THE QUESTION OF LIGHTING ON BUSES.
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Some of the Chief Factors which have to be Considered in Order that Good Results Can be Secured.
WITH the exception of those vehicles owned by the larger companies, it is no exaggeration to say that the least satisfactory part of many modern service buses is the electric lighting equipment. This does not imply that the quality of the dynamo, battery or lamps is poor, but rather that the system, as a whole, is unsuitable for the work it has to perform. The bus not only requires a more powerful lighting set than that of the average private car, but allowance has to be made for the fact that the conditions of running are more exacting than those of the vehicle used for pleasure purposes.
A poorly lit bus gives the impression that the proprietor is either indifferent to the comfort of the passenger travelling at night, or else he does not understand what kind of set should be fitted. Moreover, the owner concerned seems to forget that his bus may compare badly with the better lit vehicles of his rivals using the same route. It is still possible to see buses held up by the roadside, not because of any defect of the chassis but owing to the failure of the lighting set.
Essential Features.
The first essential of bus lighting is reliability, and the second the use of good, well-distributed lights inside and on the upper deck, if it has a covered top. The modern bus equipment consists essentially of a dynamo of 200-450 watts output, a battery of about 100 amperehours capacity and several lamps of between 12 c.p. and 15 c.p. each.
Size o f Dynamo.
ell proportioned, so that the apidly. It must be more subcomponent used on private The dynamo must lie w battery may be charged r stantial than the smaller cars as it has to stand up to practically continuous running for 14 to 16 hours daily. This means a machine which does not overheat when giving its full output for several hours, allowing for, say, halfminute breaks at fairly regular intervals.
The dynamo may hare constant current regulation ; that is to say, its output in amperes is the same when the engine is running at above about 750 r.p.m. The other type of regulation used is known as constant voltage. With this type a vibrating electromagnetic regulator is used to maintain the voltage of the dynamo constant above the minimum full-output speed. This type of regulation is unsatisfactory when used alone.
What is generally used is more correctly spoken of as voltage regulation, which, unfortunately, is often confused with constantvoltage regulation. With a voltage-regulated dynamo both the current and
the voltage are kept under control. Primarily, the voltage-regulated machine may be of the constantcurrent type, with third-brush or other current regulation, and have some form of voltage regulator which limits the voltage. Alternatively, the machine may be primarily of the constant-voltage type, wherein a vibrating regulator maintains the voltage constant so long as the current does not exceed a certain maximum limit. The latter idea Is used by the makers of the C.A.V., Bosch and Scintilla machines and the former idea by the manufacturers of the M.C,L. and others.
Estimating the Size of Dynamo.
The size of the dynamo is best determined by the total lamp current. The total lamp current is easily estimated by inspection of the bulbs. They will be found marked 9.6-watt or 12-watt ; sometimes 10-watt bulbs are used. If they he 9.6-watt, the current per lamp is .8 ampere,. and if 12-watt, 1 ampere. The 9.6watt lamp is generally gas-filled and the 12-watt lamp of the vacuum type. There are as a rule, about 16 points in a double-deck bus. This means 16 amperes for the double-deck bus at full load with vacuum bulbs and 13 amperes with gas-filled bulbs. As the dynamo output must exceed the lamp current, its output will have to be more than 200 watts for the vacuum lamps and 160 watt for gas-filled bulbs.
Dynamo Output and Lamp Current.
By how much the dynamo output should exceed the total lamp current is difficult to estimate exactly. On town bus, which makes many stops on a fairly short
• journey, the dynamo may have an output of one-and-ahalf times the total lamp current. Thus, in the above example with gas-filled bulbs, the dynamo should have an output of at least 240 watts. It is Interesting to note that on the buses of the London General Omnibus. Co., Ltd., two dynamos of 150 watts output each are employed, which allows a high margin of safety. Consequently, failure through an undercharged battery is rare. A bus used on country service may have a slightly smaller dynamo, but it is very unwise to go below Pc of the total lamp current, in case of a temporary hold-up with all lamps burning. Exceptional conditions necessitate a dynamo of 350 watts output.
A voltage-regulated system permits the dynamo to be somewhat smaller, because the machine works at overload capacity when the battery is discharged, or discharging, at a high rate.
Considering the Battery.
The class of 5attery used for bus lighting often takes the form of a miniature electric-vehicle-type battery. This type of battery is found necessary, because the cells are subject to prolonged lase near the maximum charge and discharge rates. The amount of vibration experienced is also greater than the ordinary type of plate construction will stand-. However, with the rapidly increasing improvement in the suspension of public-service vehicles this factor will become less important. The capacity is generally already settled for the purchaser, as manufacturers make only two or three sizes of battery. A capacity of 90-ampere hours is the minimum which can be safely allowed, and 180 ampere-hours the maximum for double-deckers with upper saloons. The bus accumulator is made to withstand a high rate of charge, so that it may be equal to its task if the stopping time exceeds the running time.
Types of Lamp and Their Disposition.
There are two types of bulb. The most economical, from a current consumption point of view, is the gasfilled or half-watt type. Lamps of this kind have tungsten filaments wound in a very fine spiral, and, consequently, they are more fragile than the vacuum lamp of corresponding size. The vacuum lamp is more suitable for illuminating the destination boards and the gangway of the (1601e-decker open-top bus.
The vacuum-type lamp takes 1 watt per candle-power and the gas-filled lamp approximately 4, a watt per candle-power. The actual candle-power of the bulbs has been mentioned earlier. These sizes are found to give a well-distributed light when the correct number of lamps is equally spaced along the roof. The number of lamps required to illuminate the interior is from four to eight, depending, of course, upon the seating capacity. A 14-seater single-deck bus needs only four interior lamps, whereas the 32-seater single-decker requires eight. The normal double-decker, with 48-50 seats, usually has six interior lamps, and in the case of the top-covered double-decker six lamps are usually fitted in each saloon. If a six-wheeled single-decker has 40 seats, the number of lamps would be increased to 10.
Details of the Switchboard.
The switchboard should necessarily be of very robust construction, the raised-box type being the most suitable and the easiest to mount and inspect. Usually the switchboard has at least two switches—one for the interior lamps and the other for the regulation lamps. On some systems an additional switch is necessary to control the charging of the dynamo. This is often incorporated with the main lamp switch, to ensure that the dynamo is on charge, when any lamps are on. It is always advisable to have a separate switch for the regulation lamps (that is, the exterior lamps required by law), to obviate burning out the interior lamps when the bus is running but not plying for hire. Sometimes it may be advisable to have two switches for the interior lamps, so that, in the event of dynamo failure, an economy of battery power is easily effected.
Some of the Systems in Use.
Of the many bus-lighting systems in use the salient features of some of them may be of interest.
C.A.V. systems are of two distinct types. The latest system is voltage controlled, an M or DA-type dynamo being used. Constant-current systems are used on a large number of London buses with great success, whilst a lesser number of constant-voltage systems are employed with equal reliability. Lucas and Rotax systems are of the constant-current type, whilst the M.C.L. arrangement has a constantcurrent dynamo. This has a novel method of regulation, combined with an output controller which automatically maintains the charging current at the correct value, depending on the state of the battery. Several American and Continental bus-lighting systems are in use in this country. The most common are the Bosch, S.E.V. and Remy. The Bosch system has been adepted by several British chassis manufacturers, and is a voltage-regulated system. The Bosch dynamo has a vibrating regulator, which controls both the current and the voltage of the dynamo ; thus the machine gives a constant-current output until the battery is fully charged, when the current is reduced by the operation of the voltage regulator. The S.E.V. system will be found, amongst others, on Renault chassis. The larger S.E.V. sets have voltage regulation, which the makers thoroughly recommend for commercial-vehicle service.