APPLYING MODERN IDEAS TO CAB DESIGN.
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The Use of an Engine of High Efficiency Would Give Qtecker Acceleration and Higher Speed with Greater Ease of Manipulation.
TT may seem strange to connect racing cars with itaxicabs, but, without doubt, the development of the petrol engine upon which the cab is dependent is largely the outcome of the interest created in fierce competition. Even to the non-expert modern taxicabs are built upon lines which were common practice in car design about 15 years ago, and from the number of antiquated vehicles in the streets of London to-day the modernization of the taxi service now in progress can be considered as having been long overdue.
The requirements of a taxi chassis are special, whilst such items as ease of handling, quick acceleration and good braking powers must be well developed, although maximum speed is not of extreme importance. The seating capacity for the passengers must be ample, and provision should be made for the conveyance of luggage without much difficulty being experienced in loading or unloading it.
How Engine Efficiency has Been Attained.
It might be illuminating fir.st of all to trace through the progress of the petrol engine chiefly in matters relating to racing, as from this source we might expect to receive a lead in design. During the last year or so some of the old racing giants of pre-war vintage, with engines of 10,000 c.c. capacity and over, have been handsomely beaten at Brooklands by light ears with engines coming within the light car capacity definition. Not only has the sheer maximum speed of the small car been extraordinarily high, but it is also possible to maintain that speed for very long distances, so that reliability is demonstrated to a remarkable degree. Where has this greater speed come from? Undoubtedly a large proportion is due to weight reduction, but at the same time engine efficiency must have increased almost out of all recognition. Weight, too, forms an important factor in which a considerable reduction is permissible by employing engines of a more efficient character and smaller in both external and internal dimensions.
.The modern 1,500 c.c. racing engine develops its maximum power of approximatey 100 b.h.p. in the region of 6,000 r.p.m. with the aid of a supercharger, and, assuming 25 h.p. is required for transporting a taxicab, a capacity of 375 c.c. only would appear to be amply large enough. However, the actual efficiency of an engine at the present time would be considerably less than this, although the future may See developments upon the lines of the racing engine where such high efficiencies are regularly attained in practice.
• „The Adequacy of a Small-capacity Engine. Undoubtedly, a taxicab engine should approximate closely in efficiency to those used in private cars where 1,500 c.c. cylinder Capacity .produces 30 h.p. With a suitably...designed• body, where: weight reduction has been studied; this power ,6utput -should be more than that necessitated for Ordinary city running, whilst the reliability of the present-daylight car engine is unques
tionally. good. . . . •
Now imagine a special vehicle designed with a view to incorporating.a modern type of high-speed engine in a chassis suitably proportioned with, say, a Weymann type of body. Not only would the running costs be cellsiderably lower than the present-day type, but the vehicle would possess powers of acceleration—by. virtue of the high-speed engine and lightness of the vehicle as a whole—that would completely outstrip the existing type of vehicle in steed between any two points where the road and traffic conditions were unfavourable.
A typical power curve of a 1,500 c.c. engine is shown in the accompanying figure, and when the engine is geared suitably so that the peak of the curve occurs D42 •
when the vehicle is travelling at, say, 35 m.p.h. (which is really ail that is, necessary in the case.of re,London taxi), the reserve of power throughout the whole range can be appreciated. In many cases it is impossible to travel for more than 400 yds. or 500 yds. at a stretch without stopping, or at least slowing down to a crawl, which, of course, necessitates continual acceleration and retardation.
The advantages, therefore, of the fiat top characteristic of the power curve can be utilized by the driver in obtaining high speeds on the indirect ratios of the gearbox, with consequent beneficial effect on acceleration. The present-day taxicab weighs, complete without passenger and driver, rather more than one ton (say, 22 cwt.), but by the proper all-round reduction in weight and incorporation of a modern type of •small engine there is no reason why this weight should not be brought as Iow as 15 cwt., with equal, if not improved, satisfactory running and durability.
Let us just trace through the permissible specification for such a vehicle. With a bore and stroke. of 68 mm. and Da mm. and side-by-side valves, a combustion chamber of reasonable shape could be evolved, whilst reliability should be far superior to that of any of the older type of slow-running engines. The power, curve peaks at about 3,000 r.p.m., but the power does not fall away to any marked extent between that and 3,600 r.p.m., so that, Without utilizing 'the gearbox, a,' horse-power in excess of 15 is available at speeds be tweet' 12 m.p.h. and' 42ni.p.h. ' The pressure-feed 'lubrication system to the crankshaft and big-end bearings of the connecting rod whilst; . no doubt, tending to longevity of life to the mechanism, is pethaps somewhat of a complication Which might be dispensed with, without undue loss either in reliability ; Or durability. A single-plate clutch having a very light, free member is almost a sine qua. non for rapidand smooth gear changing, whilst the gearbox components themselves need not necessarily "be so heavy or, for that matter, so robust as the equivalent parts in a slow-speed engine, although the gears themselves may': be transmitting more power in the case of the modern
type than the old type. .
Tile same reasoning applies to the propeller shaft. With the increase in revolution speed of the engine, and . consequent increase hi. speed of the gears and transmission, the actual stress in each component decreases in direct proportion, so_ that. the _high-speed type definitely requires lessbulk of metal than the low-speed type of equal strength. The rear axle, Whilst requiring rather a lower gear reduction, would not be affected from the weight stand point to any very marked extent, except by using aluminium for the casing instead of cast-iron. Brakes, of course, are very important, and almost without exception cabs of to-day are braked on the rear wheels only. In time to come they will undoubtedly have to follow the trend of private car practice, in which four-wheel brakes are standardized, and, in order to permit of this improvement, the maximum turning circle of 25-ft. radius may have to be increased.
The actual overall dimensions of the vehicle cannot be reduced to any marked• extent beyond those in use at the present time, as with a closed body providing room for four or five passengers it would be uncomfortably small if a reduction in dimensions was effected, but the mounting and method of construction could, without doubt, be made lighter without impairing its serviceability.
The Weymann type of private saloon car is becoming increasingly popular, and, by virtue of the flexible nature of the' roof, drumming may be eliminated without the need for going to enormous expense in order to obtain a high degree of silence and smoothness from the engine. First cost would_ also favour the fabric type of body, for, where an accident occurs, the damaged panel can be removed and a new one replaced easily, quickly and cheaply, so that the vehicle need not necessarily be out of commission for more than a few hours. Improvement, too, can be effected in the general lines of the vehicle as a whole without increasing the cost.
The sum total of all the foregoing suggestions is, briefly, to install a more efficient power unit in a chassis which could, without impairing durability, be made considerably lighter than those in use to-day, whilst such items as more efficient lighting systems and possibly the incorporation of an electric starter would considerably enhance the ease and comfort of driving.