7k Carburetor

[Bette Keesee]

Acarburetor (also spelled carburettor or carburetter)[1][2][3] is a device used by a gasoline internal combustion engine to control and mix air and fuel entering the engine.[4] The primary method of adding fuel to the intake air is through the Venturi tube in the main metering circuit, though various other components are also used to provide extra fuel or air in specific circumstances.

Since the 1990s, carburetors have been largely replaced by fuel injection for cars and trucks, but carburetors are still used by some small engines (e.g. lawnmowers, generators, and concrete mixers) and motorcycles. In addition, they are still widely used on piston engine driven aircraft. Diesel engines have always used fuel injection instead of carburetors, as the compression-based combustion of diesel requires the greater precision and pressure of fuel-injection.[5]

The name "carburetor" is derived from the verb carburet, which means "to combine with carbon",[6] or, in particular, "to enrich a gas by combining it with carbon or hydrocarbons".[7] Thus a carburetor mixes intake air with hydrocarbon-based fuel, such as petrol or AutoGas (LPG).[8]

The name is spelled "carburetor" in American English and "carburettor" in British English.[1][2][3] Colloquial abbreviations include carb in the UK and North America or Carby in Australia.[9]

Air from the atmosphere enters the carburetor (usually via an air cleaner), has fuel added within the carburetor, passes into the inlet manifold, then through the inlet valve(s), and finally into the combustion chamber. Most engines use a single carburetor shared between all of the cylinders, though some high-performance engines historically had multiple carburetors.

The carburetor works on Bernoulli's principle: the static pressure of the intake air reduces at higher speeds, drawing more fuel into the airstream. In most cases (except for the accelerator pump), the driver pressing the throttle pedal does not directly increase the fuel entering the engine. Instead, the airflow through the carburetor increases, which in turn increases the amount of fuel drawn into the intake mixture.

The main disadvantage of basing a carburetor's operation on Bernoulli's Principle is that being a fluid dynamic device, the pressure reduction in a venturi tends to be proportional to the square of the intake airspeed. The fuel jets are much smaller and fuel flow is limited mainly by the fuel's viscosity so that the fuel flow tends to be proportional to the pressure difference. So jets sized for full power tend to starve the engine at lower speed and part throttle. Most commonly this has been corrected by using multiple jets. In SU and other (e.g. Zenith-Stromberg) variable jet carburetors, it was corrected by varying the jet size.

The orientation of the carburetor is a key design consideration. Older engines used updraft carburetors, where the air enters from below the carburetor and exits through the top. From the late 1930s, downdraft carburetors become more commonly used (especially in the United States), along with side draft carburetors (especially in Europe).

The main metering circuit consists of a pipe which reduces to a narrows before widening again, forming a venturi (aka "barrel"). Fuel is introduced into the air stream through small tubes (the main jets) at the narrowest part of the venturi, where the air is at its highest speed.[10]

Downstream of the venturi is a throttle (usually in the form of a butterfly valve) which is used to control the amount of air entering the carburetor. In a car, this throttle is connected to the vehicle's throttle pedal, which varies engine speed.

At greater throttle openings, the speed of air passing through the venturi increases, which lowers the pressure of the air and draws more fuel into the airstream.[11] At the same time, the reduced manifold vacuum results in less fuel flow through the idle and off-idle circuits.

During cold weather fuel vaporizes less readily and tends to condense on the walls of the intake manifold, starving the cylinders of fuel and making cold starts difficult. Additional fuel is required (for a given amount of air) to start and run the engine until it warms up, provided by a choke valve.

While the engine is warming up the choke valve is partially closed, restricting the flow of air at the entrance to the carburetor. This increases the vacuum in the main metering circuit, causing more fuel to be supplied to the engine via the main jets. Prior to the late 1950s the choke was manually operated by the driver, often using a lever or knob on the dashboard. Since then, automatic chokes became more commonplace. These either use a bimetallic thermostat to automatically regulate the choke based on the temperature of the engine's coolant liquid, an electrical resistance heater to do so, or air drawn through a tube connected to an engine exhaust source. A choke left closed after the engine has warmed up increases the engine's fuel consumption and exhaust gas emissions, and causes the engine to run rough and lack power due to an over-rich fuel mixture.

However, excessive fuel can flood an engine and prevent it from starting. To remove the excess fuel, many carburetors with automatic chokes allow it to be held open (by manually, depressing the accelerator pedal to the floor and briefly holding it there while cranking the starter) to allow extra air into the engine until the excess fuel is cleared out.

Another method used by carburetors to improve the operation of a cold engine is a fast idle cam, which is connected to the choke and prevents the throttle from closing fully while the choke is in operation. The resulting increase in idle speed provides a more stable idle for a cold engine (by better atomizing the cold fuel) and helps the engine warm up quicker.

The system within a carburetor that meters fuel when the engine is running at low RPM. The idle circuit is generally activated by vacuum under the throttle plate, which causes a low-pressure area in the idle passage/port thus causing fuel to flow through the idle jet. The idle jet is set at some constant value by the carburetor manufacturer, thus flowing a specified amount of fuel.

Many carburetors use an off-idle circuit, which includes an additional fuel jet which is briefly used as the throttle starts to open. This jet is located in a low-pressure area behind the throttle. The additional fuel it provides is used to compensate for the reduced vacuum that occurs when the throttle is opened, thus smoothing the transition from the idle circuit to the main metering circuit.

In a four-stroke engine it is often desirable to provide extra fuel to the engine at high loads (to increase the power output and reduce engine knocking). A 'power valve', which is a spring-loaded valve in the carburetor that is held shut by engine vacuum, is often used to do so. As the airflow through the carburetor increases the reduced manifold vacuum pulls the power valve open, allowing more fuel into the main metering circuit.

In a two-stroke engine, the carburetor power valve operates in the opposite manner: in most circumstances the valve allows extra fuel into the engine, then at a certain engine RPM it closes to reduce the fuel entering the engine. This is done in order to extend the engine's maximum RPM, since many two-stroke engines can temporarily achieve higher RPM with a leaner air-fuel ratio.

A metering rod or step-up rod system is sometimes used as an alternative to a power valve in a four-stroke engine in order to supply extra fuel at high loads. One end of the rods is tapered, which sits in the main metering jets and acts as a valve for fuel flow in the jets. At high engine loads, the rods are lifted away from the jets (either mechanically or using manifold vacuum), increasing the volume of fuel can flow through the jet. These systems have been used by the Rochester Quadra jet and in the 1950s Carter carburetors.

While the main metering circuit can adequately supply fuel to the engine in steady-state conditions, the inertia of fuel (being higher than that of air) causes a temporary shortfall as the throttle is opened. Therefore, an accelerator pump is often used to briefly provide extra fuel as the throttle is opened.[12] When the driver presses the throttle pedal, a small piston or diaphragm pump injects extra fuel directly into the carburetor throat.[13]

In order to ensure an adequate supply at all times, carburetors include a reservoir of fuel, called a "float chamber" or "float bowl". Fuel is delivered to the float chamber by a fuel pump. A floating inlet valve regulates the fuel entering the float chamber, assuring a constant level.

Unlike in a fuel injected engine, the fuel system in a carbureted engine is not pressurized. For engines where the intake air travelling through the carburetor is pressurized (such as where the carburetor is downstream of a supercharger) the entire carburetor must be contained in an airtight pressurized box to operate.[citation needed] However, this is not necessary where the carburetor is upstream of the supercharger.

Problems of fuel boiling and vapor lock can occur in carbureted engines, especially in hotter climates. Since the float chamber is located close to the engine, heat from the engine (including for several hours after the engine is shut off) can cause the fuel to heat up to the point of vaporization. This causes air bubbles in the fuel (similar to the air bubbles that necessitate brake bleeding), which prevents the flow of fuel and is known as 'vapor lock'.

To avoid pressurizing the float chamber, vent tubes allow air to enter and exit the float chamber. These tubes usually extend into the carburetor throat, placed to prevent fuel from sloshing out of them into the carburetor.

If an engine must be operated when the carburetor is not in an upright orientation (for example in a chainsaw or airplane), a float chamber and gravity activated float valve would not be suitable. Instead, a diaphragm chamber is typically used. This consists of a flexible diaphragm on one side of the fuel chamber, connected to a needle valve which regulates the fuel entering the chamber. As the flowrate of the air in the chamber (controlled by the throttling valve/butterfly valve) decreases, the diaphragm moves inward (downward), which closes the needle valve to admit less fuel. As the flowrate of the air in the chamber increases, the diaphragm moves outward (upward) which opens the needle valve to admit more fuel, allowing the engine to generate more power. A balanced state is reached which creates a steady fuel reservoir level, that remains constant in any orientation.

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