The Fuel Tank

• The fuel tank is more than just a bucket to hold flammable liquid. Even the simplest fuel tank is a sealed system that operates under a continuous state of vacuum. As the fuel pump sucks gas out of the tank, the "empty" volume of air above it spreads out on a molecular level in order to fill the space. This causes a pressure drop that keeps gasoline vapors inside the tank from leaking out and evaporating into the atmosphere.
  
  

The Fuel Pump

• Fuel pumps come in two basic types, either submerged (in-tank) or divorced. Fuel-injected vehicles require a great deal more pressure -- about 10 times as much -- as a carbureted system, and have pump capacities to match. Fuel-injected cars generally use an in-tank fuel pump, while carbureted vehicles tend to use divorced pumps. Some vehicles use both; for instance, many older Volvos use a low-pressure pump at the tank to feed a high-pressure pump near the engine. The logic behind this approach was to make the car a little bit safer by reducing the length of the high-pressure lines in the system.
  
  

Filters

• Most vehicles have at least three filters: one in the tank, another somewhere in the line and a third either in the carburetor or the individual fuel injectors. Vehicles with in-tank pumps use a "sock-type" filter that covers the pump pickup and keeps large debris and rust scale in the tank from entering the pump internals. The in-line and carburetor filters serve as a second and third line of defense to keep debris from clogging up the fine needle valves in the carburetor. Fuel injectors use a fine screen inside the injector body to catch debris.
  
  

Carburetor

• For non-fuel-injected engines, a carburetor is the last link in the fuel system chain. Carburetors are a sort of metered fuel leak. They sit on top of the intake manifold, slightly restricting flow in order to create vacuum under the throttle blades. Engine vacuum sucks fuel from the carburetor's float fuel reservoir (aka float bowl) and through a series of finely-calibrated passages and valves that meter fuel flow. The end result is a fine mist of fuel mixed in proportion to the air flowing through the carb.
  
  

Fuel Injectors

• Injectors are nozzles used to meter fuel into fuel injected engines. Almost all fuel injectors use a needle-and-seat design, where a finely tapered "needle" moves down to plug the orifice in the bottom of the injector to close it. To permit fuel flow, an electromagnet wrapped around the top of the needle energizes, retracting the needle and allowing fuel to flow around it and through the orifice. The needle valve opens many hundreds or thousands of times per second, allowing the car's computer to accurately meter flow with millisecond precision.