Carburetion – A deep look at the Basics

WHY YOU CAN’T DRINK THROUGH A STRAW ON THE MOON.

Consider yourself standing on the surface of the moon with a glass of your favorite liquid refreshment and a straw. Just like back home on earth, you put the straw in the glass and take a long draw on the end of the straw. Nothing comes through the straw. Why not?

To answer this question, we will first return to Earth. As we approach the Earth, we enter a layer of gases that surround the entire planet and extends outward from its surface for many miles. This ocean of air is the earth’s atmosphere and it’s what we need to drink through a straw.

To understand why a straw needs an atmosphere, lets begin with the square inch to the right. Extending upward from the surface of the paper is a column of air  which extends upward for many miles. Within this column of air, there are thousands upon thousands of cubic inches of air stacked one on top of another, each adding one its own weight. The total weight, or downward SQUARE push on the square inch amounts to 14.7 pounds INCH when measured at sea level. Because the 14.7 pounds of air is resting on a square inch, we can 14.7 PSI say the column of air is exerting a force of 14 pounds per square inch (14 psi). This amounts to 2,160 pounds, or more than a ton of pressure, per square foot.

DENVER (5000 FT.) 13.0 PSI

Although we do not usually think of the air around us as having weight, it is a substance and therefore has to have weight. Just as water pressure below the surface of the ocean is caused by the weight of water, atmospheric pressure is caused by the weight of air. Pressure varies with altitude because as you go higher there is less air above you and therefore less weight.

With the tremendous pressure (2160 pounds per square foot) exerted by the atmosphere on all surfaces, you have to wonder why doesn’t everything collapse  under the weight. The things around us do not collapse because air pressure exerts it’s force equally in all directions and a balanced zero pressure condition exists on all surfaces.

Variances in atmospheric pressure from place to, place are usually due to moving air currents and storms. This sometimes causes a change in pressure of about one psi. Measurement of changing air pressure is important to meteorologists in predicting weather.

If the balance is disturbed, the weight of the atmosphere will become apparent. Consider a fuel tank with a vent which allows air to enter the tank as fuel is withdrawn by the fuel pump. The removal of the fuel creates a space within the tank that does not contain air. Without air inside the tank to balance the atmospheric pressure on the outside of the tank, the weight of the air will literally crush the tank.

It was the presence of a pressure differential between the inside and outside of the tank that allowed atmospheric pressure to crush the tank. A pressure lower than atmospheric, such as existed in the fuel tank,  is called a partial vacuum or pressure differential. Atmospheric pressure will always react to a pressure differential.

Remember It was not the partial vacuum! that sucked the tank flat. It was the weight of the atmosphere pushing on the outside of the tank that crushed the tank inward. The same principles atmospheric pressure and pressure differential cause liquids to flow through a straw. A pressure lower than atmospheric is created at one end of the straw. The atmospheric pressure acting on the surface of the liquid pushes it up through the straw. The liquid is not sucked out of the glass; it is pushed because a pressure differential exists between the two ends of the straw. And, that’s why you can’t drink through a straw on the moon. The moon does not have an atmosphere and therefore a pressure differential cannot be created with the straw.

Each time you inhale a breath of air, a muscle located below the lungs expands the chest and creates a low pressure area. Atmospheric pressure pushes air into the lungs to compensate for the pressure differential. You do not draw air into your lungs. You create a pressure differential. When you exhale the muscle collapses the chest and air is forced from the lungs.

An engine breathes in much the same way. As the piston moves downward, on the intake stroke, the area above the surface of the piston increases allowing atmospheric pressure to pus:_ air into the cylinder. When the piston moves upward, on the exhaust stroke, the area above the piston becomes smaller and the burned gases are pushed out of the cylinder.

 

Let’s see what occurs in terms of the individual air particles. When the piston moved downward the particles of air have an increased area to fill. The distance between particles became greater with piston movement. And, the greater the distance between air particles the higher the vacuum. As the vacuum increased atmospheric pressure outside the engine pushed air into the cylinder.

Each time you inhale a breath of air, a muscle located below the lungs expands the chest and creates a low pressure area. Atmospheric pressure pushes air into the lungs to compensate for the pressure differential. You do not draw air into your lungs. You create a pressure differential. When you exhale the muscle collapses the chest and air is forced from the lungs.

An engine breathes in much the same way. As the piston moves downward, on the intake stroke, the area above the surface of the piston increases allowing atmospheric pressure to pus:_ air into the cylinder. When the piston moves upward, on the exhaust stroke, the area above the piston becomes smaller and the burned gases are pushed out of the cylinder.

Let’s see what occurs in terms of the individual air particles. When the piston moved downward the particles of air have an increased area to fill. The distance between particles became greater with piston movement. And, the greater the distance between air particles the higher the vacuum. As the vacuum increased atmospheric pressure outside the engine pushed air into the cylinder.

 

 

DANIEL BERNOULLI AND THE VENTURI AFFECT

We have established that atmospheric pressure pushes air into an engine as pistons move downward on the intake stroke. During this explanation an air inlet was attached to the intake manifold. A continuous flow of air through the air inlet was possible because our multiple cylinder engine created a continuous low pressure in the intake manifold. This steady air flow through the inlet provides another opportunity to create a pressure less than atmospheric. Instead of a smooth air flow path through the air inlet we will force the air to flow through an hour glass shaped restriction called a venturi.

 

In the seventeen hundreds, when the luxury means of travel was a horse, Daniel Bernoulli knew how air would react when it was forced to flow through a restriction.

He knew that as the incoming air approached the narrow path it would have to speed up to maintain the same amount of air flow. He also knew that when the air speed increased, its pressure dropped. The increased air speed and its accompanying pressure drop is called the venturi affect.

The venturi affect is possible because air is not a continuous fluid, or substance; it consists of separate particles, or molecules. When we keep this in mind, the venturi affect becomes more easily understood. Let’s follow two particles through the venturi and see what happens.

As air enters the top of the venturi all the air particles are moving downward toward the restriction at a more or less uniform speed. However, if all particles are to move through the venturi, they will have to speed up and hurry through.

Suppose we watch two of the particles on their way through the venturi. One particle is somewhat behind the other. The leading particle, entering the venturi first, speeds up, tending to leave the second particle behind. The second particle, entering the venturi, also increases in speed.

But the first particle has, in effect, a head start. The second particle cannot catch up. They are farther apart in passing through the venturi than they were when entering the air inlet. Now visualize a great number of particles going through this same action, and you can understand that in the venturi they are somewhat farther apart than they were when they first entered the venturi. This is just another way of saying that a partial vacuum exists in the venturi. For, as we mentioned previously a partial vacuum is a thinning out of the air a more-than-normal distance between the air particles.

To understand what occurs in a real multiple cylinder engine, we have to add an intake manifold and air inlet. The manifold will confine the low pressure created by the moving pistons and route the air flow into the individual cylinders. The air inlet will be the mounting point for our carburetor when we begin to mix gasoline with the incoming air.

Because a multiple cylinder engine has continuous and overlapping piston movement, a steady low pressure is created in the intake manifold. Atmospheric pressure is constantly trying to fill this low pressure area by pushing air through the air inlet. The low pressure in the intake manifold is low enough and steady enough to create a constant air flow through the air inlet. Now we have a continuous flow of air through the engine, instead of the occasional gulp a one cylinder engine takes during its four cycles.

This steady low pressure is called intake manifold vacuum and it can be tapped to provide useful mechanical motion. If a hose is routed from a manifold vacuum tap to a sealed, flexible diaphragm the steady low pressure will extend through the hose to the diaphragm surface. We now have a pressure differential on opposite sides of a flexible diaphragm.

From our experience with the crushed fuel tank, we know atmospheric pressure will push the diaphragm and move it inward against spring pressure. This movement can be extended by attaching a link to the vacuum diaphragm. When the diaphragm moves, the link moves, providing useful mechanical motion. Remember – vacuum does not pull the diaphragm – atmospheric pressure pushes it! The spring allows the diaphragm to repeat its motion. After the engine is shut off and manifold vacuum no longer exists, the spring will push the diaphragm back to its original position. If we connect a link to the diaphragm, mechanical motion can be produced as a result of intake manifold vacuum (Note: The same motion can be duplicated by using a piston instead of a diaphragm).

COMBUSTION IS A CHEMICAL REACTION

Light a candle and then cover it with a glass. After a few minutes, the candle flickers and goes out. Before repeating this test, remember the glass is not empty. It is full of air. Now, lift the glass and light the candle. Again, the candle burns brightly until it is covered with the glass. Covering the candle sealed its flame from the surrounding air. The candle burned for a minute or two because the glass contained a small amount of air. The air around us obviously contains an element necessary for combustion.

The fact that the candle took time before going out is important. The glass contained an amount of air which included the element necessary for combustion. As the candle burned, the element contained in the jar was used to create the flame. When the limited amount of the element was used in the flame, combustion stopped and the candle went out.

The element in the air which allowed the candle to burn, is a chemical we call oxygen. The earth’s atmosphere is a mixture of gases consisting mainly of oxygen and nitrogen. Other gases are present but in small amounts. If we were to weigh the amount of each gas in the earth’s atmosphere, we would find that 75 per- cent is nitrogen, 23 percent is oxygen and 2 per- cent is trace gases. The gas, or chemical, we are concerned with is oxygen. Oxygen is one of the two ingredients the carburetor will have to mix to produce a combustible mixture.

To produce combustion we need a second ingredient which will mix with oxygen and burn when ignited. For our purposes we will mix air and gasoline. Both of the ingredients are chemicals and have to be carefully mixed or combustion will not occur.

The oxygen portion of our mixture is a basic element and occurs naturally in the earth’s atmosphere. Gasoline, however, has to be manufactured from crude oil taken from beneath the earth’s surface. Crude oil is a chemical soup consisting of two basic chemical elements called hydrogen and oxygen. Over millions of years these elements connected to each other to form hydrocarbon chains. These chemical chains have different lengths. And, when the crude oil is processed, it is chain length which will deter- mine the type of chemical produced.

The processing of crude oil occurs in a refinery. It is here the hydrocarbon chains are sorted into the various chemical lengths. The refinery can also break the chains and reform them into different chemicals. For our engine we will use a chemical chain called gasoline. This particular chain of hydrogen and carbon has the ability to burn explosively if it is properly mixed with the oxygen in the air and ignited.

 A PRECISE RECIPE

 

The two ingredients – oxygen and gasoline – have to be mixed in exact amounts or combustion will not occur. If too much air is added a lean mixture results. Too little air and we have a rich mixture. In either case the mixture will burn poorly or not at all. If we base our mixture on weight we will need 15 pounds of air to completely burn one pound of fuel or a 15 to 1 ratio. If we mix by volume we will need 9,000 gallons of air to burn one gallon of gasoline. Most engines will operate within a range of 8 to 1 (very rich) to 17 to 1 (slightly lean).

Let’s take an ounce of gasoline and see how it can be best mixed with the proper amount of air to provide an explosive mixture. If the ounce of fuel is contained in a cup and ignited, the vapors rising from the surface will ignite and burn. The liquid gasoline in the cup is evaporating and therefore has vapors rising from its surface. When a match is held over the cup, the liquid gasoline does not ignite; only the vapors rising from its surface ignite and burn. The burning vapors will produce heat but an explosive burn rate does not occur. To get a more rapid burn rate the fuel has to expose a larger surface area. If the ounce of fuel is broken into fine droplets, each droplet will be surrounded by air when ignited. This air/fuel mixture will burn with explosive force.

Mixing of fuel and air is further complicated by varying engine requirements. Mixture requirements vary with temperature, speed and load. It is difficult to provide the perfect mixture for all operating conditions. To provide easy starting in cold weather, a very rich mixture is needed. As soon as the engine starts the mixture has to lean out (more air) to prevent flooding and stalling. As the engine warms up, fuel vaporizes more easily and leaner mixtures are needed. Changes in engine speed and load also affect the air-fuel mixture. At idle, low speed and low load operation, a lean mixture is required. At medium speed and part throttle, still leaner mixtures must be supplied for good fuel economy. For maximum power, a rich mixture is needed.

FUEL BOWL AND DISCHARGE TUBE

 

At this point in our lesson, we have learned how and why air flows into an engine. And, we know an engine burns oxygen and gasoline in varying mixtures. What we don’t know is how the air and fuel were mixed before entering the engine. The mixing of our two ingredients occurs at the carburetor. Although there are many types and sizes of carburetors, they all function as a result of some basic principles. Let’s return to our air inlet and see how a few basic principles become a carburetor. As you recall, we reshaped the air inlet to provide a venturi affect. It is around this restriction we will construct a basic carburetor. The first thing we need is a source of fuel.

This fuel source is contained in a reservoir attached to the side of the venturi and is called the fuel bowl. A tube mounted between the fuel bowl and venturi will provide a connection between the air flow and f uel. The tube is called the main discharge tube. It is positioned with one end below the surface of the fuel and the other end in the center of the venturi.

The main discharge tube and fuel bowl should remind you of the straw and glass. The situations are very similar. A low pressure caused by suction, allowed atmospheric pressure to push the liquid upward through the straw. In our carburetor the venturi affect will create a low pressure at the end of the main discharge

tube. With a pressure less than atmospheric at one end of the tube, the weight of the atmosphere on the surface of the fuel in the bowl will push fuel upward in the tube where it will discharge into the incoming air. This pressure differential between the air flow and fuel is the basic principle upon which all carburetors function.

With this basic principle established we can add some refinements that will make our carburetor more like the real thing. To keep out debris, the first thing we should do is provide a cover for the fuel in the fuel bowl. The cover has to be vented. The vent is nothing more than a hole which allows atmospheric pressure into the fuel bowl.

FLOW REGULATION

The first step in refining a carburetor is to add devices that will regulate the flow of air and fuel. Beginning at the fuel bowl end of the main discharge tube, we can regulate the amount of fuel entering the tube by restricting the tube end. This restriction is called the main metering jet. And, by changing its size we can control the amount of fuel allowed to enter the main discharge tube.

At the low pressure end of the main discharge tube, we can increase the venturi affect by adding a smaller, secondary venturi. This venturi is suspended in the center of the air stream with its lower edge just slightly above the main venturi. The low pressure created by the main venturi now occurs just below the secondary venturi. To fill the low pressure created by the primary venturi affect, the air stream rushes through the secondary venturi. The air rushing through the secondary venturi creates deeper low pressure and concentrates it at the end of main discharge tube. Because it aids the primary venturi in lowering atmospheric pressure, the secondary venturi is sometimes called a “booster” venturi.

To control air flow we will add two plates; one at the top of the carburetor which we will call the choke plate and one at the bottom which we will call the throttle valve. The choke plate, when closed, will cause a rich mixture to enter the engine.

The rich mixture is required for cold engine starts and to keep a cold engine running. When the engine warms-up, the plate isopen and no longer influences the air/fuel mixture. The throttle valve, which is connected through linkage to the accelerator pedal, controls the amount of air/fuel mixture entering the engine. By controlling the flow rate, the throttle valve will determine engine speed.

ATOMIZATION AND VAPORIZATION

Atomization and vaporization are two very important principles of carburetion. Vaporization is the changing of a liquid into a vapor. If gasoline is placed in an open pan, it slowly disappears and is said to have evaporated. if the same amount of gasoline is placed in a glass evaporation will take a considerably longer amount of time. This demonstrates an important point about vaporization.

The greater the surface exposed the more rapidly evaporation occurs. If gasoline is broken into many tiny droplets, by spraying or other means, an extremely large amount of surface area is exposed and vaporization is almost instantaneous. Atomization is the term used to describe the breaking up of a liquid into a fine mist. In our carburetor we want the gasoline atomized before it enters the air flow.  Because of higher pressure, EFI breaks the fuel down into smaller droplets, and because of extreme high pressure, GDI engines break the fuel down into the smallest droplets of all.

The atomized fuel will then vaporize instantly when it enters the incoming air stream. We now have the ideal mixture of air and gasoline vapors. Remember!  The liquid gasoline in the cup. The gasoline did not burn only vapors rising from its surface ignited.

 

AIR BLEEDS

 

In a carburetor, atomization has to occur before the fuel discharges into the incoming air stream. The glass of liquid and straw, used to demonstrate the principle of atmospheric pressure, can also be used to demonstrate the principle of atomization. To properly demonstrate this principle we will limit the low pressure applied to the end of the straw.

 

The limited low pressure will be unable to raise the liquid to the top of the straw. Atmospheric pressure will push the liquid to a certain level and it will stop.

Approximately one-half inch above the surface of the liquid, we will make a small hole in the straw. When the limited low pressure is now applied to the straw, atmospheric pressure pushes the liquid through the straw in a series of small slugs. The liquid is able to reach the top of the straw because the hole provides an entry for air and atmospheric pressure. The air drawn into the straw by the low pressure breaks the liquid into small slugs. Atmospheric pressure entering the hole is now able to push the smaller slugs to the top of the straw. While the liquid is able to reach the top of the straw, it is not atomized as ice would like.

This situation is corrected by switching to a high technology straw. The air hole is lowered beneath the surface of the liquid and then extended upward to provide an air bleed. The liquid end of the straw is restricted to limit the amount of flow entering the straw. Lowering the air hole allows air to enter the straw beneath the surface of the liquid. The air now bubbles through the liquid as it enters the straw. Because the end of the straw is restricted, more of the low pressure will appear at the air bleed entry. A smaller amount of liquid will now enter the straw and mix with the additional air. The mixture will rise to the top of the straw as finely atomized liquid.

The hole in the straw and the restriction at the end have actual counterparts in a carburetor. The restriction is the main metering jet we added to the fuel end of the main discharge tube. The air bleeds will! be drilled passages that internally intersect fuel passages, and atomization will occur as demonstrated with the straw.

 

THE EARLY DAYS ENGINE DEVELOPMENT

Before the internal combustion engine, steam was used to power industry and transportation. In the early 1800’s, some inventors and tinkers began to question the efficiency of steam. They felt a great deal of heat was being wasted in generating steam to perform work. It was believed that heated air could be used to perform the same work and do it more efficiently. These early attempts were not directed at developing an internal combustion engine; but toward improving the steam engine.

Around 1819 a Frenchman named Carnot began studying steam engines and the principles upon which they worked. Through this study, he decided steam was an inefficient means of carrying heat to an engine to perform work. On paper he developed the basic theories which would later make an internal combustion engine possible. Rather than steam be believed air would be much more efficient because:

Air could more readily transfer heat (work).

While steam had to be generated in a boiler, air could be heated directly by combustion within the engine (internal combustion).

For maximum increase in temperature and volume the air must be put under pressure before heating it (compression).

Because inventors were using their experience with steam engines, these basic principles found little application in new developments. Around 1860 two events occurred that would eventually lead to the development of the internal combustion engine. A French experimenter, Lenoir built and sold an engine that drew in an explosive gas for part of the pistons stroke and then ignited it.

The explosion provided a push for the remaining piston stroke. Though inefficient and built more like a steam engine having no compression and working on only half its stroke. Two years later another Frenchman Beau de Rochas built upon Carnot’s basic principles and developed the principles of the four cycle engine. These two seemingly unrelated events were combined by Nikolaus August Otto into the first four cycle internal combustion engine. In 1861, Otto began experimenting with an engine similar to Lenoir’s.

During this experimentation he discovered the importance of compression as outlined by Carnot and De Rochas. Otto formed a partnership with Eugene Langen, an engineer, and the two men worked together to improve the lenoir engine. The experience gained while working on this engine allowed Otto to independently develop the first four cycle internal combustion engine. Being very heavy for their power output, Otto’s engines were suitable only for stationary use. But in 1882

Gottlieb Daimler, an engineer employed by Otto and Langen, left that firm to do his own development work. Within a year he was able to patent the first high-speed gas engine, capable of running at 900 rpm, and with a far more favorable power-to-weight ratio than the Otto engine. By 1883 Daimler had propelled a car, a bicycle and a boat with his new lightweight engine.

WICK AND SURFACE CARBURETION

These first internal combustion engines and the numerous adaptations that followed used many different techniques to provide an explosive air/fuel mixture. The  mixing of air with a fuel as it entered the engine was termed carburizing.  And, a carburetor was any device that saturated air with vapors of a volatile hydrocarbon.

The first carburetors were the surface type and wick type. In a wick type of carburetor, a fabric would be suspended in the tank with one end immersed in the fuel.

The fuel would saturate the fabric and air passing through it would vaporize the fuel and carry it into the engine. The air to fuel ratio could be regulated by an air control valve located at the engine intake. By moving the valve, the amount of air entering the intake could be regulated to lean out or richen the mixture.

The surface carburetor was similar to the wick carburetor in that it depended on the vapors available to create an explosive air/fuel mixture. Air entered a tank where it was either bubbled through the fuel or the fuel was agitated to create additional vapor. Like the wick type carburetor the mixture ratio was controlled by an air control valve mounted on the engine intake. These carburetors were used mainly for stationary installations and required extremely volatile fuels. Benzine, naphtha and ether were not uncommon.

The surface type of carburetor was used by Karl Benz on vehicles built at the end of the 1800’s. In this carburetor, air entered and was bubbled through benzine. Fuel  level was maintained at a constant height by a float and needle valve assembly. To increase the amount of vaporization, the fuel was heated by part of the exhaust gas which was routed through a pipe mounted on the bottom of the carburetor. Liquid fuel was. prevented from entering the engine by an extractor which would allow only vapors into the engine intake.

 

MIXING VALVES

The mixing valve was an early and simple form of carburetor consisting of a valve and a fuel inlet. When the piston moved downward, the low pressure in the engine would open the valve, allowing air to enter the engine. The opening of the valve also uncovered the fuel inlet port. With each opening of the valve fuel was pulled into the incoming air and entered the cylinder in a partially vaporized state. The amount of fuel drawn through the port, and therefore the air/fuel ratio, was controlled by a mixture screw. The tapered end of the mixture screw controlled the size of the port as it was threaded in or out of the port.

 

SPRAY CARBURETOR

Real advances in carburetion occurred as atomizing and spray carburetors began providing the required air/fuel mixtures. The modern spray carburetor in its various forms has been evolved from these early jet type carburetors. In 1892, an inventor, named Maybach, built and sold the first carburetor to direct the fuel spray from an orifice into the center of an air stream. A small float in its own chamber maintained the liquid level at. The jet by opening and closing a needle valve in the supply line to the float chamber a reduced diameter air passage at the jet increased air velocity and therefore reduced pressure to improve the spray characteristics. All of the basic elements of a modern carburetor were present.

 

PROPER AIR/FUEL MIXTURE TO ENGINE

 

When you enter a carburetor, you are entering a maze of minute and intricate passages that intersect and seem to be routed in numerous and unrelated directions.

As confusing as all this may seem each bore, hole and tube combine to provide exact air/fuel ratios.

A combination of these various passages is called a carburetor system. And, as the name implies, each system has a definite beginning and end. To trace a system  from end to end using an actual carburetor is impractical and unnecessary. Many of the passages are drilled from the outside of the casting and plugged during manufacturing, making it impossible to trace a system. To compensate for this situation, we have to use sectional diagrams that show in pictures how each system is routed and functions.

With a basic understanding of what makes a carburetor function and with a careful study of , the diagrams, the maze becomes a carefully laid I out pattern of fuel,  air and vacuum passages. The function of each passage and component, although unseen, becomes obvious as the principles and diagrams are related into a functioning carburetor. All carburetors have six functioning systems that will match precisely the air/fuel requirements for any given engine condition.

 

In the following pages we will see, through sectional diagrams, how each system functions. You will see that some systems, such as the choke system, function for specific condition

 

PROPER AIR/FUEL MIXTURE TO ENGINE

When you enter a carburetor, you are entering a maze of minute and intricate passages that intersect and seem to be routed in numerous and unrelated directions.

As confusing as all this may seem each bore, hole and tube combine to provide exact air/fuel ratios.

A combination of these various passages is called a carburetor system. And, as the name implies, each system has a definite beginning and end. To trace a system from end to end using an actual carburetor is impractical and unnecessary. Many of the passages are drilled from the outside of the casting and plugged during manufacturing, making it impossible to trace a system. To compensate for this situation, we have to use sectional diagrams that show in pictures how each system is routed and functions.

With a basic understanding of what makes a carburetor function and with a careful study of , the diagrams, the maze becomes a carefully laid I out pattern of fuel, air and vacuum passages. The function of each passage and component, although unseen, becomes obvious as the principles and diagrams are related into a functioning carburetor. All carburetors have six functioning systems that will match precisely the air/fuel requirements for any given engine condition.

 

In the following pages we will see, through sectional diagrams, how each system functions. You will see that some systems, such as the choke system, function for specific condition. While others, such as the main metering, require transitional and supplementary systems.  While others, such as the main metering, require transitional and supplementary systems.

 

CHOKE SYSTEM

A carburetor is confronted with two problems when the driver attempts to start a cold engine. First, since normal engine cranking speeds are about 100 rpm, the amount of air flowing through the carburetor is very low. With little air flow, a pressure differential cannot be created in the venturi and fuel will not leave the fuel bowl. In addition to a low air flow rate, the cold fuel does not vaporize as readily as warm fuel. In fact, the fuel and engine may be so cold that the fuel entering the engine condenses to a liquid again upon contact with the cold intake manifold. Unless the mixture is richened with additional fuel, the slow burning of partially vaporized fuel will cause power loss and stalling. If the mixture is richened, enough vapor will enter the cylinders for the engine to run smoothly once it is started.

 

As we have seen, the choke consists of a plate mounted at the top of the carburetor. When the valve is closed, the air inlet is restricted and very little air enters the carburetor. The plate literally “chokes-off” the air flow. When the engine is cranked with the choke plate closed, vacuum in the intake manifold extends into the carburetor venturi. A large pressure differential now exists in the venturi and fuel will be pushed from the fuel bowl through the main discharge nozzle. The quantity of fuel delivered and the small amount of air produces an air/fuel mixture rich enough to start the engine.

 

As soon as the engine starts, its speed increases immediately from a cranking speed of around 100 rpm to a fast idle speed that often exceeds 1200 rpm. To lean out the mixture the choke valve opens a slight amount to permit additional air flow. This slight opening of the choke is called “choke pull-off” and is necessary to pre vent an overly rich mixture that could cause stalling.

 

MANUAL CHOKE

Before the automatic choke system became a standard carburetor convenience, the driver had the responsibility to manually position the choke plate. If the engine was cold, a choke knob, usually located on the dash panel, was pulled out to close the choke plate. After the engine started the driver had to maneuver the knob to get good idle and cold driveaway. It was a situation that required some experience and a feel for what choke setting the engine required at a particular temperature.

As the engine warmed-up. the driver continued pushing the knob inward until the choke valve was fully opened. If the driver did not carefully control choke valve position, incorrect air/fuel ratios detracted substantially from cold weather driving. Lean mixtures would result in repeated hesitation, stumbling and stalling.

If the driver forgot and failed to completely open the choke valve on a warm engine, the carburetor would continue supplying an unnecessarily rich air/fuel mixture. The excessive richness caused stalling, flooding, carbon fouling, poor fuel economy and general poor engine performance.

To relieve the driver of this responsibility, carburetors were equipped with an automatic choke.

The basic element of an automatic choke is a temperature sensitive spring. This spring consists of two metal strips welded together and formed into a spring-like spiral. Because the two metals used to form the thermostatic spring expand and contract at different rates, the spring tends to wind or unwind with changing temperatures.

If we link the choke plate to the thermostatic coil, the position of the valve will change as the coil winds and unwinds. When the engine is cold, the spring has wound up enough to close the choke plate and spring-load it to the closed position. When the engine is cranked, a rich mixture is delivered to the engine. When the engine starts, air movement into the carburetor causes the choke plate to open slightly against the thermostatic-spring pressure. As the engine warms up the thermostatic coil unwinds and the choke plate gradually opens.

Although the thermostatic coil unwinds as the engine warms-up, the actual unwinding of the coil is assisted by the application of heat. The thermostatic coil can be heated in one of three ways:

exhaust manifold heat electrically generated heat a combination of electric and manifold heat.

If the coil is exhaust manifold heated, air is drawn through a well inserted into the exhaust manifold. As it passes through the well, the air is heated and then routed through a tube to the thermostatic coil housing. Other applications use a small electric heater mounted in the choke cap. When the temperature drops below a certain point a thermostatic switch closes and the heater circuit is complete.

Electric current, supplied by circuit now flows through the coil heater. Its electrical resistance to current flow generates the necessary heat. Less frequently used is a combination of hot air and electrical heat. In these applications the electrically generated heat will unwind the thermostatic coil rapidly to a certain point. When the thermostatic switch opens the heater circuit, the heated air from the exhaust manifold takes over for more gradual opening of the choke valve.

 

Choke Pulloff (or pulldown)

After the engine is started air flow into the carburetor will open the choke plate a small amount against the tension of the thermostatic coil. The choke plate will open this slight amount because it is mounted off-center on the choke shaft. This slight opening of the choke plate is necessary to prevent an overly rich mixture after the engine starts. Because the velocity of the air entering the carburetor will vary with engine speed, the choke plate is linked to a vacuum operated piston or diaphragm that provides a positive pull against the closing tension of the thermostatic coil.

The action of the diaphragm or piston is cafled choke pulldown and its purpose is twofold.

It helps open the choke after the engine starts and it controls the position of the choke plate depending on engine load. When the throttle is opened to accelerate, the air/fuel mixture has to be enriched. The action of the accelerator pump (which will be explained later) provides momentary enrichment, but additional richness is required because the engine is cold. This added richness is provided by the action of the pufldown system. The opening of the throttle valve causes a drop in intake manifold vacuum which is transmitted to the diaphragm or piston.

With the lower vacuum signal, the choke coil is able to move the choke valve toward the closed position The amount it is closed depends on how much vacuum drop occurs during acceleration.

 

FAST IDLE

In addition to controlling the position of the choke plate, the thermostatic coil is linked to a cam. When the engine is cold, the winding action of the coil will rotate this cam into a position where it will contact a lever attached to the throttle shaft. During the starting procedure, the throttle is opened and the cam rotates into the fast idle position. When the throttle is released, the cam and lever hold the throttle open to provide increased engine speed.

This fast idle speed is necessary to ensure that enough air and fuel are pushed into the engine to prevent stalling.

As the thermostatic coil unwinds its linkage opens the choke plate and rotates the fast idle cam away from the throttle lever. As the cam rotates its shape progressively reduces the fast idle speed. When the cam is rotated to the point where it can no longer contact the lever, the engine speed returns to curb idle.

 

CHOKE UNLOADER

The choke plate is mechanically linked to the throttle valve so that the choke will open if the throttle is moved to the wide open position. The purpose of the unloader is to clear the intake manifold if it receives an excessive amount of fuel during choke operation. If the engine does not start immediately, and the choke valve is closed, the overrich mixture being delivered by the carburetor will “flood” the engine.  Wet spark plugs won’t fire because the spark rides through the wet rather than jumping the gap.

Which simply means the intake manifold and cylinders become loaded with an air/fuel mixture too rich to ignite. To lean out the mixture, the driver pushes the accelerator pedal all the way to the floor, opening the throttle to the wide open position. The link from the throttle lever opens the choke plate enough to allow air flow into the engine to dry the spark plugs. With further cranking, the air flow entering the intake manifold and cylinders leans out the rich mixture to an air/fuel ratio that will ignite and start the engine.

 

VALVE SEAT

As we know, the air stream through the carburetor is continuously drawing fuel from the fuel bowl. The rate at which fuel is removed from the bowl is not constant. It will vary with many factors including engine speed, engine temperature and load. For proper carburetor operation, a specific fuel level has to be maintained in the fuel bowl. The fuel delivery and float system will maintain this level by continuously adding fuel to the bowl. The rate at which fuel is added to the bowl will exactly match the rate at which fuel is removed from the bowl.

The amount of fuel entering the fuel bowl, from the pump, is controlled by a float and needle valve. The float rides on the surface of the fuel in the bowl and is hinged at one end. The hinge allows the float to pivot as it moves up and down with the changing fuel level. As the float pivots it controls the opening and closing of the needle valve.This valve has a small flexible tip which seals against the inlet fitting, and prevents fuel entry into the fuel bowl.

If fuel enters the float bowl faster than it is withdrawn, the fuel level will rise. This will cause the float to pivot upward and push the needle valve into the valve seat. This, in turn, shuts off the fuel inlet. If the fuel level drops, the float moves down and releases the needle, opening the fuel inlet. By opening and closing the valve in response to fuel level, the float is able to maintain a specific fuel level in the bowl. The float tends to position the valve so the incoming fuel just balances with the fuel being withdrawn.

The fuel delivery system consists of the fuel tank, fuel pump and necessary interconnecting tubes or hoses. The fuel pump, which may be electrically powered or mechanically driven by the engine, draws fuel from the tank and pumps it into the fuel bowl.

 

INTERNAL VENT

All air entering a carburetor is filtered to prevent the entry of dust and other abrasive inpurities into the engine. The filter is mounted in a sealed housing and represents an air flow restriction. The amount of restriction depends upon the cleanliness of the filter. A variable restriction of this type can affect the air/fuel ratios if an internal vent is not present.

The internal vent tube extends from the fuel bowl to a point just above the choke plate. We now have a direct connection between the bowl and the carburetor side of the filter. To understand why this is important, we have to remember several things. First; the air flowing into the engine is caused by intake manifold vacuum (pressure less than atmospheric). Second; we want only the pressure differential created by the venturi affect to draw fuel from the bowl. Third; if the filter represents a restriction to air flow, the pressure on the carburetor side of the filter will be less than that on the atmospheric side of the filter.

Problems occur when the air filter becomes clogged. The intake manifold vacuum will deepen the low pressure on the carburetor side of the filter. This partial vacuum will add to the increased low pressure at the fuel discharge nozzle. As a result, fuel flow no longer matches air flow. It now flows because of the air flow venturi affect and the added vacuum created by the filter. This condition may make the air/fuel ratio too rich.

The internal vent eliminates this condition because the low pressure created by the filter is transferred to the surface of the fuel in the fuel bowl. If a reduction of atmospheric pressure occurs on the carburetor side of the filter, a corresponding reduction will occur on the surface of the fuel. With less pressure on its surface, a smaller amount of fuel will flow from the bowl, and air fuel ratios will remain constant for a given amount of air flow.

 

IDLE AND LOW SPEED SYSTEMS

Up to this point all fuel discharge has occured through the main discharge nozzle as a result of air flow and the venturi affect. As we begin varying air/fuel ratios to match engine speed and load, we will have to begin considering alternatives to the main discharge nozzle.

The throttle valve regulates engine speed by controlling the amount of air flowing into the engine. At higher engine speeds, air flow, the venturi affect and the main discharge nozzle will supply the necessary air/fuel mixture. As the throttle is closed both engine speed and air flow will be reduced. Without sufficient air flow, fuel systems which do not depend on the venturi affect are needed to supply the fuel to the intake.

IDLE SYSTEM

With the engine at idle or operating at very low speeds, the air/fuel mixture will be supplied by the idle and low speed systems. Fuel will continue to flow as a result of a pressure differential. But, instead of venturi vacuum we will use intake manifold vacuum. To make this system operational, we need a fuel passage that extends from a port below the throttle plate to the fuel bowl. For atomization the passage will be extended upward to form an air bleed.

In operation, manifold vacuum will create the pressure differential and atmospheric pressure will push fuel into the idle passage. As the fuel flows through the passage, it mixes with air and the finely atomized mixture enters the engine below the throttle valve. Although this mixture is relatively rich, it leans out as it mixes with the small amount of air that gets past the closed throttle valve.

 

The mixture can be carefully adjusted to provide the exact air/fuel mixture by turning adjusting screws located at the base of the carburetor. In later model emission-friendly carburetors, the screws, if present, were capped and provided very little adjustment range.

 

LOW SPEED SYSTEM

The idle system is limited to a specific and narrow rpm range. Once the throttle begins to open, this system is no longer capable of providing the required air/fuel mixture. A transition system, between the idle and main discharge nozzle, is required. This system does not originate at the fuel bowl. Instead it is an extension of the idle system. The system we used to provide the proper air/fuel mixture at idle has a capacity larger than that required for engine idle. Its ability to deliver fuel is limited by the size of the idle discharge port. Its additional capacity will be used to provide a transition stage from idle to the main discharge nozzle.

As soon as the throttle is opened past the idle position, air speed increases and would immediately lean out the air/fuel mixture except for the idle transfer passage. This passage intersects the idle system above the throttle plates. At idle fuel cannot discharge through this passage because it is not exposed to a pressure differential. In fact this passage contributes a small amount of air to the idle system as fuel flows by on its way to the idle discharge port. As the throttle opens the transfer port is exposed to manifold vacuum. The fuel flowing from this port mixes with the additional air moving past the opening throttle to provide sufficient mixture richness for part throttle, low-speed operation.

To tie the idle and low speed ports together here is a summary of what occurs.

With the throttle slightly open at idle, manifold vacuum is high. The pressure differential between the fuel bowl and manifold forces fuel through the idle system to the idle discharge port.

As the throttle opens and air flow increases, the transfer port is exposed to manifold vacuum. The additional fuel discharged through this port prevents a lean mixture as the transition from idle to main discharge occurs.

Further opening of the throttle increases the air flow rate, and the venturi affect begins pulling fuel from the main discharge tube.

 

MAIN METERING SYSTEM

As the throttle plate continues to open, the responsibility for supplying the required air/fuel ratio is transfered from the low speed systems to the main metering system. Actually, the low speed systems do not suddenly stop supplying the air/fuel mixture. It is more of a gradual transition. The low speed systems fade out as the main metering system fades in.

The main metering system consists of the fuel discharge nozzle, the main/booster venturis and the main discharge. These components and their operation should be familiar from our construction of our basic carburetor.

The faster the engine operates, the more fuel will flow through the main metering system. The relationship between air flow rate and venturi vacuum will result in a nearly constant air/fuel ratio as maintained by the main metering system from part to wide open throttle. The main metering system supplies a leaner air/fuel mixture than any of the other fuel systems in the carburetor. This is possible, because the main system functions when the engine is operating under a part throttle condition and is not under a heavy load or power requirements. An engine can operate efficiently on a leaner air/fuel mixture under these circumstances.

As the throttle plate moves toward the open position a sufficient amount of air flows through the carburetor to create a vacuum in the venturi. The end of the discharge nozzle is positioned in the low pressure area created by the venturi affect. A pressure differential now exists between the end of the nozzle and the fuel in the fuel bowl. This pressure differential will allow atmospheric pressure to push fuel from the bowl into the main metering system. Fuel enters the system through the main metering jet located in the bottom of the fuel bowl. The hole through the jet is called a metering orifice and it can be sized to regulate the amount of fuel leaving the bowl. As the fuel flows through the main metering system, it is mixed with air entering the system through the high speed air bleed. The mixing of the air and fuel provides a well atomized mixture at the main discharge nozzle. The mixture is vaporized as it mixes with the air stream flowing past the main discharge nozzle.

The main metering system is capable of delivering the required air/fuel mixture at a steady engine rpm. If the engine is placed under a sudden load or increased power requirement, a richer mixture is required until the extra load or power requirements are relieved. The supplemental fuel is added to the main metering system by the power enrichment system.

Because it is a supplemental fuel system that provides fuel under specific situations, the carburetor has to receive a signal indicating changes in engine demand. Once the signal is received to deliver more power, the carburetor has to respond instantly and supply the necessary air/fuel mixture.

The signal received by the carburetor can be transmitted through linkage or intake manifold vacuum. In either case additional fuel will be added to the main metering discharge system.

 

MECHANICAL POWER ENRICHMENT SYSTEM

In our basic carburetor, we installed a main metering jet to regulate the amount of fuel entering the main metering system. The size of the jet was fixed and allowed a limited amount of fuel flowing to enter the main discharge tube. To provide mechanical power enrichment, we will vary the amount of fuel allowed to flow through the main metering jet.

The mechanically operated device uses a metering rod with two or more steps of different diameters. The metering rod is suspended from linkage with the stepped end in the main jet. When the throttle is opened, its movement is transmitted through the linkage and the metering rod is raised out of the jet. At intermediate throttle, the larger diameter, or step, is in position in the metering-rod jet. This somewhat restricts the flow of fuel to the main nozzle. Enough fuel is flowing, however, to provide the proper air-fuel ratio required for an engine operating at a steady intermediate throttle opening.

If the throttle is fully opened for rapid acceleration or for increased power to overcome a load, the metering rod is raised enough to cause the smaller diameter, or step, to be lifted into the metering rod jet. The jet is now less restricted and a larger quantity of fuel can pass through it. The main metering system receives more fuel, resulting in a richer mixture.

VACUUM POWER ENRICHMENT SYSTEM

If a vacuum gauge is attached to the intake manifold of our engine, we would notice that the vacuum readings vary depending on engine speed and load. With the engine running at a steady speed, manifold vacuum is high and steady. If the engine is suddenly accelerated or placed under load, it is operating under a high power demand and intake manifold vacuum will drop. These vacuum variations can be used as power enrichment signals which will “tell” the carburetor when to add additional fuel to the main metering system.

The vacuum signal is routed through a drilled passage located below the carburetor throttle valve. This vacuum port routes intake manifold vacuum to a diaphragm operated power valve. The power valve enters the fuel bowl where it opens and closes a fuel passage depending upon the amount of vacuum applied to the diaphragm. A calibrated coil spring is attached to the valve stem and holds the valve in the open position. The valve remains in the open position until manifold vacuum is applied to the diaphragm. When vacuum is above a specified value, atmospheric pressure within the fuel bowl will force the diaphragm and valve stem closed against the seat. This stops any fuel flow through the valve.

All fuel entering the main metering system is flowing through the main metering jet When manifold vacuum is below a specified value, the power valve spring will force the valve stem off its seat and allow additional fuel to flow through into the main metering system. Only a calibrated amount of fuel will enter the main system due to the restriction in the power enrichment passage. This restriction acts as a second main metering jet and has the same function to meter the amount of fuel allowed to enter the main system during heavy load operation.

 

COMBINED VACUUM AND MECHANICAL POWER ENRICHMENT

Some carburetors use a power enrichment system that combines the vacuum operation of a power valve with the mechanical motion of a metering rod.

The metering rod is linked to the throttle so that at wide-open throttle the smaller diameter of the rod is positioned in the metering jet allowing additional fuel to enter the main metering system.

The rod is also linked to a vacuum diaphragm. When the throttle is held at a steady, intermediate position, manifold vacuum applied to the diaphragm will hold the metering rod in the lowered position. If intake manifold drops due to rapid acceleration or load, spring pressure on the diaphragm will raise the metering rod to position the narrow diameter in the main jet. With the narrow part of the rod in the jet, fuel flow is less restricted and a richer mixture results. When vacuum stabilizes, the diaphragm is pulled down and the larger metering rod diameter enters the jet. The jet is again restricted, resulting in a leaner mixture.

 

ACCELERATOR PUMP SYSTEM

If the throttle valve is suddenly opened for rapid acceleration, the engine will require an immediate increase in both fuel and air flow rates. Because it is lighter and requires much less effort to set in motion, the air flow rate will instantly accelerate to meet the increased engine demand. Gasoline, when compared to air, is heavy and requires a given amount of time to get in motion and accelerate to the required fuel flow rate. The gap between flow rates has to be filled or the air/fuel mixture will be lean when a rich mixture is actually required.

If a lean condition occurs during acceleration, the engine will seem to hesitate (flat spot), and then accelerate. A severe lean condition may even cause backfiring and stalling before the engine can catch and accelerate. The brief hesitation that occurs is the period of time the fuel requires to catch up to the air flow. To eliminate this problem the accelerator pump will provide supplemental fuel to fill the gap until the fuel can catch up to the air.

The accelerator pump system richens the mixture and prevents the lean condition by discharging a squirt of fuel directly into the carburetor venturi. The accelerating air flow grabs the gasoline as it is discharged and rushes into the engine carrying enough fuel to meet the immediate engine demands. The squirt of fuel is delivered by a simple reciprocating piston or diaphragm pump operated by linkage from the throttle.

When the throttle is opened on a piston type pump, the piston cup is forced downward in a bore against the fuel. The fuel forces the flexible cup against the bore wall and seals the fuel in the bore. Continued downward movement of the piston forces the fuel through the passage to the discharge nozzle. Although it appears different the diaphragm pump functions much the same as the piston pump. When the throttle is opened, linkage forces the diaphragm against the fuel and it discharges into the incoming air. In this type of pump a rubber check valve called an elastomer valve allows fuel to enter the pump chamber from the float bowl. When the pump stroke occurs the valve seals the hole between the pump chamber and fuel bowl.

When the throttle is released, spring pressure pushes the pump to its original position. As the pump moves to the action position the bore is refilled with fuel. With the piston type of pump the flexible cup relaxes and fuel flows past the cup edge into the bore. With a diaphragm type pump the elastomer check valve flexes to uncover the hole between the fuel bowl and pump chamber.

In either case, the check ball located at the discharge end of the passage has seated to prevent air from entering the system as the pump makes the refill stroke. This check ball and weight perform an additional function once seated. The ball and weight prevent fuel siphoning through the pump system at high air flow rates.

 

MULTIPLE VENTURI CARBURETORS

As we have seen, the air/fuel mixture flowing into a breathing engine is regulated and restricted by a single venturi carburetor mounted on the intake manifold. Our carburetor is functional and capable of providing the required air/fuel mixtures. Its size, however, can limit the amount of air/fuel mixture entering the engine. And, just as the throttle valve controls engine speed and power by restricting the amount of air entering an engine, the carburetors size will influence the amount of air entering the engine_

Of course, if air flow were the only consideration, then a single large diameter venturi could be used. But, with only a single large carburetor, venturi action would be poor, and proper air/fuel ratios would be hard to achieve under varying operating conditions. Rather than increase the size of the venturi, we can improve engine breathing by increasing the number of venturis. The increased number of venturies will provide a larger opening through which air can enter and allows better distribution of air and fuel in the intake manifold. Instead of a single venturi providing an air/fuel mixture to four cylinders we can have two venturis with each supplying two cylinders. The ultimate is to equip each cylinder with its own carburetor, and many racing engines are equipped in this manner, an eight cylinder engine would have eight carburetors.

 

DUAL VENTURI CARBURETOR

The dual venturi carburetor is essentially, two single venturis combined into a single assembly. Each venturi has its own discharge nozzle, idle port, accelerator discharge jet and throttle valve. A fuel bowl, air horn, choke and throttle shaft common to both is used. The separate throttle valves are fastened to a common shaft, so that both valves open and close together.

On a V-8 engine each venturi feeds into separate passages in the intake manifold. For example one venturi would feed cylinders 1, 4, 6, 7, while the other feeds 2, 3, 5, 8. On four and six cylinder engines both the intake manifold and venturies feed into a common passage. But the additional air flow provides better distribution the length of the manifold.

FOUR VENTURI CARBURETOR

Basically, a four venturi carburetor is a cluster of four single venturi carburetors combined into a single assembly. The low speed systems are arranged in pairs called the primary and secondary sides. This division allows the carburetor to perform as though it were two dual venturi carburetors with each carburetor supplementing the other. The primary side contains a full compliment of systems, including idle and love speed systems, high speed system, acceleration pump and choke system. It is therefore able to provide the air/fuel mixtures required for starting, low speed operation, normal acceleration and intermediate speed operation.

However, when the throttle is moved toward the wide-open position for hard acceleration or full-power operation, the secondary dual carburetor, or secondary side, comes into operation, and supplies additional air/fuel mixture. This combination provides satisfactory and economical operation from the primary side and improved full throttle operation from the secondary side. With the wide-open throttle, the passage space for air/fuel mixture to enter the engine is doubled from two to four venturis. Greater amounts of air/fuel mixture can enter the engine for improved high speed, fall power performance.

AUXILIARY COMPONENTS

The basic carburetor we have constructed and studied will function and did function as described. As vehicles became more complicated to provide driver comforts and meet clean air standards, the carburetor saw the addition of numerous auxiliary components. These components were added to adapt carburetor functions to meet new demands that were placed on the engine. Although they seem necessary to carburetor function, these components are “hang-ons” that do not alter the basic principles of carburetion. The carburetor still delivers an air/fuel mixture as required by changing engine demands.

HOT IDLE COMPENSATOR

As engines were loaded with options and vehicle packaging became smaller, underhood temperatures took a noticeable upward trend. In certain applications the increased temperature began to affect carburetor idle operation. When an engine idles for an extended period of time, such as might occur in rush hour, the heat generated can cause excessive vaporization of the fuel in the fuel bowl.

These vapors can enter the engine, through the carburetor, resulting in an overly rich mixture. As we know, satisfactory engine idle depends upon a specific air/fuel ratio. If this ratio is disturbed, rough engine idle will result. To lean out the rich mixture caused by excessive fuel vaporization, some carburetors are equipped with a hot idle compensator valve.

Like the choke thermostatic coil, the valve will react to changes in temperature because it is attached to a bimetal strip. Under cold or normal engine operating temperatures, this valve reamins closed, blocking an air passage through the carburetor to the intake manifold. As temperatures under the hood increase the bi metal will bend upward and open the valve. The passage to the intake manifold is now open and air wf71 How directly into the intake manifold. The additional air leans out the rich mixture and engine idle remains smooth.

 

THROTTLE POSITION SENSOR

As part of its interaction with the computer, carburetors in the late 80s were equipped with a throttle position sensor. The sensor is actually a potentiometer which is an electrical device capable of varying an applied voltage.

The computer applies a voltage to the sensor (potentiometer) and as the throttle shaft moves it will vary the voltage coming from the computer. These changes in voltage form a signal which is routed back to the computer. From these various voltage readings the computer “knows” the position of the throttle. For example, a certain voltage level may “tell” the computer that the throttle is almost closed or idling while another voltage reading will “tell” the computer that the throttle is wide open for maximum acceleration.

The signal sent from the throttle sensor is just one of the many the computer receives. In one of the engine control systems, the computer receives numerous signals from different kinds of sensors. The computer uses these signals to compute its own output signals. These signals are then sent to engine control devices which will modify systems related to engine emissions and performance.

SWITCHES

If a switch is mounted on the carburetor, throttle movement can be used to open and close an electrical circuit. The opening and closing of the circuit can be used to turn an electrical component on and off even though it is mounted in some remote part of the vehicle.

One such application mounted a switch on the carburetor that closed only when the throttle moved to the wide open position. The switch completed a circuit to an electrical valve mounted in the automatic transmission. At wide open throttle the switch completed a eke nit which opened the valve. When the valve opened the transmission downshifted for better acceleration.

In a more recent application, a carburetor mounted switch is used to open and close a circuit operating the air conditioning compressor clutch when the engine is idling and power steering pressure is high.

Switches of this type can be used for any application where a particular event has to occur at a specified throttle opening.

 

THROTTLE RETURN DASHPOT

In certain applications, rapid closing of the throttle has to be prevented or engine stalling will result. This is especially true when the vehicle is equipped with an automatic transmission. An automatic transmission does not have a direct mechanical link between the engine and the transmission. Unlike a manual transmission, which is mechanically linked to the engine through the clutch, an automatic transmission is linked through a fluid.

The fluid, or transmission oil, is contained in a torque converter positioned between the engine and transmission. The oil and torque converter action form the link between the engine and transmission. Because it is a fluid, the oil tends to slip under certain conditions. One such condition exists when the throttle is suddenly released for deceleration.

The slippage in the torque converter will allow engine speed to decrease rapidly even though the vehicle is decelerating from a high rate of speed. The rapid slowdown of the engine, as it was suddenly throttled down, could unbalance carburetor action enough to cause a momentary hesitation or stalling.

To prevent this situation, many carburetors are equipped with a dashpot. It contains a spring loaded diaphragm which traps air behind it when the throttle is opened. When the throttle is released, the contact arm on the throttle lever hits the rod extending from the dashpot. Since the air trapped behind the diaphragm can escape only through a small opening, the diaphragm moves slowly to the idle position.

 

THROTTLE POSITIONERS

Throttle positioners are used to increase engine idle speed. The higher idle speed aids emission control system. And, on air conditioned vehicles, prevents overheating or stalling when the compressor loads the engine or cooling system.

To help meet tougher emission control regulations, carburetor air/fuel ratios were leaned to the point where the engine would idle smoothly only at higher engine speeds. The high idle speed helped reduce emissions but it created another problem. When an engine is shut-off at a high idle, it will continue to run even though the ignition is turned-off. This situation is called dieseling and it occurs when air and fuel flow past the partially open throttle and ignite in the cylinder. To eliminate this problem, an electrically actuated plunger is positioned to hold the throttle open at a specified rpm. The circuit is complete and the plunger extends only when the ignition key is in the run position. When the key is turned to the off position the plunger retracts and the throttle closes enough to prevent dieseling.

An air conditioning system presents two problems to an engine at idle the compressor load and heat load. An air conditioning compressor requires a substantial amount of engine power when it is operating. To compensate for this load engine idle will be increased whenever the air conditioning clutch is engaged. The in creased engine speed is also necessary to increase the amount of air the engine fan draws through the radiator. Without the increased air flow, the heat load generated by the air conditioning system could cause engine overheating. By increasing the idle speed, more air is pulled through the condenser and radiator, allowing more rapid removal of heat.

The devices that increase idle speed during A/C operation, while they are throttle positioners, are more commonly called throttle kickers.

A throttle kicker may be either electrically or vacuum operated. Some electrical kickers are mounted separately while others are incorporated as part of the throttle positioner. The vacuum operated types are also mounted separately or are included as part of the carburetor assembly. An electrically operated vacuum valve, wired into the compressor clutch circuit opens and closes a vacuum passage to the throttle kicker. Whenever the clutch is engaged the valve is opened and vacuum is applied to the throttle kicker diaphragm.

 

ALTITUDE COMPENSATOR

At the beginning, we discussed how air becomes thinner with increasing altitude. And, as air becomes thinner, carburetor air/fuel ratios will become richer. Before carburetors were forced to operate on very lean mixtures as a means of reducing exhaust emissions, changes in altitude were less important. The borderline leanness of recent carburetors does not provide a range that will compensate for changes in altitude. The rich mixture can be leaned by introducing supplemental air. The air will enter the carburetor through a valve which will be opened and closed by an altitude compensator.

This device contains an aneroid capsule that moves in relation to changes in atmospheric pressure. The aneroid is a bellows like arrangement from which air has been removed, allowing it to act as a barometer. When it senses lower atmospheric pressure, the metering valve is opened and air flows through a passage to a discharge port located between the booster venturi and the throttle plate.

 

2700 / 7200 VARIABLE VENTURI CARBURETOR

The carburetor we have been studying used a fixed venturi size to create a pressure differential between the fuel bowl and incoming air. If you look into the bores of a 2700/7200 variable venturi carburetor, you will not see the familiar venturi shape only smooth walls. The venturi affect occurs between the front edge of the venturi valves and the top rear edge of the bores. A close inspection of the shape formed by the edge of the valve and the bore reveals the hour glass shape of the venturi.

The 2700/7200 variable venturi is unique because it will vary the size of the venturi to match a specific air flow rate. If the engine is operating at idle throttle valve almost closed the amount of air flowing into the engine is restricted and the venturi valves will be opened only a slight amount. As air flow increases with throttle opening, the venturi valves will be opened a corresponding amount to match valve position with air flow rate.

The venturi valves are spring loaded to the closed position. Opening of the valves is controlled by a vacuum diaphragm mounted on the back of the carburetor. The vacuum signal control vacuum applied to the diaphragm originates in the bores of the carburetor. Because the control vacuum ports are positioned between the venturi valve and throttle valve, the vacuum signal routed to the diaphragm is directly related to air flow. With the throttle at idle, very little vacuum signal exists in the bore and the venturi valves are opened a minimal amount. As the throttle opens and air flow increased the control vacuum signal strengthens, allowing the diaphragm to move the venturi valves to a position that will supply the required air/fuel mixture.

 

The carburetor we have been studying used a fixed venturi size to create a pressure differential betwen the fuel bowl and incoming air. If you look into the bores of a 2700/7200 variable venturi carburetor, you will not see the familiar venturi shape only smooth walls. The venturi affect occurs between the front edge of the ven turi valves and the top rear edge of the bores. A close inspection of the shape formed by the edge of the valve and the bore reveals the hour glass shape of the venturi.

The 2700/7200 variable venturi is unique because it will vary the size of the venturi to match a specific air flow rate. If the engine is operating at idle throttle valve almost closed the amount of air flowing into the engine is restricted and the venturi valves will be opened only a slight amount. As air flow increases with throttle opening, the venturi valves will be opened a corresponding amount to match valve position with air flow rate.

The venturi valves are spring loaded to the closed position. Opening of the valves is controlled by a vacuum diaphragm mounted on the back of the carburetor. The vacuum signal control vacuum applied to the diaphragm originates in the bores of the carburetor. Because the control vacuum ports are positioned between the venturi valve and throttle valve, the vacuum signal routed to the diaphragm is directly related to air flow. With the throttle at idle, very little vacuum signal exists in the bore and the venturi valves are opened a minimal amount. As the throttle opens and air flow increased the control vacuum signal strengthens, allowing the diaphragm to move the venturi valves to a position that will supply the required air/fuel mixture.

The 7200 variable venturi carburetor is exactly the same as the 2700 except for one important difference. The 7200 is linked to a computerized engine control system. Through a system of sensors, the computer receives data in the form of electrical signals which it processes into output signals. The electrical signals leaving the computer are directed to devices that regulate components related to emission control and driveability. By incorporating the 7200 carburetor into this system the computer is able to vary air/fuel mixture to provide lean mixtures that do not affect driveability.

A number of sensors provide data which the computer uses to determine the correct air/fuel ratio. A throttle position sensor relays information related to engine demand. Through a variable resistor it will tell the computer if the engine is at idle, part throttle or wide open throttle. Another sensor is threaded into the exhaust manifold, and measures the oxygen content of the burned gases as they leave the engine. By measuring the oxygen content, the sensor converts this information into an electrical signal that provides information to the computer about the air/fuel mixture entering the engine. Changes in atmospheric pressure and intake manifold vacuum are monitored by the barometric and manifold absolute pressure sensor. After the information from these various sensors is compared and computed the computer sends out a feedback signal to an electrical motor mounted on the side of the carburetor.

When signaled the motor will move a valve in minute steps. As the stepper motor moves the valve the air/fuel ratio will be altered. Movement of the pintle valve changes the fuel mixture in one of two ways depending on the carburetor. In one type of system, the valve moves and allows additional air to be metered into the main metering system at the discharge jets. This is the air bleed system. In another system the valve allows vacuum into the fuel bowl. The lower pressure above the fuel bowl pushes less fuel into the main metering system, resulting in a leaner air/fuel ratio, this is the backsuction system.

Like the 7200 Variable Venturi Carburetor, the 6500 2-Venturi Feedback Carburetor is part of an electronic engine control system. The computer in this system also has the capability to vary air/fuel ratios. Instead of an electric motor the computer controls the position of a vacuum actuated metering valve. The valve is linked to a control vacuum diaphragm which will position the valve in a metering jet in response to a computer generated vacuum signal.

The computer controls the amount of vacuum applied to the diaphragm through a vacuum regulator solenoid. The vacuum metering valve in the regulator is calibrated to apply the necessary vacuum in response to signals received from the computer. This results in a variable control vacuum output signal from the solenoid.

Without a control vacuum signal above the diaphragm, the valve is spring loaded to the open position, allowing the maximum amount of fuel to flow through the jet. The fuel will enter the main metering system and richen the mixture entering the engine. By applying vacuum to the diaphragm the metering valve position can be raised to restrict the amount of fuel entering

the main metering system. The amount of fuel entering the main systems depends on the position of the tapered metering valve in the jet. The regulation of the piston of the valve allows the computer to alter air/fuel mixtures when the engine is operating on the main metering system.

 

CENTRAL FUEL INJECTION (CFI)

We have advanced to the point where we are no longer discussing a carburetor but an alternative fuel delivery system. Central fuel injection does not use carburetor systems to deliver the correct air/fuel ratio. Instead, two electrically operated valves spray a fine gasoline mist into the incoming air. The amount of fuel sprayed into the airstream will vary according to engine requirements, making fuel systems unnecessary.

The fuel injection nozzles are mounted vertically on a throttle body assembly and are electrically opened. Each injector contains a solenoid operated pintle and needle valve assembly. When current flows through the solenoid windings, the magnetic force generated by the current flow raise the valve off its seat and fuel is discharged through the nozzle.

Because the injector flow orifice is fixed and the fuel supply pressure is constant, the amount of fuel discharged is controlled by how long the solenoid is energized. The injection timing is determined by a computer. Like the other engine control systems, the computer will receive sensor data in the form of electrical signals. It will process this information and calculate output signals. One of these computer output signals determines how long the injector solenoid will be energized.

Fuel is delivered to the injector from the fuel tank by an electrical pump which is located in the tank. Pressure in the fuel delivery system is maintained at 39 psi by a pressure regulator mounted on the main body. In addition to regulating pressure the pressure regulator maintains a pressurized fuel supply when the engine is shut-down. It functions like a check valve and traps pressurized fuel in the fuel line from the pump. This pressurized fuel supply eliminates the possibility of vapor formation in the fuel line and provides fuel for instant restarts and initial engine idle.

Port Injection moves away from carburetion – that’s a different study.

 

Leave a Reply

Your email address will not be published. Required fields are marked *