Injection-rate metering consists of the advance- main- and post-injection phases (known respectively as VE, HE and NE).
The injection quantities for each individual injection process are computed as functions of total quantity setpoint, engine speed and other variables and are then used to calculate the start of activation and the activation duration.
Advance injection (VE)
Advance injection helps cushion the combustion transient and thus smoothes engine operation.
The start of VE activation relative to the main-injection phase (HE) is calculated as a function of the operating point. This timing is corrected to allow for coolant temperature and air temperature.
VE is disabled as a function of the operating point if:
• VE is too far in advance of HE
• VE quantity is too small
• Residual HE quantity is inadequate
• Air mass is inadequate for the operating point
• Maximum speed is exceeded
• Engine is shut down
The result of advance-injection calculation is reported to the quantity calculation for the main-injection phase (HE).
Main injection (HE)
The majority of fuel is injected via the main injection into the combustion chamber.
The start of injection is calculated relative to TDC. The start of activation is a function of the operating point.
If HE is preceded by VE, the characteristic map used for HE injection time is not the same as that used for injection
The basic value depends on:
• Coolant and air temperatures
• Air pressure
• Speed-dependent boost-pressure fluctuation
Post-injection phase and catalytic-converter monitoring
The post-injection phase (NE) provides a supply of diesel-fuel HC to the catalytic converter. This reduces the NO and
NO2 (NOx) content of the exhaust gas.
A second temperature sensor installed downstream from the catalytic converter provides a second signal so that the
temperature differential can be calculated and used to monitor the efficiency of the catalytic converter.
The injectors have to be activated briefly so that the fuel rail can be vented for initial start and after repairs. When the injectors are activated the air can escape through the 2/2-way control vales of the injectors.
This venting process takes place automatically when the engine is started.