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The Working Principle Of Carburettor And Fuel System Of A Car

drawing of a four-stroke IC engine

Working Principle Of Carburetor

 The Working Principle Of Carburetor And Fuel System Of A Car
The Working Principle Of Carburetor And Fuel System Of A Car

In order to convert petrol from the liquid that pours from the pump in a filling station to a vapor that can be burned in the engine, it passes through a number of stages in a fuel and induction system.

In old cars, the petrol tank was mounted high up in the engine bay so that fuel could be gravity-fed to the engine. Because of the risk of fire, fuel tanks in modern cars are removed from the engine surrounding.  

To show the petrol level in the tank, there is a float which rides on the fuel and is connected by a long hinge to a variable electrical resistance. As the float rises and falls, a contact moves across the resistance and the varying current is taken to a gauge on the dashboard which indicates the fuel level.

The gauge is usually electrically damped, so that it does not fluctuate rapidly as the fuel surges inside the tank.

A pump driven by engine, or electrical motor feeds petrol from the tank to the carburetor, a device that accurately mixes the fuel with air. On some engines, fuel is accurately metered and injected into the in-going airstream by a fuel injection system which takes the place of the carburetor.

An engine running at speed consumes great quantities of air, and it is important that this contains no solid or dust particles that will cause wear or damage inside the engine.

To prevent this, the air intake is protected by a filter. It may be an oil bath type where air passes over oil before being drawn through a fine wire mesh; a metal mesh filter where an oil-wetted gauze collects any solid particles, or a paper element type with a replaceable pleated paper cartridge.

The carburetor and air filter are usually mounted on an inlet manifold, a branching pipe feeding the fuel-and-air mixture from the carburetor to the inlet ports of the cylinder.

All air drawn into the engine on the induction strokes passes through the main bore of the carburetor, known as the carburetor barrel. At one point, the diameter of the barrel is reduced by means of a waisted restriction called a venture.

When it meets this restriction, the air flow speeds up and a slight vacuum is created. Fuel pumped from the tank enters the carburetor and fills a reservoir known as the float chamber.

As the fuel level rises, a float in the chamber lifts until it closes off a valve which controls fuel entry. As fuel is used, the float falls, allowing the chamber to be replenished.

In this way, whenever the engine is running, the amount of fuel in the float chamber remains approximately the same.

If the float chamber is connected by a small drilling to the narrowest part of the venture, and the fuel level in the chamber is put slightly below the fuel outlet into the venture, the engine will suck petrol into the air stream while it is running.

This venture effect is the operating principle of all conventional carburetors used on modern engines. Fuel entering the air stream as a liquid is broken down into tiny droplets by the turbulent air flow in the barrel and is vaporized by the heat present in the manifold and cylinder head.

The speed of the engine is regulated by the amount of fuel-and –air mixture drawn in, and is controlled by a pivoted disc known as a throttle valve. The valve is mounted on a spindle passing through the lower part of the carburetor barrel.

When the throttle is moved to a vertical position, parallel with the sides of the barrel, it provides practically no restriction and the engine runs up to full speed. If the spindle is turned slowly to close the flap, mixture flow is increasingly obstructed. By altering the position of the throttle, the engine can be held at any required speed.

So far, basic principles have been explained, but even with a speed throttle, the carburetor so far described is too crude to be used on a modern car<car engine, and needs some extra refinements.



The Working Principle Of Carburetor And Fuel System Of A Car

Fuel and air will only ignite and burn efficiently if they are mixed together in the correct proportions. The precise ratio depends on a number of outside factors.

Under steady load conditions, a mixture ratio of about fifteen parts of air to one part of petrol by weight is known as the correct chemical ratio that ensures burning of the fuel.

For a cold starting, engines however; needs a mixture with a greater proportion of petrol. It should be a rich mixture of petrol as one part of air to one part of petrol by weight.

For maximum economy, a smaller proportion of fuel or a weaker mixture of around 16:1 air/fuel is needed. During hard acceleration, a richer mixture of around 12:1 must be supplied.

 The Working Principle Of Carburetor And Fuel System Of A Car
The Working Principle Of Carburetor And Fuel System Of A Car

With the simple carburetor described above, it is possible in theory to alter the bore of the petrol outlet into the airstream, eventually arriving by trial and error at the ideal mixture strength.

Calibrating the carburetor in this way, however, would give the correct mixture at only one particular engine speed, since the delivery of fuel at the venture does not automatically keep in step with the flow of air through it when the throttle is opened and closed.

If we opened the throttle and doubled the engine speed, for instance, the air flow could double but the increased vacuum could draw greater proportion of fuel from the outlet making the mixture too rich.  Conversely, halving the engine speed would reduce the fuel flow by more than half and the mixture strength would be too weak.

Over-rich and over-weak mixtures must be avoided. When the mixture is too rich, the oxygen in the air is used up before all the fuel is burned, and some unburned fuel is thrown out of the cylinder on the exhaust stroke.

 The Working Principle Of Carburetor And Fuel System Of A Car
The Working Principle Of Carburetor And Fuel System Of A Car

If the mixture is too weak, all the fuel is used while there is still oxygen available. Both condition reduce the engine output and efficiency. The mixture is correct when all fuel and oxygen are burned completely during combustion.


All carburetors are designed to provide a correct mixture strength regardless of engine speed, the most commonly used device being the emulsion tube.

Here, fuel from the float chamber passes through a main jet, which limits the rate of flow and then into a vertical well, with an outlet in the upper part opening into the venture.

In the top of this well is another jet which allows air to enter and flow down a thin emulsion tube, which mixes air with the petrol and is mounted in the center of the well. It contains cross-drillings at different heights which allow the fuel and air to mix.

Fuel is drawn from the outlet, causing the level in the well to fall below that of the float chamber. When this happens, air is drawn in through the top jet, mixing with the fuel and diluting the output.

As speed rises further, the level continues to fall, uncovering more of the air holes in the center tube, thus weakening the mixture. The jet sizes and the layout of the central tube are chosen so that the mixture ratio will be correct and constant.

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