Points in the Design of Commercial Vehicles.
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By a Member of the Society of Motor Omnibus Engineers.
The fact, that some manufacturers, who are making cornnercial motor vehicles, are using standard light-car parts n their construction, seems to show very clearly that such nanufacturers have a lot to learn regarding the conditions If commercial service. A motor omnibus, for instance, is isually geared from three to four times lower than a light :ar of the same power, and there are not many light cars onning from too to 120 miles a day, day after day ; yet notor omnibuses are expected to do this, which means that hey make from three to four times more engine revolutions n covering the same distance. A motor omnibus weighs, ay, five times more than a light car ; yet there are not a few nnnibus chassis built up with standard light car engines md clutches. The successful omnibus chassis is not necesarily the production of those builders who make a big show n light-car events, nor is it the chassis which has made ong exhibition runs in record time with a specially-selected [river. A chassis should be simple, accessible, reliable, :conomical, and silent.
Position of Engine.
The driver's seat should not be placed over the engine at he Sacrifice of ready access to the latter, for, with this irrangement, accessibility usually is sacrificed. Everything nay appear to be " get-at-able," but wait until something vants doing I The small economy in overall length is not A much use, when the bus is half blocking a crowded thormghfare, and the driver is " tying himself in knots " while :tying to make a small adjustment. Reliability is almost ;ynonymaus with accessibility, and a part which is accesAble
s more likely to receive proper attention than a part which nvolves a lot of work in getting at it. Engines should be milt up so that the bottom half of the crank-chamber will :ome away without disturbing the crankshaft and main )earings. Too many engines are put into omnibus chassis athich lack this most important feature of accessibility to )ottom end and main bearings : small repairs become the .vork of hours, instead of minutes, and involve the entire lisrnantlingof the engine and, in some cases, as where the 'river's seat is arranged over the engine, even the petrol :ank and clutch and brake pedals. When so much work is nvolved in getting at these bearings, is it small wonder that ;light knocks are not attended to, and that the chassis is eept running to see if the knock will develop. Thc knock 3f ten does develop, and .a connecting-rod will find its way :hrough the side of the crank-chamber.
Engine Bearings.
A good white metal gives the best results on these bearngs. The scoring of journals, when gun-metal or phosAior-hronze brasses are used, becomes a serious matter in he course of a year or two of constant work in public serrice. On some gunonetal brasses, which do not allow of
thick lining, white metal is put in only /-16th of an inch .hick. By carefully tinning the brasses, and heating them
o the correct temperature before running the white-metal n, no trouble has been experienced through cracking of this hin lining. A great advantage of this thin lining is that, ihould the lubrication to any bearing cease for any reason, he bearing will not seize up, with probable damage to conmeting-rod or crankshaft ; the white metal will melt, and he vehicle can be run home without fear of breakage of Ag-end bolts, or exciting the curiosity of the public by reason A any loud knocking. A bus has run on service for six lours, after melting one of these thin linings, without the oissengers being any the wiser. Forced lubrication is a good thing, but manufacturers ihould note that it is not necessary to weaken the crank;haft by drilling 1-inch holes through it to carry oil to the 7arious bearings.
Ignition.
With regard to ignition, low-tension magneto is in great avour, and not without reason. Large numbers of engines ire running satisfactorily with this system only. As a rule, dthough it may seem a paradox, on the question of icmition, t is best to rely on one system only—to out all one's irons n one fire. Where duplicate systems are fitted, it is a great inducement to a lazy man to allow one or both to get into an ineincient condition, so long as he can get along with both systems working together. When one system only is fitted, it must be kept in good condition. It is astonishing what a large number of realty first-class firms are fitting lowtension magnetos of the rotary-shuttle class (Simms-Bosch Type, MR.) to run at crankshaft speed. This does not consume much power, perhaps, but every little helps ; it is not economy to run any machine at double the necessary speed, and to throw away two peaks in every revolution, apart from the increased wear and tear. The reason why the revolvingshuttle type of magneto is used, is because, with that type, there are no revolving electrical connections; it is, therefore, more reliable.—Erel
Ignition tappets should be easily and quickly removable. There is nothing to beat the bayonet socket for this purpose, as used. on the Thornycroft engines. The tappet-block will come away with a quarter-turn of the spanner--a very different thing from the usual two or three it-inch studs with nuts in inaccessible places. Mica plugs are preferable to soapstones, and they should be so oonstructed that the spindle, or sparking-pin, can be renewed without disturbance of the mica washers. The usual meagre bearing for the rocking-lever can, with advantage, be made longer : the excessive wear on these levers in the bearing makes it very difficult to set the timing exactly. Low-tension ignition can be set to fire with a certain, fixed advance, with no detrimental effect and one control lever less for the driver,
About Valves.
Setting of valves has an all-important effect on the power and economy of an engine. To those who have not given the matter any particular attention, the effect of small differences in clearance between valve stem and lifter, qua the points of opening and closing of valves, will be an eyeopener. The diameter of valves is usually o.4 of the cylinder diameter, although some makers lose the full advantage of their valves by not lifting them high enough. For example, take two actual engines, A and B, by different makers :—
A. B.
Valve diameter divided by cylinder diameter... 0.412 0.417 Area of valve opening divided by area of piston o.137 o.o97 Now, in spite of the fact that A has a displacement of only 291.2 cc. per cylinder against B's 395.6 cc. per cylinder, A will perform the same amount of useful work as B, on the same routes, and consume 25 per cent, less petrol in doing it ! Further effects of this insufficient valve lift are dealt with in a later paragraph. An engine turned out by a first-class maker had valvesprings which pulled the valves on to their seats with a pressure of 86 lb, The valve-lifter guides were badly worn oval, in about 7,000 miles. Valve-springs exerting a pressure of 32 lb. were substituted, with no diminution of engine power, and a substantial reduction in wear and tear. These weaker springs can be lifted with one hand, without the use of any tools, and the cotter withdrawn with the other. The powerful springs commonly fitted are responsible for many barked knuckles, and lots of language. A certain omnibus, with an engine of 462.0 cc. displacement per cylinder, runs perfectly cool on service without any fan other than its fan flywheel. This engine takes only four gallons of water to fill its radiator and jackets, and is fitted with large, centrifugal, circulating pump and pipes. One secret of successful cooling appears to be a small quantity of water, rapidly circulated. This engine lifts its valves high enough to take full advantage of its valve diameter.
Engine B takes 14 gallons to fill radiator and jackets, and, in spite of this and a fan behind the radiator in addition to its fan flywheel, will not run as cool as the last-mentioned engine. In addition to engine B's having insufficient valve lift to give a free exhaust, the silencer chokes the engine, which is plainly shown by the exhaust branch getting redhot when climbing hills in low gear, or prolonged runs on the level at high engine speed. An efficient silencer can be made which will not choke the engine to any appreciable extent.
Other Points.
Ilele-Shaw clutches which have been dismantled after 30,000 miles, in omnibus service, have shown no appreciable signs of wear. This speaks for itself.
Carburetters which are simple almost to crudity are giving 38 to 44 ton-miles per gallon, in actual omnibus service, the higher figure being obtained with special parties. Much has been heard of a certain fuel controller, but, whilst giving the inventors all praise for their ingenuity, they would be the first to admit that there must he some inherent defect in a carburetter on which their device will effect a saving of io per cent. to 25 per cent. Where troubled with punctured metal floats, try wood well coated with shellac varnish : these have been in successful operation for the past five months on a particular bus service.
Aluminium is to be avoided in any part of an omnibus chassis. Thirty thousand miles or less is. usually enough to convince one that this metal is not suitable to stand the stresses and vibration inseparable from motorbuses.
A great consideration to a probable purchaser is the standard of screw-threads adopted by the manufacturer of the particular chassis which he contemplates buying. Some of the French manufacturers use standards of their own Making, and it is hardly necessary to mention the inconvenience arising out of this.
The question of ease and cheapness of renewals has a great deal to do with a vehicle's commercial success. That many 'omnibus chassis are " made to sell '' is. painfully evident to an engineer. For instance, the gears in a certain omnibus gear-box are pressed on the shafts and the shaft caulked up on each side to keep them from moving sideways. The bevel pinion is pushed up on a taper, and the shaft riveted or burred over to keep it in place. This is all right until a gear wheel wants renewing, and then the fun commences. In another case, the shaft and bevel pinion are in one ; the direct-drive dogs, second-speed gear and sliding sleeve arc all in one, which means the scrapping of a lot of expensive machine work, for, while the second-speed gear teeth may be worn out, the direct-drive dogs and slid ing sleeve may be perfectly good. The best gear-box consists of gear-rings bolted to permanent flanges on the shaft or sliding sleeves. These rings are comparatively cheap to renew. A direct drive on the gear which does most of the work is a good feature, but, to a large extent in most gearboxes, is counterbalanced by its rendering a double reduction necessary on the other gears.
Brakes should be so constructed that the actual part which forms the brake-block should consist of a rough casting only, requiring no machine work with the exception, perhaps, of drilling two holes; a brake fitted to the second-motion shaft may be very powerful, but it puts a considerable amount of extra wear and tear on the bevel gear.
The Penalty of Speed.
In connection with the actual running, speed is a vital question, for cost of up-keep increases out of all proportion to an increase in speed. There are drivers who will nurse their buses and show an intelligent interest in their welfare, but, for one of these, there are ten who will go " all out " whenever they get a chance, let the road be rough or smooth. For the benefit of these latter men, it is a good thing if the maximum engine speed can be governed to a reasonable amount, and the adjustment sealed up so that it
cannot be tampered with. Foot acceleration, if fitted, should not cut out the governor. A foot control is undoubtedly very useful when driving in traffic, and is a great aid to sweet gear-changing when properly used. Many manufacturers who are fitung engines of 4o to 4511.p. in their omnibus chassis excuse themselves by saying, that a large engine is very useful when it is running a little below par. With a large engine there is every inducement to allow it to get a little below par : drivers have been known to" hop along " for hours on three cylinders. As long as they can keep time, they don't mind. An engine of from 24 to 3oh.p. has to be kept in good condition, if it is to be kept running at all; in fact, everything which goes for economy is attended to, as a sheer necessity. Instead of increasing the size of engine, efforts should be directed towards increasing the efficiency of the small engine and the transmission.