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A carburetor (also spelled carburettor or carburetter)
Since the 1990s, carburetors have been largely replaced by fuel injection for cars and trucks, but carburetors are still used by some (e.g. lawnmowers, generators, and concrete mixers) and motorcycles. In addition, they are still widely used on piston-engine–driven aircraft. have always used fuel injection instead of carburetors, as the compression-based combustion of diesel requires the greater precision and pressure of fuel injection.
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.
The simplest carburetors work on Bernoulli's principle: the static pressure of the intake 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 diagram shown to the above right Cross-section schematic.
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 simply a derivation from the Bernoulli 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 carburettor 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).
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 usually mechanically connected to the vehicle's throttle pedal, which varies engine speed.
At lesser throttle openings, the air speed through the venturi may be insufficient to maintain the fuel flow, so then the fuel may be supplied by the carburetor's idle and off-idle circuits which will work even with a low volume of air because the narrow gap between the edge of the butterfly plate and the body gives sufficient local air speed at those jets.
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. At the same time, the reduced manifold vacuum results in less fuel flow through the idle and off-idle circuits.
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 flooded engine 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.
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.
The accelerator pump can also be used to "prime" an engine with extra fuel prior to attempting a cold start.
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'.
To avoid pressurizing the float chamber, vent tubes allow ambient air to enter and exit the float chamber. These tubes may instead extend into the carburetor air flow prior to where the fuel flows in, in order to use the Venturi effect to achieve suitable pressure difference rather than the Bernoulli principle which applies when the pressure difference is related to the ambient air pressure.
Many four-barrel carburetors use two primary and two secondary barrels. A four-barrel design of two primary and two secondary barrels was commonly used in V8 engines to conserve fuel at low engine speeds while still affording an adequate supply at high.
The use of multiple carburetors (e.g., a carburetor for each cylinder or pair of cylinders) also results in the intake air being drawn through multiple venturi. Some high-performance engines have used multiple two-barrel or four-barrel carburetors, for example six two-barrel carburetors on Ferrari V12s.
All three of these engines used surface carburetors, which operated by moving air across the top of a vessel containing the fuel.
The first float-fed carburetor design, which used an atomizer nozzle, was introduced by German engineers Wilhelm Maybach and Gottlieb Daimler in their 1885 Grandfather Clock engine. The Butler Petrol Cycle car—built in England in 1888—also used a float-fed carburetor.
The first carburetor for a stationary engine was patented in 1893 by Hungarian engineers János Csonka and Donát Bánki.
The first four-barrel carburetors were the Carter Carburetor WCFB and the identical Rochester 4GC, introduced in various General Motors models for 1952. Oldsmobile referred the new carburetor as the "Quadri-Jet" (original spelling) while Buick called it the "Airpower".
In the United States, carburetors were the common method of fuel delivery for most US-made gasoline (petrol) engines until the late 1980s, when fuel injection became the preferred method. One of the last motorsport users of carburetors was NASCAR, which switched to electronic fuel injection after the 2011 Sprint Cup series. NASCAR still uses the four-barrel carburetor in the NASCAR Xfinity Series.
In Europe, carburetors were largely replaced by fuel injection in the late 1980s, although fuel injection had been increasingly used in luxury cars and sports cars since the 1970s. EEC legislation required all vehicles sold and produced in member countries to have a catalytic converter after December 1992. This legislation had been in the pipeline for some time, with many cars becoming available with catalytic converters or fuel injection from around 1990.
A carburetor heat system is often used to prevent icing. This system consists of a secondary air intake which passes around the exhaust, in order to heat the air before it enters the carburetor. Typically, the system is operated by the pilot manually switching the intake air to travel via the heated intake path as required. The carburetor heat system reduces the power output (due to the lower density of heated air) and causes the intake air filter to be bypassed, therefore the system is only used when there is a risk of icing.
If the engine is operating at idle RPM, another method to prevent icing is to periodically open the throttle, which increases the air temperature within the carburetor.
Carburetor icing also occurs on other applications and various methods have been employed to solve this problem. On inline engines the intake and exhaust manifolds are on the same side of the head. Heat from the exhaust is used to warm the intake manifold and in turn the carburetor. On V configurations, exhaust gases were directed from one head through the intake cross over to the other head. One method for regulating the exhaust flow on the cross over for intake warming was a weighted eccentric butterfly valve called a heat riser that remained closed at idle and opened at higher exhaust flow. Some vehicles used a heat stove around the exhaust manifold. It was connected to the air filter intake via tubing and supplied warmed air to the air filter. A vacuum controlled butterfly valve pre heat tube on the intake horn of the air cleaner would open allowing cooler air when engine load increased.
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