PLD start of injection control function

W07.15-1366-79

Components for determining and controlling the optimum start of injection

4 Engine CAN bus

A3 Drive control (FR) 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 of cylinders 1 to 6

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

The start of injection is controlled primarily to compensate the ignition retard and to ensure ignition in the upper TDC range, i.e. at the highest compression temperature.

If the start of injection were not controlled then this would result in the ignition retard (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 the start of injection is primarily dependent on the engine speed and is adjusted in the direction "advance" with increasing engine speed in order to avoid the disadvantages of ignition retard at higher engine speeds.

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The fact that the start of injection is a purely calculated value means that the engine control (MR) can take account of influencing factors other than the engine speed and position of the crankshaft (A) such as the fuel temperature (D), engine temperature (E) and load condition (F). These factors also influence the ignition retard 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 the 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: 25° crankshaft angle before and up to 5° crankshaft angle after top dead center) are stored in the MR/PLD control unit (A6); this control unit is also responsible for the required delivery calculations and appropriate actuation of the individual solenoid valves in the unit pumps of cylinders 1 to 6 (Y6-Y11).
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

The signals from the crankshaft angle position sensor (B15) are the basis 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 the starting procedure or when the crankshaft angle position sensor (B15) fails that signals from cylinder 1 TDC sensor (B16) are also used. They provide information on the current position of the camshaft (B) and therefore indicate when cylinder 1 is in the compression stroke.
The actual start of injection (C), fuel temperature (D), engine temperature (E) and load condition (F) are used for final calculation of the start of delivery.

 Actual start of injection (C)
The actual start of injection (C) is determined with the aid of impact or closing recognition and is used to compensate scatter and tolerances in unit pump production by means of performance-map regulation.

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

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

 Load condition (F)
The load condition (F) is derived from, for example, the engine speed and position of the foot throttle actuator (B1).