Engine Fueling

Unleaded fuel must be used with all gasoline engines for proper emission control system operation. Using unleaded fuel will also minimize spark plug fouling and extend engine oil life. Leaded fuel can damage the emission control system, and its use can result in loss of emission warranty coverage.

All vehicles with gasoline engines are equipped with an Evaporative Emission Control ([1][2]EVAP) System that minimizes the escape of fuel vapors to the atmosphere. Information on this system is found in EVAP Control System Operation Description.

The engine is fueled by six individual 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 proper 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 its inputs and recalculating the appropriate pulse width for each injector. The pulse width calculation is based on the injector flow rate (mass of fuel the energized injector will pass per unit of time), the desired air/fuel ratio, and actual air mass in each cylinder and it 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 crank is slightly different than during 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, it begins pulsing injectors. The pulse width during crank is based on coolant temperature and barometric pressure.

The fueling system has several automatic adjustments to compensate for differences in fuel system hardware, driving conditions, fuel used, and vehicle aging. The basis for fuel control is the pulse width calculation described above. Included in this calculation are an adjustment for battery voltage, short term fuel trim, and long term fuel trim. The battery voltage adjustment is necessary since changes in voltage across the injector affect injector flow rate. Short term and long term fuel trims are fine and gross adjustments to 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.