Modern direct injection systems use a high pressure design, though some of the earlier models used a low pressure system. There are a couple of benefits to using a high pressure GDI (gasoline direct injection) system; the primary of which is greater control over the fuel.
Under a high pressure system, the fuel is pressurized at a higher pressure than a normal injection system and when it's finally given room to expand it drops in temperature and provides a cooling effect to the cylinder in which it’s injected. This allows the car manufacturer to run a more aggressive timing profile, for both fuel and injection, resulting in more power. It also, if they so choose, can make a car engine more fuel efficient by running it with less fuel but the same timings as if it was designed without GDI. The end result is that you either could gain power or gain fuel efficiency.
But, what else does it let people do? Well, the beauty of it is that it’s predictable. You know how the fuel is going to disperse, you know how much of a cooling effect it’s going to have, and you know how the car is running. If you’re loafing around on the interstate, the engine could run even leaner fuel/air mixture than normal. If you happen to be towing something, or are accelerating rapidly, and the engine starts knocking, it could run a richer than normal ratio to further cool down the cylinder.
I personally think that the future holds for us an air injection system. If the air is compressed and then expanded, we again have the cooling effects. We could change how much air is put into a cylinder and with GDI we could effectively vary the displacement as needed. If the cooling system doesn’t work as well as it should, we could continually inject air to cool the engine down, or we could alternate which cylinders get air and fuel. The possibilities are rather limitless.
Just 30 years ago we used existing throttle bodies for air and fuel injection. We then went to a central injector which, via tubes, injected fuel behind the valves. Then 20 years ago we were commonly seeing engines with a fuel injector behind every valve. Now we’re seeing fuel injectors inject directly into the cylinder. All the while the methods for delivering air for combustion have yet to change. Who knows what the next 30 years will bring for technology changes. I, for one, welcome them.