Rpm control, function

Function requirements for rpm control, general points

 Circuit 87M (engine control ON)

 Engine running

Rpm control, general

Rpm control stabilizes the idle speed and limits the motor speed to protect the engine, the drivetrain and the vehicle. The ME-SFI [ME] control unit (B70) detects the engine speed via the crankshaft Hall sensor signal (N3/10) (with direction of rotation recognition).

The ME-SFI [ME] control unit reads in the following sensors and signals for rpm limitation:

- Crankshaft Hall sensor (with direction of rotation recognition), engine rpm

- Actual value potentiometer 1 and 2 (M16/6r1, M16/6r2), throttle valve position

- Electronic Stability Program control unit (N30/4) wheel speed via the chassis CAN (CAN E)

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- Fully integrated transmission control unit for dual clutch transmission (Y3/14n4), gear range via the drive train CAN (CAN C)

The ME-SFI [ME] control unit limits the engine speed by the following measures:

- Adjustment of the opening angle of the throttle valve by the actuator motor (M16/6m1) of the throttle valve actuator (M16/6)

- Adjusting the ignition angles of the ignition coils (T1)

- Switching off of the fuel injectors (Y76) or switching off of the gas injection valves gas (Y106) over the CNG control unit (N118/2) (for rpm limitation)

- Adaptation of the injection time of the fuel injectors or the gas injection valves gas (for idle speed control)

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Function sequence for rpm control

The rpm control is described at the following points:

 Function sequence for a defective throttle valve actuator

 Function sequence for rpm limitation

 Function sequence for maximum speed limit

 Function sequence for missing variant coding

 Function sequence for idle speed control

Function sequence for a defective throttle valve actuator

In the event of a defective throttle valve actuator, the maximum engine speed is limited in idle to approx. 1400 rpm and in driving mode to approx. 1800 rpm.

Function sequence for rpm limitation

In order to protect the engine from inadmissible engine speeds during driving mode, the maximum engine speed is limited to about 6000 rpm.

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The ME-SFI [ME] control unit detects the driving mode on the "wheel speed" CAN signal, which is sent from the Electronic Stability Program control unit via the chassis CAN.

Function sequence for maximum speed limit

Upon reaching 250 km/h the opening angle of the throttle valve and the injection time of the fuel injectors are reduced by the ME-SFI [ME] control unit. These measures ensure simple maximum speed limitation through the reduction of the engine torque.

Function sequence for missing variant coding

If no variant coding exists, maximum engine speed is limited to 2400 rpm.

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Function sequence for idle speed control

The idle speed control regulates and stabilizes the idle speed depending on operating conditions and engine loads when the accelerator pedal is not actuated.

The ME-SFI [ME] control unit also reads in the following sensors and signals for idle speed control:

- Coolant temperature sensor (B11/4)

- Pressure sensor downstream of throttle valve (B28/7), engine load

- On-board electrical system battery (G1), on-board electrical system voltage (undervoltage)

- Automatic air conditioning control and operating unit (N22/7), air conditioning system status over interior CAN (CAN B) and chassis CAN

The ME-SFI [ME] control unit regulates the idle speed over the throttle valve actuator and adapts the injection time of the fuel injectors or the gas injection valves gas according to variations in the air mass.

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To assist the idle speed control, the ME-SFI [ME] control unit can adjust the ignition angles of the ignition coils by up to 20° kW (CKA) (CranK Angle) toward "retard" or 20° CKA toward "advance".

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Adjusting speed over ignition angle adjustment enables a faster response than adjusting the throttle valve.

The ME-SFI [ME] control unit regulates the idle speed in the following operating and engine load conditions:

 Warming-up phase

 Engagement of gear range "D" or "R"

 Switching on the A/C compressor (A9)

 Purge control

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Warm-up phase

In the warm-up phase, the ME-SFI [ME] control unit controls the idle speed according to coolant temperature, in order to bring the catalytic converter more quickly up to operating temperature after engine start.

Additional function requirements in the warm-up phase are:

- Coolant temperature < 40°C

- Gear range "N" or "P" engaged

The ME-SFI [ME] control unit raises the idle speed to
about 1100 to 1300 rpm for about 20 s.

After this time has elapsed, the idle speed is increased at a coolant temperature of < 70°C in line with the temperature for approx. 50 seconds by approx. 50 to 100 rpm.

Engagement of gear range "D" or "R"

When gear range "D" or "R" is selected, the idle speed is set to a lower value to reduce the tendency of the vehicle to creep.

To do this, the ME-SFI [ME] control unit evaluates the "gear range engaged" CAN signal from the fully integrated transmission control unit for dual clutch transmission.

Switching in the A/C compressor

When the engine is at idle, so that engine speed will not fall when the refrigerant compressor is engaged, the automatic air conditioning control and operating unit sends a status signal to the ME-SFI [ME] control unit before the AAC is engaged.

The ME-SFI [ME] control unit adapts the idle speed appropriately.

Purging

The idle speed control prevents any change in the engine speed due to purging.

 Diagnosis

The basic setting of the idle speed can be read out and influenced using the Xentry Diagnostics.