Porsche 911 (997) engine with direct fuel injection. Click image to enlarge
By Jim Kerr
With all the auto manufacturers working to reach new fuel economy standards set by the U.S. government for future vehicles, we are seeing new technologies, materials and techniques in current cars and trucks. One of these is direct fuel injection. It has actually been around for decades, but it was only recently that materials development and computer speeds have reached the point where it would not only work, but work within the strict emissions regulations that are in place.
So what is direct injection? Right now there are two types of injection systems common on gasoline powered vehicles: port and direct injection. Both systems use computer-controlled electric injectors to spray fuel into the engine, but the difference is where they spray the fuel. Port injection sprays the fuel into the intake ports where it mixes with the incoming air. The injectors are often mounted in the intake manifold runners, where the fuel sits until the intake valve opens and the mixture is pulled into the engine cylinder.
Direct injection has the injectors mounted in the cylinder head, so that they spray fuel directly into the engine cylinder, where it then mixes with the air. Only air passes through the intake manifold runners and past the intake valves with direct injection.
There are advantages and disadvantages of both systems. The biggest advantage of direct injection is its better fuel economy. Improvements of 15 per cent are not uncommon just by changing from port to direct injection. With tight fuel economy standards, this is the main reason we are seeing so many vehicles on the market now equipped with direct injection. Another advantage is performance: direct injection can meter the amount of fuel exactly into each cylinder for optimum performance, and because it is sprayed in under very high pressure – up to 15,000 PSI (pounds per square inch) on some vehicles – the fuel atomizes well and ignites almost instantly. With current computer controls, the injectors can be pulsed several times for each combustion stroke so the fuel can be injected over a longer time frame to maximize the power out of the cylinder.