Fuel, Exhaust & Engine Electrics
Page 2.3
Start Phase of DFI Engines
High pressure stratified charge ignition is used in the DFI systems of the
new Cayenne engines in order to optimize cold starting with regard to
fuel consumption and emissions. With this ignition system, fuel injection
occurs very late – just before the end of the compression stroke – when
starting the engine. The high pressure stratified charge ignition system
injects fuel directly only once into the specially molded piston recess so
that a stratification, which creates an ignitable mixture, is formed around
the spark plug. The piston recess ensures that the injected fuel is
directed straight to the spark plug. This reduces both the amount of fuel
required and the emissions compared to intake manifold injection.
Catalytic Converter Heating Phase in DFI Engines
Once the high pressure stratified charge ignition system starts the
engine, engine management 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 emissions temperature.
The 2nd injection of fuel into the piston recess occurs just before the
end of the compression stroke with the intake valves closed. The
air/fuel mixture is ignited very late and this increases the exhaust
emissions temperature. As a result, emissions during the start phase
are reduced and the secondary air pumps are no longer required for all
engines.
Upper Load Range of DFI Engines
Dual injection always occurs in the upper load range up approx. 3500
rpms. The amount of fuel required for combustion is distributed in two
consecutive injection processes. In the upper load range, both injections
occur during the intake stroke (intake synchronous injection) with the
intake valves open, thereby ensuring reduced fuel consumption through
improved homogenization.
Piston Recesses in DFI Engines
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. 2
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