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New thoughts on automatics
Page 49
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and aerodynamics by Paul Brockington
• The application of a hydrostatic infinitely variable transmission to a vehicle could give an improved performance and fuel consumption throughout the load /speed range, a lecturer of the Department of Transport Technology, Loughborough University of Technology, Mr. G. G. Lucas, told post-graduate students during a fiveday course last week on the performance of vehicles. An example of engine/gearbox matching providing optimum results was given by Mr. Lucas relative to a 7cwt petrol-engined van and it was shown that an efficient infinitely variable drive could improve consumption of the vehicle when laden from 55 mpg to about 60 mpg at a steady 20 mph and from 35 mpg to around 40 mpg at 60 mph.
A substantial improvement in consumption could only be obtained, however, if the efficiency of the transmission matched that of a conventional type: the lower efficiency of established types of hydrostatic drive might in part or in whole be offset in practice by the better engine /transmission matching that it offered. Moreover it would enable the transmission to be matched to the engine without a sacrifice in peak power utilization. In the case of a stepped-ratio gearbox optimum fuel consumption could not be obtained if the vehicle were geared to provide for peak engine power.
After describing the Lucas and Dowty axial-piston hydrostatic transmissions, Mr G. Mitchell pointed out that the size of hydrostatic units was proportional to the torque transmitted as distinct from horsepower. The back-to-back pump /motor unit could be preferred on this score to a pump feeding separate wheel motors because the final drive gearing would give a higher operating speed and a proportionally lower torque. Axial-piston transmissions were limited in their speed range and tended to be noisy at higher speeds. They were therefore more suitable for applications to diesel engines than petrol engines.
• During an evening talk on automatic transmissions. Dr. J. G. Giles said that the principle of the double-shunt hydrostatic automatic transmission that he had evolved had been applied to the drive of Hobson aircraft alternators, two of which were fitted to a Trident airliner. The Giles transmission provides optimum efficiency at two points in the speed range and Dr. Giles claimed that hydrostatic drive had advantages over all types of mechanical and electrical automatic drives, in part because it provided clutch operation without complication and easy control. In practical applications electric drive was relatively inefficient because of the wide range of current values involved: cooling could create difficulties and the high price of copper increased the first cost. Mechanical transmissions could not operate without very high unit loading.
Dr. Giles emphasized that the future of transmission developments depended to a considerable extent on the characteristics of future power units. It might be possible to produce internal combustion engines that developed greater power at lower speeds: although the gas turbine had this propensity some form of transmission was required. In his opinion designers should concentrate on developing a power unit the purpose of which was on the "centre-of-operation" principle. An engine ran at peak efficiency when developing about 50 per cent of its maximum power and ideally it should be matched to a transmission that enabled this to be done.
During a discussion on Dr. Giles's talk (in which a number of undergraduates took part) Dr. Giles said that a hydraulic version of the Constantinesco ratchet type of infinitely variable drive was the subject of a research project at a UK university. This operated on the hydraulic pulse principle and appeared to have a favourable future. It was doubtful, however, whether attenuation of the pulses could be avoided.
A practical example of the aerodynamics benefits of relatively minor modifications to commercial-vehicle contours and dimensions was given by Mr. F. G. Maccabee. This related to the horsepower required to overcome aerodynamic drag of a Ford gas-turbined 17,000lb double-bottom test truck at 70 mph which had been reduced from 203 bhp to 119 bhp by minimizing the cab/trailer gap, sloping the windscreen and rounding the edges of the cab, reducing the body height and chamfering the edges of the van trailers. Fuel economy had thereby been improved by 15 per cent.
In a second example given by Mr. Maccabee the maximum speed of a Volkswagen minibus had been increased from 57 mph to 68 mph by rounding the front corners of the body. This had reduced the drag coefficient of the body from 0.75 to 0.5, the lower figure approximating to the average coefficient of a popular car.
In a discussion with a member of the Commercial Motor technical staff, Mr. Maccabee suggested that "variable geometry" might well be applied to the load space of a heavy lorry to reduce drag. This could take the form of inflatable bags to give a uniform load shape at all times as near to the optimum as possible whatever type of load was being carried.
Referring to anti-locking brakes during a lecture on braking force distribution, Mr. A. W. Wildig made special mention of the Kerr hydraulic reaction valve that released the brakes momentarily when the braking force coefficient was increased above the normal value which occurred when wheel slip was 10 to 20 per cent. The valve was connected to a suitable mechanism responsive to brake reaction and sensed the difference between peak and locked wheel coefficients.
Lecturing on deceleration tests Mr. Lucas referred to a fifth-wheel device that had been developed by the university and which enabled reliable results to be obtained. Deceleration testing could more conveniently be applied to large commercial vehicles than wind tunnel testing because in the latter case a scale model had to be made and this was expensive. Fairly accurate aerodynamic drag coefficients were obtainable by the deceleration method.
In a discussion on measures that could be taken to prevent jack-knifing following the lecture given by Mr. Wildig, Mr. J. T. Clifton, of the Automotive Products Group, said that the Hope device did not cater for both the laden and unladen vehicle condition. Mr. D. J. White, of the FVRDE, claimed that the Hope system did not "get at the root cause" of jack-knifing:. as many accidents were caused by trailer swing as could be attibuted to jack-knifing.