Electricity for the farm Part 13

The principle of the gasoline engine as motive power is so familiar to the average farmer that it needs but a brief description here.

Gasoline or other fuel (oil, gas, or alcohol) is transformed into vapor, mixed with air in correct proportions, and drawn into the engine cylinder and there exploded by means of a properly-timed electric spark.

Internal combustion engines are of two general types--four-cycle and two-cycle. The former is by far the more common. In a four-cycle engine the piston must travel twice up and down in each cylinder, to deliver one power stroke. This results in one power impulse in each cylinder every two revolutions of the crank shaft. On its first down stroke, the piston sucks in gas. On its first up stroke, it compresses the gas. At the height of this stroke, the gas is exploded by means of the electric spark and the piston is driven down, on its power stroke.

The fourth stroke is called the scavening stroke, and expels the burned gas. This completes the cycle.

A one-cylinder engine of the ordinary four-cycle type has one power stroke for every two revolutions of the fly wheel. A two-cylinder engine has one power stroke for one revolution of the fly wheel; and a four-cylinder engine has two power strokes to each revolution. The greater the number of cylinders, the more even the flow of power. In automobiles six cylinders are common, and in the last year or two, eight-cylinder engines began appearing on the market in large numbers.

A twelve-cylinder engine is the prospect for the immediate future.

Since the dynamo that is to supply electric current direct to lamps requires a steady flow of power, the single-cylinder gas or gasoline engine of the four-cycle type is not satisfactory as a rule. The lights will flicker with every other revolution of the fly wheel. This would be of no importance if the current was being used to charge a storage battery--and right here lies the reason why a cheaper engine may be used in connection with a storage battery than when the dynamo supplies the current direct for lighting.

A two-cylinder engine is more even in its flow of power and a four-cylinder engine still better. For this reason, standard voltage generating sets without battery are usually of two or four cylinders when of the four-cycle type. When a single-cylinder engine is used, it should be of the two-cycle type. In the two-cycle engine, there is one power stroke to each up-and-down journey of the piston. This effect is produced by having inlet and exhaust ports in the crank case, so arranged that, when the piston arrives at the bottom of the power stroke, the waste gases are pushed out, and fresh gas drawn in before the up stroke begins.

For direct lighting, the engine must be governed so as not to vary more than five per cent in speed between no load and full load. There are many makes on the market which advertise a speed variation of three per cent under normal loads. Governors are usually of the centrifugal ball type, integral with the fly wheel, regulating the amount of gas and air supplied to the cylinders in accordance with the speed. Thus, if such an engine began to slow down because of increase in load, the centrifugal b.a.l.l.s would come closer together, and open the throttle, thus supplying more gas and air and increasing the speed. If the speed became excessive, due to sudden shutting off of lights, the centrifugal b.a.l.l.s would fly farther apart, and the throttle would close until the speed was again adjusted to the load.

These direct-connected standard voltage sets are as a rule fitted with the 110-volt, direct current, compound type of dynamo, the duplicate in every respect of the machine described in previous chapters for water-power plants. They are practically automatic in operation and will run for hours without attention, except as to oil and gasoline supply. They may be installed in the woodshed or cellar without annoyance due to noise or vibration. It is necessary to start them, of course, when light or power is desired, and to stop them when no current is being drawn. There have appeared several makes on the market in which starting and stopping are automatic. Storage batteries are used in connection with these latter plants for starting the engine. When a light is turned on, or current is drawn for any purpose, an automatic switch turns the dynamo into a motor, and it starts the engine by means of the current stored in the battery.

Instantly the engine has come up to speed, the motor becomes a dynamo again and begins to deliver current. When the last light is turned off, the engine stops automatically.

Since the installation of a direct-connected standard voltage plant of this type is similar in every respect, except as to motive power, to the hydro-electric plant, its cost, with this single exception, is the same. The same lamps, wire, and devices are used.

With gasoline power, the cost of the engine offsets the cost of the water wheel. The engine is more expensive than the ordinary gasoline engine; but even this item of cost is offset by the cost of labor and materials used in installing a water wheel.

The expense of maintenance is limited to gasoline and oil.

Depreciation enters in both cases; and though it may be more rapid with a gasoline engine than a water wheel, that item will not be considered here. The cost of lubricating oil is inconsiderable. It will require, when operated at from one-half load to full load, approximately one pint of gasoline to each horsepower hour. When operated at less than half-load, its efficiency lowers. Thus, for a quarter-load, an average engine of this type may require three pints of gasoline for each horsepower hour. For this reason it is well, in installing such a plant, to have it of such size that it will be operating on at least three-fourths load under normal draft of current. Norman H. Schneider, in his book "Low Voltage Electric Lighting," gives the following table of proportions between the engine and dynamo:

Actual watts Actual Horsepower Nearest engine size 150 .5 1/2 225 .7 3/4 300 .86 1 450 1.12 1-1/4 600 1.5 1-1/2 750 1.7 1-3/4 1000 2.3 2-1/2 2000 4.5 5 4000 9.0 10

This table is figured for an efficiency of only 40 per cent for the smaller generators, and 60 per cent for the larger. In machines from 5 to 25 kilowatts, the efficiency will run considerably higher.

To determine the expense of operating a one-kilowatt gasoline generator set of this type, as to gasoline consumption, we can a.s.sume at full load that the gasoline engine is delivering 2-1/2 horsepower, and consuming, let us say, 1-1/4 pint of gasoline for each horsepower hour (to make allowance for lower efficiency in small engines). That would be 3.125 pints of gasoline per hour. Allowing a ten per cent loss of current in wiring, we have 900 watts of electricity to use, for this expenditure of gasoline. This would light 900 25 = 36 lamps of 25 watts each, a liberal allowance for house and barn, and permitting the use of small cooking devices and other conveniences when part of the lights were not in use. With gasoline selling at 12 cents a gallon, the use of this plant for an hour at full capacity would cost $0.047. Your city cousin pays 9 cents for the same current on a basis of 10 cents per kilowatt-hour; and in smaller towns where the rate is 15 cents, he would pay 13-1/2 cents.

Running this plant at only half-load--that is, using only 18 lights, or their equivalent--would reduce the price to about 3 cents an hour--since the efficiency decreases with smaller load. It is customary to figure an average of 3-1/2 hours a day throughout the year, for all lights. On this basis the cost of gasoline for this one-kilowatt plant would be 16-1/2 cents a day for full load, and approximately 10-1/2 cents a day for half-load. This is extremely favorable, as compared with the cost of electric current in our cities and towns, at the commercial rate, especially when one considers that light and power are to be had at any place or at any time on the farm simply by starting the engine. A smaller plant, operating at less cost for fuel, would furnish ample light for most farms; but it is well to remember in this connection plants smaller than one kilowatt are practical for light only, since electric irons, toasters, etc., draw from 400 to 660 watts each. Obviously a plant of 300 watts capacity would not permit the use of these instruments, although it would furnish 10 or 12 lamps of 25 watts each.

CHAPTER XI

THE STORAGE BATTERY

What a storage battery does--The lead battery and the Edison battery--Economy of tungsten lamps for storage batteries--The low-voltage battery for electric light--How to figure the capacity of a battery--Table of light requirements for a farm house--Watt-hours and lamp-hours--The cost of storage battery current--How to charge a storage battery--Care of storage batteries.

For the man who has a small supply of water to run a water wheel a few hours at a time, or who wishes to store electricity while he is doing routine jobs with a gasoline engine or other source of power, the storage battery solves the problem. The storage battery may be likened to a tank of water which is drawn on when water is needed, and which must be re-filled when empty. A storage battery, or acc.u.mulator is a device in which a chemical action is set up when an electric current is pa.s.sed through it. This is called _charging_. When such a battery is charged, it has the property of giving off an electric current by means of a reversed chemical action when a circuit is provided, through a lamp or other connection. This reversed action is called _discharging_. Such a battery will discharge nearly as much current as is required originally to bring about the first chemical action.

There are two common types of storage battery--the lead acc.u.mulator, made up of lead plates (alternately positive and negative); and the two-metal acc.u.mulator, of which the Edison battery is a representative, made up of alternate plates of iron and nickel. In the lead acc.u.mulator, the "positive" plate may be recognized by its brown color when charging, while the "negative" plate is usually light gray, or leaden in color. The action of the charging current is to form oxides of lead in the plates; the action of the discharging current is to reduce the oxides to metallic lead again. This process can be repeated over and over again during the life of the battery.

Because of the cost of the batteries themselves, it is possible (from the viewpoint of the farmer and the size of his pocketbook) to store only a relatively small amount of electric current. For this reason, the storage battery was little used for private plants, where expense is a considerable item, up to a few years ago. Carbon lamps require from 3-1/2 to 4 watts for each candlepower of light they give out; and a lead battery capable of storing enough electricity to supply the average farm house with light by means of carbon lamps for three or four days at a time without recharging, proved too costly for private use.

_The Tungsten Lamp_

With the advent of the new tungsten lamp, however, reducing the current requirements for light by two-thirds, the storage battery immediately came into its own, and is now of general use.

Since incandescent lamps were first invented scientists have been trying to find some metal of high fusion to use in place of the carbon filament of the ordinary lamp. The higher the fusing point of this filament of wire, the more economical would be the light. Edison sought, thirty years ago, for just the qualities now found in tungsten metal. Tungsten metal was first used for incandescent lamps in the form of a paste, squirted into the shape of a thread. This proved too fragile. Later investigators devised means of drawing tungsten into wire; and it is tungsten wire that is now used so generally in lighting. A tungsten lamp has an average efficiency of 1-1/4 watts per candlepower, compared with 3-1/2 to 4 watts of the old-style carbon lamp. In larger sizes the efficiency is as low as .9 watt per candlepower; and only recently it has been found that if inert nitrogen gas is used in the gla.s.s bulb, instead of using a high vacuum as is the general practice, the efficiency of the lamp becomes still higher, approaching .5 watt for each candlepower in large lamps. This new nitrogen lamp is not yet being manufactured in small domestic sizes, though it will undoubtedly be put on the market in those sizes in the near future.

[Ill.u.s.tration: The Fairbanks Morse oil engine storage battery set]

The tungsten lamp, requiring only one-third as much electric current as the carbon lamp, for the same amount of light, reduces the size (and the cost) of the storage battery in the same degree, thus bringing the storage battery within the means of the farmer. Some idea of the power that may be put into a small storage battery is to be had from the fact that a storage battery of only 6 volts pressure, such as is used in self-starters on automobiles, will turn a motor and crank a heavy six-cylinder engine; or it will run the automobile, without gasoline, for a mile or more with its own acc.u.mulated store of electric current.

_The Low Voltage Battery_

The 30-volt storage battery has become standard for small lighting plants, since the introduction of the tungsten lamp. Although the voltage of each separate cell of this battery registers 2.5 volts when fully charged, it falls to approximately 2 volts per cell immediately discharging begins. For this reason, it is customary to figure the working pressure of each cell at 2 volts. This means that a 30-volt battery should consist of at least 15 cells. Since, however, the voltage falls below 2 for each cell, as discharging proceeds, it is usual to include one additional cell for regulating purposes. Thus, the ordinary 30-volt storage battery consists of 16 cells, the last cell in the line remaining idle until the lamps begin to dim, when it is switched in by means of a simple arrangement of connections. This maintains a uniform pressure of 30 volts from the beginning to the end of the charge, at the lamp socket.

We saw in earlier chapters that the 110-volt current is the most satisfactory, under all conditions, where the current is to be used for heating and small power, as well as light. But a storage battery of 110 volts would require at least 55 cells, which would make it too expensive for ordinary farm use. As a 30-volt current is just as satisfactory for electric light, this type has become established, in connection with the battery, and it is used for electric lighting only, as a general rule.

Batteries are rated first, as to voltage; second, as to their capacity in ampere hours--that is, the number of amperes that may be drawn from them in a given number of hours. Thus, a battery rated at 60 ampere hours would give 60 amperes, at 30 volts pressure, for one hour; 30 amperes for 2 hours; 15 amperes for 4 hours; 7-1/2 amperes for 8 hours; 3-3/4 amperes for 16 hours; etc., etc. In practice, a battery should not be discharged faster than its 8-hour rate. Thus, a 60-ampere hour battery should not be drawn on at a greater rate than 7-1/2 amperes per hour.

This 8-hour rate also determines the rate at which a battery should be re-charged, once it is exhausted. Thus, this battery should be charged at the rate of 7-1/2 amperes for 8 hours, with another hour added to make up for losses that are bound to occur. A battery of 120-ampere hour capacity should be charged for 8 or 9 hours at the rate of 120 8, or 15 amperes, etc.

To determine the size of battery necessary for any particular instance, it is necessary first to decide on the number of lamps required, and their capacity. Thirty-volt lamps are to be had in the market in sizes of 10, 15 and 20 watts; they yield respectively 8, 12, and 16 candlepower each. Of these the 20-watt lamp is the most satisfactory for the living rooms; lamps of 10 or 15 watts may be used for the halls, the bathroom and the bedrooms. At 30 volts pressure these lamps would require a current of the following density in amperes:

Candle Power 30-volt lamp Amperes 8 10 watts 0.33 12 15 watts 0.50 16 20 watts 0.67

Let us a.s.sume, as an example, that Farmer Brown will use 20-watt lamps in his kitchen, dining room, and sitting room; and 10-watt lamps in the halls, bathroom, and bedrooms. His requirements may be figured either in lamp hours or in watt-hours. Since he is using two sizes of lamps, it will be simpler to figure his requirements in watt-hours.

Thus:

Number Size of Hours Watt- Room of lamps lamps burned hours

Kitchen 1 20 4 80 Dining room 2 20 2 80 Sitting room 3 20 4 240 (3) Bedrooms 1 (each) 10 1 30 Bathroom 1 10 2 20 (2) Halls 1 (each) 10 4 80 Pantry 1 10 1 10 Cellar 1 10 1 10 ---- Total 550

Since amperes equal watts divided by volts, the number of ampere hours required in this case each night would be 550 30 = 18.3 ampere hours; or approximately 4-1/2 amperes per hour for 4 hours.

Say it is convenient to charge this battery every fourth day. This would require a battery of 4 18.3 ampere hours, or 73.2 ampere hours. The nearest size on the market is the 80-ampere hour battery, which would be the one to use for this installation.

To charge this battery would require a dynamo capable of delivering 10 amperes of current for 9 hours. The generator should be of 45 volts pressure (allowing 2-1/2 volts in the generator for each 2 volts of battery) and the capacity of the generator would therefore be 450 watts. This would require a 1-1/4 horsepower gasoline engine. At 1-1/4 pints of gasoline for each horsepower, nine hours work of this engine would consume 14 pints of gasoline--or say 16 pints, or two gallons.

At 12 cents a gallon for gasoline, lighting your house with this battery would cost 24 cents for four days, or 6 cents a day. Your city cousin, using commercial current, would pay 5-1/2 cents a day for the same amount of current at 10 cents a kilowatt-hour; or 8-1/4 cents at a 15-cent rate. If the battery is charged by the farm gasoline engine at the same time it is doing its other work, the cost would be still less, as the extra gasoline required would be small.

This figure does not take into account depreciation of battery and engine. The average farmer is too apt to overlook this factor in figuring the cost of machinery of all kinds, and for that reason is unprepared when the time comes to replace worn-out machinery. The dynamo and switchboard should last a lifetime with ordinary care, so there is no depreciation charge against them. The storage battery, a 30-volt, 80-ampere hour installation, should not cost in excess of $100; and, if it is necessary to buy a gasoline engine, a 1-1/4 horsepower engine can be had for $50 or less according to the type.

Storage batteries of the lead type are sold under a two-years'

guarantee--which does not mean that their life is limited to that length of time. With good care they may last as long as 10 years; with poor care it may be necessary to throw them away at the end of a year.

The engine should be serviceable for at least 10 years, with ordinary replacements; and the storage battery may last from 6 to 10 years, with occasional renewal of parts. If it were necessary to duplicate both at the end of ten years, this would make a carrying charge of $1.25 a month for depreciation, which must be added to the cost of light.

_Figuring by Lamp Hours_

If all the lamps are to be of the same size--either ten, fifteen, or twenty watts, the light requirements of a farm house can be figured readily by lamp hours. In that event, the foregoing table would read as follows:

Lamp hours Kitchen, 1 lamp, 4 hours 4 Sitting room, 3 lamps, 4 hours each 12 Dining room, 2 lamps, 2 hours each 4 Bedrooms, 3 lamps, 1 hour each 3 Halls, 2 lamps, 4 hours each 8 Bathroom, 1 lamp, 2 hours 2 Pantry and cellar, 2 lamps, 1 hour each 2