Automakers Embrace Direct Fuel Injection
Increased fuel economy compensates for additional cost of cutting-edge system
Gasoline direct fuel injection (GDI) has become common on many new cars.
This type of fuel injection sprays the fuel directly into the combustion chamber on top of the piston at high pressure. The other type is port fuel injection, which sprays the fuel behind the intake valve and takes it into the combustion chamber with incoming air when the intake valve opens.
While both types of injection systems deliver fuel, the differences in operation are significant.
First of all, the fuel pressures are different. Port injection may run pressures in the 40- to 60-PSI range, while GDI pressures typically operate above 2,000 PSI. This higher pressure requires the addition of a high-pressure fuel pump -- usually mechanically driven by the engine -- to complement the electric fuel-delivery pump in the fuel tank. The high fuel pressure allows it to be sprayed into the cylinder during the compression stroke of the engine, when cylinder pressures are also high.
Another advantage of high-injection pressure is the fuel droplets are broken down into an extremely fine mist as they leave the tip of the injector, so the fuel atomizes (evaporates) almost instantly. Liquid gasoline does not burn easily, but the vapours ignite rapidly. Creating a fuel spray that atomizes quickly allows all the fuel to be burned during the engine's power stroke. Fuel-economy increases of five to 10 per cent can be realized just by redesigning the engine to use direct injection.
While both port and direct injection can provide multiple injection pulses per engine-firing cycle, direct injection can be tailored to provide fuel at the optimum points of the engine's power stroke. Some of the fuel may be injected to start the power stroke, while more can be injected as the piston moves down in the cylinder during the power stroke. These secondary or even multiple injection pulses provide a more even pressure in the cylinder during the power stroke.
In port injection, all the fuel enters the cylinder at the same time and burns rapidly during the start of the power stroke. This creates a high-pressure pulse in the cylinder to push the piston down. But as the piston moves down, the force or pressure on the piston decreases. …