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BRAKES, TURBO-CONVERTERS AND HYDROSTATIC DRIVES
Page 75
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ALONGlist of advantages claimed for a Russian heavy-duty air-bag brake actuator includes uniform distribution of braking force over the drum surface, total utilization of the surface, low thermal stressing of the drum, reliability, improved efficiency, elimination of adjustment and high sensitivity. In fact the system would appear to have "all the advantages", the only admitted disadvantage being a rather high air consumption, The system was described in a recent issue of the Russian periodical, Avtomobilnaya Promishlennoist (a summary of the article has been published in a MIRA Monthly Summary) and has been extensively tested in the laboratory and in road vehicles.
A removable friction lining is fitted to the bag which occupies a relatively small space and has an oval cross-section. Backing plates with threaded hole.s are vulcanized to its inner face to enable it to be attached to a stationary drum fitted with one or more nipples.
It is claimed that an air pressure of about 50 p.s.i. produces a mean braking torque of approximately 2,200 lb. ft. In tests of a heavy multi-wheeled vehicle, drum temperatures during repeated brake applications did not exceed I00°C.
From time to time over the years, forecasts have been made that a form of compound engine would eventually supersede the conventional piston engine and would have a better performance and fuel consumption than the gas turbine. Many promising designs have failed to achieve commercial acceptance, possibly because of the complications involved, but a simple type of compounding might well be developed that would be acceptable on economic grounds.
, Most compound engines have been based on a conventional or free piston unit and an exhaust turbine which absorbs all the energy of the exhaust gases, the output shaft being connected to the turbine through a reduction gear. In this case, the piston engine is solely a gas generator.
Converting the energy of the exhaust gas produced during normal running to mechanical energy that can be relayed to the transmission possibly offers a more practical approach to compounding if the equipment is not too costly or heavy, the use of a standard power unit without modification being a favourable development factor.
It could be a preferable alternative to conventional turbocharging which increases the output of the unit and therefore the mechanical stresses and is not readily applicable to petrol engines.
In an article published in December in the German technical journal Automobiltechnische Zeitschrift (and summarized by MIRA), Mr. P. Baumgartl makes a case for the application of an exhaust-gas turboconverter to petrol engines, which he claims could reduce acceleration time to attain a speed of 80 per cent of the vehicle's maximum by 45 per cent.
In a general comment on the potential of the system, he claims that it could be used in conjunction with a simplified transmission to give a somewhat improved performance, or in conjunction with a multi-speed automatic transmission to provide a substantially improved performance.
No mention is made by Mr. Baumgartl of fuel consumption but, if his claims are valid, a very worthwhile reduction should be obtainable. A simple sum suggests that, if the acceleration rate can be nearly doubled by utilizing power that would otherwise be wasted, an overall improvement in consumption of at least 15/20 per cent could be expected.
Mr. Baumgartl observes that the torque/ speed characteristic of a converter-equipped engine is more suitable than the characteristic of a compound engine. In a typical application, a fluid coupling would be incorporated in the system.
HYDROSTATIC DRIVE
Hydrostatic drive has long been considered by many transmission authorities as the ultimate in transmission developments, but although various systems have been applied successfully to agricultural and industrial vehicles, the relatively low efficiency of established types of pump /motor unit and their high cost has militated against applications to either commercial vehicles or cars, despite the manifold advantages offered in the form of an infinitely-variable ratio and the facility with which the system can be used to provide additional braking force.
Although efficiency can be improved by splitting the torque between the hydraulic units and an epicyclic gear train, it depends in the main on using very high pressures which are also necessary to achieve compactness, but which increase production costs because of the fine machining tolerances required.
Driving auxiliary equipment by hydrostatic motor has gained ground steadily in the past few years, in competition in some cases with electric drive, its chief advantage being that the motor can be located in any position on the vehicle without complication of the drive mechanism. With this type of drive, efficiency is unimportant so long as the system is reliable.
A recent application of hydrostatic drive by Drum Engineering Co. Ltd. is of special interest because it provides for the operation of a cargo pump fitted at the rear of an articulated tanker, the pump being driven in the normal way by the power take-off of the tractive unit gearbox. A quick-fastening coupling is located on the bulkhead.
A study of this system revives thoughts on the feasibility of employing hydrostatic motors for driving a second axle of a rigid or articulated vehicle which is normally required "very occasionally" during a typical year, when extra traction is necessary to surmount a slippery slope or to negotiate a surface covered by mud in depth.
Equipping the vehicle with a second driving axle is a high price to pay for additional traction that is rarely required in both first cost and weight, and the friction of the mechanism represents a small but not insignificant wastage of power. It is pertinent that the extra traction afforded by a second driving axle is only required in an average contingency when the vehicle is travelling at a very low speed.
If hydrostatic motors were used to drive a second axle, a simple gear type would be satisfactory; and it should be possible to incorporate a free-wheeling device in the mechanisms that would disengage the motors unless they were being employed to drive the wheels at 'a low speed. Alternatively, a single motor could be coupled to a transverse pivoted shaft with end-mounted rollers that would engage the tyre threads when the shaft was moved to the operating position. In both cases, differential action would be eliminated when the drive was operational.
The motor or motors could acceptably be produced from low-cost materials, and as use would be restricted to low-speed operation of the vehicle, it should be possible to supply fluid from a conventional type of pa.o.-driven pump through a pressurereducing valve. Savings in weight and overall cost should be considerable.