There are two major methods to supplying today's gasoline engine with fuel... port injection and direct injection. 

 

Port injection has been the main method used across all manufacturers since the late 80's and early 90's.  It works great and has relatively few complications.  A fuel injector sits in the intake manifold and sprays fuel on the back of the intake valves and sequential injection provides fuel via a single injector for each cylinder.  Each injector is basically a computer controlled valve that simply opens and closes at a programmed interval to allow the proper amount of fuel to the engine.  Typical fuel pressures used in this system range from 30-60psi depending on manufacturer.

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Direct injection is a system not unlike a diesel engine where the fuel is injected directly into the combustion chamber instead of the intake as in port injection.  This system is also computer controlled and far more precise with the amount and timing of the fuel delivery.  Fuel atomization is also improved through bumping the fuel pressure up to 1700psi or more!

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The biggest con to this system is no fuel is sprayed onto the intake valves to clean them.  Without this continual cleaning action, 'coking' eventually occurs causing a host of performance problems. For preventative maintenance, it is recommended to have an intake cleaning service preformed ever 15-20,000 miles.  This procedure uses a chemical cleaner for the entire intake system dissolving any carbon deposits.