OPERATION.
Power Taken by Cars. The amount of power required in the practical operation of a car depends upon so many variable elements that many of the calculations sometimes given for determining the power required by a car are of little value. The theoretical horsepower required to maintain a car at a certain speed on a level, is evidently the tractive effort in pounds multiplied by the speed in feet per minute and divided by 33,000. What the tractive effort per ton of car will be, depends on the condition of the rail and on several other uncertain factors. For street-railway motor cars, 20 pounds per ton is the usual tractive effort assumed as necessary. A calculation of this kind, however, takes no account of the losses in the motors and gears, nor of the fact that the greater part of the power required to propel a street car in practical service is used in accelerating the car from a state of rest to full speed. In interurban service, of course, the power required in acceleration is not so great a proportion of the whole.
Fig. 93. Plotted Data of Road Test.
The safest figures to use in engineering calculations as to the amount of power required, are those taken from actual results obtained in everyday commercial service. The power required by an eight-ton car in service in a large city like Chicago, is in the neighborhood of one kilowatt hour per car-mile run. On outlying lines this figure may be reduced to .7 kilowatt hour, and in the down-town districts may run up to 1.5 kilowatt hours per car mile. Double-truck cars in city service, weighing from 20 to 25 tons, take from 2½ to 4 kilowatt hours per car mile at the power station. Interurban cars around Detroit, weighing about 32 tons, in interurban service, making 25 miles per hour, including stops, in level country, and geared to 43 miles per hour, take about 3 kilowatt hours per car mile at the power station. However, interurban railway conditions are extremely variable.
The reports of several Indiana electric railways show an average power consumption of 1.48 kilowatt hours per car mile for city cars and 5.18 kilowatt hours for interurban cars, including line and distribution losses.
An interurban car weighing 31½ tons and equipped with two 150 horsepower motors, on a test run of 50 miles at an average speed of 39 miles per hour consumed 2.20 kilowatt hours per car mile. This car made 18 stops. A similar car under the same conditions made the same run at an average speed of 26 miles per hour with 44 stops, consumed 2.44 kilowatt hours and a third car, making 12 stops and at a speed of 33 miles per hour, consumed 2.10 kilowatt hours per car mile. These individual car test figures are from measurements taken at the car and do not include line losses.
Road Tests of Electric Cars. Of late considerable attention has been given to making road tests of electric cars. The results of the tests are usually plotted in the form shown in [Fig. 93]. Time is plotted horizontally in seconds, while volts, amperes, speed and per cent grade are plotted vertically. The diagram referred to is the result of a continuous run of 6 minutes of a 32.5 ton car equipped with two motors. The line voltage, motor consumption and other readings may be obtained for any instant of time. The acceleration in miles per hour per second may be obtained by noting the increase in height of the speed curve in one second. In making such a test the necessary instruments, voltmeters, ammeters, wattmeters and speed indicators are mounted direct on the car and are read at intervals of a few seconds.
The curve of motor consumption gives an idea of the abnormal current required to get the car under headway.
Economy in Power. As already stated, a large part of the energy taken by a car in city service is used in accelerating the car. Much of this energy must be destroyed or used up in the brake shoes at the next stop. The energy stored up in a car by process of acceleration is represented by the formula:
Energy in ft. lbs. = Mass in lbs. × (Velocity in ft. per sec.)²2, which is the formula for kinetic or live energy, the derivation of which is found in any Instruction Paper on Mechanics. In performing any given schedule with frequent stops, the more rapid the acceleration the lower the maximum speed required to make the schedule, and the less the energy required in acceleration. For city street and elevated service, therefore, rapid acceleration and low maximum speeds are desirable because not only more economical but safer.
For economical operation with any given equipment and schedule, it is important to use as much of the energy stored up in the car as possible, before wasting it by applying the brakes. Motors are built of a size to yield the large horsepower required in acceleration, and consequently are lightly loaded when operating the car at maximum speed. To economize in power, current should be shut off as soon as possible after the car has attained full speed; and the car should be allowed to drift without current as long as possible before the brakes are applied. In this way the energy stored in the car will propel it at nearly maximum speed for a considerable distance between stops; there will be the smallest possible waste of energy in the brake shoes; and the losses of energy which take place when the current is in the motors will be prevented as far as possible. Practical tests as well as theoretical calculations show a possibility of very material saving in energy in the operation of an electric railway car or train, by the observance of this simple rule of drifting as much as possible and using the brakes as little as possible. Whatever energy is used up in the brake shoes is necessarily wasted. The smaller this waste can be kept while performing a given service, the greater the economy secured.
Cost of Power. The reports of 85 per cent of the railway power generating stations in Indiana show the average cost at the station per kilowatt hour to be .755 cent. This was divided as follows: Fuel .526 cents, labor .158 cents, lubricants and miscellaneous supplies .032 cents, repairs .039 cents. The lowest cost reported was .505 cents.
During 1901 the average cost of power generated at the power house of the Indiana Union Traction Company was .443 cents per kilowatt hour at the switchboard. Distributed from the substations it was .765 cents per kilowatt hour. Natural gas was used for fuel. On occasions when this failed, coal at $1.50 per ton was burned.
Sliding and Spinning Wheels. In accelerating a car, however, there is no economy in turning on current so rapidly as to spin the wheels. As mentioned in the section on “Brakes,” the tractive effort between wheels and rails falls off about two-thirds when the wheels begin to slip; and this slipping of wheels, therefore, reduces the chance of securing the acceleration which is possible. For the same reason, in making emergency stops either by the use of brakes or by reversing the motors, care should be taken not to slide the wheels, as by so doing the time required to stop the car is much increased.
In the ordinary straight air-brake equipment used on heavy electric cars, there is much higher pressure carried in the storage reservoir than it is permissible to turn into the brake cylinder, since, if the full pressure were turned into the brake cylinder, it would result in sliding of the wheels—which, it has just been shown, is something to be avoided, not only on account of making flat spots on the wheels, but also because of the reduction in the braking force as soon as the wheels begin to slide. An experienced motorman can tell from the feeling of the car when the wheels are sliding, and will instantly release the brake sufficiently to allow the wheels to begin to revolve as soon as he notices that this has taken place.
The friction between brake shoes and car wheels decreases as the speed increases. A certain pressure applied to the brake shoes upon a car running 50 miles per hour, therefore, exerts much less retarding force than the same pressure at ten miles per hour. In order to give the same braking or retarding force at higher speeds, the brakes must be applied harder than at the lower speeds. If they are applied at the maximum pressure possible without sliding the wheels at higher speeds, it is evident that this pressure must be reduced as the speed of the car is reduced, or the wheels will be “skidded.” In the Westinghouse high-speed automatic air brake used on steam roads, this reduction of pressure is automatically accomplished.