The Internal-Combustion Engine
The internal-combustion engine, commonly called a gas engine, differs from the steam engine, in that the transformation of the heat energy of the fuel into work takes place within the engine cylinder. The fuel may be gasoline, kerosene, crude petroleum, alcohol, illuminating gas, or some form of power gas.
In order to form an explosive mixture in the cylinder, air must be mixed in certain proportions with the fuel, and this can be accomplished only when the fuel is in the gaseous state, or is a mist of liquid fuel easily vaporized at ordinary temperatures. Thus the essential difference among internal-combustion engines using the various fuels is in the construction of the device for preparing the fuel before it enters the engine cylinder. If the fuel is a gas, only a stop valve is necessary between the source and the gas engine admission valve. The devices for preparing liquid fuels depend on the character of the fuel, a heavy fuel requiring heat while a volatile fuel is easily vaporized at ordinary temperatures by being broken up into a fine mist. If the fuel is in the solid form, like coal, it must be converted into a gas by the use of a gas producer, to be described later, before it can be used in the gas-engine cylinder.
After the mixture is drawn into the cylinder, it is prepared by compressing and intimately mixing the fuel with the air at one end of the engine cylinder. This highly compressed combustible mixture of air and fuel is burned within the cylinder against the face of the piston. The heat liberated by the burning gases causes these gases to expand, the pressure within the cylinder is increased and the piston is driven out toward the other end of the cylinder. The motion of the piston is changed into rotary motion at the crankshaft through the interposition of the connecting rod and crank. The crankshaft can be connected directly to the machines to be driven or through mechanical connectors, such as belts and chains.
The internal-combustion engine, in small sizes, is much more economical than the steam power plant. The average small steam power plant converts less than 5 percent of the heat energy in the fuel into useful work. A small oil engine which develops a horsepower on 1 lb. of gasoline per hour converts nearly 15 per cent. of the heat energy available in the fuel into work.
The Gas-engine Cycle
The series of events which are essential for carrying out the transformation of heat into work is called the cycle of an engine. The gas-engine cycle mostly used, the Otto cycle, comprises five events, which are:
1. The mixture of fuel and air must be drawn into the engine cylinder.
2. The mixture must be compressed.
3. The mixture must be ignited.
4. The ignited mixture expands doing work.
5. The cylinder must be cleaned of burned gases in order to receive a fresh mixture.
The above five events in the order explained are usually called: suction, compression, ignition, expansion, and exhaust.
There is another commercial gas-engine cycle, the Diesel, which is used in certain types of oil engines. The Diesel cycle also requires five events, and differs from the Otto cycle in that air without fuel is compressed in the engine cylinder to such a great pressure that the temperature resulting is sufficiently high to ignite the fuel automatically, as it is sprayed by an auxiliary pump into the engine cylinder.
The compression pressures carried in engines working on the Diesel cycle are about 500 lb. per square inch, while those carried in engines working on the Otto cycle and with the same fuels are 55 to 90 lb. per square inch.
Classification of Gas Engines
Gas engines are divided into two classes, according to the number of piston strokes required to carry out the five events of the gas-engine cycle. To one class belong all engines, which require four complete strokes of the piston, or two complete revolutions of the crankshaft to carry out the five events of the gas-engine cycle. These engines are called four-stroke cycle engines. The two-stroke cycle engine works on the same gas-engine cycle as the four-stroke cycle engine, only the mechanism is modified so as to complete the five events in two strokes of the piston.
The Four-stroke Cycle
The action of an internal-combustion engine working on the four-stroke Otto cycle is illustrated in Figs. 63 to 67.
1. Suction of the mixture of air and gas through the inlet valve takes place during the complete outward stroke of the piston, the exhaust valve being closed. This is shown in Fig. 63. This stroke of the piston is called the suction stroke.
2. On the return stroke of the piston, shown in Fig. 64, both the inlet and exhaust valves remain closed and the mixture is compressed between the piston and the closed end of the cylinder. This is called the compression stroke.
Just before the compression stroke of the piston is completed, the compressed mixture is ignited by a spark (Fig. 65) and rapid combustion, or explosion, takes place.
3. The increased pressure within the cylinder due to the rapid combustion of the mixture drives the piston on its second forward stroke, which is the power stroke. This is shown in Fig. 66. This power stroke, or working stroke, is the only stroke in the cycle during which power is generated. Both valves remain
closed until the end of the power stroke, when the exhaust valve opens and provides communication between the cylinder and the atmosphere.
3. The exhaust valve remains open during the fourth stroke called the exhaust stroke, Fig. 67, during which the burned gases are driven out from the cylinder by the return of the piston.
Gas Engine Indicator Card
An indicator diagram, taken from a four-stroke cycle engine, using gasoline as fuel, is illustrated in Fig. 68. IB
is the suction stroke, BC
the compression stroke, CD
shows the ignition event, DE
is the power stroke and EI
is the exhaust stroke. The direction of motion of the piston during each stroke is illustrated in each case by arrows. Lines AF
were added to the indicator diagram; the first is the atmospheric line, while AG
is the line of pressures. From Fig. 68 it will be noticed that part of the suction stroke occurs at a pressure lower than atmospheric. The reason for this is that a slight vacuum is created in the cylinder by the piston moving away from the cylinder head. This vacuum helps to draw or suck the mixture into the cylinder.
The engine working on the four-stroke cycle requires two complete revolutions of the crankshaft, or four strokes of the piston to produce one power stroke or working stroke. The other three are not only idle strokes, but power is required to move the piston through these strokes, and this has to be furnished by storing extra momentum in heavy flywheels. Several attempts were made from time to time to produce an internal-combustion engine by modifying the Otto or Diesel gas-engine cycles, so that the working stroke would occur more frequently. This has resulted in the so-called two-stroke cycle engine, to be explained in the next section, which completes the cycle in two strokes, requiring only one complete revolution of the crank.
The Two-stroke Cycle Engine
The two-stroke cycle engine carries out the gas-engine cycle in two strokes by pre-compressing the mixture of fuel and air in a separate chamber, and by having the events of expansion, exhaust and admission occur during the same stroke of the piston. The pre-compression of the mixture is accomplished in some engines by having a tightly closed crank-case, and in other types by closing the crank end of the cylinder, and providing a stuffing box for the piston rod. Large two-stroke cycle engines are usually made double-acting and an additional cylinder is provided for the pre-compression of the mixture.
Two-Stroke Cycle Engine
The principle of the two-stroke cycle internal-combustion engine is illustrated in Fig. 69. On the upward stroke of the piston P
, a partial vacuum is created in the crank case C
, and the explosive mixture of fuel and air is drawn in through a valve at A
. At the same time a mixture previously taken into the upper part of the cylinder W
is compressed. Near the end of this compression stroke, the mixture is fired from a spark produced by the spark plug S
. This produces an increase in pressure, which drives the piston on its downward or working stroke. The piston descending compresses the mixture in the crank-case to several pounds above atmospheric, the admission valve at A
being closed as soon as the pressure in the crank case exceeds atmospheric. When the piston is very near the end of its downward stroke, it uncovers the exhaust port at E
and allows the burned gases to escape into the atmosphere. The piston continuing on its downward stroke next uncovers the port at I
, allowing the slightly compressed mixture in the crank-case C
to rush into the working part of the cylinder W
The distinctive feature of the two-stroke cycle engine is the absence of valves. The transfer port I
from the crank-case C
to the working part of the cylinder W
, as well as the exhaust port E
, are opened and closed by the piston.
Comparison of Two-stroke Cycle and Four-stroke Cycle Engines
To offset the advantages resulting from fewer valves, less weight and greater frequency of working strokes, the two-stroke cycle engine is usually less economical in fuel consumption and not as reliable as the four-stroke cycle engine. As the inlet port I
is opened while the exhaust of the gases takes place at E
, there is always some chance that part of the fresh mixture will pass out through the exhaust port. Closing the exhaust port too soon will cause a decrease in power and efficiency, on account of the mixing of the inert burned gases with the fresh mixture. By carefully proportioning the size and location of the ports, and by providing the piston with a lip L
(Fig. 69) to direct the incoming mixture toward the cylinder head, the above losses may be decreased. In any case the scavenging of the cylinder cannot be as complete in the two-stroke cycle as in the four-stroke cycle engine, where one full stroke of the piston is allowed for the removal of the exhaust gases. The four-stroke cycle engine also has the advantages of wider use and of longer period of development.
The two-stroke cycle engine can be made to run in either direction by a simple modification of the ignition timing mechanism. This feature, and its light weight, makes the two-stroke cycle engine especially adaptable for the propulsion of small boats. For stationary purposes, in small and medium sizes, and for the propulsion of traction engines, automobiles and other vehicles, the four-stroke cycle engine is usually to be preferred on account of its reliability and somewhat better fuel economy.
Fuels for internal-combustion engines may be classified as solid, liquid and gaseous. The value of a fuel for gas-engine use depends on the amount of heat liberated when the fuel is burned, on the cost of the fuel, and on the cost of preparing the fuel for use in the gas-engine cylinder.
As was explained in the earlier part of this chapter, the fuel entering the gas-engine cylinder must be in the form of a vapor or a gas. For this reason where a gaseous fuel can be obtained at low cost, the complications of the engine mechanism are reduced. In or near the natural gas regions, no other gas-engine fuel is a competitor of the natural gas. Also in connection with certain industrial processes, certain gaseous fuels are obtained as byproducts and are utilized with good results in gas engines. Illuminating gas is usually too expensive for a gas-engine fuel.
Where solid fuels are cheap and petroleum oils are expensive, an artificial gas, suitable for gas engine use, can be generated in a gas producer. A gas producer consists essentially of a tall shell filled with coal, coke, or with some other solid fuel and supplied with a blast of air and steam. Due to the thickness of the fuel bed the combustion of the fuel is incomplete and a combustible gas is formed. The steam supplied with the blast enriches the gas and prevents the formation of clinker by keeping down the temperature of the fuel bed. Producer gas is not used at the present time as a fuel for farm motors, although experiments are being carried on with a gas producer as a possible power plant for gas traction engines.
As a portable engine for small powers, the internal-combustion engine using some liquid fuel has the greatest field of application. Such engines are especially suitable for intermittent work and are ideal for farm use.
The liquid fuels used in internal-combustion engines are gasoline, kerosene, crude petroleum, fuel oil and alcohol.
Gasoline and Other Distillates of Crude Petroleum
Gasoline and kerosene are among the lighter distillates of crude petroleum. The so-called distillates are obtained by boiling or refining crude petroleum in large retorts or stills, and condensing the vapors which are driven off at various temperatures.
The vapors which are condensed into gasoline are driven off at temperatures of 140° to 160°F. The various grades of kerosene are the condensed vapors, driven off at temperatures of 250° to 400°F., and the heavy oils are driven off at still higher temperatures.
Of all petroleum distillates, gasoline is the most important fuel for small internal-combustion engines. The yield of gasoline, however, is very small in comparison with the heavier distillates. By refining American petroleum, an average of less than 15 per cent. of gasoline is obtained and usually about 50 per cent. of kerosene. This makes gasoline more expensive than other petroleum fuels. However, as a fuel for small and portable engines it has the advantages of quick starting and greater reliability, which more than make up for the greater cost. Processes are now being perfected for extracting greater quantities of gasoline from crude petroleum, and there is little doubt that gasoline will remain for many years to come the most important fuel for small internal-combustion engines and for gasoline automobiles.
Gasoline has a flash point of 10° to 20°F. This means that it forms an inflammable vapor at that low temperature, provided a sufficient supply of air is present. For this reason care must be taken in the handling of gasoline. A good storage tank free from leaks and placed underground contributes greatly to the safety, as well as to the economical use of gasoline. When filling a gasoline storage tank or in handling gasoline, care must be taken not to have any unprotected flame nearby. In case gasoline takes fire at the engine or at the storage tank, it is best to extinguish it by means of wet sawdust. Sand or dirt will do in an emergency, but if it finds its way into the engine cylinder, it may cause considerable damage by cutting the rubbing surfaces.
Kerosene, which can be secured in greater quantities than gasoline, and which has a rather limited market, is the fuel next to gasoline, among the products of crude petroleum, for use in oil engines. This fuel is more difficult to vaporize at ordinary temperatures and presents a more difficult problem when used in oil engines than does gasoline.
The flash point of kerosene is 70° to 150°F., depending on the grade. As the flash point of oil is a measure of its safety, a kerosene of a lower flash point than 120°F. is dangerous for use as an illuminating oil in lamps. The lower the flash point of an oil the better it is for gas-engine use, as less heat is required to vaporize it ready for use in the engine cylinder.
Very light gasoline has a specific gravity of from 0.65 to 0.74.
The specific gravity of kerosene is 0.78 to 0.86, of crude oil 0.87 to 0.90, and of fuel oil 0.90 to 0.94. The specific gravity of petroleum fuels is usually given in degrees of the Baumé, hydrometer. Commercial gasoline will test from 50° to 65° Bé. This means that when a hydrometer is placed in the gasoline (Fig. 70), it will sink to a depth as will indicate 50° to 65°, the lighter gasoline showing the greater value. The relations existing between the specific gravity of various liquid fuels, the degrees on the Baumé hydrometer, and the weight of a fuel in pounds per gallon are given in Table 5.
A study of Table 5 shows that the weight per gallon of the heavier oil is greater than that of the lighter oils. Since the calorific value per pound of the various petroleum fuels is very nearly the same, and liquid fuels are bought by the gallon, it is evident that the total heat in a gallon of kerosene or in that of the still heavier oil, is much greater than the heat in a gallon of gasoline. Kerosene for farm use has the further advantages over gasoline in that it can be obtained everywhere, is cheaper, can be used for illumination in lamps and is not so dangerous.
Any good gasoline engine can be easily changed into one suitable for kerosene fuel. Such engines are started on gasoline and changed over to kerosene as soon as the cylinder walls become hot. Several types of engines, to be described later, will start on kerosene and will also operate on crude petroleum and on fuel oil. The first cost of such engines is greater than that of a gasoline engine, and these are used mainly in sizes of 25 hp. and larger for the driving of pumps in irrigation plants, and also in connection with electric light plants for towns or cities.
The various types of gas tractors, to be described in another chapter, are usually started on gasoline and operate with kerosene or with solar oil, which is a heavier distillate than kerosene.
In general, an engine running on petroleum fuel other than gasoline is more difficult to start and requires greater care and more frequent cleaning of valves and pistons. For small engines gasoline has sufficient advantages to give it the preference to the cheaper petroleum fuels.
Alcohol as a Fuel for Gas-engine Use
Alcohol as a fuel for gas-engine use has many advantages as compared with the petroleum distillates. It is less dangerous than gasoline, its products of combustion are odorless, and it lends itself to greater compression pressures than do the various petroleum fuels. Experiments show that an engine designed to stand the compression pressures before ignition most suitable for alcohol will develop about 30 per cent. more power than a gasoline engine of the same size, stroke and speed. Alcohol, when used as a fuel in the ordinary gasoline engine, and with the common compression pressures for gasoline fuel, will show a much poorer economy than gasoline, or kerosene. Engines operating with alcohol fuel are difficult to start and the operation at variable loads is less certain than with gasoline fuel.
Several years ago, when the internal revenue tax was removed from alcohol, so denatured as to destroy its character as a beverage, it was expected that denatured alcohol would become a very important fuel for use in gas engines. Its price up to this date, however, has been so much higher than that of gasoline, the most expensive of petroleum fuels, that its use in gas engines is still out of the question. It is probable that, as the cost of the petroleum distillates increases, and processes are developed for producing denatured alcohol at a low price, the alcohol engine will come into prominence as a motor for farm use.
American denatured alcohol consists of 100 volumes of ethyl (grain) alcohol, mixed with 10 volumes of methyl (wood) alcohol and with 0.5 volume of benzol.
The specific gravity of denatured alcohol is about 0.795 and its calorific value is about two-thirds that of petroleum fuels. Alcohol requires less air for combustion than do petroleum fuels. Theoretically, the calorific value of a cubic foot of explosive mixtures of alcohol and of gasoline is about the same. Actual tests show that the fuel economy per horsepower is about the same for both fuels provided the compression pressures before ignition are best suited for the particular fuel used. In gasoline engines, a compression pressure of about 75 lb. is used, while the alcohol engine gives best results, as far as economy and capacity are concerned, when the compression pressure before ignition is 180 lb. per square inch.
Essential Parts of a Four-stroke Cycle Gas Engine
Four-stroke Cycle Gas Engine
The essential parts of a gas engine are illustrated in Fig. 71. The fuel from the liquid fuel tank T
is supplied to the mixing valve or carburetor through the fuel-regulating valve G
. The air, through the air pipe A
, enters the same carburetor and is thoroughly mixed with the fuel. The mixture of air and vaporized fuel enters the engine cylinder C
through the inlet valve V
as the piston P
moves on the suction stroke. The mixture is then compressed, and ignited by an electric spark produced, at the spark plug Z
, by current furnished from the battery B
. The ignition of the mixture is followed by the power stroke. The reciprocating motion of the piston P
is communicated, through the connecting rod R
, to the crank N
, and is changed into rotary motion at the crankshaft S
. The crankshaft S
, while driving the machinery to which it is connected, also turns the valve gear shaft, sometimes called the two-to-one shaft, through the gears X
. The gear Y
turns once for every two revolutions of the crank, and near the end of the power stroke opens the exhaust valve E
through the rod D
pivoted at 0
. In larger engines this valve gear shaft also opens and closes the admission valve V
and operates the fuel pump and ignition system. As the temperatures resulting from the ignition of the explosive mixture is usually over 2,000°F., some method of cooling the walls of the cylinder must be used, in order to facilitate lubrication, to prevent the moving parts from being twisted out of shape and to avoid the ignition of the explosive mixture at the wrong time of the cycle. One method of cooling gas engines is to jacket the cylinder J
, that is, to construct a double-walled cylinder and circulate water between the two walls, through the jacket space. The base U
supports the various parts of the engine; the flywheel W
carries the engine through the idle strokes. Besides the above details, every gas engine is usually provided with lubricators L
for the cylinder and bearings, and with a governor for keeping the speed constant at variable loads.
The majority of farm gas engines are of the single-acting type. This means the combustion (burning) of the fuel takes place at one end of the piston only.
Hopper-Cooled Gasoline Engine
Vertical Gasoline Engine
The various parts of horizontal and vertical gasoline engines are illustrated and named in Figs. 72 and 73.
Carburetors for Gasoline Engines
The function of a carburetor is to vaporize the gasoline, mix it with the correct proportion of air to form an explosive mixture and then deliver the mixture to the engine cylinder.
A mixture of fuel and air in the proper proportions is one of the most important factors essential to the economical and reliable operation of a gasoline engine. If too little air is present, or if the mixture is too rich, the fuel will not burn completely. This will result in loss of power, the exhaust from the engine will be darkened and odorous, and the unburned fuel may explode in the exhaust pipe, when it meets more air. If the mixture has too little gasoline, or is too lean, it will be slow-burning. In fact, it may still be burning when the inlet valve opens on the suction stroke, and the flame, flashing back through the inlet valve into the carburetor, may produce what is commonly called "backfiring." Faulty timing of valves, or a badly leaking valve, may also cause back-firing.
In some early forms of carburetors the air was passed over the surface of the gasoline on its way to the engine and became saturated with the fuel. In another type, called the bubbling carburetor, the air was made to bubble through the fuel. The objection to these types of carburetors is that the air combines with only the more volatile portion of the fuel, leaving the heavier constituents not vaporized.
The modern carburetors are of the spray or nozzle type, that is, the gasoline is injected into the entering air through a nozzle in the form of a finely divided spray. In the best forms of spray carburetors the fuel is delivered to the nozzle at constant pressure by maintaining the fuel at a constant level in the carburetor, either by means of an overflow pipe or by a float.
To the first type belong the mixer valves, or pump-feed carburetors, in which constant pressure is obtained by a pump and an overflow pipe keeping the height of the fuel at a constant level in a small reservoir. This type of carburetor is well suited for stationary and for semi-portable engines. Pump-feed carburetors are also used to a limited extent on traction engines. This form of carburetor is well adapted for a fuel supply, which is located in a tank underground and at a considerable distance.
For automobiles, boats, portable engines and for traction engines the float-feed type of carburetor is best-adapted. In this type of carburetor the gasoline is admitted to a float chamber, by gravity, from a tank placed above the carburetor. The gasoline flows out of the float chamber by a spray nozzle, the level of the fuel in the chamber being regulated by a copper or by a cork float, which operates the gasoline valve. Most carburetors of the float-feed type are automatic in their action in that the quality of the mixture is regulated, by auxiliary air inlet valves, to suit the speed at which the motor is running.
One form of mixer valve, or pump-feed carburetor, is illustrated in Fig. 74. A pump operated by the valve gear shaft of the engine forces the gasoline through the supply pipe A
to the reservoir B
is the overflow or return pipe which maintains the fuel at a constant level in the reservoir, and slightly below the point at which the needle valve V
enters the gasoline nozzle N
. When the piston of the engine starts on the suction stroke, a partial vacuum is created in the cylinder; the inlet valve is opened and a current of air is forced by the atmospheric pressure into the cylinder. This current of air enters through the air pipe C
, attains a high velocity, and carries with it into the cylinder a portion of the gasoline vapor. This is the reason why the air passage of a carburetor is so arranged, that the velocity of the air is increased as it passes around the gasoline spray nozzle. The greater the velocity of the air at the nozzle the more vapor is carried into the engine cylinder. When starting an engine by hand with this form of carburetor, a damper or throttle in the air pipe is closed, so that the velocity of the air is increased sufficiently to admit the fuel to the cylinder.
Pump-Feed Carburetor & Engine Cylinder
The relative positions of the air throttle and mixer are illustrated in Fig. 75.
Another form of spray nozzle carburetors is illustrated in Fig. 76. Air enters at the lower opening C
, gasoline flows in at (5
), and the mixture of the air and fuel leaves the mixer valve at B
. The amount of gasoline fed is regulated by adjusting the needle valve at P
. When the engine piston moves on its outward stroke, the disc F
is raised by suction, drawing in a charge of air, through the seat opening and past the gasoline port, into the mixing chamber above F
. The lift and movement of the valve F
, and consequently the quantity of the mixture to the cylinder, is regulated by the stem (6
). The gasoline is supplied from a tank above the carburetor. This form of carburetor is much used for two-stroke cycle engines, as it facilitates easy starting, but is somewhat dangerous on account of the possibility of gasoline leakage.
In small stationary engines the form shown in Fig. 77 is often used. This carburetor consists essentially of a needle valve N
, which regulates the fuel, and a check ball valve B
which maintains the level of the fuel.
Automatic or float-feed carburetors are provided with two chambers, one a float chamber in which a constant level of the fuel is maintained by means of a float, the other a mixing chamber through which the air passes and mixes with the fuel. The float and mixing valves may be placed side by side, or the two chambers may be constructed concentric; that is, the float is placed around the spray nozzle. The concentric type keeps the fuel at the predetermined level much better than the carburetor with the chambers side by side.
The concentric float-feed type of carburetor is illustrated in Fig. 78. F
represents the float, which operates the float valve V
and regulates the amount of gasoline entering the float chamber W
through fuel inlet at G
. The air inlet to the carburetor is at A
is the gasoline-adjusting screw which regulates the needle valve. The mixing chamber around the top of the spraying nozzle
J is constructed so as to increase the velocity of the air at that point. This part is called the throat or Venturi tube of the carburetor. The amount of mixture which is allowed to pass to the engine cylinder is regulated by the throttle E
. As the throttle E
is opened and the speed of the motor increases, the velocity of the air at the Venturi passage becomes great and too much fuel is pulled in by the air. To overcome this, carburetors of this type are arranged with auxiliary valves which are controlled by the balls M
. These auxiliary valves admit more air as the speed of the motor increases, diluting the mixture before it is allowed to enter the engine cylinder.
A float-feed carburetor with the two chambers side by side is illustrated in Fig. 79. In the float chamber is placed a float F
, which operates the float valve V
and regulates the amount of fuel entering the float chamber W
. The main air inlet is at A
. When the float chamber becomes filled with gasoline to a certain level, the float closes the needle valve V
, and the flow of fuel is stopped. The fuel from the float chamber enters the mixing chamber M
, at the right, and is picked up by the air entering at A
. The mixture passes to the engine cylinder through the throttle E
The auxiliary air valve 0
is operated by a spring and regulates the quality of the mixture in proportion to the speed of the engine and in a manner similar to the ball valves in the carburetor of Fig. 78. In some forms of carburetors an enlarged main air inlet takes the place of the auxiliary valve. In others, the connection to the throttle regulates the fuel needle valve, or the air inlet, to suit the speed of the engine and the load on the engine. Two other forms of float-feed carburetors are shown in Figs. 80 and 81. The parts of these carburetors are designated by the same letters as the similar parts in Figs. 78 and 79.
The concentric type of carburetor is usually preferred on account of the fact that the pressure on the spray nozzle can be kept more nearly constant in this type than in the carburetor where the float and mixing chambers are placed side by side.
Floats for carburetors are made either of cork or of metal. The hollow metal float is more expensive and is more liable to leak. Cork floats, when covered thoroughly with shellac, will not lose their buoyancy, but there is some danger that particles may become detached from the cork and clog the passages leading to the spray nozzle.
The carburetor float chamber is usually provided with a petcock at its lowest point (P
in Fig. 79), for drawing off poorer grades of gasoline and also water.
In automobile practice, multiple-jet carburetors are sometimes used. The multiple-jet carburetor has two or more spray nozzles and this enables the engine to draw the correct proportion of fuel and air at high speeds.
The action of the carburetor, Fig. 80, is that of a multiple-jet type. In starting, this form operates as a surface carburetor, but the mixture becomes diluted as the engine speeds up.
Most float-feed carburetors are provided with some hand operated method for priming the carburetor. This is accomplished by depressing the float, so that an excess of gasoline may be allowed to enter the mixing chamber. Another method is by throttling the air.
To overcome carburetor troubles on account of climatic conditions, or where low-grade gasoline is used, the carburetor should be jacketed by hot water. A hot-air connection to the carburetor will also overcome this difficulty. In automobiles in which the thermo-syphon system of water circulation is employed, exhaust gases from the engine are used for jacketing the carburetor, instead of hot water. Hot jackets are also advantageous in cold weather and prevent the use of rich mixtures and the consequent low fuel economy.
Carbureting Kerosene and the Heavier Fuels
The various forms of carburetors described cannot be used for kerosene or for the heavier petroleum fuels, as these fuels are less volatile than gasoline at ordinary temperatures and pressures. The heavier the fuel the more heat is required to vaporize it.
A kerosene carburetor, used on some forms of traction engines, is illustrated in Fig. 82; the parts of this carburetor are designated by the same letters as similar parts in Fig. 78.
An ordinary gasoline engine will operate with kerosene fuel, if started on gasoline, but carbon deposits in the cylinder will necessitate frequent cleaning of the cylinder walls, piston and rings.
Some engines work very successfully with kerosene and the heavier distillates, if the fuel is vaporized by the heat secured from exhaust gases in a coil located entirely outside the engine cylinder. It has been found that the injection of water with the fuel reduces the carbon deposits in the cylinder and improves the operation of the engine. Water injection increases the capacity of an oil engine when operating with the heavier petroleum fuels, but decreases the economy. The supply of injection water should be cut off at light loads and used at heavy loads in amounts sufficient to prevent pre-ignition. Pre-ignition is indicated by a metallic knock within the cylinder.
Oil engines for burning petroleum fuels heavier than 35° Bé have been perfected. These engines are either of the Diesel or semi-Diesel types, and ignite the fuel automatically. The principle of construction of engines for heavy fuels will be explained in the section on "ignition."
Cooling of Gas-engine Cylinder Walls
The necessity for cooling gas-engine cylinder walls was explained in an earlier part of this chapter. In smaller engines only the cylinder or cylinder and cylinder head must be cooled. In large engines it becomes necessary to cool also the piston and exhaust valve.
Three methods are used for cooling gas engines:
An air-cooled gasoline engine is illustrated in Figs. 83 and 84. The cylinder is cast with webs, and air is circulated by means of a fan driven from the engine. In very small engines natural air circulation is used. The air-cooling system has not been found practical for stationary engines above 5 hp. Even for small engines there is no positive temperature control with this system of cooling. This often results in the decomposition of the cylinder oil and in carbon deposits on the piston and cylinder walls.
Cooling of cylinder walls by means of water is the most common method. In this case the cylinder barrel or the cylinder barrel and cylinder head are jacketed; that is, they are built with double walls and water is circulated through the space between the walls. One method of water-cooling was illustrated in connection with the hopper-cooled engine in Fig. 72. In this case the water is heated by contact with the hot cylinder walls, rises and is replaced by cooler water.
Gas Engine Water-Cooling System
Another system of water-cooling is to place a galvanized iron tank filled with water near the engine and connect the lower part of the cylinder jacket to the bottom of the tank and the upper part of the jacket at the top of the tank (Fig. 85). The cold water enters the jacket at the bottom, is heated, rises and flows to the upper part of the tank, the water circulation being similar to that of the hopper-cooled engine.
In order to definitely control the temperature of the water jacket, the forced system of water circulation shown in Fig. 71 is preferable to the two described. This system is used when a constant source of water supply is available. The temperature in the jacket is usually maintained at about 150°.
Gas Engine Water-Cooling System
Another method of water-cooling by forced circulation, used quite extensively on small stationary and portable engines, is illustrated in Fig. 86. The water from the lower part of the tank T
is forced by a pump through the jacket. The water enters the bottom of the jacket, and leaves from the top of the jacket by the pipe P. The water is then allowed to pass over the screen S
and is cooled by evaporation before reentering the tank. The advantage of this system is that the screen acts as a cooling tower and reduces the weight of water, which must be carried with the engine.
Automobiles and traction engines are provided with a cellular or tubular radiator for cooling the water from the cylinder jackets. The heated water passes through the radiator, where the rush of air to which it is exposed absorbs a portion of the heat and cools the water. A fan is arranged for inducing a cold current of air through the radiator.
Oil is being used for cooling gas-engine cylinders to a limited extent where the engines are exposed to low temperatures. The systems of oil-cooling are similar to water-cooling. In some cases natural circulation is employed, using hoppers or tanks, while in other types some form of forced cooling like the one illustrated in Fig. 71 is used. However, oil is not a satisfactory cooling medium on account of its inability to take up heat as easily as water.
In some cases non-freezing mixtures composed of water, alcohol and glycerine have been used for cooling the cylinders of gas engines. Calcium chloride and common salt solutions have also been used to some extent for the cooling of engines. These mixtures will tend to prevent freezing and the consequent cracking of the jacket and cylinder walls during cold weather when the engine is not running.
When water is the cooling medium, the engine should be provided with a drain cock at the lowest point of the jacket, so that the jacket can be thoroughly drained in freezing weather.
Gas-engine Ignition Systems
Ignition in all modern gas engines is accomplished either by an electric spark, or automatically by the high compression to which either the air or the mixture is subjected in the engine cylinder.
In some older makes of engines the hot-tube system of ignition is still employed, in which a tube, made of porcelain or of some nickel alloy, is open at one end to the cylinder and is closed at the other. The closed end of the tube is heated by a Bunsen burner. A portion of the explosive mixture is forced into the tube during the compression stroke of the piston, and is fired by the heat of the tube walls. Accurate timing of the point of ignition is quite impossible with the hot-tube system. The only points in favor of this system are the low first cost and cost of maintenance as compared with the electric system.
Electric Ignition Systems for Gas Engines
Electric ignition for farm gas and oil engines has practically superseded every other form. Electric ignition is produced by an electric spark or arc.
In one system the spark is similar to that produced when an electric circuit is broken by the opening of a switch, or when a wire connected to one pole of a battery is drawn across the other pole. This method is called the make-and-break system of ignition and is produced by contact and then quickly separating metallic points, which are located within the clearance space of an engine cylinder.
In another system of electric ignition a current of high voltage (electrical pressure) is used which jumps across a small air gap. This system is called the jump-spark ignition system.
The electric current for producing the spark in the make-and break system is usually obtained, from a primary battery of dry cells or of wet cells, from a storage battery, from a small low voltage dynamo, or from a low-tension magneto. The electric pressure required is about 6 volts and can be produced by a battery of four to eight dry cells in series or by a storage battery of three or four cells in series.
The source of current for the jump-spark system may be, a battery of dry or of wet cells, a storage battery or a small dynamo. Some form of magneto, as will be explained later, is often employed for this system of ignition.
The Make-and-break System of Ignition
Make & Break Ignition System
The principle of the make-and-break system is illustrated in Fig. 87. B
is a battery which supplies the electric current for ignition. C is an inductive spark coil, often called a kick coil. It consists of a bundle of small soft iron wires, called the core, surrounded by a coil of many turns of insulated copper wire through which the current passes. On account of the inductive action of such a coil, the spark is greatly intensified, producing a strong arc with a small current from a battery of low voltage. S is a stationary electrode well-insulated from the engine and M
is a movable electrode not insulated from the engine. Both electrodes are set in the combustion space of the cylinder. The contact points of the two electrodes are brought together by means of a cam T
operated from the valve gear shaft of the engine. When the switch W
is closed, current will flow through the circuit as soon as the contact points of the electrodes are brought together by the cam T
. A sudden breaking of the contact, aided by a spring, causes a spark to pass between the points, which ignites the mixture. The more rapidly the electrodes are separated the better is the spark produced.
The contact between the two electrodes of the make-and-break system can be made by sliding one contact point over the other, this being known as the wipe-spark igniter and is illustrated in Fig. 88. A is the movable and B
is the stationary electrode.
Another type, shown in Fig. 89, is called the hammer-break igniter. S
is the stationary and M
is the movable electrode.
The interrupter lever I
is operated from a cam on the valve gear shaft until the two contact points M
, which are located in the combustion space of the cylinder, are brought into contact. At the desired time, I
is tripped and flies back, instantly breaking the contact and producing an arc between M
. Another form of hammer make-and-break igniter is illustrated in Fig. 90, the contact points of which are designated M
Wipe-spark igniters (Fig. 88) keep the contact points cleaner than hammer-break types (Figs. 89 and 90). The hammer break igniter is more commonly used on account of the easier adjustment and less wear of the contact points.
To determine the point of ignition with the make-and-break system, the engine flywheel is turned over slowly until the igniter snaps. This is the point of ignition and should be marked on the flywheel and frame or on the piston and cylinder, so that the correct timing may be checked at any time.
To secure best results, the points of the igniter must be clean and free from carbon and corrosion, all connections must be tight, and the wires used for connecting electrodes with source of electricity must be of sufficient size to allow the current to flow freely.
The size of the inductance coil to be used in the make-and-break system depends upon the speed of the engine. For a high-speed engine, a short inductance coil should be employed, as the shorter the coil the quicker is the magnet brought to a saturated state. In the case of slow-speed engines, a larger coil can be used.
The Jump-spark System of Ignition
Jump-Spark Ignition System
The principle of the jump-spark system is illustrated in Fig. 91. A is a spark plug, the spark points E
of which project into the cylinder. These spark points are stationary, insulated from each other, and separated by an air gap of about 1
in. When the switch W
is closed, the current from the battery B
flows through the timer T
, which completes the circuit at the proper time through the induction coil I, and the induced high-voltage current produces a spark at the spark-plug gap, igniting the explosive mixture in the cylinder.
The induction coil I
, Fig. 91, differs from the inductance coil used in connection with the make-and-break system of ignition, in that two layers of insulated wire are wound on the core C
of the induction coil and only one layer in the case of an inductance coil. In an induction coil, one of the layers, called the primary P
, consists of several turns of fairly large insulated copper wire. The other winding, the secondary S
, consists of many turns of very fine insulated wire. The secondary is wound over the primary winding, but has no metallic contact with the primary. The current from the battery B
enters the primary winding P of the induction coil and induces a high voltage current in the secondary winding S
is the vibrator, sometimes called a trembler or an interrupter. The function of the vibrator R is to interrupt the primary circuit with great rapidity; this action induces an alternating current in the secondary and a series of sparks at the air gap of the spark plug. In some types of induction coils, the vibrator is omitted and but one spark is produced at the spark plug.
is known as an electric condenser. The condenser consists of alternate layers of tin foil and insulating material like paraffined paper. The condenser acts like an air chamber of a pump, in that it absorbs the excess of current at the primary winding, prevents sparking at the vibrator, and gives out this excess at the proper time to increase the intensity of the spark.
The condenser as well as the windings and the core of an induction coil are placed in a box made of wood, and the space between the parts is filled with an insulating material, usually paraffin or some similar wax mixture, in order to protect the parts from moisture. A complete induction coil for a jump-spark system is shown in Fig. 92. Induction coils operate on about 6 volts.
Fig. 93 shows inductance coils suitable for make-and-break systems of ignition.
In automobile practice, where four or more cylinders are used, induction coils are made up in units, each unit supplying a spark to one cylinder. In some cases each coil has its own vibrator; in other types one vibrator, called a master vibrator, is so connected that it breaks the current for each induction coil in turn. The system with a master vibrator produces better timing of ignition, but an accident to the master vibrator interrupts the entire system.
A spark plug used in connection with the jump-spark system of ignition is illustrated in Fig. 94. It consists essentially of two metallic points, well-insulated from each other. The central point is connected to the binding post, which receives current from the secondary, or high-tension winding of the induction coil. The other point is not insulated from the thread, and completes the circuit when the spark plug is in the engine cylinder.
Comparing the two systems of electric ignition, the jump-spark system is much more simple mechanically as it has no moving parts inside the cylinder. The make-and-break system is simpler electrically, requires less care in wiring, does not have to be insulated so carefully and the spark is more certain. It is difficult to lubricate the many mechanical parts of the make-and-break system. The make-and-break system is usually used on stationary slow-speed engines and to some extent on traction engines.
The jump-spark system is better adapted for high-speed and multiple-cylinder engines than is the make-and-break, and is used on automobiles, small stationary engines, marine engines and also on traction engines.
An ignition dynamo is a miniature direct current electric generator, built on the same plan as any large dynamo used for lighting. It has electromagnets as field magnets and is usually of the iron-clad type. One form of ignition dynamo is shown in Fig. 95. In using an ignition dynamo the internal-combustion engine must be started on batteries, as the speed developed when turning the engine by hand is insufficient to produce a spark of sufficient intensity by the dynamo. As soon as the engine speeds up, the battery current is thrown off and the spark is supplied by the ignition dynamo. Most ignition dynamos will supply a spark of sufficient intensity for a make-and-break system of ignition without an inductance coil. A 5 volt and 3 to 5 amp. generator is suitable for make-and-break ignition. For jump-spark ignition a special induction coil must be used with the ignition dynamo.
The magneto differs from the ignition dynamo in that its magnetic fields are permanent magnets. For this reason it is unnecessary to run the magneto for any length of time in order to build up its field. Magnetos can be operated in either direction and at any speed.
Magnetos may be classed under two heads:
1. High-tension magnetos which generate sufficient voltage to jump the gap of a spark plug.
2. Low-tension magnetos which include all other types and are used in place of batteries or of batteries and inductance coil.
Low-Tension D. C. Magneto
The low-tension magneto, shown in Fig. 96, is of the direct-current type and differs from the ignition dynamo (Fig. 95) in that the magneto field is a permanent magnet. This type of low-tension magneto can be used for charging a storage battery or for producing illumination on a very small scale. The direct current from the magneto (Fig. 96) is taken off by two brushes, which press on the opposite sides of a commutator. This type of magneto is usually driven by a friction wheel or by a belt, and must be operated at high speeds. Fig. 97 illustrates a low-tension alternating-current magneto. This type of magneto generates an alternating current of high frequency and can be used in connection with a vibrating induction coil for jump-spark ignition systems. It is not necessary that this type of magneto be timed with the engine.
Low-Tension A. C. Magneto
Low-Tension Low-Frequency Magneto
The low-tension magneto (Fig. 98) also generates an alternating current, but differs from the low-tension magneto of Fig. 97 in that the alternating current is of low frequency. This type of magneto is used mainly for the make-and-break system of ignition and takes the place of batteries and induction coil. The magneto of the type shown in Fig. 98 must be timed with the engine, as the current is produced only for a small portion of a revolution.
Magneto with Circuit Breaker & Distributor
The magneto illustrated in Fig. 99 is also a low-tension alternating-current low-frequency magneto, but is equipped with a circuit-breaker and distributor so that this form can be used for jump-spark system when connected with a non-vibrating induction coil. This type of low-tension magneto is often used in connection with the "dual system"; that is, with batteries for starting and magneto for operating.
Low-tension magnetos are sometimes built in the form of an oscillating magneto (Fig. 100). The oscillating magneto produces a spark irrespective of the speed of the engine, which is an advantage in starting. This form of magneto is usually of the alternating-current low-frequency type and is best adapted for slow-speed single-cylinder engines.
A high-tension magneto is shown in Fig. 101. This type of magneto is used for the jump-spark ignition systems and differs from the low-tension magnetos, in that the high-tension magneto can generate a high-voltage current without the aid of an induction coil. The armature of the high-tension magneto is provided with two windings, a primary and a secondary, carries a condenser and has a circuit-breaker at one end.
The high-tension magneto is provided with a distributor if it serves an engine with several cylinders. In traction engines and other large engines, the high-tension magneto is usually equipped with "impulse starters", which give intermittent rotation to the armature or to the rotating element of the magneto until the engine has attained a definite speed, after which time the magneto operates at constant speed.
The function of a timer is to control the flow of the low-voltage current as it comes from the battery or magneto, by closing the primary circuit of the jump-spark system at the proper time. The timer consists of one stationary and of one rotating part. Both parts are insulated electrically from each other. One part is constructed of some insulating material, such as rubber or wood fiber, and has pieces of metal, called segments, set at definite distances apart, according to the number of cylinders and number of induction coils used. As the rotating part revolves, it comes into contact with these metal segments, completing the circuit. If a four-cylinder engine has induction coils for each one of the cylinders there is a metallic contact piece on the stationary part for each cylinder.
Two forms of timers are illustrated in Figs. 102 and 103. S
is the stationary part of the timer, R
is the revolving part, and E
represents the segments which make contact as the timer revolves.
In automobiles, the timer is connected to the spark lever on the steering wheel.
Fig. 104 shows the construction of a circuit-breaker or interrupter. This form of timer is used in connection with high-tension magnetos or with a low-tension magneto and induction coil.
Fig. 105 shows a wiring diagram for a single-cylinder engine, with battery and magneto ignition.
Automatic Ignition for Oil Engines
Hot-Bulb Oil Engine
One type of oil engine, the Hornsby Akroyd, is illustrated in Fig. 106. The engine is provided with an unjacketed vaporizer A
, which communicates with the cylinder by means of the small opening B
. This vaporizer is raised to a red heat before starting, by means of a torch, and is kept hot by repeated explosions when the engine is running. This engine works on the regular four-stroke Otto gas-engine cycle. During the suction stroke of the piston only air is sucked into the cylinder and the charge of oil fuel is injected into the vaporizer by a pump. On the return stroke the air is compressed, forced in the vaporizer, mixed with the fuel and automatically ignited. This is followed by the expansion and exhaust strokes, as in other internal-combustion engines.
A modification of this type of engine is the so-called semi-Diesel type of oil engine, which is well-adapted for the burning of the lowest grades of petroleum fuels. In this case the air is compressed to about 250 lb. per square inch before the fuel is injected into the cylinder.
The Diesel engine was mentioned in the first part of this chapter. It is very economical for the burning of low grades of fuel, but the high first cost of the engine limits its field of application in small sizes.
Lubrication of Gas and Oil Engines
The selection of the proper lubricating oils and of the best oiling devices is of great importance, if reliability of operation and long life of an internal combustion engine are desired.
The extremely high temperatures which are developed within the cylinder of a gas engine or of an oil engine and the absence of moisture make the selection of the proper oil a necessity. Oils employed for lubricating steam-engine cylinders are not suitable for internal-combustion engines. Such petroleum lubricating oils should be employed as are light, are fairly thin, will withstand high temperatures, are free from acid and from animal or vegetable matter and which will leave no carbon deposits. The lubricating oil should flow freely at all seasons of the year and should not easily vaporize at the high temperatures.
Oil which will gum or form carbon deposits will tend to make the piston rings stick or may produce pre-ignition.
Graphite is satisfactory for lubricating certain parts of an internal-combustion engine outside the cylinder. In general, the gas-engine oil used for lubricating the engine cylinder will be found satisfactory for bearings and other parts, but in large engines a saving can be produced by employing a cheaper oil for the bearings.
Single-cylinder gas and oil engines are usually lubricated by a sight-feed oiler (Fig. 107). This oiler differs from the ordinary sight-feed oiler in that a check ball U
is used in order to guard the oiler during a portion of the cycle from the pressure within the cylinder.
The mechanical oiler (Fig. 108) holds a large quantity of oil, is positive in action and requires little care.
Forced-Flooded Lubrication System
In high-speed motors, the forced-flooded system of lubrication is commonly employed (Fig. 109). In this system a pump forces oil to the various bearings, keeping them flooded with oil at all times.
The splash system of oiling is usually more satisfactory with gasoline engines than with kerosene engines, as the kerosene which gets by the piston is injurious to the lubricating properties of the oil in the crank-case.
Governing of Gas Engines
Every gas engine must be provided with some governing mechanism in order that its speed may be kept constant as the power developed by the engine varies. The governing mechanism is operated by the speed variations of the engine and the speed control is accomplished by the following methods.
In this system the number of explosions is varied according to the load on the engine. This can be carried out in several ways, depending on the valve gear of the engine. In the case of small engines, where the inlet valve is automatically operated by the vacuum created in the cylinder during the suction stroke, the governor operates on the exhaust valve by holding it open during the suction stroke. The free communication of the engine cylinder with the outside prevents the formation of sufficient vacuum in the cylinder to lift the inlet valve. When the inlet valve is mechanically operated from the valve gear shaft, the governor acts on the inlet valve, keeping it closed part of the time at light loads. The governor used to accomplish this is usually some form of fly-ball governor. As the speed of the engine increases, the balls are thrown out by centrifugal force and shift the position of a cam on the valve gear shaft, preventing the opening of the inlet valve.
The hit-or-miss system of governing can also be carried out by having the governor open a switch, thus interrupting the flow of current to the igniter, as the load decreases. This method is very wasteful of fuel as the fuel drawn in at each suction stroke passes through the engine and is wasted. It should be used only in connection with one of the other methods of governing. The hit-or-miss system of governing is very simple and gives good fuel economy at variable loads. As the explosions in the engine do not occur at regular intervals, this system of governing necessitates the use of very heavy flywheels in order to keep the speed fluctuations within practical limits. The hit-or-miss system is very well-adapted for small and also for medium-sized engines where very close speed regulation is not essential.
Varying Quantity of Mixture
In this system the proportion of air to fuel is kept constant and the quantity of the mixture admitted into the cylinder is varied according to the load. This variation is accomplished either by throttling the charge or by changing the time during which the inlet valve is open to the cylinder. In fact, the two methods of varying the quantity of the mixture are similar to those used in governing steam engines.
Varying Quality of Mixture
In this case the total quantity admitted into the cylinder is kept constant, but the amount of fuel mixed with the air is varied according to the load. When gas engines are governed by varying the quantity or quality of the mixture, the speed is more uniform at variable loads. Also, since the explosions occur at definite periods, the temperatures inside the cylinder are kept more constant. The throttling form of governor is used most commonly with traction engines.
The Gasoline Engine on the Farm
Some of the uses to which a gasoline engine can be applied on the farm are illustrated in Figs. 110 to 117.
A 12-hp. gasoline engine is used for silo filling in Fig. 110.
Gasoline Engine & Sheller
Fig. 111 illustrates a gasoline engine applied to shelling corn.
Gasoline Engine & Hay Press
A 7-hp. engine driving a hay press is illustrated in Fig. 112.
Gasoline Engine & Binder
A binder driven by a 4-hp. engine is shown in Fig. 113.
Gasoline Engine & Spraying Outfit
An air-cooled gasoline engine of 2 hp., direct-connected to a
spraying outfit (Fig. 114), is capable of producing a pressure of 100 lb. per square inch or more, as compared with about 50 lb. in the case of the hand sprayer.
Gasoline Engine & Water Pump
The application of the gasoline engine to pumping water for farm use is illustrated in Fig. 115.
Gasoline Engine & Cream Separator
Fig. 116 shows the application of the small gasoline engine for the driving of cream separators.
Gasoline Engine & Wood Sawing Rig
A wood-sawing rig, Fig. 117, can be removed by loosening clamp bolts, and the engine used for grinding feed, pumping, shredding, or for any other farm work within its capacity.
Other uses to which the gasoline engine can be put include: the driving of cement mixers and rock crushers, the grinding of feed, the driving of grindstones and other tools in the farm shop, the driving of electric generators for farm lighting, and for various other work about the house, barn and dairy which require power.
Selection And Management Of Gas And Oil Engines
Selecting a Gas Engine
A gas engine should be selected large enough to do the required work, as it will stand but little overload. This is due to the fact that the gas engine develops its maximum power when a full charge of the best mixture of fuel and air, at the maximum density, has been admitted to the engine cylinder. On the other hand, an engine too large for the work it has to do will give poor fuel economy.
As the economy is very nearly independent of the size of the gas engine, it is better to buy two small engines than one large one. This applies especially to the farm, where the larger engine of 6 to 10 hp. can be used for the heavier work, such as feed grinding, threshing, wood sawing, etc., and a small engine of about 2 hp. for the many small tasks, about the house, dairy, and barn, which require but little power. An engine of 2 hp. is sufficient to drive a small dynamo, to light the house, barn, etc., and to charge a storage battery. The same engine, if portable, can be used for driving a washing machine and wringer, a tree-sprayer outfit, a house pump, a cream separator, etc.
An engine governed by the hit-or-miss principle should carry such a load as will enable it to miss one explosion in every eight, as this will keep the cylinder free from inert burned gases and will improve the economy. If an engine is worked at its maximum power the largest part of the time, the wear on the parts will be too great.
An engine for farm use must be capable of being started easily and should be simple in construction. Every gas engine must have certain parts to carry out the cycle of operations, as explained in the earlier part of this chapter, but some engines are provided with many attachments, which have good points, but which complicate the engine so that the first cost is greater and the manipulation more difficult. An engine to be of value on the farm must be sufficiently simple in construction that ordinary adjustments and repairs can be made without the aid of experts.
In regard to the method of igniting the mixture, the electric system is best for gasoline engines. It is well to provide a gasoline engine with a magneto or ignition dynamo, as with batteries the cost of upkeep is considerable and the reliability of operation uncertain. Regarding drives for magnetos, friction and belt drives should not be selected, as they are not reliable. A magneto should always be positively driven from the engine by gears.
There is very little choice between the jump-spark and the make-and-break systems of ignition. For stationary engines the make-and-break is commonly used while the jump-spark is more common on automobiles and traction engines. No matter which system of electric ignition is selected, the various wires should be well-insulated, and enclosed in some moisture-proof conduit.
For irrigation work where the cost of fuel is an important item, an engine should be selected which will operate with the cheaper fuels. For engines under 100 hp., those which will burn kerosene or solar oil will usually be found satisfactory. Such engines employ electric ignition and the fuel is vaporized in a coil entirely outside the engine cylinder. For work requiring 100 hp. and more the various engines with automatic ignition, which use fuel oil, will be found more economical.
It is essential to select an engine from a reputable manufacturer. Every engine is subject to breakage of parts and it is important that duplicate parts may be easily secured. It is also well to investigate the work done by engines of various makes before making the final selection.
The rated horsepower of an engine does not often mean the same actual power for different makes of engines. An engine rated at 10 hp. by one manufacturer may be capable of developing 10 to 25 percent more power than an engine of the same rating by another manufacturer. The purchaser should insist on a definite statement as to the actual brake horsepower, which the engine is capable of developing.
Installation of Gas Engines
It is usually best to locate a gas engine in a separate room. The room should be well-lighted and ventilated, free from dirt and dust and large enough so that there is sufficient space for easy access to any part of the engine so as to facilitate starting, oiling and inspection of all parts. In connection with gasoline and oil engines, the fuel tank should be located outside the building and preferably underground. In any case the tank must be lower than the pipe to which it is connected in the engine room.
As the mixture of fuel and air is ignited inside the engine’s cylinder, the resulting explosion produces a shock of considerable magnitude on the mechanism, which in turn is transmitted to the foundation. The foundation should be as solid as possible. If the engine is to be set on a wood floor, it is usually well to lay long timbers on or under the floor and at right angles to the joists. If the foundation is to be built of brick or of concrete it should be sufficiently heavy and should be separated from the walls of the building, so that vibrations caused by the engine will not affect the building or surrounding buildings. If the engine has to be located over another room it is best to place the engine in a corner and near the wall.
If the engine is to be connected to the machines to be driven by belt drive, the driver and the driven should be placed far enough apart, that the required power can be transmitted without running the belts too tight. A distance between pulleys equal to about eight times the size of the larger pulley will usually give good results. Open belts are preferable to crossed belts and should be used whenever possible.
The exhaust piping should be as straight and as short as possible. The exhaust gases should always be discharged out of doors, as the fumes are poisonous. Some engines are provided with exhaust mufflers, which can be located near the engine. As a rule, it is better to locate the muffler outside the building. Engines should never exhaust into a flue or chimney.
The air supply can be taken from the room in which the engine is placed or from the outside. In all cases a screen should be placed over the air pipe.
Instructions for Operating Gas Engines
Before an engine is started for the first time, all the working parts should be carefully examined and nuts and other fasteners properly tightened. The electrical connections should then be gone over and the spark plug or spark points removed from the cylinder and tried.
The operation and economy of a gas engine is greatly influenced by the proper timing of the valves and by the point of ignition.
The exhaust valve should open before the end of the power stroke. This is necessary to prevent loss of power when the piston starts on the exhaust stroke. The exhaust valve should begin to open when the crank is at an angle of from 20° to 40° before the outer dead-center. The time of opening of the exhaust valve must be earlier for high-speed than for slow-speed engines.
The exhaust valve should remain open until the crank has turned 3° to 8° beyond the completion of the exhaust stroke.
The suction stroke follows the exhaust stroke, and, in order to prevent the mixing of the fresh charge with the burnt gases, the inlet valve should open about 3° (crank rotation) after the exhaust valve closes. The time of closing of the inlet valve should be after the crank has turned 10° to 25° beyond the completion of the suction stroke.
The setting of the gas-engine valves so that they will open and close at the proper time, can be accomplished by adjusting the length of the valve push rods or by changing the timing of the cam gears. The exact setting of the valves will depend upon the engine speed, and upon the fuel used.
Ignition should be timed to suit the fuel, the compression and the speed of the engine.
In order that the entire mixture may be ignited and burning at the beginning of the power stroke, it is necessary to have the spark advanced; that is, the point of ignition must occur earlier than the beginning of the power stroke.
Proper ignition can best be determined by an indicator. The experienced operator can set the spark very nearly at the proper place by the sound of the engine. For the inexperienced operator the following approximate rules should prove of value:
For jump-spark system, turn crank and set spark mechanism so that ignition will occur, 5° ahead of dead-center, for every 100 r.p.m. of the engine speed rating.
For the make-and-break system, advance spark approximately 8° for every 100 revolutions of engine speed rating.
As an illustration of the application of the above rule, calculate the spark advance for a stationary engine operating at 350 r.p.m. If the engine has make-and-break ignition system, ignition should take place when the crank is at a position of 28° before dead-center. In case a jump-spark system is employed, the spark should occur when the crank is at a position of about 17½° before dead-center.
The gas engine is not self-starting, as is the steam engine when steam is turned on. The reason for this is that the explosive mixture of fuel and air must be taken into the cylinder and compressed before it can give up its energy by explosion. It is, therefore, necessary to set the engine in motion by some external means not employed in regular operation, before it will pick up the normal working cycle. Engines under 20 hp. are usually started by hand. This is done by disconnecting the engine from its load and turning the flywheel by hand for a few revolutions. If everything is in good condition an engine should start with two or three turns of the flywheel and should continue to run after the first explosion. An easier method of starting gasoline engines is to set the engine at the end of the power stroke, inject some gasoline into the cylinder through a priming cock, turn the flywheel backward against compression as far as possible and then quickly trip the igniter.
As it is difficult to pull over an engine by hand against compression throughout the whole stroke, some engines are provided with a starting cam, which can be shifted so as to engage the exhaust valve lever. This relieves the compression while cranking, as the exhaust port is open during the first part of the compression stroke. After the engine speeds up the starting cam is disengaged.
Gas engines larger than 25 hp. are usually started with compressed air. If the engine consists of two or more cylinders, this can be accomplished by shutting off the gas supply to one of the cylinders and running this cylinder with compressed air from a tank, in the same manner as a steam engine is operated with steam from a boiler. As soon as the other cylinders pick up their cycle of operations the compressed air is shut off and fuel with air is admitted to the cylinder used in starting. With large gas engines of only one cylinder, the compressed air is admitted long enough to start the engine revolving, when the compressed air is shut off and the mixture is admitted. The air supply for starting is kept in tanks which are charged to a pressure of 50 to 150 lb. by a small compressor, driven either from the main engine shaft, or by means of an auxiliary small engine.
In starting a gas engine the following steps should be taken, preferably in the order given:
1. The fuel supply should be examined. Cases have been known in which an operator spent considerable time hunting for faults in the ignition system, valve setting, etc., when an examination of the gasoline tank would have revealed the fact that it was empty.
2. The ignition system should be tried by closing the switch disconnecting the end of one of the wires and brushing it against the binding post to which the other wire is attached. A good spark should have a blue-white color. If the spark produced is weak, the ignition system should be put in the proper condition.
3. The lubricators and grease cups should be filled and adjusted, so that the proper amount of oil is delivered to all bearings and moving parts.
4. The load should be disconnected from the engine by means of a friction clutch or similar device, the lubricators turned on, the spark retarded to the starting position, and the starting cam moved into place.
5. The engine is now ready for starting by either of the methods previously explained. In cranking, always pull up on the crank.
6. As soon as the engine picks up, disengage starting cam, turn on cooling water, advance spark to running position and throw on the load by means of the clutch.
7. Adjust fuel supply so that the engine carries its load with the cleanest possible exhaust.
To stop an engine, the fuel valve is closed, the ignition-system switch is opened, the lubricators and oil cups are closed and the jacket water is turned off. In cold weather the water should be drained from the engine jackets to prevent freezing. The practice of draining the jackets is also advisable in moderate weather, as this tends to clean the jacket from the deposit of sediment. Before leaving the engine it should be cleaned, all parts examined and put in order ready for starting up.
Causes of Gas Engines Failing to Start
Failure to start may be due to one or more of the following causes:
1. Ignition System Out of Order;
This may be caused by the switch being left open, by a loose terminal, by a disconnected wire, by a broken wire the insulation being intact, by the ignition battery being weak if a battery is used, and by poor timing or wrong connections if a magneto is employed. Other causes of faulty ignition are due to timer slipping on the shaft, to a short circuit in the ignition system, to carbonized or broken spark points, to poor timing of the points of ignition. In the case of the jump-spark system, ignition will also be prevented if, the points on the spark plug are too far apart, the spark plug is dirty or broken, the insulation on secondary wires is poor, induction coil windings are broken or short circuited, vibrator of induction coil is not properly set.
2. An engine will not start if the mixture contains too much or too little fuel.
In very cold weather a gasoline engine may give trouble by the fuel not vaporizing. This can best be remedied by filling the jackets with hot water. Do not bring a flame near the carburetor or gas supply pipe. This is sometimes recommended for starting in cold weather, but the practice is a dangerous one.
Improper mixture may be caused by slow cranking, in which case the hand placed over the air inlet will often start the engine. Extra priming of the carburetor may also aid in starting, provided care is taken not to flood the engine with fuel.
3. Supply pipes clogged.
4. Dirt or water in the fuel.
5. Pump or carburetor out of order.
6. Water in carburetor.
7. Water in the cylinder due to leaky jacket.
8. Inlet valve poorly set or not operating due to broken valve stem, weak or broken spring, valve sticking or broken.
9. Poor compression due to leaky or broken piston rings, improper seating of valves, or to other leaks from the cylinder to the outside.
10. If the exhaust pipe or muffler is clogged, the engine will fail to start.
In any of the above cases the remedies are self-evident.
Causes of Motor Failing to Run
A motor will sometimes start, but will soon afterward slow down and stop. This may be due to:
1. Fuel tank being empty or fuel pipe becoming clogged.
2. Poor or insufficient lubrication, which may cause the seizing of the piston or of the bearings.
3. Wire being jarred loose from its terminal, timer slipping on
shaft or to some other fault in the ignition system, such as weak cells, or vibrator or induction coil becoming stuck.
4. Engine carrying too great a load.
Care of a Gas Engine
It is best to keep one man responsible for the care of an engine and in so far as possible confine the operation to one man. The engine should be kept clean and all the parts should be examined frequently to see that everything is in the best working order.
If an engine runs well at no-load but will not carry its rated load, this may be due to: poor compression, poor fuel, defective ignition, poor timing of ignition, incorrect valve setting, incorrect mixture, leaky inlet or exhaust valves, too much friction at bearings, or to engine being too small for the rated load.
The operator usually can tell whether the correct mixture is being admitted into the cylinder by watching the exhaust. Black smoke issuing from the exhaust pipe means that the mixture is too rich in fuel. This should be remedied by decreasing the amount of fuel supplied or by increasing the air supply. Insufficient fuel in the mixture, as explained in the section on "Carburetors," will cause the engine to miss explosions and may even cause back-firing.
Premature ignition, often called pre-ignition, is due to the deposition of carbon and soot on the walls of the cylinder, the compression being too high for the fuel used; by overheating of the piston, exhaust valve, or of some poorly jacketed part.
Deposition of carbon on the cylinder walls is usually caused by the use of either an excessive amount or a poor quality of lubricating oil. This will not only cause pre-ignition, but may also impair the action of the valves, igniter and piston rings. Carbon deposits will also be produced if the mixture is too rich.
Insufficient lubrication may result in abrading surfaces of piston and cylinder.
It is well not to economize when buying gas-engine cylinder oil. Due to the high temperatures developed inside the engine cylinder and to the absence of moisture, a cylinder oil should be selected which is light and thin, which will withstand high temperatures and will leave no carbon deposits. A cylinder-lubricating oil well-suited for steam-engine use will not do at all for gas engine cylinder lubrication.
For the bearings and other wearing parts outside the cylinder, a good grade of machine oil will be found satisfactory.
A blue smoke at the exhaust indicates that too much cylinder oil is being used.
Pounding in gas and oil engines is either due to pre-ignition, the causes of which were outlined above, to lost motion in some bearing of the engine, or to the engine being loose on its foundation.
In the case of oil engines using a water spray with the fuel, too little water will result in pre-ignition and consequent pounding. This should be remedied by supplying more water with the fuel. Too much water will be indicated by white smoke issuing from the exhaust pipe.
In the case of a gasoline engine, white smoke at the end of the exhaust pipe usually indicates water in the gasoline, which may be due to a leaky jacket or to some other cause.
In regard to the temperatures of the jacket water, this depends on the compression carried and on the size of the engine. With small engines of the hopper-cooled type the jacket temperature is near the boiling point of water. Ordinarily a temperature of about 150°F. will give good results. It is advisable to use cooling water over and over again, since after several circulations through the jackets, the impurities contained in the water will have been precipitated.
- Farm Motors by Andrey A. Potter 1917 pages 65-120