The engine is fueled by 6 fuel injectors, one for each cylinder, that are controlled by the PCM. The PCM controls each injector by energizing the injector coil for a brief period, generally once every other engine revolution. The length of this brief period, or pulse, is carefully calculated by the PCM to deliver the correct amount of fuel for driveability and emissions control. The length of time the 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 injector. The pulse width calculation is based on the injector flow rate, the desired air/fuel ratio, and the actual air mass in each cylinder, adjusted for battery voltage and the short term and long term fuel trim. The calculated pulse is timed to occur as each cylinder'* intake valves are closing to attain the largest duration and the most vaporization.
Fueling during crank is slightly different than during engine run. As the engine begins to turn, a prime pulse may be injected in order to speed starting. As soon as the PCM can determine where in the firing order the engine is, the PCM begins pulsing injectors. The pulse width during crank is based on the engine coolant temperature and the barometric pressure.
The fueling system has several automatic adjustments to compensate for differences in the fuel system hardware, the driving conditions, the fuel used, and the vehicle aging. The basis for fuel control is the pulse width calculation 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 changes in voltage across the injector affect the injector flow rate. The short term and long term fuel trims are fine and gross adjustments to the pulse width designed to maximize driveability and emissions control. These fuel trims are based on feedback from oxygen sensors in the exhaust stream and are only used when the fuel control system is in closed loop.
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