Injection Strategies

Injection Strategies

The piston recesses are important for high-pressure stratified-charge ignition 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 (top graph), DFI injects the fuel very late during the engine start, just before the end of the compression stroke. In the high-pressure stratified-charge ignition process, a quantity of fuel is injected into the specifically shaped piston recesses to create a stratified ignitable mixture around the spark plugs. 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.

After engine starting by high-pressure stratified-charge ignition, the engine management system switches to the catalytic converter heating phase (bottom graph). In this operating state, dual injection 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)
Single injection - idle speed
Partial load up to 3500 rpm

When the engine is at operating temperature (top graph), 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.

Dual Injection:

Full throttle up to 3500 rpm

Under conditions with high load (e.g. large valve lift), a dual injection takes (bottom graph) place up to an engine speed of 3500 rpm. The quantity of fuel required for combustion is shared between two successive injections. In the upper load range, both injections take 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.

Legend
OT - Top dead center (TDC)
UT - Bottom dead center (BDC)
KS - Crankshaft position
K - Piston position
TI - Injection
Z - Ignition
ET - Intake stroke
VT - Compression stroke







Single injection:
Full throttle above 3500 rpm

At high load and with an engine speed of over 3500 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.

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.

Legend
OT - Top dead center (TDC)
UT - Bottom dead center (BDC)
KS - Crankshaft position
K - Piston position
TI - Injection
Z - Ignition
ET - Intake stroke
VT - Compression stroke