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Dangers in Oversimplifying
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Power-to-weight By John F. Moon,
IT WOULD appear that Britain is fast reaching the point where its Ministry of Transport will introduce a form of minimum tower-to-weight ratio requirement for commercial vehicles, irobably allied with plating and certain braking requirements. lie plating and braking measures will obviously be in the lirect interests of road safety, but the minimum power requirenent also has a safety aspect in that, if it can lead to heavy ommercial vehicles having faster acceleration and hill-climbing bilities, and sufficient power to maintain a high cruising speed wer indifferent roads, such vehicles will provide less obstruction o the free flow of lighter traffic, so reducing frustration and the ikelihood of accidents occurring through impatient drivers ivertaking when conditions are dangerous.
On the face of it, therefore, a minimum power-to-weight ratio
seems highly desirable; hut is it really going to achieve the object of helping to unclutter British roads? I don't think it will if the ratio is set as low as 6 b.h.p. per ton of gross train or solo weight; it might if a figure of, say, 10 b.h.p. per ton is decided upon, It is all very well to say that 6 b.h.p. per ton seems to be working out all right in Germany, but there are various factors applicable in Germany which are not readily apparent to the uninitiated British observer. Furthermore, many engineers and senior officials in Germany are thinking seriously in terms of 10 b.h.p. per ton in order to overcome certain disadvantages of the present legal minimum power-to-weight ratio requirement.
To begin with, German vehicle manufacturers almost invariably gear their vehicles for a maximum speed of just over 50 m.p.h. so that a cruising speed of 50 m.p.h. can be maintained on long Autobahn runs, this figure being the legal limit on such roads. This means that the effective tractive effort at the rear wheels is fairly constant, irresPective of the manufacturer, when a power-to-weight ratio of 6 b.h.p. per ton is observed; so, all vehicles have similar performances, in the main, There is no point in the Germans gearing for 70 m.p.h.—with subsequent reduction in acceleration and hill-climbing performances—if such speeds are not permitted anywhere in the country and the police authorities make sure that drivers conform to the limits.
Another point is that most German heavy-vehicle engines have a governed speed of no less than 2,200 r.p.m. So, although the general rule is to match these engines to six-speed gearboxes and single-speed axles, the engine rev range is sufficient to give good acceleration performances, German drivers make full use of the available engine revolutions in the intermediate gears. The result is that top-gear lugging is rarely encountered in that country, as a driver rarely engages top gear before his engine is doing at least 2,000 r.p.m. in the next lowest ratio.
Despite all this, German heavies off the main highways still crawl up steep hills; and, even though they might be 1 or 2 m.p.h. faster than British maximum-capacity machines up the same sorts of gradient, they still can constitute a traffic block. On Autobahns, however, most German heavies appear to have better gradient performances than typical British heavies. Even so, on really steep Autobahn sections, commercial vehicles have to make their ascent in a special third lane so as not to interfere with the passage of lighter, more highly powered vehicles.
All in all, therefore, the German system is by no means a complete success, though it has introduced a certain amount of uniformity in vehicle performance characteristics. The Italians have, to my mind, a more logical approach to overcoming the problem of dangerously slow commercial vehicles. Their Road Code stipulates that vehicles must be able to ascend a 1 per cent gradient (I in 100) in top gear at a speed corresponding to an engine Speed within 10 per cent of that at which maximum power is developed.
Where it is not practicable to check this gradient performance, the regulations statethat the conditions can be reproduced by accelerating in top gear at a rate of 0-328 ft./sec.2: this rate is equivalent to taking about 90 seconds to accelerate from 10 to 30 or from 20 to 40 m.p.h., and whilst this does not sound particularly fast, it must be remembered that the Italian regulations call for the use of top gear (which in most cases would be overdrive) and that the acceleration rate has to be maintained between the engine speeds at which maximum torque and maximum power are delivered. Another important consideration is that heavy-goods vehicles in Italy are legally restricted to 60 km/h (37-5 m.p.h.), so this in itself suggests that the vehicles are going to have lower final-drive gearing than British heaVies. There is also the natural temperament of Italian drivers to consider, which in itself probably endows hill-climbing vehicles with an extra 5 m.p.h.!
To return to the basic power-to-weight question, some figures obtained in France by the commercial-vehicle engineer Pierre Lenoir make interesting reading as to the positive effects of power-to-weight ratio changes. M. Lenoir's figures were taken over a 75-mile route, half of which was hilly and half almost flat, and the vehicle used had a six-cylinder diesel engine with a gross output of 180 b.h.p. Tests were made at differing gross weights. With a ratio of 22 h.p. per ton, the average speed was 70 per cent of the vehicle's maximum speed, whilst only 40 gear changes were made When the ratio was 6-87 b.h.p. per ton, the average speed was 65 per cent of maximum but the number of gear changes increased to 120. At 35 tons gross train weight, giving a ratio of 5 b.h.p. per ton, the average speed was 60 per cent of maximum and the number of gear changes rose to 140.
Presumably no alterations were made to the gearing of this vehicle between tests, and the picture could have been different had such changes been carried out. As it was, the difference in average speed between the tests made with the highest and lowest power-to-weight ratios was surprisingly small—had the vehicle's maximum speed been 50 m.p.h., it would have meant a drop in average speed from 35 to 30 m.p.h.—but the amount of gear changing required was three-and-a-half times as much. In France, incidentally, use of a hydraulic coupling with a four-speed transmission has been found to reduce the amount of gear changing, whilst fuel consumption has increased by only about 1 per cent.
This French exercise was, of course, carried out with a fairly high-powered vehicle. When power-to-weight ratio changes result from increasing gross weight without altering engine size, as when taking a standard solo vehicle and making it into a tractive unit for use at at least 50 per cent above its designed solo weight, the performance picture comes much blacker.
A good example of this concerns two versions of a mass production vehicle which I tested. Running as a solo maehin at just under 11 tons g.v.w., and with a power-to-weight ratio e 9-6 b.h.p. per ton, this vehicle reached 30 m.p.h. from a stand still in 24-5 seconds. A tractive-unit version of (mechanically the same vehicle was subsequently tested at 18-25 tons, and 2 this weight 82 seconds were required to reach 30 m.p.h. fror rest. Here I would like to have quoted the comparative direct drive acceleration figures, but my tests showed that the articu lated outfit had no measurable top-gear performance despite theoretical power-to-weight ratio of 5-75 b.h.p. per ton. Bot vehicles, incidentally, had the same overall gear ratios, the onl significant difference at all being with respect to the gros weights.
So much for the value of a specific legal power-to-weigh ratio requirement. The ratio of this particular articulatevehicle was almost exactly the same as that of a 24-ton rigi eight-wheeler tested a few years ago with a 140 b.h.p. engin( and this vehicle took 64 seconds to reach 30 m.p.h. from re: and 70 seconds to get to 30 m.p.h. from 10 m.p.h. in top geal Going only slightly higher up the scale, the Gardner 6LX diem is normally used in 24-tonners at a ratio of 6.25 b.h.p. per to gross weight, and with the gear ratios commonly employed th 0-30 m.p.h. acceleration time averages 48 seconds, the same tim being required for the direct-drive 10-30 m.p.h. test. A typiQ German 32-ton-gross outfit with a power-to-weight ratio c 6-55 b.h.p. per gross ton can reach 30 m.p.h. from a standsti in 46 seconds.
This tends to indicate that, where big-engined heavy vehicle are concerned, the theoretical power-to-weight ratio does hay a more easily calculated effect on performance; but as engin size and gross weight are reduced the relationship become inconsistent. Still on the subject of heavies, doubling th power-to-weight ratio can double the acceleration rate also. I the case of the German vehicle referred to just now, tests mad at 16 tons (ratio 13-1 b.h.p. per gross ton) showed a 0-3 acceleration time of 24-25 seconds.
Now this raises a further thought, because this acceleratio time is just about the same as that recorded with the 11-tor gross vehicle with a ratio of 9-6 b.h.p. per ton. Doubling th gross weight of the German vehicle merely halved its accelera tion rate, yet increasing the weight of the 11-tonner by less tha 70 per cent. more than trebled the time needed to reach 3 m.p.h. from rest. Thus small engines cannot logically be dea with in the same way as big engines when it comes to definin a minimum legal road performance.
What is the answer, then? To my mind, part of the troubl lies in the absence of a speed limit on motorways, this encoural ing operators to demand (and manufacturers to build) high geared vehicles with an excessive top speed and poor gradier and acceleration performances. If a speed limit of 50 m.p.1 for heavy vehicles could be applied and enforced on motorway: this zest for speed would be damped, and designers could stai their calculations from a maximum theoretical speed of ahot 55 m.p.h. and gear their products accordingly.
I would then suggest that a minimum top-gear acceleratio performance for all vehicles be demanded by law, and thes two restrictions would make it essential to co-relate engin b.h.p. and torque outputs, engine governed speet gearbox ratios and final-drive ratios. Coupled wit more severe penalties and a closer watch on exhau: smoking and overloading, I feel that such measure would answer many of the public criticisms abot British heavy goods transport. One final point: such regulations were to be introduced, they shoul apply to all vehicles, both old and new, and irre spective of in what part of the country the vehicle were to be operated. Thus all models of a certai type would have an equivalent performance, an old vehicles would have to be plated at a reduce gross vehicle weight if they could not meet th acceleration requirements.