Bus Engine and Transmission Design
Page 50
If you've noticed an error in this article please click here to report it so we can fix it.
I HAVE found the recent pre-selective versus layshaft 'gearbox controversy most interesting. Writing simply as a passenger, it seems to me that there is something to be said for both points of view and that the importance of good and bad characteristics possessed by each depends on the .type of service and the standard of driving.
One difficulty with pre-selective transmission is that it will still oper ate when maltreated, so that a minority of bad drivers can continue grossly to miss-time gear changes, producing sickening jerks each time, for years on end, without mending their ways. With orthodox trans mission, even a slight error of this sort is liable to mean a missed gear or stalled engine, so that the most indifferent driver soon learns to do better in his own interest.
Another point is that the fluid-.
coupling slip, amounting to about 8-9. per cent. at 1,100 r.p.m. or so, at which speed most oil engines give. their' maximum torque, means that lower, gears and higher engine revs, must be-used for the best results in climbing, • as Compared with orthodox transmission, so that one often finds a preference for the latter in hilly districts.
On the other hand, in city traffic, the fluid coupling and epicyclic gearbox must make for a considerable reduction of driving, fatigue over a day,. both in • gear _ changing and in cutting out the need to "juggle". with accelerator and clutch, perhaps 60 times each hour,
when starting from rest. .
These factors; doubtless, account for the fact that, quiteapart from London Transport, with its evergrowing fluid-coupling fleet, eight out of the 10 largest municipal bus fleets have a proportion of vehicles with this type of transmission (and of these, in five cases it forms the majority of the fleet), yet. very few are to he found among the--big company fleets, which spend less time in traffic.. Incidentally, in this connection, Mr. Burford was not quite correct regarding the municipal fleet of Leeds, aS the Leylands and Crossleys, forming 30 per cent. of the double-decker rolling stock there,
have layshaft gearboxes, r
Quite closely related to this is the question of engine siie-'raised by Mr. P. G.-Tucker's article. in your issue dated August .10. Whilst it is true that some leading operators still prefer small units, they are none the less in the minority, except in respect of single-deckers, and even with these it is significant that only two makers have thought it worth while to introduce an underfloor engine in the 7-8-litre class. Of the same 10 largest municipal bus fleets referred to above, only one has put a majority of new vehicles with engines of under 8 litres into service in the past two years or so, whilst five have standardized on 9H0-titre engines, over the same period. Among the 40-odd larger " area-agreement " companies, at least 30 now specify units of over 8 litres for double-deckers, and of these about 18 . have gone in for 91-10-litre units. However, up to recently, all but a dozen or so have retained the small engine for singledeckers.
My experience does not confirm the view that "the majority of present-day drivers will use the foot hard down, no matter what power unit. is fitted." This may occur when larger-engined vehicles are first introduced, but inspectors can check any tendency to run ahead of time, and most drivers -soon seem to learn to drive
D16 . more gently. The larger engine gives a margin for delay, thus making for courtesy and safety, but on the other hand, crawling up hills in low gear with 'a " fussy " small engine turning over at high revs, on full throttle must add to driver fatigue, quite apart from being tiresome to passengers.
Regarding weight, the usual increase over pre-war standards is due chiefly to stronger construction of both chassis and body, and only very slightly to -the use of larger engines, as is shown by published figures of engine weights (with all auxiliaries), The A.E.C. 9.6-litre unit weighs 1,600 lb.—only -1 cwt.. more than the 7.7-litre A.E.C. engine, and just over 3 cwt. more than the lightest unit of all in this field, the Gardner 5LW. As the change from the 7-ft. 6-in. Width to 8 it. usually adds 5-6 cwt., it will be seen that engine size is a minor factor in this respect. Body, not engine design technique, is the important one. London, N.21. A. A. TOWNSIN.
. . .
I HAVE followed with great interest the correspondence favour of, and opposed to, the preselectiVe gearbox. Whilst I do not wish to add. to .what Mr. A. A. M. Durrant.said at Edinburgh, I would like; on behalf of London Transport, to comment on one statement in Mr. Simpson's letter in'your issue of August 10.
It is suggested that, when air-actuated, the preselective gears cannot be engaged if the brakes cease to operate. This iS not the case on London's fleet of buses of the RT family, which have separate air-storage tanks for each function. Whereas there is a common source of 'air supply, each system is independent and the failure of one cannot influence the other. Thus, a failure of the brakes—of which in any case, good warning would be given by the "stop" signal arm above the windscreen, as well as a warning light—would not render the gears ineffective on London's buses.
London, S.W.1. GEORGE DODSON-WELLS.
Chief Public Relations Officer. (For London Transport Executive.)
WHY R.H.E. GROUP MANAGER LEFT I WOULD like to draw your attention to certain mis"statements of fact in the paragraph which appeared on page 30 of your issue dated August 10, under the heading " Why B.R.S. Group Manager Threw Up His Job."
Eleven undertakings, including Mr. Arnott's, were merged together into one group comprising 118 vehicles. The clerical staff employed at this group were drawn from these undertakings, and numbered less than, the total clerical staff formerly employed on the same work. Statistical information compiled in the group manager's office is the usual basic information which every organization requires of its branches in order that adequate costing and accounting statements -can be prepared.
Lastly, Mr. Arnott did not resign; his services were dispensed with by reason of the fact that his groun was merged with a neighbouring group to forrn one unit Glasgow, C.2. J. B. FlksTit.
Divisional Manager.
• (For Road Haulage Executive.)