In order that a thing may burn, it must be provided with oxygen. Oxygen is found in air, so it is usual to say that air is necessary in order that anything may burn.

To prove this, light a candle and place an empty bottle over it, upside down; in a very short time the oxygen in the bottle will be used up, and the flame will flicker and get smoky, and finally die out. If a card is laid on the chimney of a coal-oil lamp so that it covers the opening, that flame also will flicker, get smoky and go out.

In order to deaden the fire in a stove, the dampers are closed to prevent air from entering; the fire is kept alight by the very small quantity of air that leaks in below the fire-box. When the drafts are opened the fire will burn up brightly because a plentiful volume of air can then enter.

In a similar way, air must be used in a gas engine in order that the fuel may burn. It is not possible to mix air with a liquid; the first step in making a gas that will burn is, therefore, to turn the fuel, whether it is gasoline, kerosene, distillate, or other oil, into a vapor; this vapor is then mixed with air.

For good results it is very necessary that the vapor and air be in proper proportions. In the experiment with the candle and the bottle it was seen that as the air was used up, the candle flame became yellow and smoky: this is the effect of insufficient air. If there is not enough air in the mixture, part of the vapor will not be able to burn, and will only smoke.

If, on the other hand, there is too much air, the mixture, if it will burn at all, will burn slowly, and the extra volume of air will reduce the heat.

In a mixture of the proper proportions of air and fuel vapor, the burning, or combustion, will be very rapid, resulting in the sudden production of the greatest possible amount of heat. This, of course, is what is necessary if the engine is to produce its fullest power. With such a mixture, combustion will be complete before the piston has done more than start outward on the power stroke, and the greatest possible, or maximum, pressure will then be produced.

When a mixture burns slowly, the piston will have gone through much of the power stroke before combustion is complete, in which case a considerable part of the pressure that should have been applied at the beginning of the stroke will be wasted.

A mixture that is not correct will burn unevenly; it may burn better during one power stroke than during another, which will make the engine run unsteadily.

If the mixture has too much air in proportion to the amount of vapor, it is known as a thin mixture, or a lean or poor mixture. It burns so slowly that it is quite possible for the mixture that started burning before the beginning of the power stroke to continue burning through the exhaust stroke, and for enough flame to remain in the cylinder to set fire to the fresh charge that enters during the next inlet stroke. This will produce what is known as a backfire; that is, the mixture entering the cylinder will catch fire, and in burning will blow back through the open inlet valve. This is a dangerous condition, for the flame might spread to fuel dripping from the carburetor, or to the fuel tank.

A mixture that has not enough air is called a rich mixture; the air that is present will burn part of the vapor, while the rest will go out of the exhaust unburned, or will work past the piston into the oil in the crank case. This is wasteful of fuel.

The most serious result of a rich mixture, however, is in the production of carbon, and the carbonization of the engine. The flame of a rich mixture is smoky; the smoke of this flame, as is the case with smoke from all other sources, is composed of fine particles of carbon, or soot. These particles of carbon will deposit on all parts of the combustion space: on the top of the piston, on the valves, on the spark plugs, and on the inner wall of the cylinder head. At first it is gummy, but it rapidly hardens and forms a crust that must be scraped off with a steel tool.

Carbon in an engine will reduce the power through causing preignition, or, in other words, by setting fire to the fresh charge before the proper point in the stroke. The heat of the combustion will cause the carbon deposit to become so heated that it will glow, these glowing particles being sufficient to ignite the incoming fresh charge. The remedy for this condition is to remove the carbon, which is usually done by taking off the cylinder head and scraping away the deposit.

It may be added that carbon is also formed by the use of too much lubricating oil, as will be explained in the chapter on lubrication.

Thus it is seen that if the engine is to run properly, and is to be kept in good condition, the proportions of the mixture must be very carefully maintained.

The mixture is formed in a carburetor, or mixer. This is, roughly, in the form of a tube through which air is sucked during the inlet stroke; projecting into it is a fine tube called a spray nozzle through which the fuel enters. In action it is somewhat similar to the atomizer that is used for spraying the nose and throat. By forcing the fuel to flow rapidly through this small tube it comes out in the form of spray, and the tiny drops are picked up by the current of air and are carried into the cylinder.

It is much easier to form a mixture of gasoline than of kerosene or distillate, because gasoline vaporizes more readily at ordinary temperatures. If saucers of gasoline and kerosene are placed in the sun, the gasoline will evaporate rapidly and completely, leaving only a faint oily deposit. The kerosene, on the other hand, will evaporate slowly, and much of it will not evaporate at all.

To make kerosene and distillate evaporate completely, they must be heated, just as water must be heated to make it evaporate.

In the case of a carburetor for gasoline, the current of air needs only to be warmed; the spray of gasoline will evaporate on coming into contact with the warmed air, and much of it will enter the cylinder as vapor. In order to evaporate kerosene and distillate much more heat must be provided, and it is usually considered necessary to heat not only the current of air, but the liquid fuel as well. Methods of doing this will be explained in the next chapter.

When kerosene or distillate is used, there are conditions that make it necessary to add water vapor to the mixture, which prevents the overheating of the cylinder and reduces the deposit of carbon. The difficulty of making a complete vapor of kerosene and distillate results in a tendency on the part of these fuels to carbonize the cylinders; the use of water aids in keeping the cylinders clean.

The general principle of a carburetor is shown in Figure 21, one drawing illustrating conditions when the inlet valve is closed and the other when it is open. It shows an engine cylinder connected with an inlet pipe or mixing chamber, through which there is a swift flow of air during an inlet stroke.

Projecting into the intake pipe is the spray nozzle, which is connected with a small chamber containing fuel; inside of this chamber is a float, usually made of cork, although it is sometimes a light metal box. The fuel is intended to fill the chamber to a certain height, at which the valve will be closed by the float rising on the fuel. This level is such that the fuel does not quite reach the tip of the spray nozzle.

During the compression, power, and exhaust strokes, the fuel stands at this level, for it cannot run out of the spray nozzle, and the float holds the valve closed. As soon as the inlet valve opens, air rushes through the intake pipe and sucks fuel out of the spray nozzle. This, of course, takes fuel out of the float chamber; the float in sinking opens the valve, and enough fuel enters to restore the level.

The fuel comes out of the nozzle in the form of fine spray; it is in such small drops that it evaporates quickly, and the resulting mixture of fuel vapor and air passes into the cylinder. By using a spray nozzle of the proper size, any desired proportion of fuel and air may be obtained.

If an engine runs at a single speed, a carburetor as simple as this one would be satisfactory, for if the suction is always the same, there will be little or no change in the proportions of the mixture that is formed.

To get the best results, the proportions of fuel vapor and air should be the same for all running speeds of the engine. The proportions of the mixture, however, depend on the violence of the suction, which changes as the engine speed changes, becoming greater as the speed increases. The simple carburetor illustrated in Figure 21 can be adjusted to give a correct mixture for any particular speed, but will be out of adjustment for any other speed.

The speed of a 1-cylinder engine does not change very greatly; it is built to run at practically a constant speed, and a simple carburetor is satisfactory for it. The speed of engines with a greater number of cylinders may be greatly changed, and the carburetor must be so made that it will give the same proportions of vapor and air at low speed as at high.

In the simple carburetor described, the speeding-up of the engine will result in a greater rush of air through the intake pipe, which in turn will suck out a much greater quantity of fuel. If the carburetor is adjusted to give the proper quantity of fuel for the air that passes at low speed, at high speed it will give far more fuel than will be required by the quantity of air that then passes. Thus at high speed the mixture will be too rich.

If, on the other hand, this carburetor is adjusted to give a proper mixture at high speed, too little fuel will be sucked out when the engine runs slowly, and the mixture will be too lean.

A carburetor must thus have an additional device that will keep the mixture correct, regardless of the speed at which the engine runs. This is sometimes done by changing the size of the spray nozzle so that a greater or less quantity of fuel flows out, but more usually by permitting an extra quantity of air to enter the carburetor as the engine speeds up. This is done with an extra air intake, the principle of which is illustrated in Figure 22.

As will be seen, this carburetor has two openings for air, one being the main air inlet and the other the extra air inlet. The latter is an opening provided with a valve which is held on its seat by a spring. The suction created by an inlet stroke is exerted in the carburetor, but at low speed is not sufficient to suck the extra air valve from its seat. Air then enters only through the main air inlet, and the spray nozzle is adjusted to give the proper proportion of fuel.

As the engine speed increases the mixture becomes richer; but there is also an increase in suction, which becomes strong enough to pull the extra air valve from its seat. This provides another opening into the carburetor, through which enough air enters to keep the mixture in proper proportion. The higher the speed of the engine the more the valve will open, and the greater will be the quantity of air admitted.

In order to get the fullest power from an engine, the carburetor is built to give its most perfect mixture at the usual working speed. This will be the speed at which the engine will run under ordinary conditions. As the engine will run at this speed most of the time, the carburetor should then deliver its best mixture on the least possible quantity of fuel.

As an engine is run at low speed so little of the time, it is not necessary that the mixture should then be so perfect or that the fuel should be used so economically.

The design of a carburetor is a complicated matter, because the production of mixture is due to the flow of air, which is a very changeable thing. On a cold, damp day, the air will be heavier and denser than on a day that is hot and dry, and different quantities of fuel will be necessary for the formation of the mixture. The carburetor manufacturer cannot make a commercial carburetor that will take care of such a difference as this; he strikes an average that gives good general results, and expects the user to change the adjustments when weather and temperature make it necessary.

The formation of the mixture is affected by the condition of the engine. When all of the parts of the engine are tight, the suction in the carburetor is more violent than when there is a leakage of air past the piston rings, or through a leaky valve or spark plug.

On a dry, hot day the fuel evaporates much more readily than on a day that is cold and damp; more of the fuel that flows out of the spray nozzle will be vaporized and the formation of the mixture will be easier. On a cold, damp day the fuel will not vaporize in the carburetor to any extent, and much of it will pass to the cylinder in drops that even there will not vaporize in time to form a mixture. In order to assure the vaporization of enough fuel to form a mixture under such conditions, the fuel and the air must be heated to a greater degree.

As the engine becomes heated up, more and more of the fuel will vaporize, and the amount flowing out of the spray nozzle may therefore be cut down.

With fuels like kerosene and distillate, which do not vaporize as readily as gasoline, it is not unusual to have them condense on the walls of the inlet pipe, which produces a condition known as loading. This condensation is similar to the sweating of an ice-water pitcher on a hot day. If an engine is running at a constant speed, loading does not make much difference, because the carburetor is so adjusted that it gives a proper mixture. If the engine is suddenly speeded up, however, the greater rush of air will pick up the condensed fuel, and the mixture will instantly become too rich, continuing so until this extra supply of fuel is used up. The result will be to choke the engine and make it lose power just at the time when extra power is needed.

Loading can be prevented by heating the inlet pipe to such an extent that the fuel will not condense on it.

The speed of a tractor engine is practically always controlled by a throttle, which is a valve set in the passage of the carburetor. It operates exactly the same as a damper in a stovepipe; when it is closed, it shuts the passage and prevents the flow of mixture to the engine. As it is opened, it permits a greater quantity of mixture to flow, and it follows, of course, that as the charges of mixture become larger, the engine runs with more power. A tractor carburetor usually has two throttles, one being operated by hand and the other by the governor.

It is usual for a carburetor to be fitted with a strangler, or choke, which makes it easier to form a mixture at slow starting speed. When an engine is cold, the fuel evaporates slowly; and, furthermore, when an engine is cranked by hand its speed is so low that the suction in the carburetor is not sufficient to draw out enough fuel to form a mixture. The strangler is a valve similar in every way to the throttle, but placed between the main air inlet and the spray nozzle. When it is closed and the engine is cranked, very little air can enter the carburetor; the suction is therefore very great. Far more fuel than usual is then sucked out of the spray nozzle, and of this greater amount enough reaches the cylinder to form a combustible mixture. The engine will start, but as soon as it does so, the strangler must be opened so that the normal amount of air enters. If this is not done, the excessive suction will draw so much fuel out of the spray nozzle that the mixture formed will be too rich to burn.