GRADIENTS AND THEIR EFFECT ON POWER.
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• Particularly as Regards Electrics. By Chief Engineer.
THE POWER NECESSARY to move mechanic' ally-propelled vehices varies greatly, depending on the following :— (1) Conditions of road surface. (2) Gradients. (3) Speed. (4) Effect of air resistance. • .
A good heard macadam road will require an effort of about.50 pounds per to to inn the vehicle, after it ban once been "started in motion and has attained its full speed. . The gross Weight, of course, is taken for the purpose of 'making calculations„ When the roads are in bad condition, the effort required may rise to as Much as 120 pounds per ton. With regard to gradients;. not only is.the vehicle and its load being rut along the road, but the total weight is being :lifted, and it will be found that, on-stiff gradients, the effort that has to be exerted to lift the load is far greater than that necessary merely to move the load.
Under the third heading, the power required to move the load is proportional to the speed, conditions being similar, -whether the vehicle is on a level road. or a gradient.
The effect of air resistance may be safely neglected in the case elf the heavy, slow-moving vehicle. It is important that these items should be taken fully into consideration, whatever kind of vehicle is under scrutiny, but especially in the case of the,electrio wagon, because the radius of action is limited by the capacity of the battery, which is a fixed. quantity. It is, therefore, partioularly necessary to take all these points into consideration with this class of vehicle in estimating the distance that they will .travel on one charge.
In the case of a steam or petrol wagon, the. only thing to consider is that the engine shall be capable of developing sufficient power. This item . being properly taken. care. of, it only remains to take sufficient fuel aboard.
An electric vehicle is usually Sold to run from. 35 to 40 miles on one charge on good hard level roads ; it will not, however, cover this distance if the gradients are stiff. A numerical example will show how the various conditions affect the power required. The case chosen is. that of a steam wagon weighing, with its load, 12 tons. As the legal limit is eight miles per hour, ire will assume this figure in making calculations. The.teactive force on the level on good hard roads is : If this wagon encounters a hill, with a gradient of, say, 1 in 10, the work exerted in running the load along at 8 m.p.h. is the same and, in addition to this, a weight of 12 tons has to be lifted up bodily at a rate of 70.4 ft. a minute. The power required to do this is The total horse-power required to run this Wagon up a hill of 1 in 10..at 8 m.p.h. is, -therefore, 57.3 .x 12.8 76.1.
The power required merely to propel the vehicle being actually only about, 18 per cent. of the total, the Other 82 per cent being required to lift the load.
As a matter of fact, in actual practice, so high a horse-power is not likely to be developed, the wagon automatically sloWing down, but the proportions remain the same, nevertheless.
Now, the much greater importance of these relations will be -seen in the case of the electric Wagon. A 31-ton electric-wagon, with 'itslead; will' weigh about 7 tons and will travel at approximately 8, m.p.h. on. the level. When a vehicle of this'Size is fitted with a lead battery, the capacity is generally about 260 ampere hours, and, as the consumption of current on the level is somewhere about 7.5 ampere hours per mile, it follows that, on good hard level roads,. the distance travelled per charge will be, roughly, 35 miles. If it were possible to keep up the speed of 8 m.p.h.: on a steady gradient of 1 in 10 it Would be found that the amount of discharge would 1e7.5 ampere hours per mile for propelling the vehicle, representing, roughly,. 18 per cent, of the total power required, the other 82 per cent. or 3-4 ampere hours being necessary toraise the load, making a total of 41.5 ampere hours. per mile for a steady run up a hill of 1 in 10. The total mileage on such an incline would only be about 6± miles per charge. Of course, exactly such, conditions as these are not met with inpractice, but the illustration shows the great effect that gradients' have on the. mileage per .charge of this class of vehicle.
For the sake of comparison, -a speed of 8 m.p.h. has been assumed, both on the level and up hill, but, as a matter of fact-, the electric vehicle would slow down to, probably, 3 or 4 m.p.h. on such a hill, which. would tend Lo reduce the Mileage per charge, because the motor would be working at a much slower, and, therefore, less efficient fate. There are many eases where only avery low mileage per day is required, but whore, on account/of hills, the work is very exacting for horses and whet() elect,rics would be used to niuch better advantage. The-writer knows of an instance where a two-ton 'electric is employed entirely in ta,king,goods from a factory to the station, a quarter of a mile away. It is a steady up hill pull from the factory, the gradient in places being 1 in 10. The vehicle makes, on an average, 20 journeys a day, so that the total daily mileage is only 10. The return journey consumes practically no current, except that required to start the wagon. It has performed its work most satisfactorily and shows a great saving over horses, which were originally: employed for this work, but the work happens to be of a nature particularly suitable for electrics.
It will generally be found that the consumption of fuel in bad weather is Somewhat greater than in goodsweather, when the roads are hard and dry. This has a direct effect on the mileage of the vehicle in the ease of electrics, so that, generally speaking, it will be found that the, average distance run on a charge Will be a little less (luring the winter months, the diminiehed distance being of the order of 10 per cent.