Instead of this, it was essential that if one intended to win, the motor must be faster than any other in the event.
This state of affairs encouraged people to try out experiments in the way of tuning that would make a Double-Twelve exponent shriek with horror. Compression ratios were raised to figures previously considered only applicable to Deisel engines. Revs went up to such an extent that only modern lubrication systems could cope with them for any distance. Camshafts took on contours that stretched even Terrys’ ability to provide valve springs that would make tappets follow the hectic path which had been allotted to them. Fuels gave off weird odours and approximated more than ever in their combustible properties to nitro-glycerine. All these things gave speed and power to an amazing degree, but they also gave roughness and lack of flexibility, such as the modern sports car owner could scarcely i magine possible. Who will ever forget the peculiar sobbing howl of Raymond Mays’ famous Bugatti, modified and tuned by Amherst Villiers ? Many a record and ” fastest time of the day ” fell to that car. Well over 6,000 r.p.m. was its normal rate on such work, and in those days that was something to talk about. Another illustration of the attitude towards speed and distance which then prevailed was given to one during a discussion with that great exponent of the art of sprint motoring, Capt. A. Frazer-Nash. The writer was running, at that time,
one of the overhead camshaft G.N.s, and when it went it really travelled. However, the discussion was anent the occasions on which it did not go, with special reference to the behaviour of the roller big-end. As this was doing things which were not allowed for in the instruction book, it was suggested that a plain bearing be used, and Nash asked for what distances it was required to run. It was explained that it was necessary for the car to be driven to the venues of events, no means of towing being available, and that one wished it to be safe for events up to a mile. He considered this and said, “It should be all right for about half-mile events, but you know a mile is a long way ! “
That was a fair summing up of the tuning of that time. A mile was a long way in the days when drivers used to drain their crankcases before a run to reduce oil drag, and try to get a few more revs thereby, and it seemed as if the limit of this style of tuning was being reached. Then came the supercharger in a form suitable for car engines, and drivers immediately started to try out its possibilities for getting even more speed from their sprint motors.
Once again Amherst Villiers was to the fore in the development of this, and began on the famous Vauxhall Special (again driven by Raymond Mays) the experiments which have led up to the supercharger now made by him, and fitted to the 4i-litre Bentley.
The results in the earlier stages tended to revolutionise the general ideas which had held with regard to the effects of forced induction on performance. Everyone hoped for more speed, and they got it. In fact, in many. cases they got a good deal more than the engines would stand, and much more thought had to be given to structural strength, and also to the bearings and lubrication. Everyone also expected that their engines would be even rougher to drive than they had ever been before, and this is where the first great surprise came in the development of the supercharger. Instead of being rougher, they were found to be much more flexible in spite of the • increased top speed performance.
It is this property of not only increasing the speed, but, what is far more important for general use, increasing the speed range, which has made so desirable the use of superchargers on the standard sports cars of today, and which will undoubtedly lead to their increasing adoption on the touring cars in the future. The type of blower referred to is generally known as the Roots blower, the general arrangement of which will be already familiar to most readers of MoToR SPORT. It Consists of two blades or teeth, each roughly in the form of a figure of eight, which are geared together and act as a gear pump to force the mixture into the cylinders. It is fitted between the carburetter and the engine. There are other ways of fitting blowers, such as before the carburetter, but this requires that the float chamber and petrol supply shall be pressure balanced,
and the first method is that almost universally employed on car engines. In this case the blower deals with mi*ture and not with air this is one of the reasons for the smoothness at low engine speeds. The mixture is so violently churned up by passing through the blades of the rotor, that really complete vaporisation of the fuel is made possible, in place of the usual condition of suspension which prevails when atmospheric induction is employed. A further reason for the marked increase in speed range which is gained by supercharging, is the fact that it solves, to a great extent, the problem of distribution. This problem is one that has always caused a great deal of trouble in design, and with the tendency to -increase the number of cylinders, the difficulties of distributing the mixture correctly become even more marked. Dozens of methods have been employed, many strange induction pipes have
been made, and various numbers of carburetters have been fitted. When a supercharger is used the problem virtually disappears. An ordinary induction pipe design. is permissible, and as it is kept filled with a very nearly perfect mixture at whatever pressure has been decided on when designed, every cylinder is bound to receive its correct quantity. The number and size of carburetters is merely decided by the quantity of mixture which is required to be passed through them, and the gas velocity. It may be inferred by some perhaps, that finality has now been reached, and that all that is necessary for the production of the perfect engine is that it should have a supercharger. But there are many contingencies to be taken into account with a boosted engine. The fitting of a blower increases the stresses owing to the increase in power and there are many points where an engine which has not been designed for it vill fail. The two chief sources of trouble in any increased power are temperature and mechanical weakness. owing to the fact that any heat engine gives off a comparatively small amount of power in relation to a large amount of heat from any given quantity of fuel, it follows that any increase of power means a further increase in the heat produced. A certain amount of this extra heat can be dealt with by modifications to the cooling system, but this will not, as a rule, be sufficient. There will usually be local heating of the piston and valves, of such intensity, that unless these, parts have been specially designed, the heat flow from them to the cylinder head and walls
will not be enough to avoid excessive rise of temperature and consequent failure. The requirements of mechanical strength will be found to be .concerned mainly with the crankshaft, connecting rods, and the, main and big end bearings. In most engines today the need for great rigidity and ample bearing sizes, together with forced lubrication to all bearings, is becoming so generally recognised that there is not likely to be much trouble arising from this cause. As more engines are now being designed with a view to having superchargers fitted it is not necessary to consider these point s further, and the other difficulties connected with supercharging are matters of mechanical detail in the design and construction of the instrument itself, and have mostly been over-come by the mannf act urers concerned. The earlier blowers made themselves somewhat unpleasantly prominent by the considerable noise emitted by the
drive, but now that the requirements of this drive are fully understood, this has been overcome and the modern supercharger of the permanently connected variety is practically silent.
No remarks on supercharging could he complete without some reference to the method employed on the Mercedes. On this car the blower is normally, disconnected, and the engine employs atmospheric induction up to full throttle. Then further depression of the accelerator pedal engages the clutch which drives the blower, and at the same time by an ingenious arrangement of valves changes the whole system over from atmospheric to forced induction. This has the advantage of avoiding any wear on the blower and its drive except when the extra power is actually required. The system is, however, an exclusive patent of this progressive firm.
The number of cars now marketed as standard with superchargers is very encouraging to the man who wishes to obtain a car of this type either for competitions or normal road work. Besides Bentleys, whom we have already mentioned, Lea-Francis have had a supercharged model on the market for a considerable time, while other sports cars manufacturers, such as Frazer-Nash and various foreign firms supply supercharged cars either as standard or to special order.
The day has yet to come when the general run of engines will have blowers but the tendency is certainly in that direction, and as far as sports cars are concerned supercharging has definitely come to stay.
W. S. B.