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 fuels can damage the emission control system, and the use of leaded fuels can result in the 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.

The engine is fueled by 6 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 the correct driveability and emissions control. The length of time the injector is energized is called the pulse width and is measured in milliseconds, or thousandths of a second.

While the engine is running, the PCM is constantly monitoring inputs and recalculating the appropriate pulse width for each injector. The pulse width calculation is based on the injector flow rate, or mass of fuel the energized injector will pass per unit of time, the desired air/fuel ratio, and the actual air mass in each cylinder. The pulse width is adjusted for the battery voltage and the 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 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 to accelerate starting. As soon as the PCM can determine where the engine is in the firing order, 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 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 battery voltage, the short term fuel trim, and the long term fuel trim. The battery voltage adjustment is necessary since changes in the 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 the vehicle driveability and the emissions control. The 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 Closed Loop.

Under certain stringent conditions, the fueling system will not energize the injectors, individually or in groups, for a period of time. This is referred to as fuel shut-off. Fuel shut-off is used to improve traction, save fuel, improve starting, and protect the vehicle under certain extreme or abusive conditions.

In case of a major internal problem, the PCM is equipped with a back-up fueling system that will run the engine until service can be performed.










NOTE: Do not operate the engine in the back-up fuel for extended periods. Back-up fueling will negatively impact driveability and fuel economy, and may cause damage to the emissions system.

As part of the fueling system, the PCM records and sends data about fueling to the instrument panel cluster (IPC) from which the IPC calculates the fuel economy, the range, and the fuel used displays. The PCM calculates the amount of fuel delivered to the engine through the injectors based on the injector pulse width and the flow rate. This data is accumulated and sent to the PC periodically via the data link.

The PCM controls the fuel injectors based on the information received from several information sensors. Each injector is fired individually in the engine firing order, which is called a sequential multiport fuel injection. This allows precise fuel metering to each cylinder and improves the vehicle driveability under all driving conditions.