Direct Injection Guide

Gasoline direct injection (DI) engines have been lurking in the shadows of gasoline-burning, internal combustion engine development for decades but are now becoming mainstream. This is all good, as DI engines can be tuned to unruly power levels while still exhibiting pleasant street manners and good mileage. But how does it work, and why is it good? This story is here to answer those questions.

DI?
The main aspect that defines a DI engine is the application of the fuel directly into the combustion chamber. Currently, most production gas engines use port fuel injection, where fuel is applied in the intake ports upstream of the intake valve. Port fuel injection and DI are implemented with electronic fuel injectors and an engine computer telling the injectors when to open and close to allow pressurized fuel to pass into the engine. But port fuel injection is less precise since it just sprays fuel into the intake port, which then mixes with the air in the port and rushes into the combustion chamber when the intake valve opens. The DI fuel application is a big leap forward. It allows precise timing of when fuel enters the combustion chamber and opens up a plethora of opportunities for engine tuners to make power, reduce emissions, and increase the durability of the engines-all at the same time.

Timing Is Everything
This adjustability in when the fuel is added to the cylinder is the holy grail of power production. Designers of early carbureted/distributor ignition and port fuel-injected/distributor engines only had one tuning variable that could be adjusted dynamically based on engine rpm and load: ignition timing (with counterweights on the distributor and a vacuum line from the intake manifold, respectively). Later port fuel-injected engines were developed with camshafts that could be phased (advanced or retarded) 20 or so degrees based on rpm and load. Now, DI allows the fuel application timing to be added to the cam phasing and ignition timing as another dynamic tuning tool. The DI fuel application is defined by two categories: fuel apply rate and fuel timing.

Fuel Apply Rate
The fuel apply rate is tuned via the pressure in the common fuel rail that the fuel injectors are connected to, the number of times the injector is opened to allow fuel to pass through it (during the intake cycle), and the duration of those openings. DI fuel systems are substantial in their design because they usually generate and hold fuel pressurized at a whopping 2,200 psi or more (the DI fuel rail tube often has about a 1/8-inch wall thickness to handle these extreme pressures) rather than the 40 to 60 psi common in port injection. These extremely high pressures allow the injector to flow enough fuel to achieve stoichiometric combustion (the desired 14:1 ratio of fuel and air) in a little less than half the number of degrees of crank rotation as compared with a port fuel injection engine.

Here is the explanation of that statement: The injectors on a port fuel injection engine can flow fuel for almost the entire 720 degrees of crank rotation (at lower rpm they close occasionally, but at higher rpm they can be open for as long as 720 degrees). This is acceptable as the fuel/air mixture filling the intake ports only flows into the combustion chambers when the intake valve is open.