How a carburettor works

The Carburettor

Carburetion plays an essential role in allowing a car engine to start easily, accelerate without hesitation, cruise economically, give full power and be free from stalling in traffic. Its job, briefly, is to mix the right amount of petrol with the right amount of air, so that the mixture will burn in the cylinders, and to deliver the correct amount of vaporised mixture to each cylinder.
The complete process of carburetion extends from the time the petrol is mixed with air to the time the mixture starts to burn in the cylinders. So the carburettors, inlet manifold, inlet valves and even the combustion chambers and pistons are all involved in carburetion.
Petrol for the carburettor is supplied by the fuel system which consists of a remotely mounted fuel tank, a fuel pump to force petrol up to the float chamber and several filters to prevent the entry of dirt.

Air / Fuel Ratio

With most patrols, a mixture of about 15 parts of air - by weight - to one of petrol (called the chemically correct ratio) will ensure complete combustion of fuel.
But this mixture strength, or air/fuel ratio, does not produce maximum power; nor does it in general give maximum economy. Starting in cold weather may require a mixture consisting of 1 part air to 1 part petrol. But a much weaker mixture of petrol - is needed to give maximum economy for cruising.
The mixture requirements in general are: rich for starting; less rich for slow running and idling; weak for economical cruising; and richer again for acceleration and high speeds.
Substances formed when petrol and air are burnt together include carbon monoxide, carbon dioxide, hydrocarbons and oxides of nitrogen. The proportion of these in the exhaust gases depends on the richness of the mixture.
In the United States the emission of these substances, some of them poisonous, is controlled by law; and legislation along similar lines is being considered in Britain and other European countries, which has increased the need for precise control of carburetion and combustion.
This stimulated greater research into carburetion and revived interest in what are known as fuel-injection systems.

The Principle of Carburation

Air is drawn into the combustion chambers of the engine because of the partial vacuum created when the pistons move down the cylinders on the induction stroke. The air passes through the carburettor, and the amount drawn is controlled by a pivoted flap, called the throttle valve, which is opened and closed by the accelerator.
The quantity of air drawn in depends on the engine speed as well as the position of the throttle valve. It is the function of the carburettor to ensure the correct amount of petrol is drawn into the air-stream, so that the right mixture will be drawn into the combustion chambers.
Petrol, piped from the reservoir in the carburettor, joins the air-stream in a narrow-throated passage known as the venturi, or choke. This works on the principle that as the speed of an airstream increases, its pressure drops. When air flows through the narrow part of the venturi, its speed increases, and it is in the region of low air pressure that petrol is sucked into the airstream from a jet.
The air flow will be at a maximum when the engine is running at full speed, with the throttle valve wide open; and the greater the air velocity through the venturi, the greater the flow of petrol from the pipe.
In practice, a carburettor as simple of this is not satisfactory, because petrol and air do not have the same flow characteristics. As air flows faster, it becomes less dense, but petrol remains at the same density at whatever its rate of flow. Since air and petrol must be mixed by weight (roughly in the proportion of 15:1) for efficient combustion, the mixture will become progressively richer as the airflow increased and its density dropped. Eventually, the mixture would become too rich to burn in the cylinders.
There are two ways of overcoming this problem. In a fixed-jet carburettor some air is mixed with the petrol before it leaves the jet, through an arrangement of emulsion tubes or correction jets. In a variable jet carburettor the amount of petrol leaving the jet and the size of the venturi throat are varied to maintain the correct proportions of petrol and air.
Petrol in the reservoir, or float chamber, is maintained at a constant level by means of a float-operated valve. The end of the petrol pipe in the venturi has to be above the petrol level in the float chamber to prevent fuel spilling out if the car is tilted - on a hill or on a steeply cambered road, for instance. This means that before the petrol can mix with the airstream it has to be lifted a small distance - in practice, about quarter inch. The suction produced by the partial vacuum is sufficient to lift the petrol to the top of the jet and to draw it into the venturi to form droplets.
As well as drawing in petrol and air, the carburetion system must vaporise the petrol, mix it thoroughly with the air and then distribute the mixture uniformly to the cylinders. Petrol is already in the form of droplets when it enters the venturi: with a fixed jet it has been emulsified be premixing with the air; with a variable jet the droplets have been broken up by the speed of the airstream.
When the spray of petrol and air passes through the throttle valve, it enters an area of partial vacuum created by the piston suction, so that the petrol droplets start to evaporate. The rate of evaporation depends on the degree of vacuum in the inlet manifold, and this is governed by the engine speed and the position of the throttle valve.
At high speed, when the throttle is fully open, the vacuum can be so low that most of the petrol is still in liquid form and is carried along the air, or flows along the walls of the manifold. At cruising speed, when the throttle is partially closed, the vacuum increase and most of the petrol will then be in vapour form. In engines with one carburettor to each cylinder, the fact that the mixture is part liquid is of little consequence; it will still reach the combustions chamber, where it will be vaporised by heat. But when one carburettor supplies a number of cylinders, even distribution is of primary importance; and is difficult to achieve if the mixture is 'wet'.
The addition of heat to the inlet manifold by means of an exhaust-heated or water-heated 'hot-spot' promotes further vaporisation of the petrol, and so ensures an even distribution of mixture.
Except in a cold engine, this vaporisation is completed when the mixture enters the cylinders and comes into contact with the hot exhaust valve and the cylinder walls, and with any exhaust gas remaining from the previous combustion cycle.