The Development of Petrol-Electric Systems.*
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By Percy Frost-Smith and W. A. Stevens.
For London omnibus work, and ordinary gradients, all the driving is effected on the top speed with motor armatures in parallel, and the main circuit between motors and dynamos is not broken, the control being entirely by engine speed; it should be noted that at a speed of less than soo revolutions per minute the dynamo fails to excite. The series parallel control enables the torque on the motors practically to be doubled when their armatures are in series, and, as a result, the speed is reduced in the same proportion, and the voltage across each armature is half the terminal voltage of the dynamo under these conditions. Speaking generally, it may be taken that the speed_ of the motor is proportional to the volts, and the torque proportional to the amperes.
As the conditions of efficiency in continuous current petrolelectric systems are entirely different to those obtaining in tramway practice, owing to the total energy available for the electric motors being limited by the power of the petrol engine to produce that energy, and the obvious necessity under these conditions of the voltage falling in direct proportion to the amperes demanded, as contrasted with the constant voltage supply of the tramway system, a consideration of the curves shown in the figure may be of interest, as they have a most important bearing on the efficiency, hill-climbing powers, and gear ratio, for special conditions of work. The curves are taken from the motors of our petrol-electric omnibus.
Referring to the figure, in which the vertical divisions (ordinates) on the left-hand side represent lb. torque at a radius of 20 inches, this radius being especially taken to coincide with the radius of the ordinary omnibus wheel :The horizontal divisions (abscissw) represent amperes, and the curve marked (2) the torque values at varying current densities. These torque values are, practically, constant throughout the whole of the variation of speeds of the serieswound motor; the torque with a given current at two revolutions per minute being the same as at 1,000 revolutions per minute, the speed variation at a given torque being only
affected by the voltage of supply. Considerably greater torque per ampere could be attained in a motor with a very small air gap, but such a motor would not he so efficient, at average loads, as the motor with a normal air gap. The vertical divisions on the outer right-hand side correspond with the curve (x) and are divided into volts. This curve represents the lost volts in our type of motor, due to the fall of potential, caused by the current shown in the horizontal divisions. The vertical divisions on the inner righthand side correspond with the curve (4), which represents lost kilowatts at various current densities, irrespective of the speed of the motor armature. This curve is the result of the product of the lost volts and the amperes passing through the machine, or in other words the C2R. losses. The total internal resistance of each motor, including fields, brushes and armature is .o65 ohm. It will be seen that the above curve rapidly increases in value, being proportional to the square of the current density, and its importance will be clearly shown on consideration of the efficiencies obtained with a highly geared vehicle with small engine power. As an extreme example, we may instance that of a stiff hill and a heavy vehicle, so geared as to demand a torque on each of the motors of 15o1b. at 2o-inch radius, and with a dynamo capable of generating 8 kilowatts-4 kilowatts for each motor—the efficiency of the motors would in this case only work out at 6 per cent., the remainder of the energy being absorbed in the C2R losses in the motors.
We may further mention, as an illustration of the peculiar conditions met with in petrol-electric work, that if the dynamo is capable of generating 16 kilowatts-8 kilowatts for each motor—the efficiency of the motors will be 53 per cent., this increase in efficiency being due to the voltage being doubled in the case of the 16 kilowatt supply, the amperes remaining the same, and the losses in the motors
being practically the same as in the first instance. These examples are only given to illustrate what might occur in a vehicle with a wrongly-calculated gear ratio, using motors which under better working conditions would give an efficiency of over 90 per cent. In our experience, the best gear ratio for electric motors, of the dimensions that we are employing, is from 12 : I to 16 : 1; the former for town work, and the latter for mail van, and country work generally, owing to its high efficiency on hills. With a 16 :i gear ratio, the efficiency or transmission in a petrol-electric vehicle, weighing, loaded, 7 tons on a hill of i in 12, including all electrical losses, and allowing to per cent, for gear losses, will work out at 70 per cent. from the engine to the road wheels. The efficiency of a 12 : i gear ratio under these conditions is 67 per cent. On ordinary " giveand-take" roads, the total efficiency from the engine to the road wheels averages over 70 per cent. The efficiency is, after all, best estimated by results and in petrol consumption our vehicle will compare very lavourably with the best geardriven vehicles now in operation.
Once more referring to the figure showing the curves, the curve (3) demonstrates the C2R losses in the armature, brushes and fields, of a smaller motor, which has a total resistance of .1 ohm, showing that with a current of zoo amperes, and a torque at 20-inch radius of 1171b,, a loss of 4 kilowatts in the machine would have to be made up before any external work could be done. This illustration is given to demonstrate the great loss of efficiency in a vehicle driven by small electric motors, having a small gear ratio to the road wheels.
In the matter of durability of electrical equipment, the petrol-electric omnibus possesses the important advantage over the tramcar, in that the total electrical load on the dynamo and motors, is limited by the power of the engine, and, as the dynamo and motors are, from considerations of electrical efficiency, made sufficiently large to stand 300 per cent, overload for a considerable time, to "burn out " through an overload is impossible. On a tramway system, on the other hand, this danger is ever present, the motors being at the mercy of the motorman with the full power of the generating station behind him. It is an easy matter in this case for a careless driver to switch on the full voltage of the line to his motors with both armatures and fields in parallel, when the car is starting or ascending a gradient, with the inevitable result of a " burn out."
Owing to the nature of the control on our vehicle, it is impossible for the driver to injure the electrical equipment by any means in his power. As our petrol-electric omnibus is practically free from vibration, owing to the absence of spur gearing, the dynamo and motors, fixed on a comparatively lightly sprung frame are working under conditions at least as favourable as the ordinary electric motor used for industrial purposes. All shafts run on ball bearings, and each machine is, in addition, fitted with a double ball-thrust bearing to take the side strain from the bearings due to the weight of the armatures on heavy gradients.
In concluding the account of the electrical equipment of our own vehicle, we wish to disassociate ourselves from any attempt to monopolise all the virtues appertaining to the petrol-electric system. Our competitors in this direction have our best wishes, and we fully realise the good points of their work. The field is large, and if we can get our share of the future business which will surely come in this direction we shall be well satisfied. It now only remains to give a few brief comparisons between gear-driven systems and the system herein described. This vehicle exists, and though it has only been in actual work, and on a regular London service for eight weeks—covering 6,000 miles—yet, in cornpariscFn with the recent much-talked-of R.A.C. trials, it shows up very well. The R.A.C. trials occupied 22 days, and the miles done daily were 40 : chiefly on good" give-andtake " roads and where very few stoppages were required. It is not hills and bad roads that tell on motorbuses, so much as the continual stopping, and starting, of the vehicle, the heavy application of the brakes, in the formcr circumstance, and wrong application of the clutch in the latter case. Our vehicle cannot be started from a stationary position with a jerk, it simply glides away with a gentle steady movement : this fact alone has a great bearing on the question of abnormal wear and tear. The control, from a driving point of view, is as nearly " fool-proof " as possible, and this can only be properly appreciated by actually driving. In this respect it compares quite favourably with a tramcar. It is true, the driver has to steer, and a good thing too, otherwise he would go to sleep. The field resistance of the dynamo requires a little manipulation on the part of the driver, hut not more so than a spark advance and retard lever. It must be emphasised that, for London routes, the driver never need come out of his top, or parallel, position, and thus the main electric circuit never need be broken, except in such cases when the driver leaves his vehicle, and places the controller lever in the neutral position. The night staff on a fleet of such vehicles can be very greatly reduced, and it is confidently estimated that only .5 to .75 man, per vehicle, would be required. The labour required would be, engine adjusters, brakesmen, greasers, cleaners and washers. Electrical labour has been clearly demonstrated to us to be
unnecessary on nightshifts, as the ordinary cleaning of tho commutators would be done at the most suitable point on the vehicle's route during the day, as the operation only takes a few minutes to do.
With regard to maintenance, we have the strongest hopes of effecting a most substantial saving, from the results already obtained. An engine, such as we have employed, may be safely taken to cost no more than id. per mile, on the very low basis of 30,000 miles run per annum; this mileage is more likely to approach 40,000 miles than the figure given, and at the time of writing this paper the engine has cost nothing.
On the same basis as the above the frame is estimated to cost id., and the back axle the same sum. This latter unit was stripped after the 6,000-mile run, for examination, and we wish you to accept our assurance that there was no visible sign of wear. Gears, ball bearings and other wearing parts were perfect, and the only anxiety we have regarding this unit is the possible carelessness, or want of attention, on the part of the management, or other employees. The electrical unit has shown us that with a suitably-designed
dynamo and motors, the maintenance cost would be extremely low, and certainly not more than id. per mile, though we really think that this figure, low as it is, would eventually be considerably reduced. After the above statement, we feel that it would not be out of place to give figures, showing the comparison between those working cost figures given earlier, and relating to the gear-driven types now in vogue, and the system forming the subject of this paper.
In the above table we have shown the petrol consumption to be the same as on our gear-driven vehicles, but as a matter of fact, the petrol consumption has averaged i-mile per gallon more. In conclusion, we wish to state as our opinion, that if the above figures can, as we think, be realised in practice, over a period of some years, the solution of the problem of operating a fleet of motor omnibuses with commercial success has been achieved.