Some Problems of the Motorcar.
Page 2
If you've noticed an error in this article please click here to report it so we can fix it.
Mr. Dugald Clerk, F.R.S., NE I net.C.E., in his presidential address delivered before the members of the Incorporated Institution of Automobile Engineers, at the meeting which was held on Wednesday evening last, the 14th instant, at the Institution of Mechanical Engineers, Storey's Gate, S.W., before proceeding to consider the subject which he had chosen for his address, reviewed the work of the Institution, and the progress which it had made during the past year. He stated that the Institution had now 342 members, associate members, associates and graduates, and that this figure showed an increase of see per cent, on the number as it stood at the beginning of this year. The financial position of the Institution, he also stated, was perfectly sound, and there was every reason to believe that members might look forward with confidence to a career of continuallyincreasingusefulness and importance.
The President then related sonic of the experiences of the Automobile Club's doe miles trial from Glasgow to London, in teat, and the subsequent trials at the Crystal Palace in 1903, and compared the modern motorcar with the crude vehicles which then competed. Amongst the constructional details that had undergone considerable improvement, but which still required the attention of the designer, was the clutch. On this detail, Mr. Clerk made the following statement — " It has long seemed to me possible to design a pneumatic clutch connecting the engine and the driven shaft, so that no violent shocks could reach the transmitting mechanism. Such a clutch would consist essentially of two or more cylinders mounted on the end of the driven shaft and rotating with it, and a crank on the end of the engine shaft connected to two or more pistons working in the rotating cylinders. The pistons would compress the air in the cylinders until the necessary driving torque was communicated to the driven shaft, when pistons and cylinders would rotate without further relative movement. To de-clutch it would only be necessary to open a valve controlling the cylinders, when the engine would be at once freed from the car. Such a contrivance would be easily adjustab'e 10 apply any desired torque without shock."
The interposition of an elastic air cushion between the driving and driven members was not novel, said the President, but he was not aware that it had ever been successfully applied ; if such a thing could economically be produced, it would greatly diminish the wear and tear on all the parts of the transmission system.
Change-speed gears were then reviewed by Mr. Clerk, who holds the opinion that greater and more lasting benefit would follow if more attention were paid to problems of engine design such as might ultimately lead to the removal of all necessity for any mechanical means of altering the speed of a motor vehicle. Some inventors, he stated, employed accumulators, the current from which was used to drive an electric tOol or, and by this means augment the torque of the crankshaft (the motor acting as a dynamo, and recharging the accumulators when the full power of the engine was not required for driving the vehicle); others sought to attain the same ends by fitting air pumps which charge reservoirs, and which, in turn, are used as pneumatic engines ; and still another group of inventors essayed to increase the torque by providing air pumps for supplying the air and petrol to the engine at a pressure above then of the atmosphere. Amongst the latter class are Messrs. Daimler, Dawson, and O'Gorman. After commenting on the work of these gentlemen, Mr. Clerk said :— "I have made experiments with super-compression devices of different kinds, which convince me that it is possible to arrange for the increase of the mean pressure upon the piston at the slower speeds of rotation by about fifty per cent, without increase of maximum explosion pressure. Tn do this it is necessary not only to increase the charge supply pressure as proposed by Daimler, but also to increase the volume of the compression space. By the simultaneous increase of charge pressure and compression space volume, torque is greatly increased, although, of course, expansion is diminished, and the theoretical economy is somewhat reduced. My experiments have been made on relatively large gas engines, two ermines of 22-inch diameter cylinder, one of 10-inch diameter, and another of 7-inch dismeter, but the reasoning applies to small petrol engine cylinders. . .
"If the pumping arrangements could be operated by a small separate petrol motor nicely balanced and always running, the
main engine could be arranged to act as a compressed air engine only when in traffic or on hills at slow speed. The added engine and pumps increase the complication, but it seems to me possible to reduce total weight by the reduction of main engine dimensions and the simplification of gearbox arrangement. A car which could he stopped and started in traffic without the need of the clutch and could stop on a steep hill in the same way, with the certainty of starting smoothly, would combine
the advantages of both steam and petrol engines "The flexibility of a steam engine differs considerably from that of a petrol engine, and a short consideration of what is meant by flexibility may be useful.
" As a power producer the horse must be considered as a motor of ideal flexibility ; the horse can develop his 33,000 foot pounds per minute, either by a small pull exercised through a long distance in the time, or a heavy pull through a short die. Lance. He can either give a light or heavy pull, and develop the same total power either by motion at a higher or lower velocity.
" The petrol motor cannot do this ; within wide limits of speed no substantial variation of mean pressure on the piston follows change from high speed to low speed. "Assume a petrol engine to be running at 1,000 revolutions per minute under a mean effective pressure of 90 lbs. per square inch, developing an indicated power of, say, 40 horse ; then i1 the speed be dropped to 500 revolutions per minute, the mean pressure of 90 lbs. will remain practically unchanged ; if the valves are properly proportioned there will he little change in mean pressure. The engine at half speed will then indicate only 20 horse. To comply with ideal flexibility, 40 I.Il.P. should be developed, and to do this 180 lbs. mean pressure would he required. In the sense of ideal flexibility the petrol motor is conspicuously deficient. " Assume, now, a steam engine of either the simple or compound type, to be supplied from a steam boiler capable of supplying steam at a given pressure and rate. The steam engine may consume the whole of its supply either at a high or low speed ; take the same speed variation as supposed in the petrol engine, 1,000 and 500 revolutions per minute. At the high speed, a given point of cut-off will utilise the whole steam supply, and give, say, 40 I.H.P. at 90 lbs. more pressure. At the low speed, 500 revolutions, the cut-off must be later to use the whole steam generated, and then the steam weight used per revolution will be doubled; the mean pressure will not be doubled, but it will be greatly increased. Say that it rises from 90 to 150 lbs. per square inch, then the I.H.P. at half speed will be 34 The steam engine thus possesses considerable flexibility, although it has not the ideal flexibility assumed to exist in the horse.
"True flexibility in a motor requires the property of producing maximum power at both high arid low speeds, and this property is to some extent present in the steam engine, and almost absent in the gas or petrol engine.
" What is commonly called flexibilityin a petrol motor is in reality controllability, which is a different thing.
"The steam engine could be more closely followed in this matter by supplying the petrol motor with a constant or even an increasing weight of charge as the speed of rotation falls in the manner which I have already suggested. Recent improve. merits in the petrol engine, such as very large inlet and exhaust valves and very light pistons and moving connections, have made it possible to greatly increase the total power to be obtained from cylinders of given dimensions by permitting of longer strokes and higher speeds of rotation, while keeping up the weight of charge dealt with per stroke. The famous 4-inch race has shown, it to be possible to get about 60 I.H.P. from a four-cylinder engine having 4-inch diameter cylinders, at a speed of about 2,000 revolutions per minute. The interesting new Daimler-Knight engine, in which lift valves are replaced by sliding piston ring sleeves, admits a larger charge at high speeds than is usual with ordinary valves, and so secures greater power for given cylinder dimension.
" I look forward with much interest to the coming year's experience with this fascinating form of petrol engine."
Carburetters, the nature of the products of combustion, and the questions of road clearance and its relation to dust raising, were also lightly discussed by Mr. Clerk, who concluded his address be announcing that, as the subjects of hie address were of a debatable nature, he should follow the precedent of the past president, Col. Crompton, by inviting the members to give expression to their views on any points. The discussion which followed the delivering of the address, however, will be reported by us in a subsequent issue of " THE CONIMERCIAI. MOTOR."