User:Buster40004/Sandbox/Projects/Carburetor

Purpose
The purpose of any carburetor is to provide an engine with a mixture of air and fuel that is suitable for the needs of the engine at all times. An engine requires a combustible air to fuel ratio when in operation. An engine requires a leaner or richer air fuel ratio under various operating conditions, so the carburetor must be able to meet those needs by adjusting the ratio of air to fuel as those conditions change.

When gasoline used as the fuel, the mixture will not burn when there is less than 8 pounds of air mixed with 1 pound of fuel, a condition known a a rich mixture or when there is more than 18 pounds of air mixed with 1 pound of fuel which is known as a lean mixture. The ideal air to fuel ration is about 14.6 pounds of air for each pound of fuel. If exactly enough air is provided to completely burn all of the fuel, the ratio is known as the stoichiometric mixture. This air to fuel ratio is the most economical as no unburned fuel will leave the combustion chamber with the engine exhaust. Unfortunately, a stoichiometric air-fuel mixture burns very hot and engine components are damaged when the engine is placed under high load.

For that reason, the air to fuel ratio must change to a richer (more fuel) mixture when the engine load is increased. In naturally aspirated engines powered by gasoline, maximum power is frequently reached at air to fuel ratios ranging from 12.5 to 13.3:1

The physics that govern carburetor operation
The laws of energy conservation state that when air (or any other fluid) flows through a pipe, the total energy is a function of the fluid pressure and fluid velocity. If the fluid velocity changes, the fluid pressure changes in the inverse of the velocity change, thereby maintaining a constant total energy level.

In a carburetor, as air flows through a the venturi, the velocity of the air increases above that of the air prior to entering the venturi. As the total energy does not change as the airflow speed changes, the increase in velocity will reduce the pressure, keeping the total energy constant.

The same law affects the fuel flow within the carburetor. Fuel is provided to the carburetor at a given pressure. As the fuel flows through the carburetor, one or more restrictions in the fuel passageways (called 'jets' or 'metering jets'), the fuel pressure is reduced and its velocity is increased accordingly.

The first restriction to fuel flowing into the carburetor is the fuel inlet valve. When the fuel is consumed by the engine, additional fuel is admitted, keeping a constant fuel pressure inside of the carburetor that is somewhat reduced from the supply pressure. This inlet valve thereby controls the pressure of the fuel within the carburetor to a constant, predetermined level, no matter what the rate of flow.

A second restriction to fuel flow is the restriction that exactly manages the amount of fuel flowing as it is drawn into the venturi by the reduced pressure there. This restriction must be manufactured to extremely close tolerance in both diameter and length, as both affect the resulting flow.

Under low air flow conditions, the pressure differential between the fuel pressure in the carburetor and the air pressure in the carburetor's venturi is very low, and no fuel will flow. To compensate for this, an idle system is used to connect the fuel supply to the carburetor somewhat below (or toward the engine intake manifold) the carburetor throttle plate. At engine idle, the throttle plate severely restricts the airflow through the venturi, so the air pressure is well below atmospheric pressure due to the engine drawing air into its intake system against a nearly closed passageway. A small valve is used to limit the fuel flow drawn into the low pressure area below the throttle plate to just what is required to allow the engine to idle.

When air flow into the engine is increased due to a change of throttle plate position, the pressure in the intake manifold increases toward atmospheric pressure, cutting the idle circuit flow as the pressure differential is reduced, while at the same time the airflow through the venturi increases. The faster air flow through the venturi results in lower pressure there, thereby increasing the pressure differential, causing fuel to be drawn into the venturi in proportion to the pressure differential between the venturi and the fuel pressure in the carburetor.

At this transition from idle circuit to main circuit, the air fuel ratio changes to a somewhat leaner mixture as the fuel contributed by the idle circuit is lost. An accelerator pump provides the replacement fuel need to maintain a constant air to fuel ratio as the throttle plate is opened. The accelerator pump can be operated mechanically by a linkage, or by changes in the air pressure within the engine intake manifold. Once the engine speed has increased, the control of the air to fuel ratio is returned to the main fuel circuit.

The ratio of air to fuel is also modified starting at an intermediate throttle plate setting, using what is called the enrichment system. This fuel control is automatically operated as the throttle plate moves through a specific zone, usually at a point starting at 50% to 60% open. As the throttle plate reached the enrichment zone, the enrichment valve gradually opens, providing extra fuel, richening the air to fuel ratio. The richer mixture allows the engine to provide more power while maintaining a lower combustion temperature because a richer mixture burns cooler than a lean mixture.

Fuel inlet
The pressure of the fuel provided to the carburetor is regulated by the fuel inlet valve. The fuel pressure is maintained at the designed pressure without regard to the volume of fuel flowing through the carburetor. In float type carburetors, this valve is operated by the float, holding the fuel level within the carburetor at a constant level, which results in a constant internal fuel pressure.

Main metering
The main metering system uses the pressure differential between the pressure in the venturi and the pressure of the fuel inside the carburetor. This system supplies fuel to maintain a constant air to fuel ratio as the airflow through the carburetor changes.

Idle
When the throttle plate is in the closed position only a small volume of air is admitted into the engine resulting in insufficient airflow through the venturi to draw fuel from the fuel supply within the carburetor. A small bypass passage with a needle type flow control valve routes fuel from the carburetor fuel supply to the very low pressure in the engine air intake system. The needle valve is adjusted to control the fuel flow to the best air to fuel ratio for combustion.

Acceleration
When the throttle is opened, the airflow through the venturi is increased. As the airflow through the venturi is increased, the pressure at the venturi is proportionally reduced. This change in differential pressure between the venturi pressure and the fuel pressure in the carburetor causes fuel to flow into the venturi where it mixes with the air. Unfortunately, the air to fuel ratio is too lean until sufficient airflow velocity is generated. To compensate for the initial lean air to fuel mixture, the accelerator system forces extra fuel into the airflow, maintaining a constant air to fuel ratio.

Economizer
The mixture through the main metering circuit is too lean for safe operation when the engine is operating at a high power setting, such as when the engine operates at half throttle or more. Providing additional fuel to the combustion chamber results in a lower combustion temperature. The lower combustion temperature protects engine components that are severely damaged when overheated.