Individual oil cups such as were used formerly, have been eliminated from the cylinders, and whatever sight-feeds there may be are placed on the dash in plain view of the driver. Instead of relying upon the suction of the cylinders for the positive feed to the piston, mechanically-operated pumps are used to force the oil to the various portions of the motor. In some systems, there is a separate pump for each oil lead. This is known as a mechanicaloiler, and generally consists of an oil tank located on the dashboard of the car—either in front of the driver, or under the motor hood—and connected by means of a belt or gear with some shaft of the motor. The belt or gear drives a shaft to which is connected the plungers of the various oil pumps that force the oil to the different parts of the motor. Before passing to the individual pipe, however, the oil drops through a sight-feed connected with that lead, and as all of these sight-feeds are mounted in a row within plain view of the driver, the condition of the lubricating system in part or in whole may be determined at a glance.
The parts of the motor that are lubricated by an independent feed line in this manner may vary with different motors. In general, however, it may be said that it is seldom that the oil is fed directly to the piston, but that the lubricant is first distributed to the oil wells in the crank case. Here, the splash of the cranks as they revolve in the oil is depended upon to throw the lubricant upon the exposed portion of the piston as it reciprocates below the cylinder walls. The sides of the piston thus covered carry the oil to the cylinder walls.
It is evident that if an excess amount of oilis continually carried up by the piston to the cylinder walls, a certain proportion of this lubricant will reach the open space in which the charge is ignited, and will there be burned—with the attendant formation of clouds of objectionable smoke. This trouble is overcome to a certain extent in some motors by the use of a type of ring set in the piston that prevents the lubricant from passing to the upper portion of the cylinder; but all the oil cannot thus be retained, and it therefore behooves the driver not to allow too great a quantity to be fed to the crank case if the "splash" system is used.
The main bearings on which the crank shaft revolves are generally supplied with oil by independent leads from the oiler, and when the above-described system is used they may be regulated independently of the splash feed lubricating pipes. Excess oil at the bearings will cause no damage, but each crank shaft journal does not demand as great an amount as that supplied to a piston and connecting rod bearing.
Many lubricating systems that are now in popular use employ but one pump to force the oil to the various bearings and rubbing surfaces, and regulate the supply by the size of thepipe leading to each. A satisfactory method of overcoming the possibility of excess oil in the cylinder has been adopted by some manufacturers. This consists in placing a channel, or trough, directly under the lower sweep of each connecting rod bearing. Each channel is kept filled to overflowing by a separate pipe connected with the main lead from the pump, and a constant level is consequently maintained at all speeds of the motor. An elaboration of this method consists in attaching one end of each trough to a rod operated in conjunction with the throttle, so that as the speed of the motor increases, the end of the channels may be tilted, with the result that the connecting rod scoop will dip deeper into the lubricant.
After the proper level in each trough has been reached the excess oil overflows into the bottom of the crank case. From here, it is again started on its way by the pump and is distributed to the various bearings and troughs through the different pipes leading from the pump. As a further precaution against a smoking exhaust, some designers have added a baffle plate above each crank case compartment that serves to reduce the size of the opening through which the oil may be splashed. With this combination of troughs and baffleplates the possibility of a smoking motor is practically eliminated.
All motors are not so equipped, however, and in the case of those provided with the bona-fide splash system, care must be taken to keep the separate crank case compartments filled to the proper level. Too high a level in the crank cases will cause the motor to smoke; while the supply should not be allowed to become so low that when the angle of the crank case is changed—as in ascending a hill—the lubricant will run toward the rear and will not be reached by the scoop on the connecting rod bearing. This latter danger makes it advisable to give this system plenty of oil when any touring is to be done through a hilly district.
In some lubricating systems, the oil is supplied as it is used, and either is discharged with the exhaust, or collects in the bottom of the crank case, from which it should be drained occasionally. In the circulating systems, however, which are now used on a majority of the cars, the same oil is used continuously until it becomes "worn" or filled with sediment and particles of dirt and other foreign matter. The pump used for maintaining this circulation may be either of the plunger, centrifugal, or gear type, and is generally housed in a portionof the crank case. A strainer is usually placed in the suction end of this pump for the purpose of removing all the free foreign matter from the oil before it is again started on its mission of lubrication. In these systems, the oil well is generally located in a "secondary" bottom of the crank case. From here it may be drained when the supply is to be renewed.
Another successful system by which all the bearings of the crank shaft are positively lubricated is used on many of the best cars. In this system, a continuous oil hole passes throughout the length of the crank shaft, including its "arms" and connecting rod bearings. At each bearing, one or two small oil holes connect with this main artery and extend radially to the surface. Oil is forced into the longitudinal oil hole by means of a small pump, and naturally finds its way through every radial opening to all the bearings. The excess may overflow into the individual oil wells, from which it will be splashed upon the exposed portions of the pistons as they descend.
It will be seen that, no matter what modern oiling system is used, the same kind of lubricant is supplied to all parts of the motor. This feature makes matters much simpler than was the case when one oil was used for thecylinders, another, of a different thickness, supplied to the crank case, and still a third required for the gears. By the old gravity systems, the flow of oil depended largely upon its viscosity, or thickness. Therefore, in winter, a thinner oil was required than in summer, for the more a lubricant is warmed, the thinner does it become—and vice versa. With the mechanical force systems now in use, however, practically the same kind of oil may be used throughout the year—although many motorists believe that better results will be obtained if a heavier oil is used in summer than in winter. The oil will be warmed by the motor and it will not require many minutes of operation before a lubricant made thick by a low temperature will flow freely and do its work as efficiently as a thinner oil.
But no matter how reliable a lubricating system may be in its operation, the driver must do his share and make certain that fresh oil of the proper quality is supplied when needed, and assure himself that all the passages are free from obstructions. Negligence on the driver's part may result in one or more "stuck" pistons that will either seriously injure the motor, or will put it out of commission until the trouble can be remedied. If a sufficient supplyof oil is not fed to the rubbing surfaces between the piston and the cylinder walls, a high degree of heat is generated which will tend to expand the piston until it grips the cylinder so closely that the former cannot be moved. In this event the motor will stop "dead," and cannot be started again until the piston has cooled and contracted to its normal size. Even then, however, the motor should not be run under its own power until the burned and gummed oil has been removed and the scored surfaces have been cleaned. While this may best be done by removing the piston—at which time an examination for any badly burned rings may be made—this is not always possible, and it may be necessary to run the car home or to the nearest repair shop before the proper repairs can be made.
In this case, the motor should be turned by hand until it is certain that the piston is again free in its cylinder. Liberal quantities of kerosene oil should be poured in through the spark plug opening, and if possible, the motor should be "rocked" back and forth by the flywheel to give the kerosene an opportunity to reach all parts of the piston and rings. The kerosene will serve to cut and remove much of the carbon and gummed oil and to make the way freefor the fresh lubricant, which should be poured in liberal quantities into the cylinder head. The flywheel should again be moved back and forth so that the oil will reach all parts of the piston surface, and after this—if the damage has not been too great—the motor should be ready for operation.
Toenable the parts of a motor to work well, there must be freedom of motion between all that move in contact with each other. This necessary freedom of motion is provided for to a certain extent by proper lubrication, but this is not all-sufficient. The necessity for some additional friction- and heat-reducing system can be better realized when it is understood that the temperature of the explosion in the cylinders of a gasoline engine is well over 2,600 degrees, Fahrenheit. The melting point of pure iron is less than 2,800 degrees. Therefore were there no escape for this heat, and could the motor be induced to run under these severe conditions, the cylinders would soon reach a temperature dangerously near the melting point. Long before this point could be reached, however, the intense heat would have expanded the pistons so that they would become stuck in their cylinders, and no more explosions could occur. Anominous knock in one or more of the cylinders, followed by a sudden laboring and final cessation of operation on the part of the motor, is sometimes the first intimation that the driver may have that his engine is over-heated; but serious as a "stuck" piston may seem, it is fortunate that the motor stops of its own accord, for to continue to run under these conditions of constantly increasing heat would be to wreak far more serious and permanent damage upon the moving parts than the broken rings or scored cylinders that usually result from a lack of lubrication or cooling medium.
A large amount of the heat resulting from each explosion is carried out through the exhaust pipe in the form of the burned gases, while other portions radiate into the surrounding air. These outlets are not sufficient, however, to carry away all the heat that is necessary to enable the motor to run efficiently, for proper piston lubrication is exceedingly difficult to obtain at high temperatures. There must, therefore, be more positive and direct means for carrying off this undesired heat, and to accomplish this result every internal combustion motor is provided with a cooling system of either the air or liquid (usually water) type. Motorcycle power plants and a few ofthe small and medium-sized automobile engines employ the air-cooling system; the great majority of automobile engines, stationary plants, and marine motors use water as the cooling medium.
Let us consider first the air-cooled system. The area presented by the outside of a smooth cylinder is not large enough to enable sufficient radiation to take place. That is, the heat is concentrated on a comparatively small surface, and this is much more difficult to keep cool than is the same amount of heat distributed over a greater area—for the cylinder will be exposed to a larger quantity of fresh air in the latter case. Therefore many air-cooled engines are provided with a series of grooves and flanges on the outer surface of the cylinder. The heat is conducted to all parts of this surface—flanges as well as grooves—and the area of the surface that is exposed to the cooling air is greatly increased thereby.
These grooves and flanges may extend circumferentially around the cylinder, as is the case with many motorcycle engines, or they may extend longitudinally. Another form of air-cooling system consists of pins or spines projecting radially from the surface of the cylinder. The motion of the car through theair is generally sufficient to create a circulation of the cooling medium, but in order that this circulation may continue while the car is at rest a high-speed fan is provided that draws the air from the front toward the rear of the motor. This serves also to supplement the air circulation produced by the motion of the car, and keeps the motor much cooler than would be the case were the machine run without the fan. This fan is generally attached to a bracket at the front of the motor, and is driven either by a belt or geared shaft. In some designs, however, the fan blades are included in the flywheel at the rear of the motor and the air is thus sucked over the cylinders.
One of the most effective air-cooling systems for use on an automobile motor consists of the above-mentioned longitudinal flanges and grooves enclosed in a thin jacket or casing surrounding each cylinder. These jackets are open at the top and bottom of the cylinders, and connect with large pipes, or troughs, through which air is forced. The trough into which the top of the jacket spaces open is connected with the discharge end of a large fan. The air is thus driven into the top trough, through each jacket, and into the lower trough, the farther termination of which is connectedwith the suction end of a fan included in the flywheel. The two fans serve to set up a rapid circulation of air which, by means of the troughs and jackets, is concentrated upon the surfaces of the grooves and flanges of each cylinder and none is wasted on parts of the motor that it is unnecessary to cool. Furthermore, the rear cylinders receive as much air as do the forward ones, for the trough serves to distribute the circulation equally along the grooves and flanges of each.
Inasmuch as the heat from an air-cooled motor is radiated directly into the current of air itself, the surface is very susceptible to temperature changes from the interior. Thus, if the car is run for a great distance on the low gear, and the cylinders become hot in consequence, a larger amount of heat will immediately be radiated from the cooling surfaces than is the case when the motor is running slowly. A "coast" down a short hill, however, will serve to cool the motor rapidly, for if the engine is run from the momentum of the car with the spark turned off, cool air will be drawn into the cylinders, and this, in addition to the circulation of cold air on the outside, will reduce the temperature of the engine rapidly. This is a feature of the operationof an air-cooled motor that is not possessed to so large an extent by those of the water-cooled type.
It is, perhaps, hardly accurate to apply the term "water-cooled" to the ordinary type of automobile motor. Water is merely the medium that transfers the heat from the cylinders to the cooling surface of the radiator. As air is used to cool this heated water, we see that the only difference between the two systems lies in the point of application of the actual heat-absorbing medium—which is air in both cases. Thus in the air-cooled motor the air is carried directly to the surfaces to be cooled; while in the other type, the heat is transferred by means of the water to the point where it may be effectually discharged into the air.
Each cylinder of a water-cooled motor is surrounded by a space known as the water jacket. This space is generally cast with the cylinder, although in some designs of motors the jackets are formed by the subsequent application of a copper casing that serves to retain the water. The water jackets are connected with each other by means of piping and water-tight joints so that the water will pass successively from one to the other. If thewater remained in these spaces, it would soon be warmed to a temperature far above the boiling point, steam would be formed, a high pressure generated, and infinite harm would result—both to motor and to passengers. The piping, however, does not end with the connections between the cylinders, but extends to and from the radiator.
This radiator is a large, perforated structure placed either forward of the motor to form the end of the bonnet-covering, or in front of the dash between it and the rear cylinder of the engine. The radiator is a mass of small cellular or tubular passages, each one of which possesses an exceedingly large outer surface in proportion to the amount of water that it can contain. When the hot water reaches the radiator it is distributed to these many cells or tubes, and is thus spread over a large cooling surface. A large fan is usually located directly behind the radiator, and as this serves to draw the air rapidly through the openings between the cells or tubes, cooling is greatly facilitated.
There are several types of radiators in general use. Some consist of a number of flat cells placed in such a manner that regular-shaped air openings will be formed. Each side of eachflat water cell abuts on an air passage. Such a radiator is known as the honeycomb, or cellular, the former term being applied to those whose cells resemble a honeycomb. The tubular radiator consists of a number of vertical, parallel tubes through which the water passes, and which are placed a sufficient distance apart to provide ample air passages between them. Each tube is covered at frequent intervals with fluted, circular flanges that serve to increase the radiating surface in much the same manner as do the grooves and flanges on the cylinders of the air-cooled motor. All air passages in any radiator extend directly through the width of the radiator, while the water circulates from top to bottom in a vertical direction.
The reason for this circulation of the water will be apparent if we call to mind a bit of our elementary physics. When water is heated, it expands and rises, and for this reason, we always find the surface of the water in a teakettle warmer than is that at the bottom—although the latter is closer to the fire. As the water is circulated through the radiator, it is cooled by the passage of the large amount of air through the openings between the cells or tubes. The water thus cooled sinks to the bottom of the radiator and is replaced by thewater just heated by the motor. The cooled water is conducted to the bottom portion of the end cylinder, and passes to the others in succession, gradually rising as it is heated, until it is again forced to the radiator at the top.
There are two methods of circulating the water through the cylinder jackets and radiator. The most common method consists of the introduction of a pump in the lower portion of the circulating system. In the case of automobile motors, this pump is driven by gears connected with the crank shaft of the engine. Such a pump will be either of the gear or centrifugal type, and will suck the cooled water from the lower portion of the radiator, and force it through the jackets. The second method is known as the thermo-syphon system because the circulation is automatic and depends upon the cooling of the water in the radiator. When the cooled water sinks, a syphon action is formed that tends to draw the hot water from the cylinder jackets, and the automatic circulation will thus continue as long as the successive heating and cooling take place.
Inasmuch as the pump is driven by the crank shaft of the engine, its speed will be proportional to that of the motor. The same holds true of the fan that serves to draw theair through the radiator. It will thus be seen that both the water and the air are forced at a more rapid rate when the motor runs at high speed, and that therefore the extra heat generated by the more frequent explosions in the cylinders will be counteracted to a certain extent. The increased number of explosions and the higher speed at which the fan turns also cause quicker heating and cooling of the water by the thermo-syphon system, thus forming a more rapid circulation. Inasmuch as the force exerted upon the water by its cooling and heating is not as great as that formed by a high-speed and efficient pump, the pipes and connections of the thermo-syphon system must be of ample size in order to keep the resistance to the passage of the water as low as possible. Care must also be taken in the design of this system so to construct and connect the pipes and jackets that the hot water will be allowed to rise and the cool to descend, and thus to make possible the syphon conditions on which principle the circulation is based.
The ability of the radiator to carry off the heat from the water depends upon the rapidity with which the air passes through the passages provided for the purpose. The amount of air passing through is determined by the speed ofthe suction fan and the rapidity of travel of the car itself against the wind. It has been shown that, when the motor runs at a high number of revolutions, the fan turns faster and the rapidity of circulation is increased. But if the car itself does not increase its speed in proportion to the higher revolutions of the motor, the maximum amount of air will not be forced through the radiator passages, and the excess heat will not be carried off entirely from the cylinders. This is a condition that prevails when the motor is run on low gear. The speed of the motor is increased, while that of the car is reduced; additional heat is generated in the cylinders, but the speed of the air is not increased in proportion. Therefore a motor that is driven a long distance on the low gear will have a tendency to overheat.
Water under atmospheric pressure cannot be brought to a temperature above 212 degrees Fahrenheit without being converted into steam. Therefore, when the heat from a water-cooled motor cannot be carried away sufficiently fast, the water in the circulating system will begin to boil. As long as water remains in the jackets, the temperature of these spaces cannot well rise above 212 degrees, and consequently there is small danger that awater-cooled motor will become overheated to the point at which the pistons will "seize" in the cylinders. The moment the water in the circulating system begins to boil, however, exceedingly rapid evaporation naturally takes place, and the water will soon entirely disappear in the form of steam and vapor. To run the motor under these conditions will mean that pistons and rings will soon become stuck in their cylinders, although liberal quantities of oil will sometimes delay this inevitable result.
But even when the cooling water is not brought to the boiling point there is a vapor that is constantly dispelled from it whenever its temperature is brought above that of the air. The water system of an automobile must therefore be replenished at irregular intervals, depending upon the amount and nature of the running to which the car has been subjected. The older cars were provided with an extra water tank, generally located under the seat, and connected directly with the water jackets and the radiator. The usual water-cooling system of the present-day car, however, is self-contained—that is, there is no separate tank for the storage of the water. The water is poured into the top of the radiator, and from this high point it reaches every part of the circulatingsystem. Whenever the radiator will accommodate a couple of quarts, or more, it is well to fill it, fortoo muchwatercannot be usedon the modern design of cooling system. It is true that a motor runs at its highest efficiency when its temperature is as great as that at which proper lubrication of the pistons can be obtained—for a gasoline engine is a "heat engine," and the greater its unnecessary heat losses, the less will be the power developed by it. But a motor cannot be kept at the proper temperature by reducing the amount of cooling water in its circulating system. The best method is to lessen the rapidity with which the water is cooled, and this may be accomplished by placing a leather flap, a cardboard, or other obstruction over a portion of the radiator to reduce the number of openings through which the air may pass. It should only be necessary to do this in the coldest weather, however, for the cooling system of every motor is designed to maintain the proper temperature on all except the hottest or coldest days.
It has been stated in a preceding paragraph that continued running on the low gear is the most frequent cause of overheating a motor. This is true, but it is not the only cause. Obstructions in the circulating system that reducethe flow of water will have this effect, as will also deposits on the interior of the cylinders that serve to prevent the proper transfer of heat to the water in the jacket spaces. Removal of the carbon will remedy the latter trouble, but to clear out the circulating system is more or less of a complicated matter. Stoppage in the pipes or radiator cells may be caused by a lime deposit from "hard" water that may have been used in the circulating system. There are preparations intended to remove this deposit, but such should not be used without first advising with the maker of the car or an experienced repair man. A series of battered cells in the radiator may reduce the number of cooling spaces that should be traversed by the water, and thus the hot water cannot be distributed over as great an air area as is necessary to maintain the motor at the proper temperature. Such a condition will be apparent from a marked difference in temperature between the affected portion of the radiator and the remainder. If a deposit has been formed on a certain series of cells, or if they have been obstructed in any other manner, the hot water cannot circulate through this section of the radiator, and it will remain comparatively cool.
Water is a liquid that remains in its fluidstage only through a temperature range of 180 degrees—at atmospheric pressure. At 212 degrees it boils and turns to vapor, while at 32 degrees it freezes and becomes a solid. In neither of these stages does it form a desirable cooling medium for a gasoline motor. Of the two, however, its solid stage is the more harmful to the motor. Not only will it cease to flow when it becomes ice, but the expansion of the water during the formation of the solid is liable to burst its retainer—whether it be the cells of the radiator, the pump, pipes, or even the cylinder walls themselves. It is the radiator that is the most liable to suffer from such a cause, however, for each cell contains so small an amount of water that the liquid will be brought to the freezing point before the larger volume in the jacket spaces approaches this temperature. Of course the water will be kept well above the freezing point when the motor is running, and it is only when the machine has stood idle for several hours that care must be taken to prevent the formation of ice in the circulating system.
Aside from keeping the car in a warm place whenever the motor is to be at rest more than two hours, there is only one method of preventing the cooling water from freezing, and thatis by the introduction of some chemical that lowers the point at which the liquid will turn to a solid. There are several ingenious heaters available that are attached to the circulating pipes and that serve to keep all of the jacket water warm; the use of these producing the same conditions as though the car were kept in an artificially-heated garage.
One of the most common liquids used in the cooling water to prevent freezing is alcohol. If equal parts of wood alcohol and water are used in the cooling system, the resulting mixture will not freeze until it reaches a temperature colder than 25 degrees below zero. A weaker mixture—one having 25 per cent. of wood alcohol—will freeze at about zero, and it therefore depends upon the prevailing cold-weather temperature as to the proper proportion that should be used. It must be remembered that the boiling point of alcohol is much lower than is that of water, and that therefore a mixture that will not freeze in exceedingly cold weather is liable to boil away on the first moderate day on which the car is run. The above-mentioned 50 per cent. mixture of wood alcohol and water will boil at 135 degrees, while the 25 per cent. solution will withstand a temperature 40 degrees higher before it istransformed into vapor. As the lower temperature will be reached easily if the motor is run for some time in comparatively moderate weather, it will be seen that the stronger mixture should be used only where winters are very severe. It must also be borne in mind that, as alcohol boils more readily than does water, it follows that it will evaporate more easily, as well. Therefore, in order to maintain a uniform proportion of wood alcohol to water, the former should be replenished more often than is the latter.
Glycerine is another substance that is often mixed with the cooling water to prevent the latter from freezing. A 50 per cent. mixture of this and water has a freezing point of about zero, or slightly lower, and boils at practically the same temperature as water—210 degrees. Combinations of wood alcohol and glycerine may be used—equal parts of each being the usual proportion—and thus various freezing and boiling points may be obtained.
The radiator is one of the most delicate parts of the motor car's construction, and yet it is the most exposed to flying sticks and stones that may be thrown up by the rapid travel of the car. The car owner may do well to follow the practice of many racing drivers who placea heavy wire mesh screen in front of the radiator as a protection against obstacles that may be struck by the front of the car. It would seem that sticks and stones would be thrown toward the rear of the car, and would therefore avoid the radiator by a wide margin, but experience has proved that, at high speed, such loose pieces are frequently forced forward and areruninto by the front of the car.
Therehas always been a strong prejudice in favor of the four-cycle motor for the power plant of the gasoline automobile. This may be due to the fact that designers have spent most of their time and energy on the development of this engine, and that therefore the two-cycle type has not yet been sufficiently "tried out" in the motor car to enable us to judge fairly as to its real merits. Certain it is that in the few instances in which the two-cycle motor has been used as an automobile power plant, the results have been highly satisfactory, and the present vogue of the four-cycle motor—with well over 98 per cent. of the automobiles now made adhering to this type—is largely due to popular prejudice in its favor.
As has been described in thefirst chapterof the present volume, the four-cycle motor devotes a separate stroke to each of the events of expansion, scavenging or expulsion of theburned gases, suction, and compression. The two-cycle motor, on the other hand, devotes but two strokes to these four events, and there is therefore an explosion twice as often in the two-cycle engine cylinder as is the case with the four-cycle type. But in lieu of the suction stroke of the four-cycle motor, there must be some method of forcing the charge into the cylinder of the two-cycle engine. The base, or compartment below the piston, in which the crank revolves, is used for this purpose. As the piston travels upward on its compression stroke, a partial vacuum is formed in the base, and if a passage is opened between this compartment and the carburetor, the charge will be sucked in.
All outside connections with the base are tightly closed on the down-stroke of the piston, and consequently the recently-inhaled charge will be compressed, ready for its entrance into the cylinder above the piston as soon as the connecting passage is opened. This passage is opened, as has already been described, at the bottom of the stroke and the compressed charge rushes in and fills the space in the cylinder that at that time is being vacated by the exhaust gases.
The majority of two-cycle motors are madewithout any valve mechanism, the opening and closing of the passages being entirely automatic. These passages are cast with the engine and lead into the cylinder through openings in the walls called "ports." The opening leading from the cylinder to the exhaust pipe, or exhaust port, is placed near the bottom of the stroke so that it is covered by the piston, except at the lower extremity of the travel of the latter. Just below the exhaust port, and on the opposite side of the interior of the cylinder, is placed the intake port, or opening of the passage connecting the cylinder with the base.
Now, as the piston is forced downward, it uncovers the exhaust port and an easy means of escape is furnished for the burned gases. Immediately after this, the intake port on the opposite side is uncovered by the still-descending piston, and the previously compressed charge, which is only awaiting the opportunity in the base, "blows" in. The exhaust gases are still escaping when this happens, and therefore it is necessary to prevent the incoming charge from passing directly across the top of the piston and out through the exhaust port before use has been made of its explosive qualities.
Consequently, to keep it in its proper path, a baffle plate is attached to the top of the piston which serves to deflect the incoming charge toward the top of the cylinder, and this not only prevents the loss of the mixture, but also furnishes a blast of air that helps to blow out the burned gases. On the return of the piston to the top of its stroke, it first passes over the intake port and then covers the exhaust port, effectually closing both and preventing the escape of the charge during compression. While this is going on, it must be remembered, the piston is forming the partial vacuum in the base, which serves to draw in the charge for the succeeding explosion.
If the charge is drawn directly into the base from the carburetor, a check valve must be used in the pipe connecting the two; otherwise the mixture would be forced back into the carburetor the instant the piston began its descent. A two-cycle motor drawing its charge in this manner is known as the two-port type, for there are only the exhaust and the inlet ports in the interior of the cylinder walls. The passage connecting the carburetor with the base may enter at the bottom of the cylinder, for this space and the base are the same when the piston is at the top of its stroke. Thusif this port is placed so that it is uncovered when the piston is at the top of its stroke, it will admit the charge to the base at a time when a partial vacuum has been created in this compartment by the upward movement of the piston.
This port is again covered as soon as the piston starts on its downward journey, and thus the charge is prevented from escaping until the intake port connecting the base with the top of the cylinder is opened. Such a two-cycle motor is known as the three-port type, and it will be seen that not even an automatic check valve is used in its passages—and it is consequently a "valveless" motor in the liberal interpretation of the term.
The high velocity of the charge recompenses for the short time that the port is uncovered, and consequently the base is filled with nearly as large an amount of charge as is the case with the two-port motor—which allows the incoming gases to enter the crank case during the entire upward stroke of the piston.
It will thus be seen that the piston of the two-cycle motor acts as a pump in two ways. First, the vacuum is formed that serves to draw the charge into the crank case, or base, of the motor; and second, the return stroke of thepiston compresses this recently-inhaled charge and makes it ready to be "shot" up into the cylinder as soon as the piston has uncovered the port that forms the upper terminal of the communicating passage. There can, of course, no greater amount of fresh charge enter the cylinder than is drawn into the crank case. Consequently, the amount to which the cylinder will be filled depends upon the vacuum formed and the pressure exerted upon the charge by the succeeding down-stroke of the piston. It is to be supposed that the piston rings will be tight and that none of the charge can escape by them, and therefore the vacuum formed and pressure exerted in the crank case will depend entirely upon the displacement of the piston in its travel compared with the total capacity of the crank case. In other words, if the crank case is large and the piston is small and travels but a short distance, its pump action on the entire volume will be small. But if the crank case is small and the travel of the piston alternately doubles and halves the volume, the motion of the piston will cause the pressure in the crank case to vary greatly.
In a preceding paragraph it has been described in what manner the incoming charge in the two-cycle motor was used to "scavenge"the cylinder, or rid it of burned gases, by deflecting the mixture and allowing this to force out the remaining exhaust before the exhaust port was closed by the upward motion of the piston. It is evident that the greater the force, within certain limits, with which the charge enters the cylinder, the more perfect will be the scavenging action. But there is a limit to the pressure that can be attained by the mixture when it is compressed in the crank case previous to its discharge into the cylinder. This limit is determined by the size of the space required for the revolution of the crank and "big end" of the connecting rod, and by the volume displaced by the motion of the piston. The crank must have room in which to revolve, and the displacement of the piston can only be the area of its top multiplied by its length of stroke. Thus eight pounds per square inch is about the usual limit of crank case compression with this type of two-cycle motor. This may be varied slightly one way or the other by the arrangement of the ports, but it makes slight difference whether the motor is of the two- or three-port type so far as this consideration is concerned.
Two-cycle motors have been designed which combine the principles of action of both thetwo- and three-port types. The most important departure from the generally-accepted type of two-cycle motor, however, is the design in which the charge is fed into the cylinder from a chamber that is absolutely independent of the crank case proper. This may be accomplished in several ways. There may be what is termed a "differential piston" in which a separate plunger operates in the interior of the hollow "trunk" piston, and by means of the proper connection with the crank shaft compresses the charge in the chamber thus formed at the time it is to be forced into the cylinder.
Another design for obtaining intake compression independent of the crank case consists of a collar, or circular enlargement at the base of the piston. This collar reciprocates within the lower portion of the piston in a chamber which has been bored to the exact size. The collar consequently forms a variable base for this compartment, and as the piston descends, the collar travels with it, thus drawing in a charge of the fresh mixture. On the upward stroke, this mixture is compressed by the collar as it reduces the size of the compartment. It will be seen that such a motor can be designed to compress the charge to almost any amount.
Inasmuch as the mixture, as mentioned above, is compressed on the up-stroke of the piston, it is evident that it cannot be discharged into that particular cylinder at that time—for the mixture should be delivered to its cylinder only when the piston is at the bottom of its stroke. In the case of a four-cylinder engine, however, one of the pistons would be in the proper position for the entrance of the charge, and it is into this cylinder, that the compressed mixture is forced. The compression space in each cylinder, therefore, works for its neighbor, rather than for itself.
This interchange of courtesies is obtained through the good offices of a distributor in the form of a rotating, hollow cylinder having ports cut throughout its length that register with corresponding passages leading to the various cylinders. This distributor is timed with the crank shaft of the motor, and may be driven either by a gear or by a silent chain. As the mixture is compressed in the separate chamber of one cylinder, the passage leading to the distributor is opened by the revolution of the latter, and the charge is led through this passage, the distributor, and thence through another passage—also opened by the distributor—to the proper cylinder. The cylindersthus operate in pairs, one receiving its charge while the other is about to begin its explosion stroke—and vice versa.
The force of the explosion in a gasoline engine cylinder is not only dependent upon the amount and nature of the inflammable mixture admitted, but upon the force with which it is compressed, as well. The average compression pressure of a two- or four-cycle engine of the ordinary type, is from 60 to 70 pounds per square inch. Inasmuch as this pressure, assuming that the rings and valves are tight, is proportional to the displacement of the piston stroke compared with the volume of the clearance space, the amount of compression is constant at all speeds and loads of the motor. Should it be possible to increase this compression at will, it would be found that, with a warm motor, a pressure in the neighborhood of 100 pounds per square inch would serve to generate sufficient heat to ignite the mixture before the formation of the spark—for it is one of the elementary laws of physics that a gas will become heated when compressed. It is for this reason that the compression pressure of the ordinary automobile motor is kept in the neighborhood of 70 pounds per square inch.
A method of varying compression pressureto meet individual load requirements has been devised for some motors, however, and while such types are not as yet in general use in automobiles, it is probable that the near future will find much advancement along these lines. One such two-cycle motor that has been designed especially for automobile use employs a separate air compressor driven by the engine itself and used as the clutch and variable speed transmission of the car. The amount of pressure generated in the compressor is dependent upon the resistance offered to its operation—or, in other words, it increases with additional load carried by the motor. The compression, or compressed air, rather, is carried directly from the compressor to the cylinders of the motor, being admitted at the proper time by a rotary valve driven by the crank shaft. Thus the compression in each cylinder is automatically regulated by the load, and a motor of this type possesses a high "overload" capacity.
The motor mentioned above operates on somewhat the same principles as those found in the Diesel engine, which will be, as many predict, the ultimate type of internal combustion motor. The Diesel motor is not necessarily a particular make of engine, but bearsthe name of the originator of the principles involved. These are distinct from those of the Otto cycle, which is the principle upon which practically all automobile motors operate. The Otto cycle consists of the well-known series of events in the cylinder, as follows: Ignition, followed by the explosion, or expansion of the burned charge; discharge of the exhaust gases, or scavenging; admission of the fresh charge, suction; and compression of the newly-received mixture previous to ignition and the repetition of the cycle. In speaking of the Otto and Diesel engines, it must be borne in mind that they are referred to as a class, rather than as a particular make—as one would mention poppet valve or sleeve valve engines—for there may be many manufacturers of each type.
Although the Diesel principle may be applied to either the two or four-cycle type of motor, it is to the former design that it lends itself unusually well. This motor operates a two-stage air compressor in conjunction with a storage tank. At the beginning of the compression stroke, pure air under high pressure is admitted to the cylinder. In its upward travel, the piston compresses this air to a pressure approximating 500 pounds per square inch. While it has been shown that such a pressureis about five times more than enough to generate sufficient heat to cause premature ignition, it must be remembered that, unlike the ordinary type of motor, this is only pure air that is injected into the cylinder and contains none of the explosive gasoline vapor. At the top of the stroke, however, when the compression is at its maximum, the fuel is injected directly into the cylinder without having been previously vaporized.
This is another feature in which the Diesel motor is entirely different from the Otto type, for the latter must employ a carburetor to vaporize the fuel before it can be admitted to the cylinder. But inasmuch as there is already a pressure approximating 500 pounds per square inch in the cylinder of the Diesel motor at the time the fuel is injected, there must be a force behind the latter of 750 or 1,000 pounds per square inch in order to enable it to overcome the resistance of the highly-compressed air in the cylinder. In short, the liquid fuel is sprayed directly into the cylinder at a pressure of 750 or 1,000 pounds per square inch. This tremendous pressure is sufficient, not only to vaporize the particles of fuel as soon as they enter the cylinder from the nozzle, or "atomizer," but to cause them to burstinto flame, as well. In other words, the compression of the air previously has generated sufficient heat in the cylinder to ignite the fuel immediately on its admission.
The fuel continues to be injected into the cylinder during the greater part of the down-stroke of the piston. In this respect, also, is the Diesel motor radically different from the Otto type, for the latter receives its full charge at one time and fires the entire amount in a single "explosion." In the Diesel motor, on the other hand, the ignition continues as long as fuel is admitted, and thus this engine is of the internalcombustiontype in the strictest sense of the word. It is, after all, the expansion of the gases due to the heat of combustion that produces the power in a gasoline engine, and if the fuel can be so admitted that it can burn during the greater part of the stroke, a high efficiency will be obtained.
The exhaust gases of the ordinary two-cycle motor pass out of the exhaust port as it is uncovered by the descent of the piston. Those that remain are forced out by the sudden admission of the fresh charge, which is deflected upward and is intended to scavenge the top of the cylinder. But it is claimed that thus employing the fresh mixture as a scavenging agent iswasteful of the fuel-permeated charge and does not conduce to efficient running. The system is simple in the extreme, however, and does its work well in small installations in which fuel economy is not of vital importance. But in the two-cycle Diesel type of engine, the high pressure of the pure air is used for scavenging, and as this is admitted with so large an initial force, the exhaust port may remain uncovered for a longer period than would be the case were the air to rely entirely on the up-stroke of the piston for its compression. Then too, whatever air may escape contains no fuel, and consequently efficient scavenging may be obtained without waste.
At the high pressure at which the fuel is injected into the cylinder of the Diesel engine, practically any grade of gasoline, naphtha, kerosene, crude oil, or other form of petroleum can be vaporized. The compressed air employed in the compression and injection of the fuel is also used for starting the motor, for this is not a type that is amenable to hand cranking. Thus the Diesel type of engine can be run in any weather on any grade of oil fuel, and as the carburetor and electrical ignition system are absolutely eliminated, two of the great sources of trouble of the automobilemotor are absent—and this feature, alone, even more than the superior economy of operation, will appeal to the average motorist.
Just when this type of motor will be taken up by automobile designers is difficult to state. The Diesel type of engine has proved so wonderfully successful for large stationary power plants and for marine purposes, and its reliability is so absolute on all grades of fuel, that this motor may solve the failing-gasoline-supply problem. As yet, about 100 horsepower is the smallest unit that has been made in any quantities, but it was recently announced that this type would, in the very near future, be built for motor trucks and other commercial vehicles. Consequently, it is well for all those interested in the application of the two-cycle motor to the automobile to understand the elementary principles on which this radically-different type operates.