The engine of any gasoline-powered car only needs 3 things to run:

• Fuel
• Air
• Spark

Air is pulled in through an intake manifold by the negative pressure created when post-combustion gases evacuate the engine cylinder. In contrast fuel is pushed into the fuel rail by a positive pressure created in the fuel lines that constitute the Fuel Delivery system by the fuel pump.

Fuel is atomized at the injectors before being injected into the engine cylinder for efficient combustion. In the case of computer-controlled combustion engines the onboard computer decides when to fire each fuel injector. On a four-cylinder engine like my 318ti's M44 Engine cylinders are fired in the 1-3-4-2 order.

The computer uses a mass airflow sensor to measure the amount of air that is entering the intake manifold to determine how much fuel to spray into the cylinder. In the case of the M44 this is a hot wire sensor MAF that is situated just downstream from the airbox.

Here is the pleasant video that taught me about fuel injection initially: