Small Engine Fuel System

The most common repairs involve the fuel system, which includes the fuel tank, fuel filter, fuel line, fuel pump (on some models) and carburetor. You may have heard people jokingly comment that the fuel in the tank is so low the engine is running "on vapors." Technically, they're right. Gosoline won't burn in its liquid state; it must be converted to a vapor first. The vapors that burn in your small engine are formed from a mixture of fuel (typically gasoline) and air. And you need the right amount of fuel and the right amount of air to maintain whatever engine speed you select. The best way to understand the fuel system is to begin at the tank.

FROM THE FUEL TANK TO THE CARBURETOR

Located the fuel tank on your engine. If you have an older engine, it's probably made of steel or aluminum. Newer tanks are made of plastic and are built into the molded plastic shroud over the engine. Now look for a fuel line, a hose connected to one side of the tank. The fuel lone carries the fuel to the carburetor, a mixing chamber that contains a throttle and (if equipped) a choke, attached to the equipment controls. On most engines, the force of gravity carries the fuel through the fuel line. However, if the fuel tank is mounted low on the engine, gravity may not do the trick. In this case, a fuel pump uses low pressure in the crankcase to pump the fuel. The pump is located between the tank and the carburetor or in the carburetor itself. Some engines eliminate the need for either a fuel line or pump by mounting the carburetor directly on the fuel tank and using a pick-up tube in the tank to draw fuel.

INTO THE CARBURETOR

On most engines, fuel from the fuel line enters the carburetor's fuel bowl, a resevor where a float, similar to the float ball in a toilet tank, regulates the fuel level. From there, a metering device called a jet lets fuel into the emulsion tube inside the pedestal, where fuel and air first mix. (Older models include an adjustable jet; newer models contain a fixed jet.) Fuel travels through the emulsion tube to the main passageway in the carburetor, called the throat or venturi, where further mixing occurs. If your carburetor is a tank-mounted type, fuel from the tank may be supplied directly to the emulsion tube, without the need for a float.

THE ROLE OF THE THROTTLE

At one end of the throat is a throttle plate. The throttle plate is connected to your equipment control lever (often referred to as the throttle) and opens or closes to increase or decrease engine speed. As the throttle plate opens, more air is drawn into the carburetor. Air flow, in turn, determines how much fuel is delivered for combustion. Many carburetors have an idle speed screw to stop the throttle from closing too far at low speed, and in idle mixture screw, which increases or decreases air and fuel flow to prevent a stall.

UNDERSTANDING CARBURETION

In essence, a carburetor is a passageway that drawn in air and fuel and supplies a mixture of the tow to the cylinder. It's the narrowing of the passageway, called the throat or venturi, that causes the carburetor to draw in the two components the engine nees for combustion. Basic physics tells me that air speed will increase at the narrow point and air pressure will drop. Since fluids flow to low-pressure areas, fuel from the bowl or ank is drawn into the throat, mixing with air to form a combustible vapor.

Fuel is atomized

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The more surface area of gasoline exposed to the air, the faster a given amount will burn. To produce the rapid burning required in an engine, gasoline must be broken up into tiny droplets and mixed with air. This is called ATOMIZING. Once the entire surface of each droplet of the air-fuel mixture is exposed to the surrounding air, a huge burning area becomes available. Given a spark, the entire amount of gasoline will flash into a flame almost immediately. In effect, atomization causes a sudden, explosion release of heat energy.

EXPLOSION MUST BE CONTAINED

To perform useful, work the explosive force caused by the burning gas must be contained and controlled. To illustrate this point, imagine that a metal lid is suspended on a string and held several inches from the ground. If a mixture of gasoline and air (atomized) were sprayed under it and ignited, the lid would be raised a short distance by the force of the explosion. The reason the lid hardly moved is because the explosion was not confined and directed toward the lid. Instead, the explosion exerted force in all directions, and much of the force was lost. If the gasoline and air mixture is sprayed inside a metal container with a lid, the full force of the explosion will be directed against the lid when the mixture is ignited. This will blow the lid high into the air.

Gasoline Must Burn Quickly

Gasoline placed in a container and ignited will produce a hot flame, yet it will not burn fast enough to produce the rapid release of heat necessary to run and engine. Even though a considerable quantity of fuel may be involved, a large flame will not necessarily result. As we all know, the surface area of a wick in a lighter is quite small. Vapor from the surface of the liquid, combined with oxygen, is what burns readily. If the surface of the liquid is small, relatively little vapor will be given off to provide combusion. Since the liquid must change to vapor before it is burned, it would take considerable time to use up the fuel at this rate. By placing the same amount of fuel in a shallow, wide container, more surface area will contact air and the fuel will burn rapidly. Remember, A small flame is produced, due to small area of exposed fuel. Follow this blog: Next - Fuel atomized.


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Gasoline - Continue

Modern unleaded gasoline is a complex substance. I have found that ongoing research is necessary to seek ways to produce fuels that offer efficient engine performance and meet air pollution standards. Back in the 70s, while I was still in high school, 10% etanol blend of gasoline, known as gasohol, was introduced. My best friends dad was in too this area big time. He bought a large piece of land outside of our town to store and develope in this area of new fuels. Today, this product is often sold as super unleaded or preminum unleaded gasoline, depending on the octane level. The main drawback to these gasoline blends is their ability to absorb moisture, which can pass through the fuel filter and into the combustion chamber. These fuels should never be stored in high humidity areas or used in engines that set idle for long periods of time. Gasoline containing alcohol can also corrode fuel tank linings, shrink carburetor floats and seals, increase carbon deposits, and pit metal parts. For maximum performance and engine life, only use the type of gasoline recommended by the engine manufacturer.

Gasoline

I've repaired and worked on small engines a very, very long time. You will learn alot if you read and gain knowledge from my blogs.

GASOLINE is a hydrocarbon fuel which is a mixture of hydrogen and carbon, which is refined from petroleum. PETROLEUM is a dark, thick liquid that is extracted from our earth by oil wells we see popping up everywhere. Lucky for all of us, petroleum is the second most plentiful liquid in the world; only water is available in greater quantity. Gasoline, however, cannot be recycled as water can. Therefore, it is imperative that we all conserve gasoline and use it wisely, especially today when gas prices are so high. After studing what I do, I can tell you that gasoline contains a great amount of energy. For engine use, it should:

* Ignite readily, burn cleanly, and resist detonation which is a violent explosion.
* It should vaporize easily, without being subject to vapor lock, which means, vaporizing in fuel lines, impeding flow of liquid fuel to your carburetor.
* It should always be free of dirt, water, and abrasives.

Gasoline is assigned an OCTANE NUMBER that corresponds to its ability to resist detonation. Preminum grade gasoline burns slower than regular gasoline. It has a high octane number and is used in engines with high compression. Regular grade gasoline has a lower octane number and burns relatively fast. Generally, regular gasoline is used in small, low compression, one-cylinder and two-cylinder, gasoline engines. We all know that gasoline was once available in both leaded and unleaded varieties. The use of lead compounds was the most economical way to increase gasoline's octane number. For many years, most gasolines contained TETRAETHYL LEAD.
Since way back in the 70's unleaded gasoline has replaced leaded gasoline. Instead of lead compounds, OXYGENATES which is alohols and ethers, are commonly added to these fuels to increase octane levels. The main reason that unleaded gas was introduced was to provide fuel for automobiles equipped with catalytic converters. These vehicles will not operate properly on leaded fuel.

Small Engine Construction and Principles of Operation - Part I

After reading this, you will be able to:

* Explain simple engine operation.
* Know the qualities of gasoline that make you're mower an efficient small engine.
* Know why gasoline is atomized in the small engine.
* Know the basic components of your small engine and know the function of each part.

GASOLINE ENGINES

A gosoline-fueled ingine is a mechanism desinged to transform the chemical energy of burning fuel into mechanical energy. In operation, it controls and applies this energy to mow lawns, cut trees, propel tractors, and perform many other laborsaving jobs. A gasoline engine is an internal combustion engine. Gasoline is combined with air and burned inside the engine. In its simplest form, an engine consists of a ported cyliner, piston, connecting rod, and crankshaft.
The piston is a close fit inside the cylinder, yet it is free to slide on the lubricated walls. One end of the connecting rod is attached to the poston; the other end is fastened to an offset crankpin, or jouranl, of the crankshaft. As the piston moves up and down, the connecting rod forces the journal to follow a circular path, rotating the crankshaft.

SIMPLE ENGINE IN OPERATION

When the engine is cranked, gasoline is atomized (reduced to minute particles) and mixed with air. This mixture is forced through an intake port and into the cylinder, where it is compressed by the piston on the upstroke and ignited by an electrical spark. Combuston forces the piston down to rotate crankshaft. Burning rapidly, the heated gases trapped within the cylinder (combustion chamber) expand and apply pressure to the walls of the cylinder and to the top of the piston. This pressure drives the piston downward on the power stroke, causing the crankshaft to turn. As the piston and connecting rod push the crankshaft journal to the bottom of the stroke, the pressure of the burned gases is released through an exhaust port. Meanwhile, a fresh air-fuel charge enters the cylinder and the momentum of the power stroke turns the crankshaft journal through bottom dead center (BCD) and into the upstroke on another power cycle. Tommorow: Gasoline - The Hydrocaarbon Fuel in Small Engines.