Machines and mechanics..: Carburetor

Carburetor

11:46 PM

Engines works on fuel. The earliest form of fuel supply mechanism for modern automobile is carburetor. The primary function of carburetor is to provide the air-fuel mixture to the engine in the required proportion. he goal of a carburetor is to mix just the right amount of gasoline with air so that the engine runs properly. If there is not enough fuel mixed with the air, the engine "runs lean" and either will not run or potentially damages the engine. If there is too much fuel mixed with the air, the engine runs rich and either will not run (it floods), runs very smoky, runs poorly (bogs down, stalls easily), or at the very least wastes fuel. The carb is in charge of getting the mixture just right.

Carburetor basics

A carburetor basically consists of an open pipe, a "Pengina" 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 valve — 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 and at other places where pressure will be lowered when not running on full throttle. Fuel flow is adjusted by means of precisely-calibrated orifices, referred to as jets, in the fuel path.

Fig : Carburetor Basics

Parts of carburetor

· A carburetor is essentially a tube.

· There is an adjustable plate across the tube called the throttle plate that controls how much air can flow through the tube.

· At some point in the tube there is a narrowing, called the venturi, and in this narrowing a vacuum is created.

· In this narrowing there is a hole, called a jet, that lets the vacuum draw in fuel.

Figure . Schematic Diagram of a simple or elementary carburetor

How carburetors work

All carburetors work on "the Bernoulli Principle. Bernoulli principle states that as the velocity of an ideal gas increases, the pressure drops. Within a certain range of velocity and pressure, the change in pressure is pretty much linear with velocity-if the velocity doubles, the pressure halves. However, this linear relationship only holds within a certain range. Carburators work because as air is pulled into the carburetor throat, the venturi. It has to accelerate from rest, to some speed. How fast depends upon the air flow demanded by the engine speed and the throttle butterfly setting. According to Bernoulli, this air flowing through the throat of the carb will be at a pressure less than atmospheric pressure, and related to the velocity (and hence to how much air is being fed into the engine).

If a small port is drilled into the carb throat in this low pressure region, there will be a pressure difference between the throat side of the port, and the side that is exposed to the atmosphere. If a reservoir of gasoline, the float bowl, is between the inside of the port, and the atmosphere, the pressure difference will pull gasoline through the port, into the air stream. At this point, the port gets the name of a jet in the concept of a carburetor. The more air that the engine pulls through the carburetor throat, the greater the pressure drop across the jet, and the more fuel that gets pulled in. As noted above, within a range of airflow in the throat, and fuel flow in the jet, the ratio of fuel to air that flows will stay constant. And if the jet is the right size, that ratio will be what the engine wants for best performance.

A venturi/jet arrangement can only meter fuel accurately over a certain range of flow rates and pressures. As flow rates increase, either the venturi or the jet, or both, will begin to choke, that is they reach a point where the flow rate will not increase, no matter how hard the engine tries to pull air through. At the other extreme, when the velocity of the air in the venturi is very low-like at idle or during startup, the pressure drop across the jet becomes vanishingly small. It is this extreme that concerns us with respect to starting, idle and low-speed throttle response.

At idle, the pressure drop in a 32 mm venturi is so small that essentially no fuel will be pulled through the main jets. But the pressure difference across the throttle butterfly (which is almost completely closed) can be as high as 25+ mm Hg. Carb designers take advantage of this situation by placing an extra jet, the "idle jet" natch, just downstream of the throttle butterfly. Because of the very high pressure difference at idle, and the very small amount of fuel required, this jet is tiny. When the throttle is open any significant amount, the amount of fuel that flows through this jet is small, and for all intents and purposes, constant. So it's effect on the midrange and up mixture is easily compensated for.

During startup, the amount of air flowing through the carburetor is smaller still. At least till the engine begins to run on it's own. But when it is being turned by the starter or the kicker, rpm is in the sub 100 range sometimes. So the pressure difference across the jets is again in the insignificant range. If the engine is cold, it wants the mixture extra- rich to compensate for the fact that a lot of the fuel that does get mixed with air in the carb precipitates out on the cold walls of the intake port. Bing carburetors, and most bike carburetors, use enrichener circuits. All this really is another port or jet from the float bowl to just downstream of the throttle butterfly. Except that the fuel flow to this jet is regulated by a valve that is built into the carb body. At startup, when the lever is in the full on position, the valve is wide open, and the fuel supply to the cold start jet is more or less unlimited. In this condition, the amount of fuel that flows through the cold start jet is regulated just like the idle jet is. When the throttle is closed, the pressure drop across the jet is high, and lots of fuel flows, resulting in a very rich mixture, just perfect for ignition of a cold motor. If the throttle butterfly is opened, the pressure difference is less, and less fuel flows. This is why R bikes like no throttle at all until the engine catches. However, the mixture quickly gets too rich, and opening the throttle a tad will make things better. Just like the idle jet, this cold start jet is small enough that even when the circuit is wide open, the amount of fuel that can flow is small enough that at large throttle openings, it has little impact on the mixture. This is why you can ride off with the starting circuit on full, and the bike will run pretty well-until you close the throttle for the first time, and the mixture gets so rich the engine stalls. The valve that controls fuel supply to the cold start jet allows the rider to cut the fuel available through that jet down from full during startup, to none or almost none once the engine is warm. In most cases, at the intermediate setting, fuel to the cold start jet is cut to the point where the engine will still idle when warm, although very poorly since it is way too rich.

True "chokes" are different. But very aptly named. A choke is simply a plate that can be maneuvered so that it completely (or very nearly) blocks off the carburetor throat at it's entrance ("choking" the carb, just like a killer to a victim in a bad movie). That means that the main, idle, intermediate, etc., jets are all downstream of the choke plate. Then, when the engine tries to pull air through the carb, it can't. The only place that anything at all can come in to the carb venturi is through the various jets. Since there is little or no air coming in, this results in an extremely rich mixture. The effect is maximized if the throttle butterfly (which is downstream of the big main jets and the choke plate) is wide open, not impeding things in any way. If the throttle butterfly is completely closed, the engine does not really know that the choke is there-all the engine "sees" is a closed throttle, so there is little enrichening effect. The engine will pull as much fuel as possible through the idle jet, but that is so small it won't have much effect. So a carb with a choke behaves in exactly the opposite manner as one with an enrichener. During the cranking phase, it is best to have the throttle pegged at WFO so that the most fuel gets pulled in, resulting in a nice rich mixture. But as soon as the motor starts, you want to close the throttle to cut down the effect of the choke. Even that is not enough, and most chokes are designed so that as soon as there is any significant airflow, they automatically open part way. Otherwise the engine would flood. Even "manual" chokes have this feature most of the time.

Figure. A motor cycle carburetor

Carburetor adjustment

Too much fuel in the fuel-air mixture is referred to as too rich, and not enough fuel is too lean. The mixture is normally adjusted by one or more needle valves on an automotive carburetor, or a pilot-operated lever on piston-engined aircraft (since mixture is air density (altitude) dependent). The (stoichiometric) air to gasoline ratio is 14.7:1, meaning that for each weight unit of gasoline, 14.7 units of air will be consumed. Stoichiometric mixture are different for various fuels other than gasoline.

Ways to check carburetor mixture adjustment include: measuring the carbon monoxide, hydrocarbon, and oxygen content of the exhaust using a gas analyzer, or directly viewing the colour of the flame in the combustion chamber through a special glass-bodied spark plug sold under the name Colortune for this purpose. The flame colour of stoichiometric burning is described as a bunsen blue, turning to yellow if the mixture is rich and whitish-blue if too lean.

The mixture can also be judged after engine running 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, but eventually disappeared as falling hardware prices and tighter emissions standards made fuel injection a standard item.

Where multiple carburetors are used the mechanical linkage of their throttles must be synchronized for smooth engine running.

Factors influencing carburetion

1. The engine speed ; the time available for the preparation of the mixture.

2. The vaporisation characteristics of fuel.

3. The temperature of the incoming air

4. The design of the carburetor.

Types of Caburetors

1. Open choke type

Here, the main orifice known as the choke tube or venturi is of fixed dimensions, and metering is affected by varying the pressure drop across it.

2. Constant vaccum type

In this type of carburetor the area of the air passage is varied automatically while the pressure drop is kept approximately constant.

Basic forms of Carburetor