Principles of Carburetion

Carburetors function The carburetor works on Bernoulli's principle: the faster air moves, the lower its static pressure, and the higher its dynamic pressure. The primary function is to produce an air fuel mixture to operate the engine The carburetor has several functions: 1) it combines gasoline and air creating a highly combustible mixture, 2) it controls the ratio of air and fuel, and 3) it controls the engine's speed.

Carburetion Con’t The carburetor is a much more complex system that allows for the operation of the engine during these conditions Cold or Hot Starting Idling or slow running Part Throttle (light load) Acceleration High Speed Operation (at full throttle)

Working of Carburetor

A carburetor basically consists of an open pipe through which the air passes into the inlet manifold of the engine. The pipe is in the form of a Venturi: it narrows in section and then widens again, causing the airflow to increase in speed in the narrowest part. Below the Venturi is a butterfly valve called the throttle valve — a rotating disc that can be turned end-on to the airflow, so as to hardly restrict the flow at all, or can be rotated so that it (almost) completely blocks the flow of air. This valve controls the flow of air through the carburetor throat and thus the quantity of air/fuel mixture the system will deliver, thereby regulating engine power and speed. The throttle is connected, usually through a cable or a mechanical linkage of rods and joints or rarely by pneumatic link, to the accelerator pedal on a car or the equivalent control on other vehicles or equipment. Fuel is introduced into the air stream through small holes at the narrowest part of the Venturi and at other places where pressure will be lowered when not running on full throttle. Fuel flow is adjusted by means of precisely calibrated orifices, referred to as jets, in the fuel path.

Types of Carburetors There are three different types of carburetors These carburetors are named after the direction of airflow from the outlet to the engine manifold They are as follows Natural (Side) draft Updraft Downdraft

Natural Draft Carburetor Used when there is little space in top of the engine Air flows horizontally into the manifold This type will be the most common on the small engines that we are working on.

Updraft Carburetor Air / Fuel Mixture must be forced upward into the engine Needs high air pressure

Downdraft Carburetor Operates with lower air velocities Gravity assists the air fuel mixture flow into the engine Can provide large volumes of fuel for high speed and high power operation

Basic Parts of Carburetor

Choke System Since cold fuel is hard to vaporize, the choke is used during cold engine starts to provide a rich mixture to the carburetor in order to get the engine started. The choke is a round disc mounted on a shaft located at the intake end of the carburetor.

Throttle System The throttle is a round disc mounted on a shaft beyond the main fuel nozzle in the carburetor. It regulates the amount of air-fuel mixture entering the cylinder. The throttle can be connected to a governer The governer will open and close the throttle to regulate engine speed

Venturi Principles The carburetor creates a partial vacuum itself by using this principle By restricting the passage that the air is flowing through, the velocity of the air will increase This increase in velocity will decrease the pressure behind the venturi This low pressure will be filled by fuel Basically, a venturi is a restriction in an air flow passageway

Venturi

What it looks like all together

Load Adjustment The amount of fuel entering the main discharge nozzle is sometimes regulated by a load adjusting needle. In some carburetors, there is no adjustment needle. These have a fixed flow that is designed for that specific engine

Air Cleaners / Filters As an engine operates, it needs an enormous amount of air passing through the carburetor into the combustion chamber This incoming air needs to be clean (this means that all particulate needs to be removed!) If this grit were to enter the combustion chamber, it would cause rapid wearing of the precision machined parts

Air Cleaners / Filters Under severe dust conditions, engine life can be reduced to minutes To keep this dust from entering the engine, we use different types of cleaners / filters Oil wetted Dry types Dual element

Oil Wetted Air Cleaner Uses a filtering element (crushed aluminum and polyurethane foam) dampened with engine oil Air is drawn through the oil wetted filter The filter in return removes any debris or particulate This type of filter can be reused! Clean with soapy water Dry Re-oil

Dry Type Air Cleaner These are designed for 1 time use! You can only clean large particulate from the filter Most dust however will stay in between the fibers of the filter

Dry Type Cleaner Cont A new style is the pleated paper design. This style uses a porous paper filter material to allow air flow to the engine, while allowing the filtration of debris This design provides great surface area to collect particles This is common to most automobiles

Dual Element Air Cleaners These filters are used on engines that operate in dustier than normal conditions Can you guess why its called a dual element air cleaner?

Its because they use both an oil wetted cleaner foam filter, and a dry type filter This offers more protection to dust and particulate The oil wetted filter acts as a pre cleaner The paper cartridge then cleans out any missed debris These come in many shapes and sizes, the most common is a cylidrical filter Dual Element Air Cleaners