DFI Characteristics
Characteristics Of 911 Carrera Direct Fuel Injection (DFI)
- Homogeneous operation
- Better cylinder charging
- Reduced knock sensitivity
- Higher compression ratio
- High-pressure stratified charge injection at startup
- Multiple fuel injection
The direct fuel injection (DFI) used in the new 911 flat-six engines is based on homogeneous direct injection. In this process, electro-mechanically controlled fuel injectors inject the fuel directly into the combustion chamber at a pressure of approx. 580 psi to 1740 psi (40 to 120 bar) and with millisecond accuracy as required by the respective driving state. The mixture of air and fuel is distributed as evenly as possible in the combustion chamber, thereby allowing optimal combustion.
The spray cone angle and injection-jet orientation have been optimized to achieve optimum homogenization in the entire operating range. Within the injector, the fuel jet is twisted (rotated around the longitudinal axis). This rotation forms a conical cloud of fuel. The fine atomisation produced in this way allows faster evaporation of the fuel.
The heat energy required to evaporate the fuel is absorbed from the combustion air, thereby cooling the air. This reduces the cylinder charge volume and additional air is drawn in through the open intake valve, which in turn improves cylinder charging.
Lowering the temperature reduces susceptibility to knocking and additionally allows the compression ratio to be increased. The higher compression ratio in turn increases the engine efficiency. This reduces fuel consumption, particularly in the partial-load range.
NOTE: In case of intake manifold injection, fuel is deposited on the intake manifold, cylinder walls and valves during the intake process and is therefore no longer available for combustion. This is particularly the case during the starting phase of the engine at low temperatures. The consequence of this is that the amount of fuel used exceeds the amount of fuel that is actually needed for combustion.
Starting phase of DFI engines with high-pressure stratified charge ignition
High-pressure stratified charge ignition is used in the DFI flat-six engines in order to optimize cold starting with regard to fuel consumption and emissions. In this process, fuel is injected once into the specially shaped piston recess just prior to the end of the compression stroke when the engine is being started. This forms a stratified charge around the spark plug to ensure that an ignitable mixture is formed.
This reduces both the amount of fuel required and the emissions during the starting phase compared to intake manifold injection.
Catalytic-converter heating phase of DFI engines with dual injection
Once the high-pressure stratified charge ignition starts the engine, the engine management system switches to the catalytic converter heating phase. In this operating state, multiple injection helps to bring the catalytic converter to the temperature required for optimal conversion of pollutants as quickly as possible.
To this end, the first injection of fuel takes place during the intake stroke and the second injection of fuel occurs into the piston recess when the intake valves are closed, just before the end of the compression stroke. The slightly lean air/fuel mixture (lambda value approx. 1.05) can then be ignited at a very late point, thus increasing the exhaust temperature. The resulting delayed combustion and the continued exothermic oxidation of unburnt fuel (excess oxygen) even after the exhaust valves are opened produce a considerably higher exhaust temperature directly with the use of dual injection to heat the catalytic converters.
The high exhaust temperature quickly heats the catalytic converters to the temperature for optimal conversion of pollutants. As a result, emissions during the warm-up phase are significantly reduced and the secondary air pumps are no longer required for all engines.
Upper load range of the DFI engines
Engine operation at low rpms and high engine load leads to special requirements for the mixture-forming process within the engine. Here, triple injection takes place in the upper load range up to 2,500 rpm. Dual injection is performed up to 3,200 rpm. The amount of fuel required for combustion is distributed over two or three successive injection processes during the intake stroke (injection in synchronization with intake). This injection strategy further improves homogenization of the cylinder charge in the stated map ranges.
In the other engine map ranges and in the range near idle, the engines are operated with conventional single injection during the intake process.