SUPERCHARGING IN THEORY AND PRACTICE.
By H. HAGENS. (Chief Engineer of the British Anzani Engine Co., Ltd.)
MOST people who take an interest in the importance of the application of supercharging to automobiles are aware that the system was first introduced in connection with aeroplane engines, for as the aeroplanes were called upon to operate at enormous altitudes, the need for maintaining the engine power in the rarified atmosphere at thousands of feet above sea-level, became very apparent.
In the early days of flying, before supercharging was attempted, it was usual to design an engine to develop a certain horsepower at a given altitude. For example, if an engine was required to develop 350 h.p. at moo() ft. above sea-level, it would be necessary to give a far greater power output when working at sea-level. The air-pressure at sea-level being about 15 lbs. per sq. in., whilst at an altitude of 10,000 ft. it drops to about to lbs. per sq. in.; it follows that if an engine developed 350 h.p. at sea-level it would only give out about 234 h.p. at mom ft.
The latter conditions led tá the introduction of supercharging in aeroplane engines; and it was not long before automobile engineers began to realise the possibilities of the same method for increasing the power output for motor car engines.
Early Methods of Supercharging.
One of the earliest methods of producing a supercharging effect was introduced on racing motor cycles, where the velocity of the machines was utilised for forcing air into the engine for the purpose of producing a slight ” cramming ” action. With this object in view the air inlet to the carburettor was fitted with a trumpetshaped mouth, or wind scoop, so located that, as the speed of the machine increased, air was forced into the
engine. In actual practice, however, the benefit proved to be almost negligible, as the pressure of the air only increases to about 32 lbs. per sq. ft. when a speed of 100 m.p.h. is attained, which means, if the engine developed io h.p. at atmospheric pressure, it would only give an increase of 0.153 h.p. when supercharged by the method just described, which is obviously hardly worth taking into account. But when one recollects that some of the larger twin motor cycle engines develop about 40 h.p., an increase of .612 h.p. is obtainable, which may make all the difference in winning a race or breaking a record.
Even this apparently simple method of supercharging is not without its little complications, such as the need for balancing the supply tank and float chamber with the same pressure as that derived by the scoop, and this is done by taking a pipe from the latter to the respective parts and sealing it. Adaptations of this method have been applied to racing cars, and may be described as the earliest method of supercharging.
Progress in Supercharging Methods.
The next real step in the development of supercharging was the introduction of mechanically operated centrifugal fans, similar in principle to those used for blacksmiths’ fires. To obtain any appreciable pressure from such fans it is necessary that they should revolve at extremely high speeds, and to adapt such a contrivance to a motor car engine, where the overall dimensions must be kept small, a speed of from io,000 to 40,000 r.p.m. is essential. Since a fan of this type does not produce any appreciable pressure when running at low speeds, it follows that an engine to which it is
fitted does not begin to supercharge until it attains a high number of revolutions. Supposing such a fan were so geared as to operate at low engine speeds, its revolutions would be enormous when the engine was running fast, with the result that there would be great danger of mechanical breakdown.
One method of obviating the difficulty of high fanspeeds consists in connecting a series of small blowers in “cascade,” which means that the outlet of one blower is connected to the inlet of the next ; but even this does not altogether do away with the difficulty of obtaining sufficient pressure to supercharge at low speeds. The next method adopted was that of driving the blower independently, and probably the most successful of the early methods was that of operating the blower from the exhaust of the engine through the medium
of a turbine, this idea having been introduced by M. Rateau in connection with aeroplane engines.
The problem of supercharging motor cars for road or track work is all the more complicated on account of the advantages to be derived from the application of the system at all engine speeds between r,000 and 5,000 r.p.m., and for this purpose a special type of blower becomes necessary. It would be a comparatively simple matter to design a reciprocating pump to fulfil the required conditions, but as it is not desirable to introduce any extra reciprocating parts in any high speed mechanism, or to increase the weight of those existing, this type of pump is not regarded too favourably.
The Advantage of the 44 Roots ” Type Blower.
The nearest approach to the action demanded by a supercharging pump is found in the ” Roots ” type blower, which is capable of delivering air at medium pressures when working at a comparatively slow speed.
This class of apparatus also works fairly efficiently up to 15 lbs. per sq. in., and one designed to give this pressure at about 5,000 r.p.m. will deliver air at 2.3 lbs. per sq. in. when revolving at 1,500 r.p.m. The exact output from a blower of this type depends to some extent upon the working clearances, the latter forming one of the more important of the practical problems of the constructor.
The power required to drive a supercharging blower for a i litre engine is approximately 7 h.p., but against this it will be realised that a supercharged engine is relieved of its pumping losses, i.e., the amount of power absorbed by an engine in charging itself when working under ordinary conditions. Assuming the pumping losses for a i litre engine to be 5 lbs., it is then apparent that 2 h.p. more is required for supercharging, but it has been proved
that by the use of supercharging a i litre engine can easily develop from between 80 and 90 b.h.p., so it is obvious that the practical advantages are enormous.
Arrangement of Supercharging Systems.
Though experiments have been conducted with different lay-outs for supercharging apparatus, designers now appear to be agreed as to the benefits gained by adopting one of two systems, which are : (1) Arranging the carburettor between the engine and the blower ; and (2) Arranging the blower between the carburettor and the engine. Of the two, the former lay-out is the more complicated, partly on account of the need for balancing the air-supply to the fuel tank and the float chamber of the carburettor. Furthermore, as the supercharged air pressure increases with the engine revolutions, the petrol supply must be increased proportionately. The latter condition is fulfilled by the provision of a diaphragm pump driven irc•ni the engine, which forces
petrol from the supply tank to the float chamber, its action being regulated by the pressure of air delivered from the blower. Any failure in this part of the system causes the engine to ” hunt ” or run irregularly, due to the choking of the inlet system with excessive quantities of liquid petrol, instead of atomised vapour.
The alternative system of arranging the blower between the carburettor and the engine simplifies the problem very considerably, but when the mixture is drawn from the carburettor through the blower the difficulty of lubricating the moving parts of the latter is encountered. The usual practice, however, is to admix a certain amount of lubricating oil to the fuel, which does not appear to impair the value of the latter, provided that the proportions of the mixture are carefully predetermined.
Mechanical Defects in Blowers.
From the foregoing remarks it will be realised that the success of any supercharging system depends to a very large extent upon the design and construction of the blower, which must be characterised by first class workmanship and very close working tolerances. For
example, with the ” Roots ” type of blower, which creates a working pressure by the displacement of air between two revolving parts, the efficiency depends upon the amount of air passing back to the low pressure or inlet side, and this varies with the speed of the blower and the working clearances.
The higher the speed of the blower, the greater must be the working clearances, but, provided these clearances are not overdone, they can be fairly generous, without decreasing the actual air pressure obtained.
In the design and construction of a blower particular care must be taken to eliminate chances of rapid wear in the bearings and the gears used for actuating the revolving portions, for if the latter kind of wear takes place the revolvers will come into contact with each other and start hammering, with the result that the whole of this part of the apparatus soon becomes a candidate for the scrap heap.
Supercharging in Relation to Engine Design.
In attempting to apply any approved form of supercharging to an existing engine, various details need
very careful consideration. In the first place, if an engine has, under ordinary conditions, a compression ratio giving a compression pressure of 90 lbs. per sq. in., this pressure increases immediately supercharging is applied. The increased compression gives a greater maximum explosion pressure, and consequently a greater mean effective pressure, quite apart from the extra power gained due to a greater charge. In short, the engine must be considered as one of considerably increased power, therefore the efficiency of the cooling system must be augmented to avoid overheating, and as the working parts of the engine will be more heavily stressed, the loads on the bearings will be greater. All these, and other practical considerations known to the designer, limit the extent to which a standard engine can be supercharged without rendering it liable to mechanical breakdown. As the question of cam setting has been dealt with so admirably by Mr. P. T. Bersey in the last issue of this journal, I need not go very far into that subject in this article ; but I should like to say that, with an efficiently supercharged engine, alterations to the cam settings
are not so essential as they are in the case of an atmospherically charged engine, as one can make up for any loss by increasing the supercharging pressure to the extent of a pound or so.
Nevertheless, if the two are worked in conjunction with one another, improved results may be obtained, as it is possible to secure better scavenging effects and improved volumetric efficiency by the aid of certain valve settings worked out for supercharging.
It is not absolutely necessary that an engine must be designed on purpose to be supercharged, though naturally with such an engine one can carry the benefits to a far greater extent than would otherwise be possible. My personal experience with supercharging has brought me to the conclusion that the ” Roots” blower has advantages not possessed by any other form of pressure creating device, but I would not go so far as to say that finality has been reached in this direction. What one really requires is some form of apparatus capable of delivering air at a more or less uniform pressure through the speed range of the engine, this pressure being variable at will if desired. The blower
should be driven direct from the engine, not from any separate source of power. It should be light, compact, of high efficiency, and not require continual repair or overhaul. In fact a blower for this purpose should be extremely reliable, and operate with the minimum of attention.
Supercharged Racing Cars.
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machines similarly equipped, and motorists in this country have already witnessed the extraordinary success of the Talbot-Darracq cars in last year’s Two Hundred Miles Race at Brooklands.
Two of my supercharged engines competed in this race, but being, unfortunately, late in finishing off the machines, there was very little time for the final tests and adjustments. One does not expect to succeed in such circumstances, but, two days after that particular race, my supercharged engine in Major Coe’s Horstmann car lapped Brooklands at 103 m.p.h. Since then I have carried out exhaustive experiments with supercharging, and feel confident of being able to throw some interesting light on the problem during the coming racing season.
In conclusion, I would like to offer a few remarks on the future application of supercharging to standard as well as to sporting cars. By the aid of supercharging it will be possible to build light and compact engines capable of developing very high power, which should assist materially in reducing manufacturing costs, notwithstanding the additional apparatus required. There is no reason why the system should not be applied with equal advantage to motor cycle engines, and I think that we shall see some interesting developments in the latter direction in the very near future.