Camshaft Adjustment, Function - GF05.20-S-0003TRJ
Engine 274 in model 447.6/7
with code XM0 (Facelift)
Function requirements for camshaft adjustment, general points
- Circuit 87M ON (Engine management ON)
- Engine running.
Camshaft adjustment, general points
With the camshaft adjustment, the two camshafts can be adjusted in a continuously variable manner by an up to 40° crank angle (CKA). This means the valve overlap in the event of a gas exchange can be varied within wide limits.
This optimizes engine torque characteristics and improves the exhaust characteristics.
Valve overlap
The intake valves open before the exhaust valves close.
For camshaft adjustment, the ME-SFI [ME] control unit (N3/34) reads in the following signals over direct line:
- Intake camshaft Hall sensor (B6/29)
- Exhaust camshaft Hall sensor (B6/30)
- Coolant temperature sensor (B11/21)
- Intake manifold pressure sensor (B28/24)
- Crankshaft Hall sensor (B70/2)
Function sequence for camshaft adjustment
The function sequence is described in the following steps:
- Function sequence for release of the camshaft adjustment
- Function sequence for oil pressure
- Function sequence for camshaft adjustment
- Function sequence for adjustment range
- Function sequence for start position
- Function sequence for valve overlap
- Function sequence for camshaft positions monitoring
- Function sequence: diagnosis
Adusting the intake camshaft (3)
Function sequence for release of the camshaft adjustment
Camshaft adjustment is enabled by the ME-SFI control unit dependent on engine speed and engine oil temperature.
The ME-SFI [ME] control unit determines the engine oil temperature based on different operating data (e.g. coolant temperature, time, engine load) and a saved temperature model.
The temperature and thus the engine oil viscosity is taken into account in order to provide sufficient oil pressure (> 1.5 bar) for the camshaft adjustment also in the case of hot engine oil.
Enable of adjustment of the exhaust camshaft relative to the intake camshaft only a higher rotational speed compared to. In this way it is ensured that the locking position is reached against the "retarded" reaction torques of the exhaust camshaft, even at a low oil pressure level. A return spring (1/3) is located in each camshaft positioner (2) for support.
If both camshafts are adjusted, adjustment of the exhaust camshaft takes place after a delay (later). Oil supply problems are prevented and secure functioning of the locking mechanism achieved.
Release of the camshaft adjustment occurs load-dependent:
- For an engine oil temperature of 80°C from approx. 600 rpm
- For an engine oil temperature (inlet side) of 120°C from approx. 800 rpm
- For an engine oil temperature (exhaust side) of 120°C from approx. 1050 rpm
Function sequence for oil pressure
The engine oil pressure is regulated via the engine oil pump valve (Y138) in order to ensure an adequate oil supply and to lower the oil pressure if necessary (a saving on fuel).
Remove camshaft positioner (2) for intake camshaft (3)
Function sequence for camshaft adjustment
The intake camshaft solenoid (Y49/10) and the exhaust camshaft solenoid (Y49/11) are actuated by the ME-SFI [ME] control unit using a pulse width modulated signal (PWM signal).
The position of the control plunger (1/2) and therefore the oil filling capacities for the camshaft positioner (2) are controlled over the characteristics dependent duty cycle.
In the camshaft positioners (2) the vane-cell piston (2/4) which is firmly connected to the camshaft is turned by the oil pressure.
Function sequence for adjustment range
- Intake camshaft (3): 4° CKA before top dead center (TDC) up to a 36° CKA after top dead center (intake opens)
- Exhaust camshaft: 25° CKA BTDC up to 15° CKA after TDC (exhaust closes)
Function sequence for start position
- Intake camshaft (3): 36° CKA after top dead center (intake opens)
- Exhaust camshaft: 25° CKA BTDC (exhaust closes)
- The camshafts are locked by the catch bolt (2/1) in a fixed position for the start. This start position is unlocked for the first adjustment request hydraulically.
Oil flows in the camshaft positioner (2) of intake camshaft (3)
The upper part of the picture shows filling of the oil reservoir for timing retard (A), open the oil reservoir for timing advance (B).
The upper part of the picture shows filling of the oil reservoir for timing advance (B), open the oil reservoir for timing retard (A).
Function sequence for valve overlap
At low engine speed and load, the ME-SFI control unit sets a large valve overlap in order to produce internal exhaust gas recirculation. Less fresh air is admitted, as exhaust gases with low oxygen content are still present in the cylinders. The combustion temperature thus decreases, the formation of nitrogen oxides (NOx) is reduced.
The intake air mass is reduced by the amount of exhaust gases present. The ME-SFI control unit shortens the injection period accordingly.
The smallest valve overlap for gas exchange occurs if the exhaust camshaft is adjusted to the maximum before TDC (advanced) and the intake camshaft (3) to the maximum after TDC (retarded). The resulting increased fresh air content produces more engine torque and engine power.
Function sequence for camshaft positions monitoring
The camshaft positions are detected by the intake camshaft Hall sensor and the exhaust camshaft Hall sensor through the position of the pulse wheel and monitored by the ME-SFI [ME] control unit.
Function sequence: diagnosis
During diagnosis of the camshaft adjustment, the ME-SFI [ME] control unit checks whether the camshafts are in start position at engine start and whether the requested adjustment has been reached after the engine has been running for a short time. Output stage errors in the camshaft solenoids (integrated with the ME-SFI [ME] control unit) and defective camshaft Hall sensors are also detected.
| Function schematic for camshaft adjustment | PE05.20-S-2500-97TRH | ||
| Overview of system components for gasoline injection and ignition system with direct injection | GF07.70-S-9998TRJ |