Starting procedure, function

W07.16-1139-79

General

The engine start procedure is initiated when a corresponding start request through the drive train CAN (CAN 4) is present, e.g. when starting the engine using the electronic ignition lock (EIS) (S1) or through the parameterizable special module (PSM) control unit (A22), or through corresponding operation of the engine start and engine stop button (S600).

Preconditions

 Voltage supply (terminal 30 and terminal 15) on the engine management (MCM) control unit (A4) (ignition switched on)

 Electronic immobilizer deactivated by DAS and start enable issued by electronic ignition lock (EIS) (S1), i.e. the inserted transmitter key is authorized

 Starter lockout canceled, i.e.:

 Transmission in neutral (only for start request using engine start and engine stop button (S600))

 Power take-off, where present, switched off

 Engine stopped or engine speed is <50 rpm

Function

If all requirements are met and if the engine start and engine stop button (S600) is pressed or a corresponding message with the start request is received over the drive train CAN (CAN 4), e.g. when the start-stop button on the electronic ignition lock (EIS) (S1) is pushed to the start position (Stage 2) and held there or the parameterizable special module (PSM) control unit (A22) sends a corresponding message with the start request, the engine management (MCM) control unit (A4) actuates the starter's starter solenoid (M1) through a series relay. The starter (M1) sets the engine in motion and the engine management (MCM) control unit (A4)checks whether the starter (M1) is turning the engine at the specified minimum speed. It also waits until it can recognize from crankshaft position sensor signals (B600) when cylinders 1 and 6 are at top dead center (TDC). Only then can the actuation begin for the fuel injectors for cylinder 1 and 6 (Y608 and Y613), calculated while taking account of the current operating condition of the engine. At the same time, the engine management (MCM) control unit (A4) determines the required injection quantity for combustion and initiates the injection of this at the correct point in time into the combustion chambers of cylinders 1 and 6 through appropriate actuation of the quantity control valve (Y643), the pressure regulating valve (Y634) and the fuel injectors for cylinder 1 and 6 (Y608 and Y613). This so-called double ignition serves to accelerate the starting procedure. It is continued (afterwards follow cylinders 5 and 2, 3 and 4, then further with 1 and 6, etc.), until the ignition TDC of cylinder 1 is recognized with the aid of the camshaft position sensor (B601).

 If the camshaft position sensor (B601) fails to return any pulses, then in each case a portion of the double firing is cutoff, until the dip in speed, which then occurs, when a piston is not in ignition TDC, enables ignition TDC to be recognized. Once ignition TDC is recognized, the engine continues to run in unchanged state. Nevertheless, the maximum torque of the engine will be limited to protect the engine from damage. The engine management (MCM) control unit (A4) can also regulate the limp-home speed.

Determining the injection quantity:

Determination of the injection volume independently of the accelerator pedal position in order to achieve a secure engine start with the lowest possible emission of pollutants. The injection quantity selected for the starting procedure is primarily geared to the coolant temperature, which is determined by the engine management (MCM) control unit (A4) via the exhaust coolant temperature sensor (B606). Further influencing factors for the injection quantity during the starting procedure are the current engine speed as well as the geographical altitude at which the vehicle or engine is located.

 There is an atmospheric pressure sensor in the engine management (MCM) control unit (A4) over which the air pressure of the ambient air is determined for determination of the altitude and the current altitude of the vehicle or engine can be deduced.

The engine starts with this initial, primarily temperature-dependent, start injection quantity. If the engine does not start with the start injection amount, then the injection quantity will be continuously increased until the engine starts or the limit value for the max. start injection quantity is reached. This is then maintained until the starter (M1) actuation is interrupted either by itself or after a specific time by the engine management (MCM) control unit (A4), to protect the starter (M1) against mechanical or thermal overload (start time limit). If the engine starts, while the start-stop button on the electronic ignition lock (EIS) (S1) or the engine start and engine stop button (S600) are still being pressed, and if it reaches a specific speed then the actuation of the starter (M1) is interrupted (starter dump speed).

The pinion is pulled back out of the ring gear on the flywheel, while start actuation of the injection quantity changes over to the idle speed control. This changeover is known as starter disengagement which exclusively results in a change of the calculation method. Because the cold start ability of diesel engines at low outside temperatures is restricted by the engine's increasing resistance to rotation, lower combustion chamber temperatures and unfavorable fuel atomization conditions, under certain conditions a pilot injection is conducted. This contributes significantly to shorter starting times, to faster smooth engine operation, to improved throttle response, to lower white smoke emission, to less pollutant emissions and lower noise emissions.