An engine cannot run on air alone. It needs a fuel system to produce enough power to move a car down the road. Modern cars have an electric fuel pump located in the gas tank. Fuel is pushed through lines, a filter and the fuel rail on its way to the cylinders. Fuel is introduced to the intake system through a fuel injector. A fuel injector is an electronic valve whose opening and closing is controlled by the computer, or engine control module. The computer precisely determines the amount of fuel that gets delivered based on inputs from the multiple sensors found on the engine.