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creased over 120 lb. per square inch, it will be exploded by the heat derived from compressing the charge.
Q. What is the difference between "explosion" and "combustion”?
A. The difference is mere y a matter of time, as an explosion is rapid or practical.y instantaneous combustion.
Q. Why does expansion of gas produce power?
When the gas charge which has been compressed to a point ranging from 60 to 90 lb. per square inch before ignition is exploded its tendency is to expand and regain the volume it occupied before compression. As the increase in temperature due to burning produces an actual increase in volume, considerable pressure is directed against the sides of the container in which the gas was exploded. If one of these walls is a yielding member it will be forced outward by the pressure of the gas which ranges from 300 to 400 lb. per square inch if the charge has been fired normally, and even higher than this if the charge has preignited, or exploded before the proper time.
Q. What are the main elements of any gas engine?
A. The main elements of a gas engine are a containing chamber in which the gas is exploded, which is called the "cylinder,”and a yielding member termed the “piston” against which the force of the explosion is directed. The piston is joined to a crank shaft by means of a movable link called the connecting rod which transforms the reciprocating or back and forth motion of the piston to a rotary or turning movement of the crankshalt. (Fig. 11)
Q. What is the two-stroke cycle principle?
A. In the two-stroke cycle engine the piston makes but two strokes and the crankshaft one revolution for each explosion in a single cylinder motor. On the upward stroke of the piston a gas charge which has been previously transferred to the interior of the cylinder is compressed by the upward movement of the piston and at the same time a charge of fresh gas is drawn into the engine base, due to the suction of the piston as it moves upward in the cylinder. When the piston reaches the top of this stroke the gas above it is compressed ready for ignition while the cylinder and crankcase below it are also filled with gas. The electric spark takes place just as the piston reaches the top of its upward movement and explodes the gas under compression in the combustion chamber. This explosion drives the piston down and at the same time it compresses the charge of gas in the engine base.
Just a little before the piston reaches the bottom of its stroke a port in the cylinder wall is uncovered by the piston and the burnt gases escape. When the greater part of the heat of the exhaust gas
Fig. 12.-Two and Three_Port Two-Cycle Engines Compared.
A–Typical Two-Port Design With Automatic Valve in Crank
Controlled by Piston Movement. has been disposed of the piston uncovers another port on the opposite side of the cylinder which communicates with the engine base interior and the charge of fresh gas which is under compression at that point is transferred through a bypass or transfer passage from the lower part of the piston to the portion of the cylinder above the piston top. This entering stream of fresh gas strikes a deflector plate interposed between the inlet and exhaust port across the top of the piston so the fresh charge of gas is directed to the upper portion of the cylinder instead of passing out through the now fully opened exhaust port. The piston is moved up to the top of the cylinder again by means of momentum or energy stored in the engine flywheel and operations of compressing the gas above the piston top and charging the lower portion of the engine base with gas are again effected. When the piston reaches the top of the cylinder the gas is again exploded and this cycle of operations continues as long as fuel is supplied the cylinder and a spark is provided to ignite it at the proper time. (See Fig. 12.)
Q. What is the four-stroke cycle principle?
A. The four-stroke cycle engine, commonly termed the “Four cycle” or Otto cycle, differs from that previously described in that four strokes of the piston are necessary to obtain an explosion in the cylinder. The operation of this form of engine is clearly outlined at Fig. 13. The first stroke of the piston which is shown at A is a down stroke or movement from the closed to the open end of the cylinder. During this suction or intake stroke a vacuum or suction is produced in the cylinder and an inlet valve member opens to admit a charge of gas which is drawn in by the pumping action of the moving piston. This inlet valve remains open during the entire first down stroke, and closes when the piston reaches the end of its downward movement.
The second stroke of the piston is depicted at B. This is an upstroke in which the piston moves from the open to the closed end of the cylinder. The intake valve, which has been open on the first stroke, is closed, as is also the exhaust valve which is utilized to close the passage through which the burnt gases are expelled. During this upward movement the charge of gas which has been previously drawn into the cylinder is compressed. At the end of this stroke, an electric spark takes place to explode the gas.
The third stroke which is outlined at C is termed the "explosion or power stroke.” In this the piston is driven down by the expanding gas with a force depending upon the area of the piston top and its position in the cylinder. At the instant of combustion a pis
ton with an area of 100 square inches will receive an impulse varying in value from 10 to 15 tons. As the piston goes further down on its stroke, and the burnt gases expand more, the pressure becomes reduced to a point where the gases have so little pressure that it is better to dispose of them than to take further advantage of their expansive force.
The power of a gas or gasoline engine is usually figured on a basis of mean effective pressure of 80 to 100 lb. per square inch, which is considered as existing during the entire stroke of the piston. When the piston reaches approximately seven-eighths of its stroke following the explosion, the exhaust valve is raised by a cam member and the inert gases escape through the open exhaust port by virtue of their pressure. The next stroke of the piston is outlined at D and is termed the "exhaust” or scavenging stroke, and it is devoted exclusively to clear the cylinder of the burnt gases. These are pushed out by the upward movement of the piston, and when the piston reaches the end of its stroke the exhaust valve closes and the inlet valve opens again to admit a fresh charge of gas during the suction stroke.
It will be seen that in a four-cycle engine the piston must make twice as many strokes as in a two-cycle. This is because all the work is done in the upper portion of the cylinder and by the piston top. One entire piston stroke is necessary to fill the cylinder, and a full piston stroke is devoted to clearing the cylinder of burnt gas. In a two-cycle engine, these two operations are performed at practically the same time.
Q. Name advantages of two-cycle engine.
A. Owing to the more rapidly occurring series of explosions, a two-cycle engine has a much more even torque, which is synonymous to more uniform power application. A two-cycle engine is much simpler than four-cycle, as in its simplest form it comprises practically only the essential elements absolutely necessary to secure power from the explosion of gas. There are but three moving parts, namely, the piston, connecting rod and crankshaft. As the ports through which the gas enters and leaves the cylinders are opened and closed by the piston the use of valves which are necessary with a four-cycle engine is dispensed with.