Fuel injectors are devices that spray fuel in a very fine mist into an internal combustion engine, mixing the fuel with air to allow it to combust properly. Modern electric fuel injectors are computer-controlled; they monitor the engine's demand for fuel to determine how long each should remain open to inject fuel into the system. The system can also alter the spray to react to changes in the air-fuel mixture.
Before fuel injectors, cars used carburetors to mix fuel and air to feed engine cylinders. These systems are much simpler than fuel injectors, but lack the capacity to individually regulate fuel flow to each cylinder. Since the fuel and air are mixed in a single chamber and then fed into the engine, each cylinder receives the same ratio whether or not the load is perfectly balanced. Carburetors are generally less efficient and produce more emissions than fuel injection systems, but still find use in some simpler engine designs.
One common problem fuel injection systems experience is blocked injectors caused by dirty fuel or residual debris in the fuel system. This can block individual fuel injectors, reducing their ability to properly deliver the fuel-air mixture to the engine and causing ragged performance.