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carbon mixture. As the air which is compressed to this high point cannot burn, the fuel is introduced into the cylinder head under still higher compression than that of the compressed air, and as it is injected in a fine stream, it is immediately vaporized because of the heat. Just as soon as the compressed air becomes thoroughly saturated with the liquid fuel, it will explode on account of the degree of heat present in the combustion chamber. (Figs. 61, 62.)
Q. What are the advantages of the injection method?
A. The injection method of fuel supply has the important advantage of permitting the use of very cheap fuels which could not be utilized in ordinary forms of vaporizers on account of the high temperature needed to vaporize them.
Q. What are the disadvantages of injection engines?
A. Owing to the extremely high compression employed in injection engines, these must be made very substantial and strong, and this principle is suitable only for the comparatively slow acting power plants of the stationary or marine type. An injection engine is not as flexible as the electrically ignited carburetor supplied power plants used for automobile service, and this fact also militates against the injection principle for this use.
I ESSON SIX
TYPES OF CARBURETORS AND THEIR ACTION
Q. What are the simplest forms of carburetors?
A.: The old patterns of evaporation or surface carburetors were the simplest forms of carburetion devices and the modified device in which wicks were used was but slightly more complicated.
Q. Describe action of evaporation or surface type.
A. This form of carburetor consisted of a simple tank or container for the liquid and the air was drawn in and across the surface of the gasoline in order that it might become saturated with the vapors constantly present at that point. These rich gases were drawn into the engine through a simple form of mixing valve which permitted the entrance of an auxiliary supply of air from the outside of the container to dilute the rich gas and make it of a proper composition to insure energetic combustion.
Q. Describe action of wick type vaporizer.
A. The wick form of carburetor is essentially the same in construction as the simpler surface type except that the mixing compartment through which the air flows is separated from the fuel containing portion by means of a wall of absorbent material, such as wicks, which feed the gasoline up into the mixing compartment by capillary attraction and by spreading it over more surface make it easier for the air stream passing over the wicking to pick up gasoline vapor.
Q. Describe action of bubbling type carburetor.
A. The bubbling type differs from the other simple forms previously described in that the air enters at the bottom of the device and bubbles through the liquid to reach the mixing chamber from which it is drawn to the engine cylinder.
Q. Name objections to these forms.
A. Devices of the nature previously considered had great defects which would militate against their general adoption at the present day. They are only suitable for use in conjunction with high grade gasoline which has high evaporating value. It is doubtful if they would give satisfactory results with the low grade fuels available today. In many cases these devices were as large as the cylinder of the motor to which they were applied, and this factor would also be considered a disadvantage.
Q. What is the principle of spraying carburetor action?
A. The spraying form of carburetor was evolved to eliminate one of the great disadvantages present with the simple evaporation types. As these were used, the fuel contained therein became heavier, because only the lighter and more volatile constituents evaporated. After a motor had been running for some time, it was necessary to drain out the residue, and admit a supply of fresh fuel from the main container, because the heavy matter left after the more volatile vapors had passed into the engine could not be vaporized by an air stream merely brushing over its surface or passing through it. In the spraying type of carburetor the fuel is drawn into the entering air stream through a small jet or stand pipe which causes it to issue into the form of a spray which soon turns into vapor. With the spraying principle every particle of the fuel is used because the heavier portions are sprayed into the air stream at the same time that the lighter constituents are, and as the liquid enters the air stream in a finely divided state or as a mist it is almost immediately vaporized and turned into an explosive gas.
Q. Describe a simple mixing valve.
A. A simple form of mixing valve consists of a brass body portion having three outlets. The largest of these, which serves as the main air entrance, is normally closed by a mushroom valve which is held against its seat by means of a coiled spring in such a way that it can only open when acted on by the suction of the engine. The fuel is connected to the device at a point where a small passage leads from the fuel inlet to the poppet valve seat. Whenever the valve is closed, the fuel spray opening is covered by the valve and the fuel cannot flow. The third opening forms an outlet for the mixture and is connected to the engine. The action of a device of this kind is easily understood. Every time the piston of the engine draws in a charge of air, it lifts the mushroom valve from its seat and a small quantity of liquid fuel is injected into the air stream as it brushes over the small orifice in the valve seat through which the liquid issues. The liquid spray and air become mixed together in the body of the device and flow to the engine in the form of an explosive mixture.
Q. How are mixture proportions regulated?
A. As a general rule the amount of gasoline supplied may be regulated by a small needle valve controlled by a knurled thumb nut while the air supply may be altered by controlling the lift of the air valve by suitable means.
Q. Name defects of simple mixing valves.
A. One of the most important disadvantages of the simple mixing Valve is that it does not provide as good a mixture as modern high speed engines demand, because it does not vaporize all of the liquid, and some of the fuel is present in liquid form until taken into the hot cylinder. At low engine speeds particles of liquid fuel will adhere to the walls of the manifold and when the engine speed is augmented and the velocity of the gases passing through the manifold increases, the air stream picks up some of the liquid previously deposited in the inlet pipe and the result is a mixture excessively rich in fuel is supplied the motor. Another disadvantage is that after the device has been in use for a time the air valve might wear its seat enough so that it does not prevent the gasoline from running out all the time and flooding the air inlet opening with the liquid.
Q. What is a float feed carburetor?
A. A float feed carburetor is a device in which the fuel is maintained at a certain level in a stand pipe or jet. This level is so proportioned that the liquid does not overflow the stand pipe, and thus the fuel will be sprayed into the mixture only when drawn out of the jet or nozzle by means of the air stream induced by engine suction. The level in the spray nozzle is maintained by a simple automatic valve mechanism in which a float controls the admission of fuel to the device. The essential elements of a simple form of float feed carburetor are clearly outlined at Fig. 63.
Q. Name two important parts of the float feed carburetor.
A. All float feed carburetors are composed of two separate chambers or compartments, one of which is called the mixing chamber and the other the float chamber.
Q. What is the mixing chamber?
A. The mixing chamber is the portion of the carburetor in which the spray nozzle is placed and through which the air stream passes before it can reach the inlet manifold.
Q. What is the float chamber?
A. The float chamber is that part of the carburetor to which the fuel is first admitted and which serves as a container for the float which regulates the level of fuel in the standpipe. Whenever the fuel level falls, the float which is supported by the liquid falls and opens a valve which permits more of the liquid to flow into the
Fig. 63.-Defining Elements of Simple Float float bowl from the main
Feed Carburetor. fuel container. When the level reaches the proper height the float shuts the valve and the fuel supply is stopped.
Q. What are floats, made of?
A. The float may be solid and composed of some light substance that will float in the liquid or it may be of hollow metal of any form that will displace more than its weight of fuel. Cork is the material generally used for solid floats, while sheet copper or brass serves for the hoʻlow members.