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INTELLIGENT COMPROMISE

27th May 1966, Page 124
27th May 1966
Page 124
Page 125
Page 126
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Page 124, 27th May 1966 — INTELLIGENT COMPROMISE
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Which of the following most accurately describes the problem?

TDEALLY, the engine and transmission of a vehicle should be selected to cater for the conditions of operation as specified by the operator. But although this ideal is approached, for example, by Birmingham and Midland Motor Omnibus Co. Ltd. in the case of its motorways coaches—which are produced by the operator—the normal target is an intelligent compromise. particularly with regard to the transmission of maximum-load goods vehicles.

Intelligent compromise covers a multitude of variables. In the first place, given that engine power is adequate, the suitability of a particular transmission will depend on the torque curve of the unit and, of closely related significance, the speed at which maximum power is produced. Normally the low-speed torque of a high-speed engine is inferior to that of a low-speed unit and, therefore. an increased number of ratios will be required. But it is notable that the output of a number of slow-speed engines on the market is inadequate for a typical application and the use of a multi-ratio box is necessary to make the best use of the limited power output.

Gearboxes and axles are available for most applications that offer an appropriate range of ratios, the main problem of vehicle manufacturers being to accommodate varying powers and torques without undue multiplication of transmission types and without imposing an unacceptable weight or price penalty when a hightorque box is coupled to an engine with a lower torque output.

If a high-powered engine is employed to improve gradient ability and, with the optimum final-drive ratio, provides a top speed in excess of requirements (which the average driver will exploit— and thus increase wear and tear—without profiting his employer in any way), the operator normally would prefer a ratio that reduced the vehicle's maximum speed to an acceptable m.p.h., despite a concomitant increase in fuel consumption and possible engine wear at the lower speed. In this case using the higher ratio obviously would be profitable if a maximum-road-speed governor were fitted to limit the speed obtainable without affecting acceleration or gradient ability. Catering for a high ratio of laden-to-unladen weight is, of course, the most knotty problem of any that the vehicle designer has to contend with, not least with regard to the transmission. The approximate value of this ratio is generally known, and if the traffic is such that the vehicle runs laden in one direction and empty on the return trip, the operator may be advised to specify a twin-range transmission providing a low range suitable for the laden condition and a high range for the return run.

In practice, the more intelligent driver would use both ranges to obtain the best ratio at every stage of the rum However, such "optimum usage" is rare and the high incidence of this form of driver deficiency is behind the interest of many vehicle manufacturers in the operational potential of semi-automatic and fullyautomatic transmissions. It is axiomatic that the near-optimum use of a lesser number of ratios is preferable to the misuse of a larger number of ratios.

If a vehicle is continuously operated on the motorways, the availability of the correct ratio for each stage of the journey is relatively of far greater importance than such availability on giveand-take routes. This is because the distance covered on each stage with little variation of load or speed is much greater, and this encourages the driver to be more appropriately selective in his choice of ratio. Close spacing of upper ratios offers obvious advantages on typical motorway operations.

The sustained high speeds that are now normal on the motorways give added point to the plea that too little regard is paid to the effect of windage losses on a van body. At 60 m.p.h., well over 30 per cent more power may be required to propel a van-bodied vehicle than a platform type and the same final-drive ratio (and possibly other ratios) cannot be right for both.

These comments represent random thoughts on what is, after all, a subject that prompts conjecture on the part of the vehicle maker because of the unpredictability of a substantial proportion of operators (mainly of goods vehicles) both at home and overseas.

In substantiation, the chief engineer of one of the major manufacturing companies criticizes operators generally because they dom consider whether the transmission system of the vehicle :y purchase offers the combination of options that is best suited their requirements. This is particularly so in the case of a box th a splitter auxiliary or a two-speed axle. Operators, too, quently fail to educate the driver in the use of the gearbox. As a consequence of the increasing demand for higher powers d speeds, both at home and overseas, the vehicle maker is faced th a complexity of transmission problems; it is largely the ;ponsibility of the operator, in liaison with the distributor, to Ike the right choice of transmission options as well as vehicle d engine options. The intelligent operator can be of great assistee to makers in planning current production and future policy. Whilst a number of manufacturers contend that there is no :essity to simplify ratio changing if the vehicle is used on motory or trunking runs as distinct from routes beset by congested .ffic, and the constant-mesh gearbox is regarded by the majority a perpetuating entity, the largest producer of heavy coinTcial vehicles has other ideas.

The virtual extinction in due course of the crash-type gearbox forecast by Mr. A. T. Webster, assistant managing director of Le Leyland Motor Corporation Ltd., with the qualification that nventional transmissions may be retained indefinitely in the case short-distance tipping vehicles engaged on quarry work and on. It is considered that for heavier goods vehicles, as well as p.s.v., the semi-automatic epicyclic gearbox with fluid flywheel offers unchallengeable advantages over conventional and fully-automatic transmissions and that eventually it will be specified by the majority of operators, including long-distance hauliers.

As exemplified by the Pneumocyclic gearbox, the semiautomatic provides ease of control and minimal delay in gear changing, and in contrast with a fully-automatic box it enables the driver to change to the most suitable ratio in anticipation of a condition, such as the need for maximum acceleration when overtaking.

Mr. Webster emphasizes that a good driver is a far better judge of road and traffic conditions than any sensing device that could be developed for an automatic, and that a sensing device reacts to need some time after the need has arisen. Moreover, it reacts to a transitory condition and this may result in loss of performance at a critical moment.

In applications for which multiple ratios offer fuel-saving and performance advantages, given that the ratios are used intelligently, it is claimed that the semi-automatic with a splitter auxiliary (in the form of a two-speed axle or an attachment) has the highest practical potential because frequent gear changing does not overtax the driver's ability or patience. Possibly, however, the most important potential of the type is the facility it would give for providing matched combinations of engine and transmission without undue complication or multiplication of the components.

Two or three basic epicyclic units could possibly be employed in conjunction with splitter auxiliaries to cater for all types of heavier vehicle, with consequent benefits in terms of production costs and maintenance.

Whilst the use of a fluid flywheel incurs a small fuel-consumption penalty during stop-start running, the lock-up characteristic of the clutch obviates any penalty when the vehicle is running without stops. On some types of route the ease with which the driver can select the most appropriate ratio is favourable to fuel consumption, notably if a two-speed axle is fitted.

Following this endorsement of the virtues of the semi-automatic, it is perhaps appropriate to mention that David Brown Industries Ltd., Huddersfield, credits the constant-mesh gearbox with an indefinite life expectancy and considers that no form of automatic unit will be commercially acceptable for standard goods vehicles until a suitable type of infinitely-variable transmission is available. A hydrostatic split-torque gearbox, known as the Transmatic, an of the type described in the August 27, 1964, issue of COMMERCIAL MOTOR, is being developed by the company.

The 650 gearbox, which has a torque rating of 650 lb. ft., is produced in prototype form by David Brown and will be available as a six-speed and 10-speed unit. It is a noteworthy example of progress in recent years in the design of higher-torque constantmesh units catering for the increasing outputs of modern engines, partly by virtue of its lightness and favourable dimensions.

The 6-650 six-speed box and the 10-650 10-speed unit are based on identical aluminium casings having an overall length, including the bell-housing, of about 36 in., whereas the box the 10-650 supersedes measures approximately 50 in. overall. Of possibly greater merit, both the 650 boxes weigh 525 lb. compared with the older weight of 1,060 lb.

The two 650s are identical apart from the inclusion in the 10-650 of a splitter mechanism that enables the torque to be transmitted to the layshaft through the second train of gears instead of the first train by employing a dog clutch which can be used to lock the input pinion of the first train, or the mainshaft pinion of the second train, to the jack shaft.

Referring to the drawing, overdrive is engaged by locking pinion A to the spigot shaft with sliding dog E and pinion D to the mainshaft with dog F. The drive is then transmitted through layshaft pinions B and C. Direct drive is obtained by locking pinion D to the spigot shaft with dog E and to the main shaft with dog F. In high range, A is locked to the spigot shaft with dog E and power is transmitted to the layshaft through pinion B, whilst low range is engaged by locking D to the spigot with dog F and relaying the torque through pinion C.

In contrast, the predecessor of the 10-650--the 561A—was a five-speed unit fitted at the rear with a two-speed epicyclic gear.

Detailed features of the 650 include gears that are shotpeened to provide an artificial compressive stress which increases the load capacity by 15 per cent and thus enables the face width to be reduced a corresponding amount. The forward ratios of the 10-650 are equally spaced and the upper ratio provides an overdrive of 0.758 to I. Whilst credit is given to the two-speed axle for fulfilling a need, it is emphasized that the range of available two-speed axles only enables ratio spacing to be provided in a limited number of cases.

Pending the production of a stepless automatic gearbox, David Brown technicians consider that the fully automatic is unsuitable for heavier vehicles, firstly because its power-to-weight ratio is reduced by as much as 70 per cent with the addition of the load, and secondly because it would be necessary to cater for diesels with an idling-to-maximum speed range as low as 900 r.p.m.

It is claimed that governors of sufficient sensitivity cannot be produced to give proper control when such a load variation applies or the speed range is of this order. With the vehicle unladen, automatic control is liable to initiate excessive power surge at lower speeds which can be dangerous in traffic.

Moreover, it is affirmed, producing the type of multi-ratio box that is required for the motorways would necessitate unacceptable complications in the case of an automatic.

The lack of appreciation on the part of operators of the benefits of a multi-ratio gearbox—notably the users of medium-sized vehicles—is shown by the absence of interest in the front-mounted splitter auxiliary and the rear-mounted overdrive unit offered as optional equipment by the company for fitting to a number of gearboxes in the range.

According to Mr. D. Thaw, chief engineer, ENV Engineering Co. Ltd., London N.W.10, the compound variable-boost engine combined with an exhaust turbine could be a highly flexible unit capable of producing a useful torque over a wide speed range (without ratio variation). He emphasizes that the losses inherent in the operation of fully automatic torque-converter and hydrostatic variable-pump-delivery types of transmission applied to conven al engines would necessitate the use of larger engines to avoid active-effort penalty even in cruising conditions.

4r. Thaw observes that adoption of any of these systems is kely until their cost can be reduced and that it will be necessary ::ontinue development of conventional multi-ratio gearboxes. reference to transmissions for higher-speed engines, Mr. Thaw its out that their design necessitates careful attention to bearing ication and tooth form. Of topical interest operationally he 3 the hypothetical example of a 10-speed twin-layshaft type of -box coupled to an engine developing 185 b.h.p. at 3,300 r.p.m. a torque of 318 lb. ft. at 2,000 r.p.m. installed in a 28-ton-gross cie. Assuming a wheel radius of 19 in., the rear-axle ratio Id be about 6.2 to I if the gearbox were of the direct-drive-top

or 8 to 1 if an overdrive unit were employed, which would Me overdrive to be used with advantage for light running and ct drive for cruising fully laden.

L good example of intelligent compromise is provided by Guy :ors Ltd., Wolverhampton. The company employs an ENV , five-speed, direct-top gearbox for torque outputs up to lb. ft., and a six-speed overdrive box for torques up to lb. ft. These two basic units cater for a very wide range of ne types. The forward ratios of the 542 comprise a bottom of 6.51 to 1, a second gear of 4.3 to 1, a third gear of 2.51 to 1 a fourth gear of 1.48 to 1 in addition to the top gear of 1 to 1, the unit can be mated to a two-speed back axle equipped with Eaton 18802 driving head.

orward gear ratios of 7.15 to 1,4.45 to 1, 2.54 to 1, 1.33 to 1, 1 and 0.75 to 1 are afforded by the overdrive box, which is d with a synchromesh splitter at the front, an underdrive :ter being also available with a ratio of 1.25 to 1. The under e splitter is operated by depression of the clutch pedal following election by button control.

Vhen the latter unit is applied to the Cummins V8 265 b.h.p. .TE engine, it will be modified to include constant-mesh overt splitter auxiliary in place of the standard overdrive gear, ch will give 10 forward speeds without an increase in the th of the assembly. This is of importance with regard to onalization of propeller-shaft length. According to Mr. E. F. Clarke, assistant chief engineer of Guy Motors Ltd., it is pertinent that the engine-transmission systems of the company's standard vehicles are designed to provide startability on gradients up to but not in excess of 1 in 4.5. This is more than adequate for the great majority of applications and the imposed limitation obviates the use of an overweight axle. Units giving startability on steeper gradients are optionally available.

The extent to which the higher-speed diesel could enable the weight of the gearbox to be reduced if the units were suitably matched is of academic interest only at this stage of development because compromise is still a ruling factor. But it is interesting to note the views of an expert, who is not associated with the production of the types of vehicle under consideration, on the potential advantages of diesels operating at very much higher speeds.

In the view of Mr. M. R. Dunn, chief engineer, vehicle division, Alvis Ltd., Coventry (which produces fighting vehicles), an existing type of power unit in the 200 b.h.p. category operating at a maximum speed of 2,500 r.p.m. weighs about 40 per cent more than would an engine running at 4,000 r.p.m., whilst the weight of a matched gearbox is 15-20 per cent greater.

Quoting from experience of a torque-converter, six-speed, semiautomatic transmission of American origin, Mr. Dunn states that it provides a good fuel consumption and reduces driver fatigue. The weight increase involved is assessed to be about 30 per cent and the cost increase about 50 per cent.

In contrast with the American gearbox mentioned by Mr. Dunn, the SRM fully-automatic unit produced by Bristol Siddeley Engines Ltd., Coventry, relies entirely on a converter and auxiliary control mechanisms to provide torque multiplication, and the layout of the units is notably simple and compact. A torque-multiplication efficiency of up to 87 per cent is achieved, whilst in lock-up direct drive the efficiency is better than 98 per cent.

The SRM features a converter that acts as a fluid coupling at idling speeds by virtue of the guide vanes being released to rotate freely, but when the engines accelerate under load an epicyclic gear mechanism driven by the output shaft contra-rotates the blades and the high reaction torque developed increases torque multiplication to approximately double that normally obtainable. At higher speeds, the blades are locked and the converter performs as a conventional unit Torque multiplication up to 9.6 to 1 is obtainable and the latest DS model weighs only 520 lb.

As mentioned in the August 6, 1965, issue of COMMERCIAL MOTOR, a 420-mile test was made by Ron Cater, of the journal's technical staff, of a Scottish Brewers' ERF eight-wheeler, equipped

• with an SRM DS transmission, in which the fuel consumption was 9.25 m.p.g. on the outward run at an average speed of 24.5 m.p.h. with the vehicle grossing at 21 tons 12 cwt. The consumption was reduced to 13.4 m.p.g. on the return run following removal of the load of 14 tons 4 cwt. and the vehicle averaged 30.5 m.p.h.

The converter of the SRM can be employed as a retarding brake (for decelerations up to about 30 per cent) which is regarded by many potential users of the unit, as well as Scottish Brewers, as a major asset. Its use can provide a large saving in lining-replacement costs and promotes safer driving.


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