Bearings in a Bentley
IN certain quarters stories have circulated to the effect that that fine car, the 4¼-litre Bentley, is liable to bearing maladies if driven fast for prolonged periods, a matter referred to by Cecil Clutton in his highly controversial article published in MOTOR SPORT last August, so it is worth harking back to our experience of this car when we road-tested it in August, 1938. We took out a Vanden Plas drop-head foursome coupe and averaged 50 m.p.h. with it from London to John O’Groats. To do this the car was driven hard for the whole 700 miles, with only one break of any length, and the makers said nothing whatsoever about keeping the r.p.m. or road speed down. Eighty m.p.h. was held wherever it could be in safety, and the engine was run up to its maximum in third gear after fast corners. In all, over 1,500 miles were covered in this manner and speeds of more than 90 m.p.h. were reached on more than one occasion. No trouble of any sort was experienced, certainly no bearing trouble, and as this run was as stiff a test as any private owner could well impose, we have never suspected the Bentley of suffering from weak big-ends. The big-end bearings in use when we tried the car were die-cast in A.C. 9 aluminium tin and they ran for 48,176 miles in this car, of which 24,980 were those fast Continental miles, after which the engine was taken down and the condition of the bearings was found to be far superior to that of white-metal and lead-bronze linings subjected to identical tests. The shells were almost unmarked, particularly those from Nos. 1, 2, 4 and 5 connecting rods. Various lubricants were used, purchased in the ordinary way, with by-pass filtration. This bearing metal followed Rolls-Royce’s R.R. 56 alloy, and is composed of 5.5-7.0 per cent. tin, 1.5-1.8 per cent. nickel, 0.6-0.9 per cent. copper, 0.7-1.0 per cent. manganese, 0.15-0.3 per cent. silicon, 0.2-0.45 per cent. iron and 88.5-91.3 per cent. aluminium. The specific gravity is about 2.95-3.0 and the coefficient of expansion 0°-150° C., 0.0000225. The heat conductivity is about 36 c.g.s. units, or approximately three times that of steel and five times that of babbit. A.C. 9 has excellent anti-corrosive qualities, and resists oil corrosion, in contrast to cadmium and lead-bronze. It gives a brinell hardness figure of from 45 to 50, and after heat treatment for twelve hours at 180° C., this figure rises to between 60 and 75.
A Solution to a Problem?
One of the big worries that faced the supercharger people when forced induction began to be considered not unreasonable for ordinary cars, was to obtain a boost sufficient to give a useful increase in acceleration at low engine speeds, while not so excessive as to blow the engine up when it really got going. Boost pressures could be controlled by the size of the blower and its speed relationship to that of the crankshaft, but a little blower had to run excessively fast to produce any worthwhile blow low down, and a bigger blower, geared to avoid excessive pressures, ran too slowly to be much good low down the speed range. What was wanted was a variable speed device between engine and blower so that the blower speed changed in inverse proportion, or thereabouts, to that of the engine, and that was a thing outside the realms of practical engineering. Mercédès-Benz had early decided to use their famous clutch operation, which only permitted the boost to come in at big throttle openings, and therefore for short periods only under ordinary conditions, though still enabling the supercharge to be employed to improve either acceleration or maximum speed, at will. On the same principle Stutz and Atalanta have used hand-controlled clutches for the supercharger drive. McEvoy and Pomeroy used other tactics, believing in partially meeting the problem by using vane-compressors, which, giving their maximum boost at lower speed than the Roots blower, could be higher geared to prevent excessive pressures at the upper end of the engine speed range. Even so, they experimented with the McEvoy-Pomeroy piston boost control, reminiscent of the simple by-pass used in a few of the very early supercharged engines, and just before he forsook the spanner for the pen, Pomeroy re-attacked the problem with his Velox variable-vane compressor. Nowadays, the increased efficiency of blowers and compressors provides a partial solution of the difficulty, because improved output at low speeds enables the ratio between engine and supercharger speed to be kept small. Nevertheless, the problem is far from solved, and the commercial flop of supercharging for ordinary cars can largely be laid at its door, for the early troubles associated with the supercharger as a component have been overcome and only its detrimental effect on the engine, weighed against the amount of useful performance increase offered in return for its cost of £25–£65, stands against it. Apparently any form of diaphragm control of induction pressure via an oil servo acting on the carburetter throttle, is impractical for car engines. But it seems possible that the supercharger drive of the Daimler-Benz DB 601A aero-motor, as examined in shot-down Messerschmitt ME 109 and certain Heinkel He 111 K enemy aeroplanes, holds the key. In this engine the supercharge is driven through an hydraulic coupling on the lines of the Daimler fluid flywheel. The coupling has oil-filled rotors and a degree of slip can be introduced by varying the oil-flow to the coupling. An altitude capsule controls the flow from the delivery pump and the drive does not become solid until the rated height of 6,000 m. (19,685 ft.) is reached, but the amount of “slip” is progressively decreased as this height is approached. In an air-battle an unlucky shot resulting in an oil leak would mean a serious loss of boost, but for car supercharged installations the system would seem to be of considerable interest, the method being reversed, so that the coupling drove the blower at full speed at low engine speeds, the oil flow being diminished automatically as engine speed rose, by a centrifugal governor or similar means, so that an excessive supercharge would never be reached.
In the old sane days, Christmas was a time when lots of happy, long-distance, rather hurried journeys were necessary to enable family reunions to happen, and the sports-car and the skilled driver were in their element. Those persons who have known many such runs will be content to make the best of things this coming Christmas, perhaps even to spend the holidays working in the national interest, if a long run home is out of the question. But they will not grudge the younger generation the pleasure that good motoring with a purpose, under the wintry conditions of the good “old-fashioned” Christmas, can provide, in cases where this is possible. No doubt, by combining November basic rations with those for December, and even January if need be, quite a lot of young sportsmen and sportsgirls will contrive to motor big mileages at this exciting time of the year. In case the next issue of MOTOR SPORT gets held up through unforeseen circumstances, we take this opportunity of wishing them good motoring during the festive week— may every seat be occupied by a cheerful enthusiast; and may the run enable them to forget temporarily that we can no longer argue whether the world was a better place in Dickens’s time than it is to-day with any sincerity . . . .