COMMERCIAL-VEHICLE SUPERCH/ ERS AND WHAT THEY CAN DO
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ACONSIDERABLE volume of literature is. now available on the problems relating to the supercharging of commercial vehicles and the benefits to be obtained therefrom. Some recent papers are listed in an appendix.
The writers of these papers and many other authorities have agreed that there is a strong case, technically and economically, for the use of superchargers on oil engines for road vehicles, but it is not the purpose of this article to prove this point or to go over again the ground covered in the above papers. One may summarize the matter as follows :— A gain in torque and power of at least 25 per cent. can be readily realized by supercharging; hence performance can be improved to a corresponding extent, or a smaller, lighter and cheaper engine can be employed, running on a higher load factor. The designer and user thus have a wider choice between load-carrying and economy than they normally have, whilst in some classes of vehicle the substitution of a smaller and lighter engine may lower the weight to a point where the vehicle falls into a lower tax class, thereby giving the operator substan tial annual savings. ` These results can, as a general statement, be obtained without excessively raising maximum cylinder pressures, with little or no increase in specific fuel consumption and with small increase in maximum internal temperatures. The difficulties of supercharging lie not in the principle of supplying air at more than atmospheric pressure, but in the purchase and driving of a reliable blower which is efficient and reasonably priced.
By reason of excessive bulk, it is impossible to use piston-type displacers to supply air to the manifolds of a road vehicle. Some form of rotary pump must necessarily be employed and certain special problems — arise from the widely varying duties they have to perform.
Supercharging is a fait accompli of stationary and marine plants, largely because the conditions under which these operate, both in respect of speed and load, do not widely vary. The contrary is the case on a road vehicle, for the speed range extends in a ratio of at least 5 to 1 and, irrespective of this, there may be a load variation of 5 to 1 at any given engine speed.
Dealing first with the question of speed effects, it must be realized that these have a considerable influence on the efficiency of the rotary pump. All pumps as now constructed have certain working clearances — which are not positively sealed. In consequence, they B4
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have a leak from the delivery to the intake sides, which is primarily dependent upon the total area of the clearances and the pressure difference in the pump. The total volume pumped, however, will be dependent on the speed of the blower ; hence, the proportional leak will fall rapidly as the speed of the total volume pumped goes up.
In practice this means that over a speed range of 5 to 1 the pumping efficiency of a typical blower will vary from 70 per cent. at the lowest speed to 90 per cent. at the highest. This has an important effect on the size of blower needed and on the pressure variations in the engine.
Taking a typical engine of 7 litres capacity, a supercharge of 25 per cent, gives it an effective swept volume of 8.74 litres. To obtain this output with a blower having a volumetric efficiency of 90 per cent., the latter component must have a theoretical swept volume of 9.72 litres. If it be achieved with the lowest efficiency of 70 per cent., the theoretical swept volume must he 12.5 litres and the volume delivery at high speeds with 90 per cent. efficiency will be 11.25 litres.
Under the latter conditions, therefore, the supercharge to the engine will have risen to nearly 00 per cent. and it will he receiving approximately 30 per cent_ of excess air. Moreover, this excess volume of air will be delivered at a supercharged pressure of 10 lb. per sq. in., as against the desired supercharge pressure of approximately 4 lb. per sq. in.
Although, as we shall see later, the disadvantages arising from this state of affairs can be ameliorated, there is no doubt that power is lost by supplying more air than is necessary at a higher pressure than is required. A fundamental, therefore, of blower design is to attain as good a volumetric efficiency as possible over a wide range of speed.
This consideration naturally lea is us to another, which is the weight and bulk of the supercharger itself. Clearly the more efficient any given design is as a pump the greater will be its merits in this respect. Additionally, however, it must be pointed out that various designs of rotary pump differ considerably, the output being affected by the maximum r.p.m. permissible and the displacements of the pump for fixed outside dimensions.
Finally, in considering the matter 'of cost, it can be said that the relative merits of various types have not yet been decided on the basis of large-scale production. None of the designs I shall describe later is inherently expensive to make, provided production is seriously tackled on a scale comparable with that of the engine itself.
Making reasonable allowances for the high standards of workmanship which are admittedly required, and for the fact that, in consequence, the blower is likely to cost more per pound than the engine, the economic case is still favourable.
Having thus outlined some of the generalities which must be borne in mind when considering the subject under review; I will now deal more specifically with the types of supercharger at present available _for use on commercial-vehicle engines. I propose to divide them into three classes. First, the types which are not covered by patent protection and can be used freely by any designer ; secondly, proprietary types which can be purchased in sizes of varying outputs, or which can be manufactured under licence from the patentees; and, thirdly, designs which are patented but which are not in production, the patentees desiring to work on a licence basis only.
Two well-known forms of blower come into the first category—the Roots-type blower and the turbo compressor. The former is a specialized development of the gear pump and acts as an air dispIacer, there being no compression within the pump casing.
For this reason the adiabatic efficiency of such a blower is poor under conditions of high pressure, but for the normal supercharges of 3 to 7 lb. per sq. in. it is reasonably high, as indicated in the curves shown in an accompanying graph, reproduced from The Engineer dated December• 2, 1938.
The volumetric efficiencies are also high (as is shown B5
in another graph, reproduced from the same source), these figures having been obtained on a Marshall-type blower, which will be described in detail later. The Roots blower is used extensively in marine Diesel practice, and on the two-stroke engine developed for road vehitles by the General Motors concern in the U.S.A. In the latter case an attempt has been made to improve certain characteristics of the blower by using an unusual foam of rotor. Three lobes are employed and these are, so to speak, twisted throughout their length. This reduces the pulsations in output which are normal to the two-lobe rotor, and the refinement is of special interest. The Roots blower is manifestly simple and, provided the centres for the shafts are accurately cut, there is no rubbing friction between any part, so that none of the internal mechanism requires lubrication. It is, therefore, inherently reliable, even when run at very high *speeds which give it outstanding outputs for unit size. Mechanically it is quite feasible to run Roots blowers up to 18,000 r.p.m., but on large types there are certain pneumatic or aerodynamic difficulties which in practice restrict the tip speed of the rotor. In addition to production by engine manufacturers, the Roots blower is made in England as a proprietary article by Sir George Godfrey and Partners, Ltd., Hanworth, Middlesex, to the designs of Mr. J. W. Marshall. It is of more than academic interest tb note that nickel alloys are freely used in this design, the rotor shafts being 60-ton tensile-strength nickel steel and the gears made from case-hardening nickel-chrome steel. The rotors are joined to these shafts by electric welding and are themselves made from solid-drawn tubing and are accurately ground to the required involute form. At each end the shafts are located in ball or roller bearings and are related and driven from a pair of spur gears in which the maximum backlash, with the blower cold, is 0.0005 in. Both the General Motors blower and the Marshall may be used in conjunction with a by-pass device. As the output of the engine is raised the air output can be correspondingly increased by closing the blow-off valve. In this way it is possible to avoid power losses when the engine is running on part load conditions and this is an important factor on road vehicles which rarely run at maximum torque. In addition to the Roots positive-displacement type of blower, another type, free for any designer to use, is the turbo compressor which is universally employed for the supercharging of aero engines. This has, however, certain disadvantages which have so far restricted its use on road vehicles, viz., the need for providing a considerable stepup in r.p.m., a 10-to-1 gear being needed to obtain the 30,000 r.p.m. rotor speed necessary for high efficiency. This entails gearing problems, whilst an even greater difficulty is the tremendous variation of output in relation to speed which is a mathematical characteristic of such a blower. With the exception of the Bilchi, the compressors I shall now describe are all of the vane type. It may, therefore, be appropriate at this stage to consider some fundamentals of their construction. A cylindrical casing has running within it an eccentric drum or rotor and projecting from and sliding through the drum are blades, usually four or more in number. A diagram shows these essential elements, and it will he realized that as the mechanism turns so gas is sucked through the inlet port, trapped in the space A, compressed into space B and delivered through the outlet port. The degree of compression varies with the position of this port and can range from zero up to three atmospheres or even more. For constant-pressure supercharging it is theoretically desirable for the pre-compression to be the same as the manifold pressure, because when this is realized the lowest power required to drive can be obtained.
In its simplest form such a blower has a solid drum rotor in which slots are milled, the blades being inserted in these and sealing agaiest the inside of the case by centrifugal force; possibly aided by springs. The friction losses associated with this scheme make it impracticable for high-speed blowers. For high speed it is essential that the blades should be controlled so that their tips do not contact with the case. There are basically two methods whereby this can be achieved.
The most commonly employed system, used in the Arnott. Centric and Velox blowers, is to have a central shaft on which radial blades oscillate, being attached thereto by blade carriers which fit over the shaft. The shaft being concentric and the drum or rotor being eccentric, such a construction involves an angular motion of the blades as they pass in and out of the rotor, as can be seen horn another sketch, Some form of trunnion hearing has, therefore, to be provided, and early types of this hlower always failed because any metal used for the trunnion has excessive loading at high speeds owing to centri
fugal forces. Non-metallic trunnion hearings are now available which overcome this difficulty.
The mounting of the blades on the shaft can either be with ball bearings which reduce lubrication problems to a minimum,_ or plain bearings which save space, allow a greater eccentricity of the rotor and thereby increase the volume of the pump for given outside dimensions.
Within the limits of pressure which would normally be employed for supercharging commercial vehicles, the gas loadings on the trunnion with this type of blower are well within safe limits. Rotational speeds are, however, limited by the inertia forces set up in the oscillating vanes, the tendency of these to bend and the loading imposed on the trunnion.
The volumetric efficiency, on the other hand, can be high for the blade clearances can be very accurately determined. It has, however, been found from experiment, that this feature has less influence on volumetric efficiencies than the clearances at the end of the rotor.
In this latter respect the vane type of blower has a big advantage compared with the Roots, for it is possible to recess the rotors into the end covers. This eliminates a delicate assembly operation to get the right clearance and enables an effective gas seal to be provided.
The alternative blade type has diametrical blades passing from side to side of the rotor through flat bearing plates. The blades are located by having feet machined at each end, these feet fitting over an internal cam.
The Zoller compressor embodies this construction which has the advantage of considerable bearing area between the blades and their supports and of eliminating the oscillating blade with its inherent inertia weaknesses. The Zoller type has, on the other hand, problems of its own, notably the loading on the cams due to the out-of-balance effect of the blades as they rotate, and the fact that the feet become offset relative to the cam and hence liable to distortion. The lubrication of this cam track is, therefore, a matter of considerable importance.
The Arnott supercharger is produced by Carburettors, Ltd., Grange Road. London, N.W.10, and is a straightforward oscillating-vane type using synthetic-material trunnions. The shaft holding the blades is of the stub type, being located on one end cover, whilst interposed between it and the blade carriers are ball or roller bearings.
The Centric compressor made by Messrs. Dilworth and Carr, Preston, also has sliding vanes. These components oscillate and are centralized on a shaft passing through the rotor. For smaller sizes this shaft is a'stub extending into the rotor from one end, and in the larger types suitable for commercial vehicles, a better support is given by an ingenious arrangement, shown in the drawing. • It will be seen that the driving shaft is supported in each end cover and must, therefore, pass through the rotor. The INDUCED VOLUME A
The principles of operation supercharger are simply e
latter being eccentric to this shaft, some ingenuity must clearly be exercised to connect the driving shaft with the rotor. It will be seen, in fact, that an internal and external gear are used for this purpose. In order to reduce lubrication problems to a minimum the blades swing around the shaft on roller bearings and oscillate through trunnions of a plastic material.
This blower enjoys all the advantages normally associated with vane types and on which comment has been previously made.
. The maintained efficiency of this blower both adiabatically and volumetrically can be seen from two accompanying curves and reliability has been established by extensive testing by a number of leading manufacturers.
Certain designs of blower are not manufactured pro prietary articles, but are available to engine builders under licence on terms which can be negotiated with their respective patentees. These include the Zoller, Velox, and Brichi, The rights in the Zoller compressor are handled in England by M. A. McEvoy, Elstow House, Eistow, Staffordshire. It. is a type which has been extensively developed in England for touring and racing cars and was in manufacture in this country during the period of 1931-1936. Much experience has, therefore, been gained, and Fetters, Ltd., Yeovil, manufactures it under licence for its scavenge twocycle engines.
A typical design applicable to commercial vehicles is shown in an illustration. As the blades are turned with the rotor the feet not only circle around the cam track but also slide across it, and the motion of the blade tip is not a circle but a concoid or epicycloid. In order, therefore, to give a constant clearance between the tip and the casing, the latter must be machined to a form and cannot be bored in a simple fashion. By using a cam tool the required machining can be undertaken fairly simply. Another point is that the rotor is recessed into the case at the bottom, thus forming a surface seal between the pressure and induction sides of the pump.
Mr. M. A. McEvoy is also largely responsible for the design and is joint patentee for the Velox. This compressor has not yet been manufactured in England, but experimental types have been produced by F. Faudi, Falkenstein, Taunus, Germany.
So far as the pumping elements are concerned, it is of the radial-blade type. An endeavour has, however, been made to raise the output for a given capacity (by giving the utmost eccentricity to the rotor), also to provide a control over the delivery by a special arrangement which will be later described.
An illustration shows that the oscillating blades run through trunnions, as is normal practice, but they are located on the central shaft not by ball or roller but by plain bearings. These can be lined with white metal or an oil-impregnated bush and, in the case of three blades, oscillate around the shaft, They do not rotate around it because one blade and its carriers are keyed to the shaft, which component is, therefore, turned at the same speed as the rotor, although not, of course, at the same angular velocities.
A high volumetric efficiency is realized throughout the speed range, whilst by the elimination of ball bearings the output for a given size is increased by approximately 25 per cent. Going farther, the designers realized that an ideal would be a blower in which the output could be changed in accordance with the engine torque, or, alternatively, to provide a flat boost curve irrespective of volumetric. efficiency variations.
The former characteristic is of importance from an economy viewpoint with petrol engines, as the change of torque is accompanied by throttling the air intake. This is not so on compression-ignition engines as used for com mercial vehicles, but nevertheless only about half the work used to drive the blower when giving a 5-1b. boost is recovered by virtue of th: m.e.p. realized on the " induction " stroke and by the elimination of pumping losses.
In the Velox, control over output is secured by coring the casing so that. for approximately half the length of the blower it stands well clear of the rotating blades for a considerable angle of travel. With such a casing no air would be positively delivered, because it would all by-pass through the large clearance existing between the blade tip and the outer casing, although air could be sucked direct through into the engine. From this condition of zero a charge for any required output up to the maximum available can be provided by interposing a ported sleeve which rotates in part of the casing and covers the cored passage as required.
The considerable angular motion required from such a sleeve makes it imperative to place the outlet port in a position that is staggered in relation to the sleeve, but there is no difficulty in securing reasonable port area with a compressor which is as long, or longer, than its diameter.
A delivery variation of 30 per cent, can be congortably achieved with a four-bladed blower of this type.
Another blower which has been developed on the Contint and widely used on stationary engines is the Btichi. The owners of the patents are BUchi Syndicate, Winterthur, and successful applications have been made on commercial vehicles.
Of the turbo-compressor type, it constitutes an alternative and widely used means whereby the volume of air delivered can be proportionate to the work required from the engine. Bfichi blowers normally consist of two turbo elements directly coupled together. One of these operates in the exhaust track and the other constitutes a pumping element for charging the engine.
Supercharging on the Buchi system involves special design of the valves and valve timing. This, however, presents no difficulties and a considerable gain in efficiency may follow. The power required to drive the compressor is derived from a potential which would otherwise go to waste, and from this follows the important advantage that a high degree of scavenging can be obtained without loss of efficiency. It has, in fact, been found that pressure charging may be carried up to 50 per cent. without increasing the heating interchange from engine to cooling water, and at lower gains in power there may actually be a reduction in exhaustvalve temperature. These results can be obtained without increasing the back pressure on the exhaust system to any undesirable figure; in fact, with an efficient turbo compressor, the surpercharge is always appreciably higher than the mean gas pressure required to drive the turbine. This can be readily understood when one, realizes that the major part of the driving power is derived from the kinetic or velocity energy of the exhaust impulses.
Road tests on a Saurer vehicle using this system show that a slmeed of 30 m.p.h can be obtained up a gradient which limited the speed to approximately 21 .m.p.h. with the normal engine.
The thichi supercharger is highly loaded in respect of thermal stressing and inertia loading, because the driving element is placed in the exhaust stream and the rotors run at high r.p.m. In both turbine and blower the blades are made from nickel alloy steels, the turbine blades being of a high-temperature-resisting material. It is hoped that this review will demonstrate that there is a large number of superchargers available if and when designers decide to take advantage of the present data, They are now in a thoroughly practical stage where they can be installed to give calculated results and where they ran be run for long periods without trouble. The average supercharger will deliver, say, 50 per cent. morz. air than the normal engine will induce, although weighing a twelfth or less of its -weight. From this it will be obvious that the mechanism will be comparatively highly stressed, and it is therefore of undoubted metallurgical interest to note that practically every manufacturer or designer uses or recommends nickel alloy steels for the essential parts. Indeed, it is safe to say that without them the lot of the supercharger designer would be appreciably more difficult.