CARL H. CLARK
Before the gasoline can be used in the cylinder of an engine it must be converted into vapor and mixed with the proper proportion of air. The devices for performing these functions are termed vaporizers or carburettors. Although a certain distinction may be made between the two, their functions are the same and their methods of operation quite similar.
The fundamental principle of their operation is the picking up of the gasoline by a current of air; the gasoline in practice flows out of a small orifice direct into the stream of air which is drawn in by the suction stroke of the engine. The relative proportions of gasoline and air must, of course, be nicely regulated, as the proper mixture is far more effective than one either too weak or too rich. All vaporizers or carburettors to properly perform their duties should be provided with means for regulating the flow of both the gasoline and air.
The simplest form of vaporizer, or mixing valve as it is sometimes termed, is shown in Fig. 19. It consists of a brass casting containing the valve V and its seat, and having the openings, G, A and I. It is attached to the crank case of the engine by the threaded end I.
The gasoline enters at G, flowing around the needle valve N and into the very small opening at O. The valve V has a spindle extending upwards and fitting the hole in the cover C. This stem acts as a guide to the valve and assumes its seating correctly after being raised. The spring S is inserted to return the valve to its seat quickly. The needle point N may be moved in or out by the thumb nut W, thus regulating the flow of the gasoline. .The valve V when seated covers the needle opening O preventing the escape of the gasoline.
This form of vaporizer is most commonly fitted to two cycle two port engines. The suction created in the crank case by the upward strokes of the piston causes the valve V to raise and the air to rush in through the opening. The raising of the valve uncovers the needle opening O, and allows the gasoline to flow out into the incoming stream of air, which im-mediately absorbs it. The mixture thus passes into the crank case and is ready for use. As soon as the piston has reached nearly the top of its strokes and the suction has ceased, the valve V returns to its seat by its own weight, aided by the pressure of the spring s, and prevents the escape of the gases on the downward compression stroke; this valve V is the non-return valve mentioned on page 211 of Chapter I.
This vaporizer has no means for adjusting the supply of air. only the gasoline supply being variable. It is, consequently, not very sensitive. It is, however, owing to its simplicity, suitable for small engines, where it gives good results. For engines of large size this type of vaporizer is not sufficiently sensitive.
A more approved type of generator is shown in Fig. 20. Its principle of action is the same as that just described, but in addition to the parts already named, has the wheel R, which screws down on the stem of the valve and thus regulates its lift. It also has a sort of shutter or throttle valve T, consisting of a disc which may be turned to partially or wholly close the opening. . , .
The proportion of the mixture may be varied as follows : The gasoline opening may be varied by the nee-dle point N and the thumb wheel W, giving a weaker or richer mixture. The air supply is regulated by the screw R, which varies the lift of the valve V. These adjustments allowthe regulation to suit the different conditions of running the engine. The spring S is fitted in order to make the valve seat more quickly without loss of crankcase compression, and thus allow the enine to run at higher speed than would otherwise be the case.
For regulating the speed of the engine a throttle, consisting of the disc T is placed in the inlet. It may be turned by the small handle outside and thus regulates the amount of the mixture without change in its proportion. The same result may, of course, be accomplished by the manipulation of the screws R and W, but in doing this the best proportions of the mixture may be lost and readjustment required. Some of the cheaper generating valves for this reason are not furnished with the throttle, but it is far more satisfactory to have one with this fixture.
The cap C may be unscrewed allowing the valve V to be taken out.
Vaporizers of this type are suited to two cycle engines of the two port type, which require check, or non-return valve on the crank case. For three port and four cycle types some form of float feed carburettor may be used to advantage.
Although, as before stated, there is no difference in the function of vaporizers and carburettors, those of the types just described are commonly called vaporizers or generating valves, while those governed by a float and having no check valve are usually termed carburettors.
Fig. 21 shows the elements of the float feed carburettor which consists of a chamber containing the float
F. Leading from this chamber is the small opening containing the needle valve N. The gasoline enters at
G. At V is a small valve attached to the float which closes the opening of G when the float rises; the float is guided by the two stems as shown. The gasoline passes through the needle valve V and up the vertical tube to the opening O; the height of the opening O is so adjusted that the level of the gasoline is just below It when the float is in its highest position and the valve V is closed. The air, drawn in by the suction stroke of the engine, rushes past the opening O, draws up and absorbs a portion of the gasoline. At w w is a cone of fine wire gauze which catches any gasoline not taken up by the air current and holds it suspended ready to be taken up at the next stroke.
At T is a sort of shutter or throttle which may be turned by the handle D and wholly or partially close the opening J and thus regulate the amount of the mixture passing to the engine and consequently the speed; the gasoline supply is regulated by the needle valve N, as before. A small arrangement is sometimes fitted below the needle valve to regulate the air supply independently.
The float F is made usually of copper, although sometimes of cork; it is guided by the small stems as shown. At P is a small plunger for forcing the float down for the purpose of flooding the opening 0 and making certain of the flow of gasoline. The cover C unscrews to allow access to float chamber.
In operation, as the gasoline is drawn up by the air, the level in the float chamber falls slightly, finally opening the valve V, admitting gasoline and restoring the level, whereupon the float rises, closing the valve V and stopping the flow. Thus the level is maintained constant and the same amount of gasoline drawn out at each stroke. The proper mixture is obtained by the proper relation of the air and gasoline supplies.
Fig. 22 shows another form of carburettor, whose operation is similar to that of the one just described but which has, however, some points of difference. The gasoline is admitted at G as before, the valve V, however, is a separate piece and is held down to its seat by the spring s; the lever L - which is pivoted at p is forked and rests just below the projection on the valve V; the float F is free and rests just above the long end of the lever.
When the float F settles, owing to the using of the gasoline, it strikes the end of the lever L and presses it down by its weight, thus raising the valve V and admitting more gasoline. When the level lias risen sufficiently the float ceases to press in the valve Y, and it closes. The usual needle valve is at X and the orifice at 0 as before. At I is a disc which may be turned across the opening and act as a throttle to regulate the speed of the motor. At i is a similar disc to regulate the air supply; by means of this disc t and the needle valve X the correct proportion of air and gasoline may he maintained.
Although the actual details of the different carburettors will differ greatly, the principles governing their action will be found to be similar to the above, and the several parts will be found in one form or another.
It is necessary for proper performance that means should be supplied for the regulation of both gasoline and air supplies. Owing to varying atmospheric conditions the relative adjustments will require changing from time to time. Even for starting the engine a different adjustment may be required from that under which the engine will run alter being warmed up. Any carburettor not having these adjustments is likely to give trouble in starting the engine.
The advantages of the float feed carburettor over the mixing valve are: The check valve is done away with, avoiding the noise and loss of the suction necessary to raise it; a more uniform mixture is obtained, as the gasoline is always at the same level; this is especially true in a sea, as there is a sufficient body of gasoline in the float chamber to draw upon at times when the boat is pitching and the tank is perhaps lower than the vaporizer, and the flow would otherwise be interrupted, causing the engine to work irregularly ; the speed may be more easily controlled, and the engine may be run at a slower speed where the suction would not be great enough to raise the check valve of the vaporizer.
The vaporizer or generator valve is fitted to two cycle two-port engines which require a check valve. For two-cycle three port engines some form of float feed is used. The float feed may also be fitted to the two-port type by the addition of a check valve between the carburettor and the engine. Some forms of float feed carburettor are fitted with a check valve for this purpose. The four-cycle engine is commonly fitted with some form of carburettor.
Warm air will absorb a greater amount of gasoline than cool air; for this reason the air inlet A is usually connected to a pipe which can draw air from some warm place, such as around the exhaust pipe or between the cylinders. A very common idea is to encircle a portion of the exhaust pipe with a perforated sleeve of thin iron with an outlet leading to the air pipe; in this way the carburettor is supplied with warm air. The vaporizing of the gasoline also draws heat from the body of the fluid and causes the carburettor to become quite cold, even so cold as to cause some trouble; this is avoided by drawing warm air from some source.
The air inlet should be protected from spray, as the presence of water will spoil the mixture. It should also be provided with a screen of wire gauge to prevent the sucking in of particles of waste, or other light sub-stance which would cause trouble.