The Avoidance of Smoke.
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By Douglas Mackenzie.
An explanation was given, in my article of last week, of the improvements made in motor omnibuses with a view to the removal of all reasonable objection to their use on the score of noise. It was shown, that motor-omnibus engineers had taken full advantage of the eighteen months' lessons now available, and that they had, enormously, improved their chassis, it is interesting to note how the accumulated experience has been utilised in the avoidance of smoky vapour, the second cause of complaint, and one which is so ruthlessly exploited by the enemies of " the poor man's motor." The smoke emitted by Motor vehiclescannot arise from a number of causes, as is the case with noise, but is, invariably, due to the presence of burnt lubri.. eating oil in the exhaust gases. The scat of the trouble is, therefore, apparent at once, but the remedy is far from easy. There is a great
RISK THAT THE BEARINGS MAY SEIZE,
if the supply of lubricating oil is reduced, and this would be a worse evil than the showing of a little smoke; in fact, all drivers of motor vehicles would far rather err on the side of an excess of lubricant than the reverse.
The lubrication of all engines, and machinery, was effected, previous to the year 188o, or thereabouts, by the filling of a cup, on each bearing, at regular intervals of time. These cups were, occasionally, fitted with wick syphons, so that the oil passed slowly from the cup to the hearing, through a turn of worsted; in another pattern, the hole at the bottom of the cup was nearly filled by a needle, so that the lubricant could only find its way out very slowly. The oil was allowed to work through the bearing, and then to make its way out along the shaft, or down the pedestal, and, so, to run to waste. Such methods as these were the reverse of economical, and, in addition, required constant manual attention, so that really high-speed engines were impossible until something better was devised. A " something better " was employed by Winans, in the Winans and Robinson high-speed steam engine, and the system was termed splash lubrication. All the motor-work of the engine was enclosed, and the crank chamber was half-filled with oil. The cranks, and big-ends of the connecting rods, struck the oil at every revolution, and splashed it about the casing so thoroughly that everything was, practically, working in
AN " ATMOSPHERE OF OIL.
The two details that required careful attention, were the provision of cups to catch the flying oil, to lead it into the oil-holes, and efficient oil-ways to allow of its travelling all along the bearing, or journal. The oil that, eventually, worked through a bearing found its way out, and dropped into the casing, from whence it was, in due course, splashed into some other bearing, so that waste was reduced to a minimum.
Splash lubrication opened up quite a new field in machine construction, for, not only were high speeds rendered practicable, but the need for skilled attention was removed. Cases are known where Winans' engines have run, continuously, night and day, for as long as six months, the engine-house being unlocked only once a week for the fireman to put an extra quart of oil into the engine casing, in order to replace that lost by friction, evaporation, and fractional leakages. It is safe to say that splash lubrication made applications of the modern petrol engine possible, for, certainly, speeds of a thousand revolutions per minute would have been impossible without it. Different forms of splash lubrication are, now, used for every important moving part of many chassis : engine, gear box, countershaft, live axle, and even for the steering segment, although a higher level, and a stiffer oil, or grease, is employed where the distributing effect of the crank is absent.
This is an age of progress, and white motors have been developed from the early 1.111.p. tricycle to the 40114p. motor omnibus, the science of lubrication has advanced independently. Careful observation has proved that the correct point to introduce oil to a bearing is at the
LINE OF MINIMUM PRESSURE.
This point has been calculated for all revolving shafts, big-ends, gudgeon pins, etc., and it seldom proves to be at the top of the bearing. In the best practice, the oil is led to oil-ways at this line of minimum pressure, whence it is carried round by the shaft, and performs its proper function. A scientific study has been made of various oils, and of their suitability for different classes of bearings, so that an engineer is, now, in possession of data showing the best oil for any particular purpose. An improved system of distributing oil to bearings is, to-day, coming into more general use, and this is termed " forced lubrication." It is, perhaps, descending to very elementary principles to explain that lubrication consists in keeping a film of oil between the fixed and moving surfaces, so that fluid friction is substituted for solid friction. Water would be applicable to this purpose, could it be kept between the fixed and moving surfaces, but the peculiar clinging, or viscous, property of oils, which is, colloquially, described as their "greasy " feeling, causes a film to adhere to both surfaces, and to separate them to a minute degree. Great pressures have the effect of squeezing much of the oil out of the bearings, and the pressure may even he sufficient to expel the film completely, causing a metallic contact, heating, and, perhaps, seizure. There have been special cases in which the length of a bearing had to be reduced unduly,
FROM CONSIDERATIONS OF SPACE,
and this, at once, necessitated an improved method of lubrication, as the shortening of the bearing meant an increase of pressure on the available frictional area, and it would have been impossible to have kept the rubbing surfaces lubricated had any ordinary system been used. The newer method, as previously mentioned, is called " forced lubrication," and consists in forcing the oil into the bearing by a pump, and so keeping the journal floating in oil The oil pump distributes the oil, through separate pipes, to the different bearings, and, as there is, naturally, considerable pressure upon the oil, it soon finds its way out from the ends of each bearing, and drains to the bottom of the crank chamber, where, if it were not immediately sucked up by the pump and again circulated to the bearings, it would collect. This system was patented in 1896, and was applied, first, to high-speed steam engines; its complete success opened a further field in machine construction. It was seen, quickly, that forced lebrication in an internal-combustion engine would prevent seized bear ings, and the Delaunay-Belleville engine (Fig. 1), which came out at the latter end of 1904, was fitted with this system* instead of splash lubrication. The base chamber was .so shaped, that the oil drained to the back end, where it was taken up by a specially-designed pu.mp, which delivered it to each bearing of the crankshaft. The crankshaft was made hollow, throughout its length, the oil-passage extending up the webs, and through the crank pins. A large annular groove, which formed an oil reservoir, was turned in each bearing brass, and these annular grooves communicated with the interior of the hollow crankshaft by holes drilled radially in the journals. The oil was, thus, forced into the hollow crankshaft, and other ducts, in the crankpin journals, allowed the oil to escape from the centre of the crank pins to the annular grooves turned in. the centre of the big-end bushes. "Ishe connecting rods were also hollow, the central passage communicating with these annular grooves in the big-end bushes, so that the oil was forced up the connecting rods to the gudgeon pins, and past the pins on to the cylinder walls. The designers had, therefore, provided for the lubrication of every bearing throughout the engine, by oil supplied under pressure, and there is no doubt that the system left nothing to be desired.
Now as to the connection between lubrication and smoke. The pistons are kept tight within the cylinders by two, or more, piston rings, and it is necessary, in order to prevent the products of combustion from passing the rings, that considerable pressure should exist between the rings and the cylinder walls, though the friction must be minimised by keeping a film of oil upon the cylinder walls. This requirement was easy with splash lubrication, because there was so much oil flung about that some of it was sure to adhere to the lower part of the trunk piston, when this protruded at the bottom of the stroke, and, from the piston, to the cylinder walls when the piston was at the top of its stroke. In point of fact, too much oil, not infrequently, lodged on the cylinder walls : this excess was
BURNT ON THE FIRING STRONE,
with the result that it loaded the exhaust with decomposed lubricating oil. Some of the resulting products, being only partially oxidised, irritated the olfactory nerves, and others offended the vision, as smoke. The burnt oil was lost to the engine, and the deficiency had, constantly, to be made up by a drip-feed or other oil supply, whilst the driver was always in the unhappy position of knowing that, although slightly too much oil meant smoke, too little, probably, meant a seized big-end bearing. The driver would, naturally, choose the lesser of the two evils, and the result was a more, or less, constant emission of smoke.
The introduction of forced lubrication promised a reliable solution of the above difficulty, because, with it, there is no longer a quantity of oil flying about inside the engine casing, as the oil is drained away, and re-circulated through the bearings. Experience showed that it was unnecessary to provide special means for oiling the gudgeonpins, and pistons, as, even with forced lubrication, there was still sufficient oil flying about to meet this purpose, and
DOI.LOW CONNECTING RODS
and gudgeon-pins were not essential.
I mentioned, in the article quoted at the outset, the many detail improvements made by Mr. R. Bell, the engineerin-chief of the London Motor Omnibus Company. He has carried these modifications to the point which enables him to make his own engines complete, and to fit them to Milnes-Daimler and Scheibler chassis, as the original engines, with splash lubrication, are removed for extensive overhaul. Fig. 2 is from a photograph of one of these L.M.O. engines, and Fig. 3 is from a drawing of the crankshaft, showing the ducts drilled for forced lubrication : the former shows the ribbed aluminium casting (A), which forms the sump on the base chamber, to which the oil drains, and the Albany pump (B) that forces the oil to each of the three main bearings. It will be noted that the pump spindle is vertical; it is driven by skew gearing at the end of the adjoining camshaft, where it is concealed behind the brass casing (C). The timing wheels on the Milnes-Daimler engines are not encased, but the L.M.O. Company encloses these wheels in an aluminium casing (D), which receives the oil that is forced out of the forward crankshaft bearing, the surplus finding its way back into the base chamber. These gear wheels are, thus, running in oil, which is kept in constant circulation. It can be seen that the oil-ducts lead through the crankshaft from both the end and centre bearings, and that the crank pins are drilled out to twice the diameter of the oil ducts, so as to form eflicient oil reservoirs. The main bearing bushes are, also, shown in longitudinal and cross sections. The
GROOVES ARE NOT OPPOSITE
to one another; they correspond with the ends of the slots in the journals, so that the oil is distributed, along the length of each journal, by the clearance space at the edges of the two half-bushes. The fact that the grooves are not opposite to one another ensures more even wear than with a plain annular groove. The company is applying this method of forced lubrication to the 1925 and 1906 models of Milnes-Daimler engines, as well as to its own engines. The engineers are satisfied with the effective lubrication resulting from this system, and are certain that they have, practically, abolished smoke from the exhaust of these particular omnibuses. An equally valuable feature of the improvement has been the saving in lubricating oil. It stands to reason, that oil must be saved if it is no longer burnt away, and, in fact, omnibuses which, before the conver
sion, used four gallons per day, require, now, only six pints for 7oo miles.
It is probable that a little further experience of forced lubrication will prove it to be so superior to others, that it will become almost universal on motor omnibuses. The ever-advancing progress of scientific invention may bring forward a better system, but forced lubrication, which has been adopted on the new 1907 Milnes-Daimler omnibuses, the row Leyland omnibuses, and others, will pre:dominate at the March and April shows.
The system on the 1907 Milees-Daimler chassis,
the first notice of which appeared in "THE COMMERCIAL. MOTOR" of October r8th, 1906, is very similar to that on. the L.M.O. engines, but the circulation is maintained by a. force pump, of 20MM. stroke, which is driven direct, by a short crank, from the end of the camshaft. There are, of course, still. a few people who grumble about smoke and smell, even from cars with forced lubrication ; but the great economy in oil consumption, which has been quoted, proves that smoke 10 can hardly be possible, and, unquestionably, cannot be emitted in sufficient quantity to ben nuisance_