Motors Part 6
The original method of utilizing what is called _Internal combustion_ Motors, was to employ a fixed gas. A _fixed_ gas is one which will remain permanently in that condition, unlike a vapor made from gasoline.
The difference may be explained as follows:
Fixed Gases.--If the vapor of gasoline, or petroleum, is subjected to a high heat, upwards of 1500 degrees, it is so changed chemically, that it will not again return to a liquid state. This is called _fixing_ it. Gas is made in that way from the vapor of coal, and fixed, producing what is called illuminating gas.
Although the temperature of fixing it is fully three times greater than is required to explode it, the fact that it is heated in closed retorts, and oxygen is prevented from mixing with it, prevents it from burning, or exploding.
Gas Engines.--Such a gas has been used for many years in engines which were usually of the horizontal type, and were made exceedingly heavy and c.u.mbrous, and provided with enormous fly wheels. Gases thus made are not as rich as those generated direct from the hydro-carbon fuels, because, being usually made from coal they did not have a large percentage of hydrogen.
Energy of Carbon and Hydrogen.--When a pound of carbon is burned, it develops 14,500 heat units, and a pound of hydrogen over 52,000 heat units. a.s.suming that 85 per cent. of a pound of petroleum is carbon, and 15 per cent. is hydrogen, the heat units of the carbon would be 12,225, and the heat units of the 15 per cent. of hydrogen would be 12,800. The combined value is, therefore, 25,025, which is almost double that of coal gas.
This fact makes the gasoline engine so much more efficient, and for the same horse power the cylinders can be made smaller, and the whole structure much lighter in every way.
Gasoline motors are of two types, one in which an explosion takes place at every revolution of the crank, called the _two-cycle_, and the other the _four-cycle_, in which the explosion occurs at every other turn of the crank.
The terms _two-cycle_ is derived from the movement of the piston, as that moves downwardly during the period when the crank is making a half turn, and returns in its upward stroke when the crank completes the turn, or that two half turns of the crankshaft complete the cycle.
Four-cycle engines have two such complete movements at each impulse, or require four half turns of the crankshaft to complete the cycle.
The Two-Cycle Type.--In order to clearly distinguish between this and the four-cycle, it would be well to examine the diagram, Fig. 23. For a clearer understanding the drawing is explained in detail.
The cylinder A, within which the piston works, has a removable cap B, and at its lower end a removable crank case C. The case is designed to entirely close the lower end of the cylinder so that it is air tight, for reasons which will be explained.
The outer jacket, or casing D, at the upper end of the cylinder, is designed to provide a s.p.a.ce E, for the circulation of water, to cool the cylinder during its working period. The crankshaft F pa.s.ses through the crank case, the latter having suitable bearings G for taking care of the wear.
[Ill.u.s.tration: _Fig. 23. Two-cycle. First Position._]
The piston H is connected up with the rod I, the latter being hinged at a point within the piston, as shown. The crank case has an inlet port, provided with a valve which opens inwardly, so that when the piston moves upwardly the valve will open and air will be drawn into the crank case and s.p.a.ce below the piston.
At one side is a vertical duct K, which extends from a point directly above the crank case, to such a position that when the piston is at its lowest point gas can be discharged into the s.p.a.ce above the piston.
On the opposite side of the cylinder, and a little above the inlet port of the duct K, is a discharge port M. The inlet port and the discharge port, thus described, are both above the lower end of the piston when it is at its highest point.
The spark plug is shown at N. On the upper end of the piston, and close to the side wall through which the inlet port K is formed, is an upwardly-projecting deflecting plate O, the uses of which will be explained in the description of its operation.
Fig. 23 shows the piston at its highest point, and we will now a.s.sume that ignition takes place, thus driving the piston downwardly until the upper end of the piston has fully uncovered the discharge port M, as shown in Fig. 24. This permits the exhaust to commence, and as the piston proceeds down still further, so as to uncover the inlet port K, the gas, which at the down stroke has been compressed in the s.p.a.ce below the piston, rushes in, and as it strikes the deflecting plate O, is caused to flow upwardly, and thus helps to drive out the burnt gases remaining at the upper end of the cylinder.
[Ill.u.s.tration: Two-cycle Engine.
Fig. 24. Second position. Fig. 25. Third position.]
This action is called scavenging the cylinder, and the efficiency of this type of engine is largely due to the manner in which this is done.
It is obvious that more or less of the unburnt gases will remain, or that some of the unburnt carbureted air will pa.s.s out at each discharge, and thus, in either case, detract from the power of the subsequent explosion.
As the piston now moves upwardly to complete the cycle, the piston closes both of the ports, thus confining the gas which was previously partly compressed, and as the piston proceeds the gas is still further compressed until the piston again reaches the upward limit of its motion.
Advantages of the Two-Cycle Engine.--This kind of engine has several distinct advantages. It has less weight than the four-cycle; it gives double the number of impulses for a given number of revolutions of the crankshaft; and it dispenses with valves, springs, cam-shafts, stems and push rods.
More or less danger, however, attends the operation of a two-cycle engine, princ.i.p.ally from the fact that an explosive mixture in a partially compressed condition is forced into the s.p.a.ce which the instant before was occupied by a flame, and it is only because the expansion of the burst gases at the previous charge has its temperature decreased so far below the explosion point, that the fresh gas is not ignited, although there have been occasions when explosions have taken place during the upstroke.
The Four-Cycle Engine.--The most approved type is that which is known as the _four-cycle_. This will also be fully diagrammed so as to enable us to point out the distinctive difference.
[Ill.u.s.tration: Four-cycle Engine.
Fig. 26. First position. Fig. 27. Second position.]
Figs. 26 and 27 show sections of a typical four-cycle engine, in which the inlet and the exhaust valves are mechanically operated. The cylinder A is either cast with or separate from the crank case B, and has a removable head C. The upper end of the cylinder has a water s.p.a.ce formed by the jacket D.
The inlet port E and the discharge port F are both at the upper end of the cylinder. The crank shaft G pa.s.ses horizontally through the crank case, and it is not necessary, as in the case of the two-cycle-engine, to have the case closed tight.
The piston H is attached to the connecting rod I, which is coupled to the crank, as shown. The crank shaft has a small gear J, which meshes with two gears of double size on opposite sides of the crank shaft, one of the gears K, being designed to carry the cam L for actuating the stem L', which opens the valve M in the port that admits the carbureted air.
[Ill.u.s.tration: Four-cycle Engine.
Fig. 28. Third position. Fig. 29. Fourth position.]
The other large gear N is mounted on a shaft which carries a cam O that engages the lower end of a push rod P, to open the valve Q in the discharge port F. It should be observed that the stems L', P, are made in two parts, with interposing springs R, so the valves may be firmly seated when the stems drop from the cams.
The spark plug S is located in the head, close to the inlet port. The character of the igniting system is immaterial, as the object of the present diagrams is to show the cycle and method of operating the engine at each explosion, and to fully ill.u.s.trate the manner in which it is distinguished from the two-cycle type.
A fly wheel is necessary in this as in the other type, and in practice the two gear wheels, K, N, are placed outside of the case B, and only the small gear, and the cam shafts, on which the cams are mounted, are within the case.
The operation is as follows: In Fig. 26 the piston is shown in a position about to commence its downward movement, and we will a.s.sume that the ignition has just taken place. Both valves M, Q, are closed, as it will be noticed that the cams L, O, are not in contact with the lower ends of the push rods.
The explosion drives the piston down to the position shown in Fig. 27, when the cam O begins to raise the stem P, and thus opens the discharge valve Q, permitting the burnt gases to escape as the piston travels upwardly to the position shown in Fig. 28.
At this position the valve Q closes, and the cam L opens the inlet valve M, so that as the piston descends the second revolution, the carbureted air is drawn in until the crank has just turned at its lowest limit of movement, as shown in Fig. 29.
The upward stroke of the piston now performs the work of compressing the carbureted air in the cylinder, and it is ready for the ignition the moment it again reaches the position shown in Fig. 26.
The Four Cycles.--The four distinct operations thus performed are as follows: First, the explosion, and downward movement of the piston.
Second, the upward movement of the piston, and the discharge of the burnt gases. Third, the down stroke of the piston, and the indrawing of a fresh charge of carbureted air. Fourth, the upward movement of the piston, and the compression of the charge of carbureted air.
The order of the engine performance may be designated as follows: 1.
Impulse. 2. Exhaust. 3. Admission. 4. Compression.
Ignition Point.--While the point of ignition, shown in the foregoing diagrams, represents them as taking place after the crank has pa.s.sed the dead center, the firing, in practice, is so adjusted that the spark flashes before the crank turns past the dead center.
The reason for this will be apparent on a little reflection. As the crank turns very rapidly the spark should be _advanced_, as it is called, because it takes an interval of time for the spark to take effect and start the explosion. If the sparking did not take place until the crank had actually pa.s.sed the dead center, the full effect of the compression and subsequent explosion pressure would not be had.
Advantage of the Four-Cycle Type.--The most marked advantage in the four-cycle type is its efficiency. As it has one full stroke within which to exhaust the burnt gases, the cylinder is in a proper condition to receive a full value of the incoming charge, and there is no liability of any of the unburnt gases escaping during the exhaust from the previous explosion.
The next important advantage of this type is in the fact that it can be operated at a higher speed than the two-cycle type, and this is a great advantage, notwithstanding the less number of impulses in the four-cycle type.
The Loss in Power.--The great disadvantage in all engines of this cla.s.s is the great loss resulting from their action. The explosion which takes place raises the temperature to fully 2000 degrees of heat, and unless some provision is made to keep the cylinder down to a much lower temperature the engine would soon be useless.
High temperatures of this character absolutely prevent lubrication, a thing which is necessary to insure proper working. For this reason a water jacket is provided, although there are engines which are cooled by the action of air.
In any event, the heat imparted to the cylinder is carried away and cannot be used effectively, so that fully one-half of the power is dissipated in this direction alone.