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PowerPedia:Carburetor

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The carburetor, carburettor, or carburetter (also called 'carb' or 'carbie') is a device that mixes air and fuel for an internal combustion engine. It was invented by Hungarian scientists Donát Bánki and János Csonka in 1893. Carburetors are still found in small engines and in older or specialized automobiles such as those designed for stock car racing. However, fuel injection, first introduced in the late 1950s and first successfully commercialized in the early 1970s, is now the preferred method of automotive fuel delivery. The majority of motorcycles still are carburated due to lower weight and cost, but as of 2005 many new models are now being introduced with fuel injection.

Carburetors measure the airflow of the engine, deliver the correct amount of fuel to keep the fuel-air mixture in the proper range, and mix the two finely and evenly.
Carburetors measure the airflow of the engine, deliver the correct amount of fuel to keep the fuel-air mixture in the proper range, and mix the two finely and evenly.

Introduction

The carburetor was invented by the Hungarian engineer Donát Bánki in 1893. Frederick William Lanchester of Birmingham, England experimented early on with the wick carburetor in cars. In 1896 Frederick and his brother built the first petrol driven car in England, a single cylinder 5 hp (4 kW) internal combustion engine with chain drive. Unhappy with the performance and power, they re-built the engine the next year into a two cylinder horizontally opposed version using his new wick carburetor design. This version completed a 1,000 mile (1600 km) tour in 1900 successfully incorporating the carburetor as an important step forward in automotive engineering. The word carburetor comes from the French carbure, meaning 'carbide'. George Kingston invented the Carburetor in 1902, known as the Kingston carburetor, in Kokomo, IN. The Kingston company also manufactured such things as Roller Skates and Radios. To carburete means to combine with carbon. In fuel chemistry, the term has the more specific meaning of increasing the carbon (and therefore energy) content of a fuel by mixing it with a volatile hydrocarbon.

Most carbureted (as opposed to fuel-injected) engines have a single carburetor, though some engines use multiple carburetors. Older engines used updraft carburetors, where the air enters from below the carburetor and exits through the top. This had the advantage of never "flooding" the engine, as any liquid fuel droplets would fall out of the carburetor instead of into the intake manifold; it also lent itself to use of an oil bath air cleaner, where a pool of oil below a mesh element below the carburetor is sucked up into the mesh and the air is drawn through the oil covered mesh; this was an effective system in a time when paper air filters did not exist. Beginning in the late 1930s, downdraft carburetors were the most popular type for automotive use in the United States. In Europe, the sidedraft carburettors replaced downdraft as free space in the engine bay decreased and the use of the SU-type carburetor (and similar units from other manufacturers) increased. Small propeller-driven flat aircraft engines still use the updraft carburetor design.

The carburetor works on Bernoulli's principle: the fact that moving air has lower pressure than still air, and that the faster the movement of the air, the lower the pressure. The throttle or accelerator does not control the flow of liquid fuel. Instead, it controls the amount of air that flows through the carburetor. Faster flows of air and more air entering the carburetor draws more fuel into the carburetor due to the partial vacuum that is created.

Carburetors Type

Carburetors are either:

Fixed Venturi
the varying air velocity in the venturi alters the fuel flow; this architecture is employed in most downdraft carburetors found on American and some Japanese cars
Variable Venturi
(Constant Depression) the fuel jet opening is varied by the air flow to alter the fuel flow. This is done by a vacuum operated piston connected to a tapered needle which slides inside the fuel jet. The most common variable venturi (constant depression) type carburetor is the sidedraft SU carburetor and similar models from Hitachi, Zenith-Stromberg and other makers. The UK location of the SU and Zenith-Stromberg companies helped these carburettors rise to a position of domination in the UK car market, though such carburetors were also very widely used on Volvos and other non-UK makes. Other similar designs are used on some European and a few Japanese automobiles.

Conditions and functions

The carburetor must under all engine operating conditions:

  • Measure the airflow of the engine
  • Deliver the correct amount of fuel to keep the fuel/air mixture in the proper range (adjusting for factors such as temperature)
  • Mix the two finely and evenly

This job would be simple if air and petrol (gasoline) were ideal fluids; in practice, however, their deviations from ideal behavior due to viscosity, fluid drag, inertia, etc. require a great deal of complexity to compensate at exceptionally high or low engine speeds. A carburetor must provide the proper fuel/air mixture across a wide range of ambient temperatures, atmospheric pressures, engine speeds and loads, and centrifugal forces:

  • Cold start
  • Hot start
  • Idling or slow-running
  • Acceleration
  • High speed / high power at full throttle
  • Cruising at part throttle (light load)
  • In addition, modern carburetors are required to do this while maintaining low rates of exhaust emissions.

To function correctly under all these conditions, most carburetors contain a complex set of mechanisms to support several different operating modes, called circuits.

Basic elements and operations

A carburetor basically consists of an open pipe, a "throat" or "barrel" 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 — 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. Fuel flow in response to a particular pressure drop in the venturi is adjusted by means of precisely-calibrated orifices, referred to as jets, in the fuel path

Idle circuit

When the throttle valve is closed or nearly closed, the carburetor's idle circuit is in operation. The closed throttle reduces the airflow through the venturi to a level which cannot overcome the resistance to flow of the fuel, but it also means that a fairly significant vacuum occurs behind the closed butterfly valve. This manifold vacuum is sufficient to pull fuel through small openings placed after the butterfly valve (and in SU and similar sidedraft carburetors to pull the piston and metering rod up).

Only a fairly small amount of air and fuel can pass through in this manner. Since this small volume of fuel/air mixture can generate so little force to keep the engine turning, keeping it running at idle is more difficult than keeping it running at higher speeds. Since the airflow is too low for the carburetor to respond at all, it cannot compensate for fluctuations;instead, idle airflow is set manually by the technician or mechanic, adjusting a screw which opens the throttle a tiny fraction to allow a minimal amount of air to pass, and another screw which serves as a valve in the idle fuel circuit to adjust the volume of fuel delivered. These adjustments interact with each other, as well as affecting manifold vacuum which affects distributor spark advance which in turn affects idle speed, so that adjusting the idle to optimum (highest manifold vacuum at the specified engine idle speed) is not a completely trivial operation. While experts often claim the ability to set the idle perfectly by ear, most individuals do a better job using a tachometer and vacuum gauge. Since the advent of emissions controls on production automobiles, the idle fuel flow is typically set at the factory on the "lean" side of optimal, by restricting fuel flow so that idle speed falls by 100 — 150 rpm from where it was when optimally adjusted, in order to reduce unburned hydrocarbons and carbon monoxide with some slight loss in reliable and smooth idling; the idle fuel adjustment is typically sealed at the factory to prevent tampering, so that adjustment when age and wear cause a large deviation from proper operation requires drilling out a plug over the adjusting screw or some similar modification to gain access.

Off-idle circuit

As the throttle is opened up slightly from the fully closed position, the throttle plate uncovers additional fuel delivery holes slightly higher in the carburetor throat; these allow more fuel to flow as well as compensating for the reduced vacuum that occurs when the throttle is opened, thus smoothing the transition to metering fuel flow through the regular open throttle circuit.

Main open-throttle circuit

As the throttle is progressively opened, the manifold vacuum reduces since there is less restriction on the airflow, reducing the flow through the idle and off-idle circuits. This is where the venturi shape of the carburetor throat comes into play, due to Bernoulli's principle (i.e. as the velocity increases, pressure falls). The venturi (sometimes a second or "booster" venturi is placed inside the venturi shaped into the carburetor throat to increase the effect) raises the air velocity, and this high speed and thus low pressure sucks fuel into the airstream through a nozzle located in the center of the venturi.

As the throttle is closed, the airflow through the venturi drops until the lowered pressure is insufficient to maintain this fuel flow, and the idle circuit takes over again, as described above.

Power valve

For open throttle operation a richer mixture will produce more power, prevent detonation, and keep the engine cooler. This is usually addressed with a spring loaded "power valve", which is held shut by engine vacuum. As the throttle opens up, engine vacuum decreases and the spring opens the valve to let more fuel into the main circuit.

Accelerator pump

Similarly, the greater inertia of liquid gasoline, compared to air, means that if the throttle is suddenly opened, the airflow will increase more rapidly than the fuel flow, causing a temporary "lean" condition which causes the engine to "stumble" under acceleration (the opposite of what is normally intended when the throttle is opened). This is remedied by the use of a small mechanical pump (often just a simple plunger which pushes down through a small tube filled with gasoline which feeds into the carburetor throat) which injects an additional amount of fuel as the throttle is opened to cover this lean period; this is usually adjustable for both volume and duration by some means, sometimes just bending the linkage. Often the seals around the moving piston parts of the pump wear out, so that pump output is reduced; this loss of accelerator pump action causes the characteristic stumbling or bogging under acceleration often seen in old, well worn engines until the seals on the pump are replaced. Specialized aftermarket kits are widely available for this purpose. Other variations of pump also exist, such as diaphragm based pumps.

Choke

When the engine is cold, fuel vaporizes less readily and tends to condense on the walls of the intake manifold, starving the cylinders of fuel and making the engine difficult to start; thus, a richer mixture (more fuel to air) is required to start and run the engine until it warms up.

To provide the extra fuel, a choke is typically used; this is a device that restricts the flow of air at the entrance to the carburetor, before the venturi. With this restriction in place, extra vacuum is developed in the carburetor barrel, which pulls fuel through the venturi to supplement the fuel being pulled from the idle and off-idle circuits. This provides the rich mixture required to sustain operation at low engine temperatures.

In addition, the choke is connected to a "fast idle cam" or other such device which prevents the throttle from closing fully, which could starve the venturis of vacuum and cause the engine to stall. This also serves as a way to help the engine warm up quickly by idling it at a higher than normal speed. In addition, it increases airflow throughout the intake system which helps to better atomize the cold fuel and smooth out the idle.

In older carbureted cars, the choke was controlled by a cable connected to a pull-knob on the dashboard (GB — facia) operated by the driver. In most carbureted cars produced from the mid 1960s onward (mid 1950s in the United States) it is usually automatically controlled by a thermostat employing a bimetallic spring, which is exposed to engine heat. This heat may be transferred to the choke thermostat via simple convection, via engine coolant, or via air heated by the exhaust. More recent designs use the engine heat only indirectly: A sensor detects engine heat and varies electrical current to a small heating element, which acts upon the bimetallic spring to control its tension, thereby controlling the choke. A choke unloader is a linkage arrangement that forces the choke open against its spring when the vehicle's accelerator is moved to the end of its travel. This provision allows a "flooded" engine to be cleared out so that it will start.

Some carburetors do not have a choke but instead use a mixture enrichment circuit, or an enrichener. Typically used on small engines, notably motorcycles, enricheners work by opening a secondary fuel circuit below the throttle valves. This circuit works exactly like the idle circuit, and when engaged it simply supplies extra fuel when the throttle is closed.

Classic British motorcycles, with side-draft slide throttle carburetors, used another type of "cold start device", called a "tickler". This is simply a spring-loaded rod that, when depressed, manually pushes the float down and allows excess fuel to fill the float bowl and flood the intake tract. If the "tickler" was held down too long it also flooded the outside of the carburetor and the crankcase below, and caused a few fires in the process.

The main idea behind these devices is that extra fuel (a rich condition) is necessary to get a "cold" engine started and running for a short period of time. Either the air is restricted (choke), or more fuel is added (enrichener and tickler).

Other elements

The interactions between each circuit may also be affected by various mechanical or air pressure connections and also by temperature sensitive and electrical components. These are introduced for reasons such as response, fuel efficiency or automobile emissions control. Various air bleeds (often chosen from a precisely calibrated range, similarly to the jets) allow air into various portions of the fuel passages to enhance fuel delivery and vaporization. Extra refinements may be included in the carburetor/manifold combination, such as some form of heating to aid fuel vaporization.

Fuel supply and float chamber

To ensure a ready supply of fuel, the carburetor has a "float chamber" (or "bowl") that contains a quantity of fuel at near-atmospheric pressure, ready for use. This reservoir is constantly replenished with fuel supplied by a fuel pump. The correct fuel level in the bowl is maintained by means of a float controlling an inlet valve, in a manner very similar to that employed in toilet tanks. As fuel is used up, the float drops, opening the inlet valve and admitting fuel. As the fuel level rises, the float rises and closes the inlet valve. The level of fuel maintained in the float bowl can usually be adjusted, whether by a setscrew or by something crude such as bending the arm to which the float is connected. This is usually a critical adjustment, and the proper adjustment is indicated by lines scribed into a window on the float bowl, or a measurement of how far the float hangs below the top of the carburetor when disassembled, or similar. Floats can be made of different materials, such as sheet brass soldered into a hollow shape, or of plastic; hollow floats can spring small leaks and plastic floats can eventually become porous and lose their flotation; in either case the float will fail to float, fuel level will be too high, and the engine will not run well unless the float is replaced. The valve itself becomes worn on its sides by its motion in its "seat" and will eventually try to close at an angle, and thus fails to shut off the fuel completely; again, this will cause excessive fuel flow and poor engine operation. Conversely, as the fuel evaporates from the float bowl, it leaves sediment, residue, and varnishes behind, which clog the passages and can interfere with the float operation. This is particularly a problem in automobiles operated for only part of the year and left to stand with full float chambers for months at a time; commercial fuel stabilizer additives are available that reduce this problem.

Usually, special vent tubes allow air to escape from the chamber as it fills or enter as it empties, maintaining atmospheric pressure within the float chamber; these usually extend into the carburetor throat. Placement of these vent tubes can be somewhat critical to prevent fuel from sloshing out of them into the carburetor, and sometimes they are modified with longer tubing. Note that this leaves the fuel at atmospheric pressure, and therefore it cannot travel into a throat which has been pressurized by a supercharger mounted upstream; in such cases, the entire carburetor must be contained in an airtight pressurized box to operate. This is not necessary in installations where the carburetor is mounted upstream of the supercharger, which is for this reason the more frequent system. However, this results in the supercharger being filled with compressed fuel/air mixture, with a strong tendency to explode should the engine backfire; this type of explosion is frequently seen in drag races, which for safety reasons now incorporate pressure releasing blow-off plates on the intake manifold, breakaway bolts holding the supercharger to the manifold, and shrapnel-catching ballistic nylon blankets surrounding the superchargers.

If the engine must be operated in any orientation (for example a chain saw), a float chamber cannot work. Instead, a diaphragm chamber is used. A flexible diaphragm forms one side of the fuel chamber and is arranged so that as fuel is drawn out into the engine the diaphragm is forced inward by ambient air pressure. The diaphragm is connected to the needle valve and as it moves inward it opens the needle valve to admit more fuel, thus replenishing the fuel as it is consumed. As fuel is replenished the diaphragm moves out due to fuel pressure and a small spring, closing the needle valve. A balanced state is reached which creates a steady fuel reservoir level, which remains constant in any orientation.

Multiple carburetor barrels

While low performance carburetors may have only one barrel, most carburetors have more than one venturi, or "barrel", most commonly a two barrel, with 4 barrels being common in higher performance larger displacement engines, to accommodate the higher air flow rate with larger engine displacement. Multi-barrel carburetors can have non-identical primary and secondary barrel(s) of different sizes and calibrated to deliver different air/fuel mixtures; they can be actuated by the linkage or by engine vacuum in "progressive" fashion, so that the secondary barrels do not begin to open until the primaries are almost completely open. This is a desirable characteristic which maximizes airflow through the primary barrel(s) at most engine speeds, thereby maximizing the pressure "signal" from the venturis, but reduces the restriction in airflow at high speeds by adding cross-sectional area for greater airflow. These advantages may not be important in high-performance applications where part throttle operation is irrelevant, and the primaries and secondaries may all open at once, for simplicity and reliability; also, V configuration engines, with two cylinder banks fed by a single carburetor, may be configured with two identical barrels, each supplying one cylinder bank. Similarly, in the widely seen V8 and 4-barrel carburetor combination, there are often two primary and two secondary barrels.

Similarly, multiple carburetors can be mounted on a single engine, often with progressive linkages; three two barrel carburetors were frequently seen on high performance American V8s, and multiple four barrel carburetors are often now seen on very high performance engines.

Carburetor adjustment

Too much fuel in the fuel-air mixture is referred to as too "rich"; not enough fuel is too "lean". The "mixture" is normally controlled by adjustable screws on an automotive carburetor, or a pilot-operated lever on piston-engined aircraft (since mixture is air density (altitude) dependent). The (stoichiometric) air to petrol ratio is 14.6:1, meaning that for each weight unit of petrol, 14.6 units of air will be burned. In theory this the most efficient regarding the power/consumption ratio. But as combustion chambers in use on engines are not able to allow complete combustion of the gasoline with stoichiometric mixture, a mix around 18:1 will give better results regarding fuel economy and pollution, the excess air allowing more complete combustion. A richer mixture around 11:1 will deliver more power as the excess fuel will cool the cylinders and pistons but the price to pay is increased consumption and environmental pollution.

Carburetor adjustment can be checked by measuring the carbon monoxide, hydrocarbon, and oxygen content of the exhaust gases. The mixture can also be judged by the state and color of the spark plugs: black, dry sooty plugs indicate a too rich mixture, white to light gray deposits on the plugs indicate a lean mixture. The correct color should be a brownish gray. See also reading spark plugs. In the early 1980s, many American-market vehicles used special "feedback" carburetors that could change the base mixture in response to signals from an exhaust gas Oxygen sensor. These were mainly used to save costs (since they worked well enough to meet 1980s emissions requirements and were based on existing carburetor designs), but eventually disappeared as falling hardware prices and tighter emissions standards made fuel injection a standard item.

Catalytic carburetors

There are persistent rumours that appear to extend into the realm of urban legend or even into conspiracy theory of extremely efficient carburetors. However, there may be some basis for these rumors or claims. A catalytic carburetor mixes fuel fumes with water and air in the presence of heated catalysts such as nickel or platinum. The fuel would break down into methane, alcohols, and other lighter-weight fuels. The original catalytic carburetor was introduced to permit farmers to run tractors from modified and enriched kerosene. The U.S. Army used catalytic carburetors in World War II in the North African desert campaign, it has been said, to achieve substantial logistic surprise and thus tactical and strategic advantage against the Germans.

However, it is known that less than two years after commercial introduction of the first catalytic carburetor, in 1932, tetraethyl lead was introduced as an additive to raise gasoline's resistance to spontaneous combustion, thereby permitting the use of higher compression ratios. Also in that time, the price differential between a thermal calorie of gasoline and kerosene was ended. Tetraethyl lead had the effect of poisoning catalytic carburetors. Many modern gasolines appear to have additives for "cleaning" which perform the same effect by producing varnishes or gums in the presence of water, which of course, is not a recommended use. Gasoline/petrol is an impure mixture of linear heptane and octane and other miscellaneous light alkanes. Commercial gasolines usually contain additives to clean engines, artificially lower evaporation points, and (conjecturally) poison catalytic carburetors (an effect that is certainly real, but might be accidental).

Famed NASCAR mechanic Smokey Yunick spent many years working on a high fuel economy "vapor carburetor". The detailed operation is not widely disseminated, but the general principle is to heat the fuel with waste engine heat to enhance vaporization and improve the fuel's combustion characteristics. This was reasonably effective compared to normal carburetors of the time, but had implementation difficulties. During the years he was developing it, the average production engine moved from centrally located carburetors to electronic fuel injection, wherein the fuel is delivered right to the intake port. This dramatically reduces fuel condensation and puddling in the intake manifold and runners, making Smokey's design a solution to a problem which no longer existed so it was never commercially developed.

Carburetor icing and heat

Carburetor icing is an icing condition which can affect any carburetor under certain atmospheric conditions. While it applies to all carburetors, it is only really a problem in association with piston-powered aircraft, particularly small single-engined light aircraft. Carburetor heat (usually abbreviated to 'carb heat') is a system used in piston-powered light aircraft to help prevent or clear carburetor icing. It is usually manually controlled by the pilot. It consists of a flap which diverts warm air from around the exhaust manifold into the engine's air intake. The warmer air will usually clear any icing present within the carburetor. Applying carb heat as a matter of routine is built into numerous in-flight and pre-landing checks. Engines equipped with fuel injection do not require carb heat or an analogous system as they are not prone to icing. Some car intake manifolds had this, for the same reasons that aircraft do. Many Chevrolets from the early 50's had it, but so do a number of aftermarket performance manifolds from Clifford Performance.

Aircraft powered by carbureted engines are equipped with carburetor heat systems to overcome the icing problem. In cars, carb icing can occasionally be a nuisance but isn't usually a huge problem, as the inlet manifold and parts of the carburetor often have warm water circulating through them from the water cooling system. Motorcycles can also suffer from carb icing, although some engine designs are more prone to it than others. Air cooled engines may be more prone to icing, though it is mostly in aircraft that the phenomenon is a significant problem.

Carb icing occurs when there is humid air, and the temperature drop in the venturi causes the water vapour to freeze. The ice will form on the surfaces of the carburetor throat, further restricting it. This may increase the venturi effect initially, but eventually restricts airflow, perhaps even causing a complete blockage of the carburetor. Icing may also cause jamming of the mechanical parts of the carburetor, such as the throttle butterfly valve. For information about when there is a chance of carburetor icing, consult a graph (compiled by the British CAA) that can be downloaded here: Carburetor icing chart.

Icing occurs in certain conditions due to the venturi within the carburetor, which raises the velocity of the air in the carburetor, which lowers its pressure (see Bernoulli's principle) and hence temperature (Boyle's Law). If the outside air is already at a low temperature, the temperature in the carburetor can drop below the freezing point of water, and if the air is humid, ice can form inside the carburetor, narrowing the aperture of the throat, which can create an even stronger venturi effect, and so forth. If left unchecked, the carburetor will eventually malfunction which will cause an engine failure, an emergency situation. Temperature drops of 20 degrees C or more are often encountered within the carburetor, and so icing can occur even on relatively warm days. Also, the adiabatic lapse rate (temperature drop) is around 4 degrees C per thousand feet, so it is really the humidity of the air which is the more important indicator of potential icing conditions.

Perhaps paradoxically, winter flying is often less prone to icing, since cold weather is rarely associated with high humidity, and the air temperature can drop so far below freezing that there can be little or no water vapour in the air to begin with. Diverting warm air into the intake will usually clear any icing present, though in some conditions still may not be sufficient. The wise pilot will not attempt to fly into known icing conditions if his aircraft is not equipped to deal with it. The diversion of warm air into the intake reduces the thermodynamic efficiency of the engine, which will be manifest as a slight reduction in power while carb heat is applied. The reduction in power indicates to the pilot that there is no icing present, a reassuring piece of information. If there is icing, applying carb heat may not show this initial reduction, and as the ice clears there may be an increase in power. Again, the pilot will note this as evidence that icing conditions are present.

If carb icing results in an engine stoppage, one of the first things the pilot will do is apply carb heat in an attempt to clear the icing, though as the engine will not be running, it is possible that the exhaust will cool sufficiently quickly that clearing the icing will not be possible. In any case the pilot will be carrying out the emergency landing procedure, including a possible engine restart. The outcome depends on the conditions, prompt action and skill. Again, avoiding icing is far better than trying to clear it in an emergency.

SU carburetors

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SU carburetors (named for Skinners Union, the company which produced them) were a brand of sidedraft carburetor widely used in British (Austin, Morris, Triumph, MG) and Swedish (Volvo, Saab 99) automobiles for much of the twentieth century. Originally designed and patented by George Herbert Skinner in 1905, they remained in production through to the 1980s by which time they had become part of the BMC/British Leyland Group. Hitachi also built carburetors based on the SU design which were used on the Datsun 240Z and other Datsun Cars. While these look the same, they are different enough that needles (see below) are the only part that fits both.

su-hs4-section.gif
SU HS4 Carburetor Tuning
(Image from Library: SU HS4 Carburetor Tuning)

SU carburetors featured a variable venturi controlled by a piston. This piston has a tapered, conical metering rod (usually referred to as a "needle") that fits inside an orifice ("jet") which admits fuel into the airstream passing through the carburetor. Since the needle is tapered, as it rises and falls it opens and closes the opening in the jet, regulating the passage of fuel, so the movement of the piston controls the amount of fuel delivered, depending on engine demand.

The flow of air through the venturi creates a reduced static pressure in the venturi. This pressure drop is communicated to the upper side of the piston via an air passage. The underside of the piston is in communication with atmospheric pressure. The difference in pressure between the two sides of the piston creates a force tending to lift the piston. Counteracting this force are the force of the weight of the piston and the force of a compression spring which is compressed by the piston rising; because the spring is operating over a very small part of its possible range of extension, the spring force approximates to a constant force. Under steady state conditions the upwards and downwards forces on the piston are equal and opposite, and the piston does not move.

If the airflow into the engine is increased - by opening the throttle plate, or by allowing the engine revs to rise with the throttle plate at a constant setting - the pressure drop in the venturi increases, the pressure above the piston falls, and the piston is sucked upwards, increasing the size of the venturi, until the pressure drop in the venturi returns to its nominal level. Similarly if the airflow into the engine is reduced, the piston will fall. The result is that the pressure drop in the venturi remains the same regardless of the speed of the airflow - hence the name "constant depression" for carburettors operating on this principle - but the piston rises and falls according to the speed of the airflow.

Since the position of the piston controls the position of the needle in the jet and thus the open area of the jet, while the depression in the venturi sucking fuel out of the jet remains constant, the rate of fuel delivery is always a definite function of the rate of air delivery. The precise nature of the function is determined by the profile of the needle. With appropriate selection of the needle, the fuel delivery can be matched much more closely to the demands of the engine than is possible with the more common fixed-venturi carburettor, an inherently inaccurate device whose design must incorporate many complex fudges to obtain usable accuracy of fuelling. The well-controlled conditions under which the jet is operating also make it possible to obtain good and consistent atomisation of the fuel under all operating conditions.

This self-adjusting nature makes the selection of the maximum venturi diameter (colloquially, but inaccurately, referred to as "choke size") much less critical than with a fixed-venturi carburettor. A two-inch SU carburettor is a useful device to have in the workshop when experimenting with engines, as it is possible to bolt it onto more or less any engine and the engine, if in good order, will burst into life without the need for complex carburettor adjustments to get it to start.

To prevent erratic and sudden movements of the piston it is damped by light oil in a dashpot which requires periodic topping up. The dampening is asymmetrical; it heavily resists upwards movement of the piston. This serves as the equivalent of an "accelerator pump" on traditional carburetors by temporarily increasing the speed of air through the venturi, thus increasing the richness of the mixture.

The beauty of the S.U. lies in its simplicity and lack of multiple jets and ease of adjustment. Adjustment being carried out by altering the starting position of the jet relative to the needle on a fine screw. At first sight, the principle appears to bear a similarity to that used on many motorcycles where the main needle position is raised and lowered by a direct connection to the throttle cable rather than indirectly by the depression in the venturi. However, this apparent similarity is misleading. The piston in a motorcycle-type carburettor is controlled by the demands of the rider rather than the demands of the engine, so the metering of the fuel is inaccurate unless the motorcycle is travelling at a constant speed at a constant throttle setting - conditions which are rarely encountered except on motorways. This inaccuracy results in the wasting of fuel, particularly as the carburettor must be set slightly rich to avoid damaging leanness under transient conditions. For this reason Japanese motorcycle manufacturers ceased to fit slide carbs and substituted constant-depression carbs which are essentially a miniature Japanese SU. It is also possible - indeed, easy - to retro-fit SU carbs to a bike which was originally manufactured with a slide carb, and thereby obtain improved fuel economy and more tractable low-speed behaviour.

Weber carburetors

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weber_carburetor-big.jpg
Weber carburetors
(Image from http://www.racepages.com)

Weber carburetors were originally produced in Italy by Edoardo Weber as part of a conversion kit for 1920s Fiats. Weber pioneered the use of twin barrel carburetors with two barrels (or venturi) of different sizes, the smaller one for low speed running and the larger one optimised for high speed use. In the 1930s Weber began producing twin barrel carburetors for motor racing where two barrels of the same size were used. These were arranged so that each cylinder of the engine has its own carburetor barrel. These carburetors found use in Maserati and Alfa Romeo racing cars. In time, Weber carburetors were fitted to standard production cars and factory racing applications on automotive marques such as Abarth, Alfa Romeo, Aston Martin, BMW, Ferrari, Fiat, Ford, Lamborghini, Lancia, Lotus, Maserati, Porsche, and Triumph. In modern times, fuel injection has replaced carburetors in both production cars and motor racing. Weber fuel system components are distributed by Magneti-Marelli's After Market Products and Services.

Zenith Carburetters

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Zenith Carburetters was a British company making carburetors. In 1955 they joined with their major pre-war rival Solex Carburetters and over time the Zenith brand name fell into disuse. The rights to the Zenith designs was owned by Solex UK (a daughter company of Solex in France). The big products of Zenith were the Zenith-Stromberg (used in MGs, early Saab 99, Saab 90, early Saab 900, some 1960s Volvo 140s, and 1960s Triumphs, for instance the Triumph Spitfire) and Zenith IV carburetors. The Stromberg carburetor features a variable venturi controlled by a piston. This piston has a long, tapered, conical metering rod (usually referred to as a "needle") that fits inside an orifice ("jet") which admits fuel into the airstream passing through the carburetor. Since the needle is tapered, as it rises and falls it opens and closes the opening in the jet, regulating the passage of fuel, so the movement of the piston controls the amount of fuel delivered, depending on engine demand.

zenith32.jpg
Zenith Carburetter Co. Ltd
(Image from http://www.356registry.org/Tech)

The flow of air through the venturi creates a reduced static pressure in the venturi. This pressure drop is communicated to the upper side of the piston via an air passage. The underside of the piston is in communication with atmospheric pressure. The difference in pressure between the two sides of the piston creates a force tending to lift the piston. Counteracting this force is the force of the weight of the piston and the force of a compression spring which is compressed by the piston rising; because the spring is operating over a very small part of its possible range of extension, the spring force approximates to a constant force. Under steady state conditions the upwards and downwards forces on the piston are equal and opposite, and the piston does not move.

If the airflow into the engine is increased - by opening the throttle plate, or by allowing the engine revolutions to rise with the throttle plate at a constant setting - the pressure drop in the venturi increases, the pressure above the piston falls, and the piston is sucked upwards, increasing the size of the venturi, until the pressure drop in the venturi returns to its nominal level. Similarly if the airflow into the engine is reduced, the piston will fall. The result is that the pressure drop in the venturi remains the same regardless of the speed of the airflow - hence the name "constant depression" for carburettors operating on this principle - but the piston rises and falls according to the speed of the airflow.

Since the position of the piston controls the position of the needle in the jet, and thus the open area of the jet, while the depression in the venturi sucking fuel out of the jet remains constant, the rate of fuel delivery is always a definite function of the rate of air delivery. The precise nature of the function is determined by the tapered profile of the needle. With appropriate selection of the needle, the fuel delivery can be matched much more closely to the demands of the engine than is possible with the more common fixed-venturi carburettor, an inherently inaccurate device whose design must incorporate many complex fudges to obtain usable accuracy of fuelling. The well-controlled conditions under which the jet is operating also make it possible to obtain good and consistent atomisation of the fuel under all operating conditions.

This self-adjusting nature makes the selection of the maximum venturi diameter (colloquially, but inaccurately, referred to as "choke size") much less critical than with a fixed-venturi carburettor. To prevent erratic and sudden movements of the piston it is damped by light oil in a dashpot (under the white plastic cover in the picture) which requires periodic topping up.

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