What is Gasoline Direct Injection?

Paul Scott

Gasoline direct injection (GDI) is a fuel delivery system in gasoline internal combustion engines. The system represents the latest technologies in fuel injection and utilizes a high pressure common rail accumulator assembly which injects the fuel mixture directly into the engine's combustion chambers. GDI systems offer improved fuel efficiency, higher output power values, no throttle losses, and more flexible engine timing capabilities. In conjunction with computerized engine control units (ECU), a gasoline direct injection system can deliver a range of fuel/air mixture combinations not possible with carburetor and older multi-point fuel injection engines. These factors mean GDI equipped vehicles are not only more powerful and economical, but far more responsive throughout the full range of possible driving conditions.

Fuel injection systems supply fuel directly to the cylinders of a vehicle's engine.
Fuel injection systems supply fuel directly to the cylinders of a vehicle's engine.

Fuel injection systems function by pumping fuel under pressure through a small nozzle into various points in the engine's air intake section. The atomized fuel then enters the engine cylinders where it burns during the combustion cycle. Older single point, multi-point, and continuous injection systems introduced the fuel into the air stream via throttle controls or at a point adjacent to the cylinder intake valves. Gasoline direct injection systems differ in that they inject the atomized fuel directly into the cylinder combustion chamber.

The individual injector jets or nozzles are all connected to a distribution manifold known as a common rail. The fuel is pumped into the common rail under very high pressure and with the rail acting as an accumulator to maintain a constant pressure on the fuel ready for injection. When signaled to do so, the injector valve opens and allows an atomized jet of fuel to enter the combustion chamber. These valves may be conventional solenoid or piezoelectric types and controlled by the vehicle's ECU.

The ECU controls the fuel delivery pressure and the eventual fuel/air mixture for the combustion cycle. These parameters allow gasoline direct injection systems to very precisely match the fuel mixture to the prevalent driving conditions, thereby delivering optimal performance and economy. GDI units typically deliver fuel/air mixtures in three ratios: ultra lean, stoichiometric, and full power. Each of these are suited to particular sets of driving conditions. Gasoline direct injection systems are also well suited to a range of associated engine technologies which include variable valve timing and variable intake manifold length systems. GDI systems are also particularly effective on two stroke gasoline engines, thus eliminating the efficiency and pollution issues associated with conventional two stroke designs.

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