DME Injection Strategies V6 DFI
DME injection strategies V6 DFI
Characteristics of direct fuel injection
- High cylinder charging
- Low knock sensitivity
- High compression ratio
- High-pressure stratified charge injection
- Multiple injection possible
- Homogeneous operation (=1) with engine at operating temperature
In the case of direct fuel injection, the fuel is injected directly into the combustion chamber, so that mixture formation takes place almost entirely within the combustion chamber.
The direct fuel injection system used in the Panamera engines is based on homogeneous operation when the engine is at operating temperature. The mixture of fuel and air is distributed as evenly as possible in the combustion chamber, thereby enabling optimal combustion. In this process, the fuel is injected directly in the combustion chamber at a pressure of up to 1740 psi (120 bar). The jet and cone angles of the injectors have been optimized to achieve optimum homogenization across the entire operating range. Within the injector, the fuel jet is swirled (rotated about the longitudinal axis). This rotation forms a cone-shaped cloud of fuel. This results in fine atomisation which in turn enables quicker evaporation of the fuel.
The heat energy required for this is removed from the air by the fuel evaporation and this cools the air as a result. This reduces the cylinder charge volume and additional air is drawn in through the open intake valve, which in turn improves cylinder filling. The reduced temperature level also helps to create the prerequisites for the high compression ratio in all Panamera engines, since knock sensitivity and engine efficiency have been improved.
Injection strategies
Engine start with single injection (top graph)
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 ignition, 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 molded piston recesses to create a coating around the spark plugs which produces an ignitable mixture. The piston recess ensures that the injected fuel is channelled directly to the spark plug. This reduces both the amount of fuel required and the emissions compared with intake manifold injection.
Catalytic converter heating with dual injection (bottom graph)
Catalytic converter warm-up
After engine starting by high-pressure stratified-charge ignition, the engine management system switches to the catalytic converter heating phase. In this operating state, a dual injection system helps to bring the catalytic converter to the temperature required for optimal conversion as quickly as possible by increasing the exhaust gas temperature. 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.
Engine at operating temperature (homogeneous operation)
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 at idle speed and partial load up to 3,500 rpm (graph on next page)
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.
Single injection at idle speed and partial load up to 3,500 rpm (top graph)
Triple injection at full load 1,000 rpm to 1,900 rpm (bottom graph)
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 1,000 to 1,900 rpm. The quantity of fuel required for combustion is shared between 3 successive injections.
Dual injection at full load 1,900 - 3,500 rpm (top graph)
Under conditions with high load (e.g. large valve lift), a dual injection takes place at an engine speed from 1,900 to 3,500 rpm. The quantity of fuel required for combustion is shared between two successive injection processes.
Single injection at full load above 3,500 rpm (bottom graph)
At high load and with an engine speed of over 3,500 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.