Engine Fueling

The engine is fueled by 6 individual fuel injectors, one for each cylinder, that are controlled by the PCM. Each fuel injector is fired individually in the engine firing order, which is called a sequential multiport fuel injection. The PCM controls each fuel injector by energizing the fuel injector coil for a brief period once every other engine revolution. The length of this brief period, or pulse, is carefully calculated by the control module in order to deliver the correct amount of fuel for proper driveability and emissions control. The period of time when the fuel injector is energized is called the pulse width and is measured in milliseconds, thousandths of a second.

While the engine is running, the PCM is constantly monitoring the inputs and recalculating the appropriate pulse width for each fuel injector. The pulse width calculation is based on the fuel injector flow rate, mass of fuel the energized fuel injector will pass per unit of time, the desired air/fuel ratio, and actual air mass in each cylinder and is adjusted for battery voltage, short term, and long term fuel trim. The calculated pulse is timed to occur as each cylinders intake valves are closing to attain largest duration and most vaporization.

Fueling during an engine crank is slightly different than fueling during an engine run. As the engine begins to turn, a prime pulse may be injected to speed starting. As soon as the PCM can determine where in the firing order the engine is, the PCM begins pulsing the fuel injectors. The pulse width during the crank is based on the coolant temperature and the engine load.

The fueling system has several automatic adjustments in order to compensate for the differences in the fuel system hardware, the driving conditions, the fuel used, and the vehicle aging. The basis for the fuel control is the pulse width calculation that is described above. Included in this calculation are an adjustment for the battery voltage, the short term fuel trim, and the long term fuel trim. The battery voltage adjustment is necessary since the changes in the voltage across the fuel injector affect the fuel injector flow rate. The short term and the long term fuel trims are fine and gross adjustments to the pulse width that are designed in order to maximize the driveability and emissions control. These fuel trims are based on the feedback from the oxygen sensors in the exhaust stream and are only used when the fuel control system is in a Closed Loop operation.