Carburetion

• The fuel system on a small engine consists of several components;

Carburetor

Fuel Lines

Fuel Filter

Fuel tank

Carburetion

• The function of the carburetor is threefold;– It breaks up or atomizes the fuel into a fine spray

and it mixes with air to make a mixture that will burn readily

– It regulates the ratio of fuel to air– It regulates the amount of the fuel-air-mixture

going into the combustion chamber

Carburetion• The ratio of fuel air is controlled by adjusting needle valves

and this allows you to select the proper mixture for the conditions under which you operate your engine

• Too lean a fuel-air mixture may result in hard starting, overheating, pre-ignition & valve burning

• Too rich a fuel-air mixture may result in excessive fuel consumption, carbon build up in the cylinder and cause pre-ignition

Carburetion: Principles of Operation

• As the piston moves down a partial vacuum is created in the cylinder

• Atmospheric pressure pushes air through the carburetor air intake to equalize this pressure

• The air speed increases in the venturi (the narrow passage in the air intake passage)

Carburetion: Principles of Operation

• As the air speed increases the pressure is lowered.

• Because the pressure is lowered atmospheric pressure in the fuel bowl pushes fuel through the pipe to the venturi and into the air stream

Carburetion: Principles of Operation

• The speed of the air in the venturi and the turbulence past the venturi atomize the fuel and mix the tiny droplets with air.

• Once the air & fuel are mixed, the next job of the carburetor is to provide a means whereby the amount of mixture that enters the cylinder can be controlled

Carburetion: Principles of Operation

• A butterfly (throttle) valve in the manifold controls the fuel air mixture

• If you want your engine to run fast open your butterfly (throttle) valve, the more fuel and air that gets delivered the faster it will run

Carburetion: Principles of Operation

• A “Choke” valve is used to aid in cold starts

• It helps provide a better vaporization of the fuel–air and it provides more fuel-air (a richer mixture)

• The choke valve is similar to the butterfly except it is placed on the air-intake side of the carburetor.

Carburetion: Principles of Operation

• When the choke is closed air entering is restricted.

• The pressure inside the carb. & cylinder is further reduced and this increases the vaporization of the fuel.

• Some engines use a primer setup instead of a choke valve

Carburetion: Principles of Operation

•

Carburetor Types• Briggs and Stratton uses

many different types of carburetors for their many different types of small gas engines

• The principal of operation of each is the same but there are many variations

Carburetor Types

There are basically three different types of carburetors used in small gas engines depending on how fuel is supplied from the tank to the fuel chamber in the carburetor.

The Float Type

The Suction – Lift Type

The Diaphragm Type

The Float Type• These are so called because

the fuel level in the fuel chamber is maintained by a float – controlled valve.

• There are several different styles of float-type carbs such as Updraft and, Side-draft types

The Float Type• The float type carburetor has features which provide for

adjustment and regulation of the fuel air to meet different operating conditions.

• When a sudden load or acceleration is demanded , a richer mixture (more fuel-air) is required

• These carburetors have what is called an accelerating well that surrounds the lower part of the fuel discharge nozzle and remains full of fuel while the engine is operating under normal load.

• When there is a sudden demand for power a governor opens the throttle valve and air moves past the nozzle much faster thus picking up more fuel

Natural Draft Carburetor

• This carburetor is used where there is little space on top of the engine. The air enters horizontally into the manifold.

Updraft Carburetors

• This type is placed low on the engine and use a gravity fed-fuel supply. In other words, the tank is above the carburetor and the fuel falls to it.

Downdraft Carburetors

• This carburetor operates with lower air velocities and larger passages. This is because gravity assists the air-fuel mixture flow to the cylinder.

Suction-Lift Carburetor• This type is usually mounted on

the top of the fuel tank.• Vacuum from the engines intake

stroke causes a low pressure in the venturi.

• Atmospheric pressure forces fuel up through the tube into the low pressure area of the venturi and then into the engine.

• This type of carburetor will not work with larger engines and tanks

Diaphragm – Type Carburetor• This type uses a spring-

loaded diaphragm for regulating the fuel flow into the carburetor fuel chamber

• The diaphragm serves the same purpose as a float in the float type carb.

Diaphragm – Type Carburetor• The main difference

between this and the float type is that the use of the diaphragm carburetor will allow the engine to work on any angle. For this reason the diaphragm type is used a lot in multi-positional engines.

Constant vacuum carburetor