Unsolved Problems in Motor Engineering.*
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I will now take a few of the minor paints connected with the engines of both systems. In the internal-combustion engine, we must admit that much and satisfactory progress has been made in the development of the carburetter. There remains the important matter of introducing a satisfactory carburetter for the heavier oils, for although it is true that a great many of the existing engines still burn paraffin once they are heated, it is not a satisfactory solution of the problem to carry petrol to start the engine. Next come the ignition devices. Much time and thought have been spent on these, and the use of the magneto is on the increase. As an electrical engineer, I have some special knowledge on the subject of the magneto machine itself, and I cannot help thinking that, in the
FEW CASES WHEN IT CAUSES TROUBLE, it is an extremely difficult piece of apparatus for anyone to put in order who is not skilled in electro-technical matters. The puzzles and worries to a car user which are connected with the electrical or electro-magnetic appliances on his car are very great indeed; so great as to certainly put them outside the province of the ordinary driver. It follows that an expensive car may be disabled and rendered useless from very simple electrical causes, although everything else may be in perfect order. This trouble, which practically amounts to a defect in design, is not felt so much in large towns where intelligent repairers are easily found, but it is a real difficulty when the driver or owner is touring in places where such intelligent assistance is not available.
I now come to the much-discussed question of the number of cylinders. It appears to be the general opinion that the advantages of four cylinders are now so acknowledged that no car should have less than four cylinders, and serious efforts have been made to convert the public to the idea that six or even eight are necessary to ensure satisfactory running. I think the thoughtful engineer who does not look at this from the purely trade standpoint will admit that this desire for multiplicity of cylinders really gives away the case of the internal-combustion engine. Even if works are so organised, and the machinery for turning out interchangeable components is perfected, so that the additional cost of the extra cylinders does not tell much on the cost, and consequent selling price, of the car, yet the extra cylinders, with their valves, ignition devices, and other parts, increase the complexity and risk of stops on the roads almost in propor
tion to the increase of the number of parts. For many years it used to be the proud boast of English designers of machinery of all classes that, whereas French and German designers seemed to take pride in adding parts, in order to obtain a given •result, the pride of the English designer was to obtain the same, or even better, results, at the same time reducing the number of parts. We see this very plainly in the great simplicity, both in the outside appearance and in reality, of an English locomotive or English marine engine as compared with the same class of engine designed and made on the Continent or in America. I have not time to enlarge further on the subject of the engines themselves, about which so many papers have been already read, and so much said. The various organs and details of the Otto cycle internal-combustion engine have also been so much discussed that I do not propose to go further into that matter now. Little or nothing, however, has been said on the great advances that have been made in the steam boiler and engine used for motorcar purposes ; but in this case, although much has been done, still more remains to be done. Serpollet, White, Miesse, Clarkson, and others, by their improvements in the flash and semi-flash boilers, have converted the boiler from being the most troublesome organ, requiring perpetual attention and care in maintenance, to the part which
PRACTICALLY REQUIRES NO ATTENTION; but there is still great room forimprovement in the means of vaporising and burning the liquid fuel. Petrol and benzolene are easy to deal with, leaving little or no residue, but the difficulties of vaporising the paraffin and still heavier oils remain very great.
There is, I regret to say, far too much feeling on the part of the users of petrol cars against the steam car ; they would like to see no more of the steam car. A more short-sighted policy I cannot imagine. There are enormous quantities of liquid fuel in the world which can be provided at extremely low prices, and which merit the closest attention of those interested in the cheap working of commercial motors. It is beginning to dawn on engineers who understand the steam question that superheated steam at a temperature of 000 degrees Fahrenheit presents an entirely different problem from steam as ordinarily used in our locomotives or marine engines, and there are already indications that, with properly-constructed engines, in the near future, a fuel economy approaching, if not equalling, that of the internal-combustion engine might be obtained. I take this opportunity of ap pealing to all interested in automobilist') to treat this matter from a far more catholic point of view than has hitherto
been the case. Manufacturers and users of petrol vehicles are really indirectly interested in the success of steam-driven vehicles. It would be disastrous if the only fuel available for motor vehicles were petroleum spirit. As the use of this increases, without corresponding use of the heavier distillates, the price of the spirit is certain to increase, whereas a great increase in the use of the heavier distillates for fuel purposes will assuredly lower the price of the spirit. lit fact, I may carry this matter a step further, and point out that the use of all possible forms of fuel is to be encouraged. Among solid fuels the choice lies between Welsh coal and coke, but any advance in the production of compressed or powdcz-ed solid fuels must be to the advantage of the users .of other fuels. I press these considerations before you, as I regret to say that, in discussing police and other regulations, the petrol men have assumed the position that they need not take care of the interests of the steam people. whereas I believe the interests of all are so bound together that those interested in one class should help the other class. I must now turn to the all-important question of the gearing. The self-propelled road vehicle differs from the locomotive on rails in that the former must always have gearing interposed between the crank shaft and the road wheels, whereas in a locomotive on rails the driving axle is the crank shaft, and this is the true direct drive, or, as the traction-engine people call it, a one-shaft drive; in fact, the SO-CALLED DI REcr DRIVES
for cars are two-shaft drives in the case of steam ears, and are never less than three-shaft drives in the case of petroldriven cars, for with the petrol engine it is necessary to have an additional shaft for change speed and for reversing. Wherever chains are used the petrol engine demands a fourshaft drive. This multiplicity of shafts and gear wheels is a feature of the modern car which I feel confident will not long survive. In order that the internal-combustion engine may arrive at equal simplicity of drive with the steam engine, it is necessary that a radical departure should be made in the whole design of the internal-combustion engine : instead of being content with the Otto cycle, which gives only one useful effort per cylinder for every two revolutions, we must work out the old Brayton idea of burning the mixed gases steadily as they flow into the cylinder, so that the pressure from the commencement of the stroke to the point .of cut-off, as shown by indicator diagrams, may be approximately as uniform as that observed in any steam-engine cylinder. We may be far off this solution of the question, but it is evidently worth striving after, and I therefore give it the prominence it deserves; but, taking internal-combustion engines as they are, I think one of the most urgent problems is the simplification of the gearing : the reduction of the shafts to three, i.e., the driving axle, the cardan shaft driving it, and a clutch shaft, combined with a means. .of reversing which will not involve movement of the idle portions at the time that the engine is driving the cardan shaft direct.
Next as to the gearing itself. In the majority of cases bevel wheels are interposed either between the crank shaft and counter shaft when chains are used, or between the cardan shaft and the live axle. In both cases, by very perfect workmanship, bevel gear has been produced which works for a very long period silently and without apparent wear. This has been effected by the accurate adjustment of the relative position of the two shafts at right angles to one another, by the use of ball-thrust bearings. This accurate adjustment is of immense importance, as a very small movement end-ways of either shaft changes the line contact of the perfectly-adjusted bevel gear to a point contact, and this point contact can only again become line contact by the wearing down of the parts. Theoretically, the best form of gearing to be interposed between two shafts working at right angles to one another is the worm gear, which has been so well developed by Lanchester. There is no question that worm gear can be so made that true surface contact is obtained when new, and can be easily maintained after wear, the advantage being that the contact surfaces, with worm gear, can be made large enough so that under ordinary stresses the oil film interposed between them need not he squeezed out ; such worm gear can, therefore, be run for lengthened periods without any wear, and this is beautifully shown in some worm gear that has been made for electrical driving by the fact that clear oil put into the worm remains clear after many hours of running. The defects of the ordinary type of gearing, usually called the
Panhard " type, are commencing to show themselves in the motor omnibuses that are now running in such large numbers in our streets. Although the best manufacturers in France, Germany, and England state that this gear can be made to run for long, and remain reasonably silent throughout, yet the fact remains that these same manufacturers, when they supply a chassis for a motor omnibus, fail to obtain a 'life for the third-speed wheels, on which the bulk of the wear comes, of more than a few months, so that the rattle and noise of this worn gear is already becoming a nuisance, and would, in the absence of improvement, when the horsed omnibus is completely superseded by the' motor omnibus, render life in our streets intolerable. It appears to me that the solution of this gear difficulty lies in the development of the worm drive and in the abolition of intermediate speeds. I will now break my rule and give a suggestion. Hitherto I have been playing the easy role of critic. I have gone somewhat carefully into the question of the driving of motor omnibuses, and I believe it will be found that the motor omnibus chassis of the future can be satisfactorily driven with no change speed, but only one reverse shaft, and an engine running always at the rate of eight to one of the driving axle, the transmission being effected by worm gear. The size of cylinders has, of course, to be increased to enable the engine to start away on the one speed answering to the present top speed. I do not shut my eyes to the fact that this change in the arrangement of the gearing presents many difficulties– possibly in the arrangement of the seating and entrances of the omnibus; but I do not think these difficulties are insurmountable. At any rate, I present them to designers as a problem worthy of the closest attention. Passengers would have to sit back to back in the centre of the bus facing outwards instead of with their knees inwards. That would probably not be a difficult thing to accomplish, but the English public is very conservative. (Hear, hear.) Enough has been said on the subject of gearing. I now turn to the wheels, tyres, springs, and axles, which must he taken together. The man who has influenced most profoundly the motorcar is the late R. W. Thomson—(applause)—through his invention, not only of the pneumatic, but also of solid rubber tyres. (I am glad to see his son in the room. I worked under him ; and there is another gentleman here who worked under him.) He did more for road locomotion than any other man of the last century. (Applause.) [Col. Crompton here handed round several photographs showing the solid rubber tyres used in the seventies, remarking, amid laughter, that such masses of rubber the world had never dreamt of since.1 The early motorcars built by Benz, Peugeot, De Dion, Panharcl, and others commenced either with rigid-tyred wheels or with solid rubber. In both cases they were compelled to use wheels of considerable diameter. As soon as the pneumatic tyre became practically universally used, the cost of the rubber, and of manufacture, at once cut the diameter of the wheels down to the present figure, rarely exceeding 34 inches. This reduction in diameter greatly increased the strength of the wheels themselves to stand side shocks, so that the ordinary artillery wheel is quite good enough for the purpose, but the scarcity of rubber, in proportion to the demand for it, has so increased its price that attention has been attracted to the older type of wheels, and certainly the question of wheel and tyre upkeep of the commercial vehicle is not the least important problem that I have to put before you. For the past 50 years inventors have endeavoured to make a metallic spring wheel, and several such wheels have been made sufficiently successful to warrant their use, but, although they undoubtedly do to some extent reduce the shock which is transmitted from the road to the axle and parts below the main springs, yet they do not to any noticeable extent reduce the noise of the metallic tyre crushing the surface of the roadway. This noise is of such importance that we must contemplate the possibility of the speed of commercial vehicles, such as omnibuses, being limited by regulation unless a noiseless covering for the rim is used. The weight and cost of this covering will, however, depend on the excellence of the spring arrangements interposed between the axle and the rim of the wheel. In producing spring wheels some inventors have proceeded on a line which, according to my own experi
ence and knowledge, is entirely wrong—that is to say, they attempted to use an elastic material, such as rubber, nearer to the hub of the wheel, and in this way hoped to reduce the quantity and cost. I think, however, that it is impossible in this way to obtain a satisfactory spring wheel, for just in proportion as the weight of rubber interposed between the axle and the road is reduced, so will the amount of work which each cubic inch of rubber, so interposed, has to perform, in absorbing shocks, be increased; and I have had plenty of experience during my nine years' working with solid tyres in India to know that there is no gain to be obtained by such reduction. The same law— namely, that a certain weight of material is absolutely necessary to absorb shocks, is equally applicable to metallic springs, whether in the wheels themselves or when used as the main springs of a vehicle. If spiral or volute springs are used, they must be of such cross section and consequent weight as will fully absorb the work put into them. As an instance of what I mean, I mention the wheel designed by Prof. Robert Smith. His design may be taken as diagrammatic—namely, that you cannot get sufficient elasticity from straight radial spokes, but you can get it if each spoke is lengthened and thickened in the form of a long spiral, but such long spiral radial springs cannot be used for the two-fold purpose of absorbing road shock and also of transmitting the driving stresses from the engine. These must be provided for by a second set of springs and linkage, as he has done, and this principle of providing ample elastic material must be followed throughout if spring wheels are to be designed to be satisfactory. While on the subject of spring wheels, I must call the attention of their designers to the great difficulties caused by our road authorities in exaggerating the camber of the cross curvature of our roadways. A well-made road 30 feet wide ought not to have a total rise at the centre of more than 4 inches, and this, when evenly distributed over the entire width of the road, makes it look, to the eye, nearly. flat, and with such a road there is no difficulty in getting a broad tyre to bear evenly all over its surface. Many of our London macadam roads have been measured by me, and I find roads of this width having a total camber of zo inches, the bulk of the slope being on the two flanks close to the gutter : it is impossible for a broad, rigid-tyred vehicle to work on its own side of the road without bringing an unusually large share of the total weight on to the inner edge of the rigid tyre. This not only causes heavy cross strains on the wheel itself, but damages the road. This EXCESSIVE AND IRREGULAR CAMBER of our roads is one of the chief difficulties that designers of road wheels have to contend against, and it is, undoubtedly, a chief cause of the rapid deterioration and heavy wear of our pneumatic tyres. Whenever a vehicle provided with pneumatic tyres is driven where the camber or slope is large, great strains are brought on the covers close to the rim attachment, and this at the very point where the duty required of the fabric of the cover is heaviest, due to its deformation as it rolls on the road. The strains on the fabric at these points are extremely complex and difficult to follow, but they nevertheless exist, and that the tyres endure them so successfully is due to the immense amount of inventive talent that has been put into the tyre question, and about which the average car owner knows little or nothing. In fact, there is no doubt that the time has now come when the question of motor and road design must be considered as a whole. In railway engineering the co-relation of the road bed, rails, and locomotive has been long acknowledged. Alteration in one cannot be carried out without profoundly affecting the other. The requirements of modern mechanical propulsion demand, similarly, that the road authorities should confer with us so to design and construct their roads as to minimise our difficulties of design; we, on our side, to do all that is necessary to minimise the wear and tear of the roads themselves.
A problem of great importance in connection with spring wheels is that of the connecting pins or other attachments at which rubbing contact takes place, and which must exist in order that the rim of the wheel may rise and fall in relation to the axle, for although, theoretically, the best position for these attachments is as close as possible to the rim of the wheel, yet this is the very position where heavy wear and tear must take place, owing to the dirt and mud found there. For this reason any spring wheel which uses sliding devices at this point must be avoided. I cannot afford now to devote time to the important matter of the springing of vehicles of all classes. The springing of the pleasure car is already very luxurious. The French greatly excel in this matter, and the comparatively recent introduction of three-point suspension, which can be accomplished by a cross spring at the back of the cars, is to be commended. I think the only error that has been made in spring design has been too much desire shown to keep down the weight of the springs, and this is incompatible with efficiency. A highly-important matter requiring reconsideration is that of the height of the under-surface of the vehicle above the road surface, and the shape of this under-surface, in order to minimise eddy currents at the back of the car, which, undoubtedly, are the MAIN CAUSE OF EXCESSIVE DUST RAISING.
This question is so important that it merits a paper to itself, fully illustrated with diagrams. I can only say, now, that the efforts of the committee who for several years have been examining this question, on behalf of the Automobile Club, have been so far attended with success that they are now able to show that, if cars are designed with reasonable clearance between the underside of the machinery, and if the casing of that machinery is properly shaped, the currents of air passing underneath the car will not be deflected up at the back, will not form eddies so as to lift the dust and cause annoyance by its being raised to excessive heights. It has been shown that, if it were not for this air draught passing underneath the car, the dust raised by the wheels would, in the majority of cases, quickly fall again, and the dust cloud attending a car driven over a dusty road would not rise more than five or six feet from the road, instead of, as at present is the case, form a column zo or 30 feet high. Stated briefly, the proper form of the under-surface of a tar should be that of the lower half of a fish, if the fish were divided by a horizontal plane. We require a comparatively blunt nose, but fine stern lines, in order to reduce the eddy currents at the back to a mininium.
Another point that has been clearly shown has been that much may be done by having adjustable flaps on the front mudguards, so that these may be much higher from the ground in dusty weather than in muddy weather, but, apart from the question of dust raising, it is probable that better results in the future will be obtained for the ordinary vehicle by the use of wheels of greater diameter and by arranging the whole of the car higher from the ground. It can be shown that, as we increase the diameter of the wheel, so do we decrease the stresses on the fabric and rubber at the two points where it suffers most—namely, where the tyre deforms as it rolls on to the road, and again resumes its original form when it leaves the road. By increase of diameter we obtain the double advantage, firstly, that for a given road speed there are fewer of these changes of form per minute, and, secondly, the angular movement of the rubber when these changes take place is greatly lessened by such increase of diameter. I have nowhere seen any record of experiments on this important question, but I think it can be shown that a given weight of material can be better disposed in a pneumatic tyre of larger diameter and less cross-section than that of those that have been hitherto employed, and that if some of the shocks can be taken up by improving the design of the wheel itself, so that it becomes a spring wheel, still further REDUCTION IN THE COSTLY RUBBER can take place. I think that this wheel and tyre question, whether we look at it from the point of cost, reliability, or general convenience, is the most important one before the designers of all classes of vehicles, whether they be those used for pleasure, or for the motor omnibus, or for the vans which transport goods throughout our streets, for it is evident that goods cannot be carried at high speeds on rigid wheels. These wheels must be elastic and must be provided with some class of tyre which will deaden the sound.
I have now touched on as many points as is possible during the limited time at my disposal. I have purposely chosen some of these points, as I know them to be controversial, and that they will, therefore, give rise to animated discussion. In this way I hope I have succeeded in interesting you on matters which I am convinced lie at the root of future progress. (Loud applause.)