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Petrol Motor Omnibuses.*

21st March 1907, Page 6
21st March 1907
Page 6
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Page 6, 21st March 1907 — Petrol Motor Omnibuses.*
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Which of the following most accurately describes the problem?

By W. Worby Beaumont, hi.I.Mech.E., M. Inst. C.E.,

Within the last two years the number of petrol omnibuses in greater London have increased from a negligible number of small vehicles to 795 in actual commission. These at the present rate of patronage are carrying about 185 millions of passengers per year, which is equal to carrying the whole population about 37 times in the year. The omnibuses run from 90 to 120 miles per day, or from 30,000 to over 40,000 miles per year. In one year the engines of these omnibuses will make from about 230,000,000 to 250,000,000 revolutions per year, and at the end of that time the cylinders of many are in perfect working condition. Considering that this means about 2,000 millions of strokes of the four pistons in every engine, it may be fairly claimed that the performances of many of these delicate engines far exceeds that of any prime-mover ever before put to any kind of work, and that it is more than astonishing when the conditions under which the work is done are remembered.

On all the London motor-omnibuses seats are provided for 34 passengers, not including the driver and seat for an occasional learner at his side. There are now in the possession of the several companies in London alone over 800 motor-omnibuses, nearly all of which have been made during the last three years. During this period very few firms in this country were able for different reasons to undertake the construction of more than a few of these large vehicles, those best prepared being already very busily occupied with the production of either light passenger-cars or commercial vehicles of different types. Although such a large proportion of those motor-omnibuses which have performed most satisfactorily have been the product of a few of the larger Continental works, it is peculiar that the countries producing them do not use them, or are only now commencing to use them at all extensively; and it was left to this country, and more particularly to London, to show sufficient enterprise and to bear the heavy cost of the initial experimental work, represented by the use of a new and untried mechanical means, of continuing and developing the passenger transport work hitherto carried on by the large London companies by means of horse-haulage. A relatively short period of service of some of the earlier, and at the time better, designed vehicles sufficed to show that, even with somewhat higher maintenance costs than then appeared probable or even possible, there was some prospect of running the motor-omnibus profitably, and of providing an improved road service.

It is now evident to all, instead of to the few who could anticipate development, that just as the horse-drawn tramcar has gradually been replaced by the improved mechanicallypropelled car, so will the mechanically-propelled omnibus supersede the horse-drawn omnibus. The transition stage has now been reached, accompanied, as might be expected, with problems arising largely from the substitution of the new things for the old. When some consideration is given to the volume of traffic now dealt with by the London omnibus companies, the magnitude of the change may be appreciated. The favourable reception of the motor-omnibus by the public, despite the occasionally just but more often impatient and un• reasonable objections raised by those to whom it does not happen to prove a convenience, led to so rapid an increase of the demand for more omnibuses that the available supply was soon exceeded. The difficulties that have been experienced in connection with the running and maintenance, in proper running order, of practically all of the many types of omnibus now in use, may be largely attributed to this rapidly increased demand, and to the similar and less readily met demand for competent and trustworthy men to either supervise, drive, or repair them. It is the natural result of a change of considerable magnitude and of rapid occurrence, one of the results of the abnormally rapid growth of this new industry.

There are in use now not less than twenty-four different makes of omnibus constructed by a similar number of builders, and of this number twenty-two are driven by petrol engines and two. by steam engines, or considered numerically, there are 95 per cent, of the former type to 5 per cent, of the latter. No electrically-propelled omnibus has yet been successfully run, and those of the petrol-propelled type, with different forms of electric transmission-gear, have yet to be used to show to what extent the improvements expected will be realised. Apart from the relative merits of the different systems which may be adopted, it becomes evident that interest at present attaches principally to the petrol-propelled types, whether that interest be propor tional to the number of different makes in use or to the numerical proportions already stated. There is in general appearance great similarity in external design of the complete omnibus ; a similarity that is necessarily aided by the use of bodies of nearly uniform type, made to comply with regulations equally applying to all. There are, however, in the design of the underframes and in the details of the mechanism very considerable differences of method and of form, which, considered with reference to the result obtained, are of interest.

Single.deth Omnibuses.—A few of the earlier omnibuses of small power and weight were constructed with single-deck bodies, and, as long as they were in use, the practical. indications were encouraging ; but the period of service was short and the number of vehicles insufficient to allow definite conclusions to be drawn as to their successful commercial working. Although some of these small omnibuses only carried twelve passengers, and the proportion of passenger load to total load was low, there is no doubt that running expenses would also be low, and the earnings per omnibus-mile sufficient to ensure successful working in some districts.

Dozthle-deck Omnibuses.—The types of double-deck omnibuses now used weigh in working order from 31 to 41 tons, and the weight of full load of 16 inside and 18 outside passengers with the driver and conductor averages 2 tons 8 cwt. The total load on the road wheels is therefore from 6i to 6i tons, and may occasionally reach 7 tons. The load on the driving wheels with the omnibus fully loaded is in the most-used types about 0.6 of the total load, and it may therefore be a little over 4 tons, or say 2,4 tons per wheel.

The average weight of the bodies as used in London is 29 cwts. The overall dimensions* are as follows :— Extreme length varies from 18 feet 6 inches to 22 feet 6 inches. Width over wheel caps 6 feet 3 inches to 7 feet 2 inches, the maximum allowed.

Height to top rail of body 11 feet 6 inches to 11 feet 9 inches. The weights of the various under-carriages illustrated are approximately as follows :—

The conditions of service of these heavy vehicles are immeasurably more severe than those applying to any other type of motor vehicle. They are continuously used during good and bad weather, and they frequently have to be run over indifferently maintained and sometimes extremely bad road-surfaces. Not only is the road speed high and the total weight to be moved usually greater than 6 tons, but a large part of the period of service is occupied in constantly repeated starting and stopping. This very interrupted running is continued for 16 or 18 hours every day, and the daily mileage frequently reaches the high ;maximum already mentioned. It is not therefore surprising that many of the earlier omnibuses were soon found to be in some respects unsuitable for the heavy work they were required to perform. Although, as has already been stated, earlier experience has been helpful in determining the design of the motor-omnibus, subsequent experience has shown that often the severe conditions of service were not anticipated.

The dimensions adopted for many parts, although sufficiently liberal to give reasonable durability for periods or rates of service hitherto customary, were totally inadequate when the rate of working previously constituting a year's service became, in the case of the motor-omnibus, equivalent to only from two to three months' work. To the failures due to rapid wear and breakage, condensed into the shorter period, had to be added other and more serious troubles occasioned by causes even now incompletely understood, if we may judge from the comparatively recently completed designs of some makers. It would seem that the designer had concluded that an engine capable of delivering, for instance, 30b.h.p. and speed-reduction gearing already found to give satisfactory results in conjunction with it, when used as parts of the equipment of a light high-speed passenger-car would prove equally satisfactory when required to haul the much heavier but slower speed vehicle. This expectation was of course doomed to disappointment. Parts of the transmission mechanism and gearing have been rapidly destroyed, and the engines, although in a more favourable position, have been shaken and pulled to pieces, as a re stilt of the heavy and repeated inertia stresses experienced when starting and stopping the omnibus. Members of this Institution will recognise that the nature of the conditions of working of the light and heavy motor vehicles of similar powers is analogous to that experienced with, for instance, the stationary engines used in the one case for driving electric-lighting generators, and in the other for driving electric-traction generators ; and many will remember the difficulties that occurred when, the engine and generator that proved quite satisfactory for the first purpose were found to be unsuitable for the second, although the horse-power of the engine was the same. The increased severity of the working conditions is not due to any increase of horse-power, but to the continual fluctuation from a low minimum to a high maximum of the rate and speed of working, and to the heavy inertia shocks that occur as a result of that variation. In both cases stresses due to inertia of motion are experienced, and, although so far as the engine is concerned, these stresses or shocks may be limited by the energy of its moving parts, intensified in the case of the internal-combustion engine by the effect of explosion efforts, the whole of the intermediate machinery or transmission parts has to resist very severe efforts controlled by the amount of the resistance or weight overcome or driven and by the energy of the engine. Differently expressed, it may be said that whereas in the one case the load to which the engine and transmission gear is connected does not exceed two tons, it in the other case approaches seven tons. With the heavier load representing the conditions obtaining with the omnibus, the frequently repeated and longer periods occupied in acceleration have to be endured, and it is these periods that are distressing to the engine and transmission parts.

The speed of the engine may be suddenly reduced, without inconveniently high rate of acceleration of the vehicle, and its energy delivered at a high rate to the transmission gear ; and a similar effect may and does often occur when the engine speed becomes reduced by overloading during hill-climbing, before the c,onditons are altered by use of the change-speed gear. Reduction of the severity of the stresses prod:Iced in this way depends very much upon the capability of the driver, but it is necessary to provide for relatively high rates of acceleration, and to so increase the strength or stiffness of the working parts that their endurance may be satisfactory, even though there be no increase of the horse-power of the engine. Stresses of even greater magnitude have to be resisted by -the whole of the parts included between the driving wheel tires and the brakes. Prominence has been given to the cause and occurrence of these destructive stresses because of frequently expressed surprise, and because, although often unrecognised or underestimated, they are responsible for much of the trouble that has been and is now being experienced.

Power.---The question of the amount of power that may be usefully employed is one that requires very careful consideration, for on its correct determination depends the character of design and to a large extent successful working. There are in service to-day omnibuses with engines of only 20h.p., a large number with engines of 24h.p., and an increasing number with engines developing between 30 and 40h.p. The larger powers have been employed after considerable experience with the smaller engines, and there remains a decided tendency to use the higher powers mentioned with the designs of undercarriage now in common use. Engines of as much as 50h.p. have been tried, but only to a very small extent, and it is unlikely that there will be any further increase in their use. The increasing use of engines of not less than 30h.p directs attention to the reasons that have led to their adoption, and invites investigation of the soundness of those reasons. The greater engine-power makes more rapid acceleration of the omnibus possible, and without necessarily increasing the maximum speed, increases the average speed. Higher speeds of hill-climbing are maintained, and the change-speed gears are not so frequently brought into and out of action. The work of driving is consequently simplified, and when there is slight temporary loss of power due to one or other of the occasionally unavoidable causes, the omnibus may be kept running in service without marked inconvenience until completion of the journey. The addition to the weight resulting from the use of the larger engine is not in proportion to the increase of power.

Against these possible advantages have to be set the disadvantages of increased weight of the more powerful complete omnibus, and the power of running at high road-speeds, inevitably attended by more rapid wear and failure of the rubber tires. Increased consumption of petrol and lubricating oil has also to be considered. The effect of high road-speed is not however confined to increased wear of the tires. With 'greatly increased severity of the road shocks, there is necessarily greater wear of all the active parts of the under-carriage. It has already been remarked that engines of about 24h.p. have been largely used and continue to be used, and the question arises as to whether the omnibuses with these engines have shown themselves to -be inferior to those of higher power. speeds considerably in excess of that permitted by the Local

Government Board Regulations, namely, 12 miles per hour, can he and are maintained on level or nearly level roads ; but on the hills to be found on some of the London routes it is frequently difficult to run at the speeds at which the horsedrawn omnibus is usually driven. If the hills are to be climbed at high speed, and a speed of more than 12 miles per hour is to he maintained on the level roads then the engine of higher power becomes necessary ; but in this connection it must he remembered that disecretion must be used as to the relative importance of rapid acceleration and of economy of fuel. At the present time omnibuses constructed to run at 12 miles per hour are now run at considerably higher speeds, and this entails injurious racing of the engines and noisy working. On the whole, experience tends to show that a 30h.p. engine is ample for all London use, especially if anything like proper observance of the regulation speed properly allowable to such vehicles is to be maintained, and this speed regulation should be followed in the interests of the owner of the vehicles as well as in that of the public. In order to prevent this high-speed running under any circumstances, a system of automatic control is required, which, while limiting the maximum speed, will yet leave it in the power of the driver to accelerate rapidly up to the predetermined limit. With a 281up. engine the mean rate of acceleration to 12 miles per hour is about 1.25 feet.

For the purpose of illustration of this paper, drawings of some of the most frequently used under-carriages have been selected, to represent the general practice now obtaining as regards the general disposition of the machinery and the forms which it commonly takes. Those which are here given differ in some of the important details, and some of them can he considered representative of the methods adopted by nearly all of the many makers now engaged upon the construction of petrol motor-omnibuses.

Framework of Under-carriuge.—The framework of the undercarriage is generally similar, so far as the use of a main frame for the support of the whole load and a secondary or underframe for the attachment or support of the engine and some of the transmission gearing is concerned, but there are three methods of construction which differ considerably. These methods may be classed as those in which

• (1) The main longitudinal and transverse members are formed of rolled-steel channels and angle steel of uniform depth • of section from end to end.

(2) The longitudinal and some of the transverse members are • of pressed steel, with a depth of section of the main longitudinals greater at the centre and tapering towards • the ends.

• (3) The longitudinal members arc made of wood encased or stiffened by steel channels or flitch plates with steel transverse members.

For purposes of comparison, sections of maximum depth of the three types of main frame arc shown by Fig. 1. The relative powers of resistance of these frames to bending are respectively in the order given 1.37, 1.78, and 1.0, from which it may be inferred that type 2 is that which should give the best results in service, so far as strength of this part as an element is concerned.

Type 1 is representative of that used by the principal makers for some years. It has been found of insufficient strength to resist the bending stresses experienced in service. Nearly all of them showed signs of distress by bending at a position a little forward of the front end of the omnibus body, and it is obviously the part of the frame which would most severely feel the bending efforts and efforts productive of contrary flexure. In front of this critical part of the frame the load is distributed between the points of attachment of the front springs, and behind it considerable stiffening effect is obtained by fixing it to the comparatively rigid omnibus body. The severest bending stresses undoubtedly occur when a fierce clutch is suddenly dropped in, or when the brakes are instantaneously applied, and aggravate the effect of the pitching that may be observed when the omnibus is travelling over a bad road surface. Nearly all these frames have now been stiffened or reinforced, either by the use of truss-rods or stiffening plates of various forms. The type of frame represented by No. 2 has not been so niuch used as No. 1, but it has been adopted by at least one maker of great experience. There is a growing tendency to use it, although it is a more expensive form of construction than the heavier form represented by No. 1. From the point of view of reduction of weight and correct use and disposition of the material, it is an improved form, hut as a deeper section of lighter scantling is employed the junctions of frame-members have to be very well designed and made to resist loosening by working or bulging and tearing away. Some of these frames now in use and made in France are showing signs of distress as before, by bending of the longitudinal members. The type of frame represented by No. 3 has been used with only three types of omnibus, but the particular form shown has proved to be very successful as used with the De Dion omnibus, Plate 3. Its strength, judged merely from the section and with the combination of materials taken into consideration, is much less than either of the other types, but there is no doubt that it is a livelier or more elastic frame and capable of undergoing greater deflection than the other types before deformation occurs. A frame that is rigid, or is of a section from which little deflection can be expected without permanent change of form, must he of greater weight than the more flexible frame, and for these reasons the armoured-wood frame is good.

Two examples of rolled-steel frame construction are shown by Plates 1 and 2, as used with the Straker-Squire and the Dennis omnibuses. In the former the reinforcing plates referred to may be seen in the plan ; and in both types the form and arrangement of the transverse members is shown, and the nature of the attachments to the longitudinal members. In both these designs it will be seen that a secondary frame, hung from the main frame, carries the engine and change-speed gear-box. In Plate 1 the position of this frame is indicated at D, and in Plates 2 and 3 at II.

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