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work, usually have one horsepower for each 50 pounds car weight. Medium speed touring cars and runabouts usually have one horsepower for each 75 pounds car weight. Delivery wagons designed for speedy service usually employ a power plant of such size that one horsepower is provided for each 100 pounds, while designers of heavy motor trucks that operate at very low speeds find that one horsepower to two or three hundred pounds weight is adequate.

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Fig. 11.-Sectional View of Overland Engine Showing Important

Parts.

LESSON TWO

ACTION OF THE TWO AND FOUR STROKE CYCLE

MOTORS

Q. Describe the operating principle of the gas engine:

A. Power is obtained in a gas engine by exploding some combustible vapor or gas in a closed cylinder which is provided with an internal yielding member moved by the force of the explosion. The moving member converts the expansive power of the rapidly burning gases to mechanical energy by suitable leverage. The gas engine is a form of heat engine just as the steam or hot air motor.

Q. How does the gas engine differ from the steam engine?

A. The gas engine, while it embodies some of the main elements used in a steam engine, differs greatly in general design. The cylinder is provided with a water jacket and most of the gas engines used as automobile power plants have the connecting rod attached directly to the reciprocating member or piston in the cylinder instead of to a crosshead as used in a steam engine Most forms of steam engines are double acting. That is to say, the steam is admitted first to one end of the cylinder, and then to the other, and both ends of the piston take a power impulse. All gas engines used in automobiles or similar vehicles are single acting, which means that the expansion of gas that produces the power takes place in the upper end of the cylinder and against the top of the piston only.

The power of a steam engine can be increased within certain limits by augmenting the pressure and quantity of steam supplied the cylinder. The effective foree obtained by the explosion of gas in a gasoline engine attains a certain maximum volume which cannot be increased. It is possible to start a steam engine under load,

while a gasoline engine can be started under these conditions only by means of some mechanical starting arrangement that will enable the crank shaft to be turned until a charge of gas has exploded in the cylinder. A steam engine starts as soon as steam is admitted to the cylinders. A gas engine as used in automobiles will not start unless the crank shaft is rotated by a starting handle or some other mechanical equivalent. A steam engine is an external combustion motor while the gas engine is an internal combustion type.

Q. Define "external" and "internal" combustion.

A. External combustion means burning the fuel which produces power outside of the cylinder as is the case when water is heated in a steam boiler, which is separate from the steam engine. Internal combustion is what takes place when the gas that produces the heat from which power is obtained is burnt or exploded directly against the piston in the cylinder. An engine which depends upon the internal combustion principle is more efficient than an external combustion form because loss of heat is less.

Q. How is fuel supplied to gas engine cylinders?

A. The gas, which may be derived from the burning of coal in a gas producer, manufactured by the usual methods of making illuminating gas or taken from natural underground sources as natural gas, is supplied to the cy inders of a gas engine directly from the source of supply through the medium of piping which leads to the combustion chamber of the motor. When liquid fuel such as gasoline or kerosene is used, it is first transformed into a gas by atomizing or spraying a small quantity of the liquid into an entering charge of pure air which changes the liquid spray to a vapor that afterward mixes intimately with the air to form an inflammable gas.

Q. What is necessary before the gas in the cylinder can be exploded ?

A. Before exploding the gas charge it is customary to compress it into from one-third to one-fifth of the cylinder volume. Acharge of gas that is not compressed will burn comparatively slow and will not produce much power. The more the gas charge is compressed the greater the force of the explosions. There

There is a limit to the amount of compression, because if kerosene or gasoline vapor pressure is in

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 practically instantaneous combustion.

Q. Why does expansion of gas produce power?

A. 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 eng'ne 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 crankshaft. (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 be

low 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

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Fig. 12.–Two and Three Port Two-Cycle Engines Compared.

A-Typical Two-Port Design With Automatic Valve in Crank
Case. B—Conventional Three-Port Design With Charging Port

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

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