A carburetor (also spelled carburettor or carburetter) is a device used by a gasoline internal combustion engine to control and mix air and fuel entering the engine. The primary method of adding fuel to the intake air is through the Venturi effect or Bernoulli's principle or with a pitot 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), motorcycles, and smaller outboard engines. In addition, they are still widely used on piston-engine–driven aircraft. Diesel engines have always used fuel injection instead of carburetors, as they do not compress an air-fuel mix and instead require fuel injection much later in the compression stroke.
Etymology The term "carburetor" is derived from the English verb carburet, meaning "to charge with hydrocarbons"; in early motoring books the product of the carburetor is carburetted air, i.e. air charged with hydrocarbon vapour. This meaning superseded that from the late eighteenth century in which carburet was a noun for which we now use the word carbide. A carburetor charges the intake air with vapour from a hydrocarbon-based fuel, such as petrol or autogas.
The name is spelled "carburetor" in American English and "carburettor" in British English. Colloquial abbreviations include "carb" in the UK and North America or "carby" in Australia.
Operating principle
Air from the atmosphere enters the carburetor (usually via an air cleaner), has fuel added within the carburetor, then the air fuel mixture leaves the carburetor and enters the inlet manifold, then through the inlet valve(s), and finally into the combustion chamber. Most engines use a single carburetor shared among all of the cylinders, though some high-performance engines historically had multiple carburetors and/or throats.
The simplest carburetors work on Bernoulli's principle: the static pressure of the flowing air at the fuel entry point, which can be in a tube which is constant diameter, reduces at higher speeds compared with the pressure in the float chamber which is vented to ambient air pressure, with the pressure difference then forcing more fuel into the airstream. If the tube is a constant diameter the configuration is slightly simpler than in the cross-section diagram shown above right.
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.
Bernoulli's Principle applies (apart from friction and viscosity and turbulence etc.) to both the air and the fuel, so that the pressure reduction in the air flow tends to be proportional to the square of the intake airspeed, and the fuel in the main jets will obtain a speed as the square root of the pressure reduction so the two will be proportional to each other. If the pressure reduction is taken as from a reduction of area along the air flow rather than from ambient pressure to the fuel entry point the effect can be described as the Venturi effect, but that is still derived from Bernoulli's principle at two positions. If the pressure reduction is taken as from a dynamic pressure (facing the incoming air) within the air entry point rather than from ambient pressure instead the carbureter can be described as the Pitot tube type, but that is also still derived from Bernoulli's principle at two positions.
The actual fuel and air flows are more complicated and need correction. This might be done variously at lower speeds or higher speeds, or over the whole range by a variable emulsion device to add air to the fuel after the main jets/s. In SU and other (e.g. Zenith-Stromberg) variable jet carburetors, it was mainly controlled 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).
Fuel circuits
Main metering circuit The main metering circuit usually consists of barrel/s which reduces to a narrow part where the air is at its highest speed, forming a choke or venturi. Fuel is usually introduced into the air stream at that narrow part through small tubes leading from the main jet. The change in area is only necessary for a Venturi carburetor but in other configurations the barrel/s can be of constant area with the fuel entry anywhere along it.
Downstream of a choke or 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 usually mechanically connected to the vehicle's throttle pedal, which varies engine speed.
At lesser throttle openings, the air speed may be insufficient to maintain the main fuel flow, so then the fuel may be supplied by the carburetor's idle and off-idle circuits usually near the narrow gap between the edge of the butterfly plate and the body to give sufficient local air speed.
At greater throttle openings, the speed of air increases, which lowers the pressure of the air and draws more fuel into the airstream. At the same time, the reduced manifold vacuum results in less fuel flow through the idle and off-idle circuits.
Choke 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.
Idle circuit
The system within a carburetor that meters fuel when the engine is running at low RPM. The idle circuit is generally activated by vacuum near the (near closed) throttle plate, where the air speed increases to cause 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.
Off-idle circuit 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 caused by the high air speed near the (partly closed) 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.
Power valve 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.
This is not to be confused with the unrelated exhaust power valve arrangements used on two-stroke engines.
Metering rod / step-up rod 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 flow through the jet. These systems have been used by the Rochester Quadrajet and in the 1950s Carter carburetors.
Accelerator pump 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. When the driver presses the throttle pedal, a small piston or diaphragm pump injects extra fuel directly into the carburetor throat.
The accelerator pump can also be used to "prime" an engine with extra fuel prior to attempting a cold start.
Fuel supply
Float chamber
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 or by gravity with the fuel tank located higher than the carburetor. A floating inlet valve regulates the fuel entering the float chamber, assuring a constant level. In some small engines that may instead of a float chamber just use a fuel tank close below the carburetor and use the fuel suction to supply the fuel.
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. 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'.