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VEHICLE EFFICIE\C
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0. Is the Ultimate in Sight?
HOW often has one heard the expression, "a very efficient vehicle," when reference is made to any up-to-date transport chassis? Immediately, thoughts of efficiencies of 90 to 95 per cent. are conjured up, for the overall ratio of power output to power input. Yet how wide of the mark such estimates are.
• If one considers the energy stored in a gallon of petrol or oil file], and makes a calculation showing the possi bilities of turning that stored energy into useful . work, the lowness of the overall efficiency of the "machine "— a petrol or oil-engined chassis—would astound even experienced operators. When it is realized that the loss, • caused solely by the heat dissipated through the medium • of the radiator, may amount to 50 per. cent. or more, besides the pumping and frictional losses which mount up to a considerable figure, the position will be better understood.
It should be stated here, however, that the study of . thermal efficiencies is so extensive as to warrant a. . separate article. Accordingly, our investigations in this
• review are to be confined to considerations of mechanical efficiencies, making particular reference to the strides that are being made towards an improved ratio between the power available at the road wheels and the power obtainable from a given cylinder capacity.
Broadly speaking, performance may ba rated in terms of ton-m.p.g., but this is a loose equation favouring large vehicles. Apart from dimensional considerations, such as the size of engine used for a given chassis, taking into account its load capacity, there is a point often overlooked, ' namely, that the largest vehicles-10-ton to 14-ton sixand eight-wheelers—are usually driven at a relatively low speed of a more or less constant nature over long distances.
Legislation regarding speed has, . indirectly, had a considerable influence upon the normal rate of revolution of such engines, and it so happens that these operate, for a big percentage of the total mileage, at speeds around the point of innumum fuel consumption.
Lighter chassis, used for general purposes in short . distance haulage, delivery work and so forth, usually make a, great number •of stops during the day, with a consequent deleterious effect upon efficiency. It is for this reason that the ton-mpg. figures are not being ' taken into consideration.
Considerations of Engine Efficiency.
• The efficiency of the engine can be considered in two ways: in terms of b.h.p. per litre of piston-swept volume, or of the power obtained fcir a given weight of fuel. Both are important, whilst from an operator's point of view They are intimately connected by a third factor—first cost. .
. For relatively heavy power plants, that is, units installed in vehicles of 5 tons capacity and over, there can be no question but that for an. equal first cost the oil engine wins, and even allowing for a substantially higher purchase price; the compression-ignition unit still shows to advantage.
c32 The b.m.e.p. of the average oil engine is not so high as that of the equivalent petrol engine,consequently, for a given performance, the piston-swept volume of the former has to be greater than that of the 'latter, and although the disparity between the two types is gradually becoming smaller, the difference cannot be ignored. On the other hand, fuel consumption based on the b.h.p.-hour principle gives the oiler a distinct advantage.
It is interesting to make a comparison between the power units of 10 years ago and those of to-day. Petrol engines of 1925-1926 vintage seldom give a better fuelconsumption figure than 0.7 lb.-per-b.h.p.-hour, whereas to-day fuel consumption of 0.6 lb.-per-b.h.p.-hour is quite normal in engines suitable for road transport. The consumption of fuel of certain aeroplane engines is now around 0.5 lb,-per-b.h.p.-hour.
The Advantage of the Oil Engine.
For the compression-ignition engine. 0.4 lb. of fuel per-b.h.p.-hour was a good figure five years ago and is still considered satisfactory, although recently figures around 0.37 lb.-per-b.h.p.-hour have been attained.
Calculated on the other basis of efficiency—b.h.p.-perlitre—a similar improvement in petrol engines is to be noted. Ten years ago an average figure of 14 b.h.p.-perlitre would have been representative, but to-day 18 b.h.p.per-litre can be adopted as a basis for comparison. Naturally, there are wide variations between the average and the best possible power-to-volume ratios. For example, commercial aero engines are in existence from which a power output as high as 35 b.h.p.-per-litre has been obtained. Again quoting aero engines, the output per litre of a compression-ignition unit is about half that of a petrol unit.
A. representative figure for thermal efficiency in modern engines suitable for commercial chassis is approximately 23 per cent. This compares favourably with engines of a decade ago when a figure of 18 per cent. was considered to be reasonably good.
In a water-cooled engine, the' boiling point of the cooling medium is the deciding factor for the general wall temperature of the combustion chambers, but even so, there are hot spots, notably around the exhaust valves, where metals having a higher heat conductivity than cast-iron would allow increased temperatures to occur without risk of 'fracture of the material or of its distorting.
The heat lost is also intimately connected with the area of the combustion chamber and cylinder walls, and it requires little mathematical knowledge to realize that the minimum superficial area is produced by a hemispherical combustion chamber with the valves following the general contour of the head.
• The higher turbulence prevailing in a compact corn
• bustion chamber, as compared with a pocketed type. promotes rapid burning of the mixture with an attendant decrease in the tendency towards pinking, so that -a higher compression ratio can be utilized with a still further increase in b.h.p. per unit of volume.
That is one side of the picture. The other side • requires a rather different analysis,, for it is concerned c33
20%
OF rutL vALL/f chiefly with the mechanical losses occurring in a chassis. If one considers the auxiliaries in an engine with their attendant drives and shaft bearings, there is still a considerable drop from the b.h.p. available in the cylinders to the b.h.p. available at the flywheel.
As a typical example, one may quote a loss of as amebas 3 b.h.p. for the air fan alone, whilst a 12-volt 50-amp. dynamo (such as is used on double-deck buses) having an efficiency of about 55 to 60 per cent. Would take at least b.h.p. to drive, in addition to a slight loss due to the bearing and tooth friction of the driving media. A water, pump and an oil pump can easily account for 1 b.h.p.1 between them—more in certain instances—betides considerable loss dun to crankshaft-bearing friction, piston friction and air pumping action.
The efficiencies of the transmission from the engine to the road wheels are much easier to compute, because they involve purely mechanical considerations and specialist manufacturers have made careful analyses of the conditions obtaining in such items as bearings, shafts and gearing of all types, and of the losses due to mal-alignment caused by overloading. In a normal type of friction clutch there is, of course, a heavy loss during the period of slipping, but, when once the friction surfaces arc fully engaged, the drive virtually becomes solid, so that apart from a drop in efficiency of, perhaps, 1 per cent., the full flywheel power is available at the gearbox input .shaft. Now, in top gear, there is only one loss that is at all serious between the input and the Stern-wheeI shaft, oil churning being responsible for a drop of about 4 per cent. With an indirect gear engaged, however, there is probably a toss of about. 1 per cent., through churning and 1i to 2 per cent, from each driving train. It will be seen, therefore, that at the universal joint behind the gearbox there is available about 94 per cent, of the power at the flywheel.
A Loss for the Sake of Silence.
There is something of an anomalous position with regard to gearbox efficiencies, for, despite much more accurate manufacturing methods, ground teeth and so forth, the input-output ratio is not now so high as in the past. This is entirely due, however, to the quest for silence, large teeth being employed to enable more accurate profiles for the driving and driven elements to be .obtained.
The efficiency of the gear train itself is not markedly different from the alder type of box in which pinions having fine-pitch teeth were used, but the oil-churning losses have increased in almost direct proportion to the depth of the teeth—hence the drop in overall efficiency. There can be no doubt about the frictionless characteristics of a normal propeller-shaft layout, for hall or roller bearings are used throughout and even the crosspins Of the universal joints are usually mounted on needleroller bearings. It is doubtful Whether a drop of 1 per cent. would be shown even on a long chassis with a divided shaft.
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In the iear axle, however, matters are much more complicated, because there is such a wide variance of speed and load so, that to obtain a really accurate measure of loss for all conditions of service becomes difficult.
There are, at present, two types of final reduction gearing used in commercial chassis—worm and bevel. The efficiencies, considering the relatively heavy loading of the reduction gearing, are surprisingly high, typical figures for good worm gears being from 95 to 96.5 per cent., the higher efficiency being obtained when speed and power are greatest.
An Exceptional Figure.
Ten years ago an overall efficiency of 97 per cent was recorded during tests conducted at theNational Physical Laboratory, but such a result, although practicable, is . academic in a certain measure, as precautions were taken to keep the oil-churning losses as low as possible. The efficiencies mentioned as being representative td-day include churning losses such as might be experienced in a rear, axle of normal design.
:The losses in spiral-bevel gearing are much the same, being about 4 to 5 per cent. with a normal layout, but where an exceedingly small pinion is called for the efficiency would not be much greater than 934 per cent.
While dealing with the subject of rear-axle design, the following facts may prove of interest. Improved manufacturing methods developed during the past five years or so have permitted worm gearing to be more heavily loaded than was the case, say, 10 years ago, without any sacrifice of efficiency. For example, 12 years ago a worm and wormwheel layout having 7-in, centres was the usual size for a 30-cwt. lorry of air tons gross laden weight. Nowadays, similar centres are employed on a double-deck-bus chassis, weighing, fully laden, 9A tons.
Rigidity of mounting for the worm wheel is of paramount importance. An increase of over 50 per cent, in the loading factor has been found possible after strengthening the differential housing and disposing the bearings in such a -manner that side deflection of the worm wheel is avoided. Furthermore, the material from which the worm wheel is manufactured is also a matter of moment. .Another 50-percent. increase in load-carrying capacity became available when worm wheels were machined from centrifugally cast bronze as distinct from chill-cast material of the same composition. A recent development in tooth form has tended to give an increased length of line contact between the worm and the teeth on the wheel.
For all practical purposes, losses in differential gears may be ignored, because on a straight road these are inoperative. The wheel bearings, however, are not entirely frictionless, 'so that one might expect 'a loss of, perhaps, -1 per cent. on top of the reduction-gear loss.
The position may be summarized in the following, manner,. Allowing for a dynamo, an air fan and the usual engine auxiliaries, the efficiency at the flywheel would probably work out around 22 per cent. in a unit of 23-percent. thermal efficiency. At the stem-wheel shaft of the gearbox, when using top gear, the mechanical loss would not nowadays -amount to more than about 1 per cent. of the heat value Of the .fuel, whilst at the worm shaft in the rear axle it is doubtful whether another 1 per cent:" of loss would he recorded on a well-designed and accurately
manufactured. chassis. . . , In other wOrds, fundamentals cut down the thermal
efficiency of the fuel to 23 per cent., hut the mechanical losses in the transmission, together with the power absorbed in driving the auxiliaries, do not reduce the figure appreciably—actually from 23 to about 20 per cent. The figure is, of course, somewhat lower when hill-climbing with the drive passing through a secondary train in the gearbox.
What of the future? Our investigations have shown that . production methods ensuring extreme accuracy have " contributed in no small measure towards attainment of highly creditable figures for mechanical efficiency. To enhance these is not insuperably difficult, although the margin of permissible improvement is small. The figures quoted are, of course, estimates from known data, and good conditions of operation are presumed. '
In practice, many chassis will fall short of the percentages shown and there is, therefore, more scope for improvement in the maintenance of high initial efficiencies' than in enhancing the efficienCies.,themselves. Lightly loaded bearings, robust—although not necessarily-heavy-shafts, housings ensuring correct alignment under all conditions of service and so forth will help to keep a chassis in its pristine condition,