Electrical Transmission for Motor.Vehicles.*
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In view of the somewhat barbarous method of gear-changing at present in vogue on most petrol-driven motor vehicles, and of the fact that, crude as it is, this method continues to hold its own against all other mechanical devices, it is not surprising that attention has been turned to electrical arrangements for arriving at what is required, especially in the case of the heavier description of vehicles. Apart from the mere transmission of the motion of the engine crankshaft, usually with a considerable reduction in the number of revolutions per minute, to the road wheels, the chief requirements are : (t) an easily operated and preferably automatic variability of gear-ratio, so that full advantage may be taken of the maximum effort of which the engine is capable without over-running in obtaining the highest possible acceleration and speed for the vehicle under varying conditions of gradient, load, and surface and other resistances ; (2) perfect control of the speed of the vehicle under different conditions beyond the variations of which the petrol engine is capable ; (3) ready disconnection of the engine from the road-wheels, so that the former can continue to run while the latter are at rest, coupled with means for gradual reconnection so that the vehicle may start smoothly and without shock.
Important considerations that have to be taken into account in comparing e]ectrical and mechanical methods of arriving at the above requirements are ease of working, reliability, and freedom from breakdown and derangement, weight, efficiency in terms of the power lost in the transmission, first cost, cos-f of upkeep, quietness and freedom from vibration.
Several arrangements of electrical transmission have been adopted, and, with varying details, may be shortly described as follows :— (i) The engine, which is preferably fitted with a governor, drives a dynamo which supplies energy to an electric motor, which in turn drives the road-wheels through the usual mechanical differential gear. An electrical controller of some description is fitted so that the electric current can be varied as to regulate the speed of the motor, while, either be means of the controller, or automatically by special windings on the dynamo and motor, the full power of the engine at its most efficient speed can be employed to generate constant watts made up of either a large current at low voltage, such as is required for starting and on steep gradients, or a small current at a higher voltage for running the vehicle at high speed on the level, with corresponding variations under intermediate conditions.
(2) Similar to No. t, excepting that the axes of the dynamo and motor being arranged in line, a clutch of some description is arranged between them so that at top speed they can be mechanically connected together and electrical driving and all consequent losses dispensed with, the armatures of dynamo and motor acting merely as flywheels. In this arrangement the electrical drive is only employed for starting, and at other times when the power of the engine is insufficient to propel the car on the direct drive. (3) The engine drives a dynamo as in the previous cases, but instead of one motor two or more are employed, each driving a separate wheel. The mechanical differential is
thus dispensed with, but the direct drive as in No. 2 cannot be arranged for. The two or more motors can either be
arranged in the hubs of the wheels, when all gearing is dispensed with, or can drive the wheels through chain Or toothed-wheel reduction-gear. The latter are the arrange ments usually employed, as unless some form of speedreduction is used the motors are of necessity relatively very heavy. The employment of two or more motors enables the series-parallel system of control, so much used on electric tramcars and trains, to be adopted.
(4) A seorage-battery of comparatively small capacity, but capable of giving large discharges for short intervals, can be employed with any of the above-mentioned systems, the arrangement being such that for starting, or on hills, the battery discharges and assists the engine and dynamo in driving the motors, while at other times a part of the power K)f the engine is absorbed in charging the battery, the change being automatically effected by the relative voltage of the dynamo on the one hand and of the battery on the other.
(5) In another method, in which a storage battery is used to assist the engine when required, there are no separate dynamo and motors, but on the shaft of a mechanical transmission is fixed the armature of a machine which performs the functions of both dynamo and motor as required, automatically charging the battery when the engine has power to spare and the speed increases, and taking a discharge from the battery and acting as a motor so as to assist the engine when the latter becomes overloaded and the speed diminishes.
These systems with auxiliary batteries possess the advantage that, as the battery affords a ready means of starting the engine, the latter can always be stopped when the vehicle is at rest.
Where storage batteries are employed, continuous currents are necessary, and, apart from this, most of the systems so far used have been on the continuous-current principle, which gives large changes in gear-ratio more readily. On the other hand multi-phase alternating currents have already been employed with some measure of success, and are a matter of considerable interest, as with them commutators and brushes, and their attendant troubles and expense, are avoided, except in the case of the comparatively small exciter dynamo that is usually employed.
The chief scope for electrical transmission on motorvehicles seems to he on heavy vehicles such as motoromnibuses, lorries and the like, and especially on largemotor-coaches on rails, such as several of the railway companies are now running on suburban lines. Though considerable success has already been attained' with the electrical system of transmission in its various. forms, there is obviously still much room for development._