The job of the carburetor is to mix combustible fuel (usually petrol (gasoline) or kerosene) with sufficient air to make a readily combustible mixture which is drawn into the combustion chamber to be ignited in the power stroke. The simple carburetor we will deal with in this article uses the vacuum of the intake stroke of a piston to draw fuel though a small hole which is partially blocked making a spray of fine droplets - a vapour. This vapour mixes with air drawn through another port during the same cycle then sucked through the intake valve and burnt. Sounds simple? It is. The only complex part of carburetion is achieving a balance between three things, vacuum, air and fuel.
Why is the carburetor required? Putting it simply petrol is not flammable. This might sound ridiculous but it is true. Petrol in its liquid form will not burn without a great deal of heat to ignite it. What is highly flammable is petrol vapour, petrol in its gaseous form. When mixed with oxygen it takes very little to ignite it. The carburetor converts liquid and air to vapour. According to Dyke's Automobile and Gasoline Engine Encyclopedia Instruction No. 13 the best air to fuel ration is 14 to 1 when maximum power is desired or 17 to 1 for maximum economy.
You can study this using a non-explosive fuel such as methylated spirit. If you pour a small amount of meths into a clear pyrex beaker and attempt to ignite it with a match you will get a blue flame which hovers just above the fluid. As the flame warms the fluid more of it converts to gas and the flame grows in size. Do not try this with petrol as it is explosive when it reaches its flash point the remaining liquid will convert to gas extremely quickly and ignite all at once creating an explosion.
The illustration shows an example of a simple early carburetor much like those found on most old stationary engines. There are three key areas to look at:
- The mixing chamber;
- The needle valve; and
- The suction valve.
The mixing chamber is a large empty space where the fuel vapour and the air are mixed together to form the easily combustible mixture.
The needle valve is between the fuel and the carburetor proper; it has two jobs: To govern the amount of fuel drawn into the mixing chamber; and to convert the liquid fuel into a vapour (or at least a spray of fine droplets). This is the most critical function. It is imperative that the fit between the needle valve and and the body of the carburetor is exact and tight. If there is any leakage the fuel will spurt still in liquid form into the mixing chamber and will not properly mix leading to poor or no running. The needle valve is adjustable (usually) by screwing it in or out. Screwing it in closes the fuel intake hole and screwing it out opens it wider allowing more fuel into the unit, but also making bigger droplets within the spray and eventually if opened wide a flow which will not combust, but will flood the engine.
The suction valve governs how much air is drawn into the mixing chamber during each intake. It will only allow air in when the vacuum reaches a point where it overcomes the strength of the spring on the valve. The valve is sucked inwards allowing air to pass into the mixing chamber. Many older engines do not have this valve, and these are often the hardest engines to make work reliably. Without this valve there is nothing to help achieve the balance between suction on the fuel pipe and suction of air. A carburetor without this valve will have a much smaller air intake in attempt to maintain balance. It is advantageous to have the valve since you can make adjustments to allow for changes in air pressure and air conditions.
An additional feature seen in many carburetors or at least within the intake manifold is the butterfly valve. This is a circle of metal which can completely block the intake stopping the process partially or entirely. This valve is controlled by the engine governor to manage the engine's speed.