PLD start of injection control function

W07.15-1354-79

Components for determining and controlling the optimum start of injection

4 Engine CAN bus

A3 Vehicle engine control (FMR) control unit

A6 MR/PLD control unit

B1 Foot throttle actuator

B10 Fuel temperature sensor

B15 Crankshaft angle position sensor

B16 Cylinder 1 TDC sensor

B65 Coolant temperature sensor

B111 Charge air combination sensor (temperature, pressure)

Y6-Y11 Unit pumps for cylinders 1 to 6 (engine 906.9, 926.9, )

Scatter of the NOx and HC emissions depending on the start of injection

x Start of injection in °CA

y Emission in %

α Optimum start of injection

- advanced

+ retarded

Factors influencing controlling of the optimum start of injection (α)

A Engine speed and the position of the crankshaft (crankshaft angle)

B Position of camshaft

C Actual start of injection

D Fuel temperature

E Engine temperature

F Loading condition (partial loading, full loading)

General information

Controlling start of injection occurs primarily by comparing the ignition delay and for securing ignition in the area of the upper TDC, that is at the highest compression end temperature.

If start of injection were not controlled then this would result in ignition delay (time from start of injection to start of ignition) having a negative effect on the course of the combustion, particularly at high engine speeds. Pollutants emissions and noise levels would increase while the power output of the engine would reduce.

Thus start of injection is primarily dependent on the engine speed and is adjusted in the direction "advanced" with increasing engine speed in order to avoid the disadvantages of ignition delay at higher engine speeds.

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The fact that start of injection is a purely calculated, the engine control (MR) can take account of other influences other than the position of the crankshaft (A) such as the fuel temperature (D), engine temperature (E) and loading condition (F). These factors also influence the ignition delay and can therefore also contribute to increased pollutant emissions.

Exact calculation and appropriate shifting of the start of injection allows the engine control (MR) to balance out these effects and to lower the pollutant emissions. This allows one to significantly reduce nitrous oxide (NOx) which is generated when start of injection occurs too early and hydrocarbons (HC), which are generated when the start of injection occurs too late.

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1 Controlling the start of injection

Start of injection is controlled over the start of delivery and is constantly adjusted to the current operating conditions of the engine.
Start of delivery, which takes place somewhat earlier than the actual start of injection is the point in time at which the solenoid valve in the unit pump is fully closed and the fuel can be compressed or delivered. Start of delivery can be fully and freely regulated within mechanical limits (limited by the design of the cam) by the engine control (MR).
The limits (max. start of delivery a 25° crankshaft angle before through to a 5° crankshaft angle after top dead center) are stored in the MR/PLD control unit (A6) through which also computation and corresponding actuation of individual solenoid valves in the unit pumps of cylinders 1 to 4 (Y6-Y9) for 4-cylinder engines or cylinders 1 to 6 (Y6-Y11) takes place for 6-cylinder engines.
The constantly changing operating conditions of the engine means that the start of delivery must be recalculated after every injection process.

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2 Calculation of the start of delivery

Signals from the crankshaft position sensor (B15) are decisive for calculating the start of delivery. These allow one to determine the current engine speed and the position of the crankshaft (A). It is only during start-up or when the crankshaft position sensor (B15) fails that signals from the cylinder 1 top dead center sensor (B16) are also used. They provide information about the current position of the camshaft (B) and therefore when cylinder 1 is in the compression cycle.
For final computation of the start of delivery the actual start of injection (C), the fuel temperature (D), the engine temperature (E) and the load condition (F) are used.

 Actual start of injection (C)
The actual start of injection (C) is determined with the aid of the impact or closing recognition (C), which serves to determine scatters and tolerances in manufacture of the unit pumps and to balance these out using performance map regulation.

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 Fuel temperature (D)
The fuel temperature (D) is determined from values of the fuel temperature sensor (B10).

 Engine temperature (E)
The engine temperature (E) is determined from the values of the coolant temperature sensor (B65) and the charge air combination sensor (temperature, pressure) (B111).

 Load condition (F)
The load condition (F) is deduced amongst other things from the engine speed and position of the foot throttle actuator (B1).