Direct Fuel Injection (DFI)
Direct Fuel Injection (DFI)
Like in the current 911 models, mixture formation in the new 911 Turbo engine is based on homogeneous direct fuel injection. The mixture of air and fuel is distributed as evenly as possible in the combustion chamber to enable optimum combustion, which thus reduces fuel consumption and exhaust emissions while increasing torque. In the new 911 Turbo, the fuel is injected directly into the combustion chamber through multi-port injectors (6 ports) at a fuel pressure of up to 1957 psi (135 bar) (911 Carrera: 1740 psi/120 bar). The higher pressure and multi-port injectors offer a further increase in the maximum amount of fuel injected and support mixture formation with even finer atomization of the fuel.
Fuel injection through multi-port injectors creates individual cone-shaped clouds of fuel, which enable very fine atomization and quick evaporation of the fuel. The heat energy required to evaporate the fuel is extracted from the combustion air. This cooling reduces the cylinder charge volume and additional air can be drawn into the cylinder through the open intake valve. In turbo engines in particular, this effect improves cylinder charging and therefore engine performance.
Lowering the temperature also reduces knock sensitivity and additionally allows the compression ratio to be increased from 9.0:1 in the previous model to 9.8:1 in the new 911 Turbo. The higher compression ratio improves engine efficiency and reduces fuel consumption as well as exhaust emissions.
Characteristics of direct fuel injection:
- Low knock sensitivity
- High compression ratio
- High-pressure stratified-charge injection
- Multiple injection possible
- Homogeneous operation (l=1) with engine at operating temperature
Injection Stratagies
The piston recesses are important for high-pressure stratified charge injection and for dual injection during the catalytic converter heating phase. They allow late injection of fuel in order to create an ignitable air/fuel mixture around the spark plug for late ignition.
With high-pressure stratified-charge injection, DFI injects the fuel very late during the engine start, just before the end of the compression stroke. In this so-called high-pressure stratified-charge injection process, a quantity of fuel is injected into the specifically moulded piston recesses to create a coating around the spark plugs which produces an ignitable mixture. The piston recess ensures that the injected fuel is channeled directly to the spark plug. This reduces both the amount of fuel required and the emissions compared with intake manifold injection.
Once the engine has started, the engine management system switches to the catalytic converter heating phase. In this operating state, the combination of dual injection and secondary air helps to bring the catalytic converter to the temperature required for optimal conversion of pollutants as quickly as possible. The objective here is to achieve the latest possible ignition of the fuel-air mixture in order to obtain high exhaust-gas temperatures in the exhaust port.
For this purpose, the second injection is made into the piston recess with closed intake valves shortly before the end of the compression stroke. The fuel-air mixture is ignited very late, thereby increasing the exhaust-gas temperature. This reduces the emissions during the starting phase. Intensive post-oxidation of uncombusted hydrocarbons and a further rise in the exhaust-gas temperature on its route to the turbocharger are achieved in combination with the secondary air supplied here.
Engine at operating temperature (homogeneous operation)
Single injection - idle speed
Partial load up to 3200 rpm
When the engine is at operating temperature, injection into the cylinder takes place only during the intake stroke. In this so-called homogeneous operation there is uniform mixture formation for the entire cylinder charging process.
Single injection for idle speed/lower partial-load range:
At speeds close to idle speed and in the lower partial-load range, the engine is operated with a single injection in the intake stroke since this relatively short injection time cannot be divided up.
Multiple injection:
In the upper load range, the quantity of fuel needed for combustion is shared between three or two successive injection operations per cycle. This multiple injection takes place during the intake stroke (synchronous intake injection) with open intake valves, thereby ensuring better homogenization (spatial distribution in the combustion chamber) in order to save fuel and increase power output.
Triple injection:
Under conditions with high load (e.g. large valve lift), a triple injection takes place at an engine speed from 1000 to 2600 rpm. The quantity of fuel required for combustion is shared between 3 successive injections.
Dual injection:
Under conditions with high load (e.g. large valve lift), a dual injection takes place at an engine speed from 2600 to 3200 rpm. The quantity of fuel required for combustion is shared between two successive injection processes.
Single injection:
At high load and with an engine speed of over 3200 rpm, injection takes place only once in the intake stroke, since there is not enough time to divide up the injection time at high engine speeds.