For an internal combustion engine to operate, two things must be present - the fuel mixture and the spark. The fuel mixture is simply petrol and air mixed in the correct proportions. If the mixture is incorrect the engine will often run, but performance and efficiency would be very poor.
The petrol is stored in the tank which is usually at the rear of the car, often under the boot (or trunk) or inside it behind the rear seats. The petrol is then sucked by the petrol pump through an in line filter into the carburettor. The petrol pump can be of two types. Some are driven by very small electric motors, but most are driven by the crank shaft as it rotates. When the petrol reaches the carburettor it is pushed through a fine nozzle, or jet, into a float chamber. When there is enough petrol in the chamber a float rises and closes off the jet.
Here the petrol is mixed with air in the correct proportions before it enters the combustion chambers. When the engine is cold, it requires a richer mixture and thus the air intake is closed down. This is normally done manually by operating a lever inside the car, called a choke lever, although many cars now have automatic chokes. As the engine warms up it requires a weaker mixture and the flap is then opened to allow more air to mix with the petrol.
The jets inside the carburettor are very fine and easily blocked. This is the purpose of the in line filter often fitted between the tank and the pump. All the particles of dirt are trapped here to prevent them reaching the carburettor. In order to ensure that the mixtures are correct, cars need to be regularly tuned.
