PFI/Di Fuel System: Notes

Port Fuel Injection/Direct Injection (PFI/DI) fuel systems will be introduced in the 2017 MY on the 3.5L GTDI F150 and 3.5L GTDI Ford GT. The benefits of using both a PFI and a DI fuel injection system on an engine are fuel economy, emissions and performance.

Fuel economy can be improved by running in PFI mode during light load conditions. In this region, the energy required to compress the fuel for use within the DI system is not utilized which equates directly into improved fuel economy. Performance can be improved by running DI as engine speeds and loads increase. By injecting the fuel directly into the cylinder, more heat from the air can be transferred to the fuel. This charge cooling effect results in an increase in torque by using DI injection. Emissions can be improved by running in PFI mode at idle. DI fuel rail pressure must be lowered at idle to avoid running at fuel pulse widths that are very small. The variability in delivered fuel at small fuel pulse widths leads to higher Particulate Matter (PM) emissions. Running in PFI mode at low loads reduces PM emissions. PFI/DI Maintenance Modes

DI Injector Maintenance: 

The DI injector can be damaged if exposed to excessive heat, which can occur by running PFI only at higher loads. To avoid damage, the DI system is commanded on sufficiently enough to cool the injector via the fuel flow.

DI Rail Pressure Maintenance: 

While running PFI only, the pressure within the DI fuel rail will increase due to thermal expansion. Higher pressures reduce the ability of the DI system to accurately meter small fuel masses when needed. The minimum pulse width of an injector is set by electrical and mechanical injector characteristics and fuel pressure. As pressure increases, the minimum fuel mass that can be delivered also increases. Pressure maintenance mode is used to ensure that the minimum requested fuel mass can be delivered, if needed. Pressure relief can be done in two ways. The first is to use the pressure bleed valve within the high pressure pump. This is a passive, mechanical valve which opens around 3, 500 psi. The second is an active approach where the DI injectors are commanded on for a few engine cycles. The delivered fuel mass will lower the rail pressure and will lower the rail temperature as fuel is replenished with cooler fuel from the tank that has not be subjected to engine heat.

DI High Pressure Pump Maintenance: 

While running PFI only, the temperature of the DI pump will increase due to heat transfer from the cylinder head and due to friction, as the pump's piston is still actively moving. To avoid pump damage, the control system will turn on the DI injectors to allow fuel to flow through the pump. This cooler fuel flow replenishes the lubrication film on the piston and transfers out of the pump.