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Transmissions: a period of change
Page 47
Page 48
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by Martin Hayes
CONSIDERABLE changes in attitudes towards the subject of city bus transmissions have been taking place in the past few years. British bus operators, spurred on in many cases by their crews, have begun to accept the principle that city buses should be fitted with fully automatic transmissions as standard.
This review of current developments in bus gearbox thinking well illustrates the point that we are in a period of transition in transmissions. While a few operators are sticking to manual vehicles, others have moved on to semiautomatics while still more have gone the whole way and are specifying fully automatics.
Many major fleet operators have realized, having witnessed the non too happy experiences of others, that semiautomatics can cause difficulties. To avoid this they have resisted this form of transmission and are holding out for a foolproof, fully automatic gearbox.
The desire for simplicity of control is something which has come to the fore since the troubles with semi-automatics. Often major failures have resulted from abuse of the electronic control switch by the driver which has resulted in the whirling pieces of metal 30ft or so behind him doing each other severe damage.
One of the difficulties facing operators and manufacturers alike is the attitude of crews — and their unions — to full, automatic control. There is a belief on the part of some drivers, that part of the "dignity" of the job is reduced if control over gear changes passes from man to machine.
Many drivers think — often mistakenly — that they can change gear faster than any automatic control. In practice, of course, the reverse is usually true. This is something I have often proved when road testing vehicles at MIRA against the stop watch. Even by abusing the transmission and overspeeding engine and drive line components it is very rare that manual changes can produce better acceleration times than for the same type of vehicle with automatic control.
This anti-automatics attitude on the part of drivers continues in many areas though it is now being replaced by a more tolerant approach as the benefits of such devices in reducing driver fatigue are appreciated. Not surprisingly, perhaps, this new attitude is strongest in fleets which do not have obstructive and time-wasting door interlock systems built into their transmission set-ups. It is often these which give the transmission such a bad name.
Probably the biggest share of British buses on the road today are fitted with the Self Changing Gears pneumocyclic semi-automatic unit. Adapted initially to meet the needs of the new generation of rear-engined buses which required simplified control systems, this unit has now been considerably refined. Many of its early difficulties, in fact, were due to operators failing to appreciate that it needed more sensitive treatment than the crash box it replaced. Often fleet engineers serviced it no more regularly and as a result there were severe problems. This tended to tarnish the image of semi-automatics in general.
Given all this it is not surprising, perhaps, that the SCG unit is at present the subject of major efforts by both SCG and CAV to develop simple and cheap full automatic control systems. 1 SCG C83 system uses modulated pressure to make gear changes at correct engine speed. Sensors g information about road speed E throttle position and the air pressur varied to give a smooth change.
Throttle dipping
The CAV 488 system produces rather similar effect in a different w The automatic control is achieved b control board with six separate lo panels. An important feature of CA V system is its throttle-dipp facility. This uses a compressed cylinder to override the throttle pe position and close — or dip — throttle during gearchanges. This p duces a quite acceptable change with any skill on the part of the driver. Al needs to do is to hold the throttle pe on the floor and the 488 does the n One company which is working to prove basic pneumocyclic control further is Martin Transmiss Developments. For £15 it is mark etir bolt-on device which fits on the g selector and is claimed to take "bump" out of bus restarts. When di is engaged on a standard vehicle tltends to be a bump as first geat selected from neutral or the door o position. The Martin device impos+ brief spell in top gear to lessen bump.
One of the more technically inter ing developments in transmissions year was the introduction of Fo ESTA device. Standing for E lically Synchronized Transmission embly, ESTA is only available in Li-automatic form at present but will developed into a fully automatic ;ion later. The device combines elec. tics, pneumatics, hydraulics and ,hanics which makes it just too tplicated for some operators.
owever, the basic principle of a i-computer which controls throttle gearbox operation is worthy of ntion. Control of down changes is 1 electronic with the throttle being )matically opened at the relevant t. A conventional clutch is retained this is perhaps the source of some :ion among operators towards 'A.
,Ithough it is still too early to be fully am, it seems likely that in terms of tr numbers on the road, first place in automatic league table is likely to be m by British Leyland's new and :nious G2 automatic. Now being d to Leyland Nationals in increasing ibers and offered as an option on iy of the Corporation's other bus lels, it represents a significant step vard in transmission development. he important points about G2 are it can be matched to five-speed, e ratio Leyland gearboxes and, ke the SCG and CAV units — ntially bolt-on extras — it is an ;rated part of the transmission. ugh it obtains road and engine d signals as well, the G2 also takes account engine torque loading it is claimed, an altogether more listicated response to differing con ■ ns.
wnhil I performance
tother point about the G2 is its rmarice on downhill gradients. n, the general tendency with auto;s is to change up to the highest ble gear — exactly the reverse of is required. The G2 resists this ncy.
ving talked of the British transon devices available, now is the time to turn to the imported models. Probably most numerous are the transmissions produced by Allison, a General Motors subsidiary. The AT540 model fitted to Seddon's midibus and the new Marshall Camuter has received plenty of publicity. This is a well proven unit with millions of miles behind it in the States.
When driving a Greater Manchester midi fitted with the transmission I was impressed by the smooth upwards changes but less than happy with the complete lack of any down changes to aid the brakes during deceleration. Larger Allison models work on a similar principle with the same drawbacks. However, they are readily available and have the advantage of being independently produced. This is why Alexanders is hoping to fit one soon to its midibus which uses Ford A series running units and which is in urgent need of an automatic which Ford cannot — or will not — supply.
The Allison is a purpose-built automatic as are the two German makes available in the UK. Both ZF and Voith have been involved on the British market for some years but neither have so far had significant automatic gearbox sales. Both are now seeking to redress that situation with new models incorporating significant new approaches to the situation. As well as conventional automatic transmissions for buses, which have not been seen on the UK market, ZF has a range of "Hydromedia" gearboxes with a twostage torque converter and its WSK transmission which combines a hydraulic torque converter, with a selector clutch and a synchromesh gearbox. This new device, shown at the Earls Court show, should be introduced to the UK next year. This is said to allow a combination of manual and automatic which protects the driveline from undue shocks. Significantly, perhaps, CAV is understood to be working on a control system giving similar results at present.
Voith has a number of its fully automatic transmission on trial service in the UK. Most notably, it is fitted to Ailsa's Volvo 859 demonstrator and the two Mercedes-Benz 0.305 city buses on trial with Greater Manchester PTE. The transmission on the Voith incorporates a torque converter with the facility to lock-up into direct drive in either of two stages.
The more interesting Voith model, shown for the first time at Earls Court, incorporates an integral hydraulic retarder. Called the DIWA D851 the transmission is a three-speed with the first speed on the torque converter and the other two in direct drive. Changes are acceptable, though inclined to be a little rough on power downchanges, but the most impressive thing about the. transmission is the retarder.
The lesson which can be learnt from this review of developments is that there is plenty going on. As the theme of transition in transmission design goes on, there will be increasing calls for sophistication. Operators will want to be satisfied, hOwever, that the new units will be reliable and will produce benefits which they can quantify in terms of hard cash as far as savings in component life is concerned.
B15 secret
Fuel consumption too cannot be neglected. One hopes that British Leyland had borne this in mind during the design stages of its HIS double-decker — due to enter service next year — which will feature a radically different and still secret concept of hydracyclic transmission. The aim of this, I am told, is to give a good performance and a long life in the low speed, stop-start conditions in which British buses spend their time.
Still a long way off, but nevertheless interesting, is a system of hydrostatic drive being developed by the National Engineering Laboratory in East Kilbride. Not only should this be mechanically more efficient but it could offer significant fuel savings as well. As long as its designers achieve those goals of durability and reliability that British operators rightly put so high on their list of priorities then it could have a productive future.