Camshaft Adjustment, Function - GF07.10-P-1016MRS
Engine 264.9 in Model 205, 253
Function requirements, general
- Engine runs
The ME-SFI [ME] control unit (N3/10) detects the Engine running via the evaluation of the crankshaft Hall sensor signals (B70). The ME-SFI [ME] control unit directly reads in the signals of the crankshaft Hall sensor.
Camshaft adjustment, general points
With the camshaft adjustment, the two camshafts can be continuously adjusted by up to 40° CKA (crank angle) to "advanced" and "retarded".
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.
The ME-SFI [ME] control unit evaluates the following signals to implement camshaft adjustment:
- Intake camshaft position
Intake camshaft Hall sensor (B6/15)
- Exhaust camshaft position
Exhaust camshaft Hall sensor (B6/16)
- Coolant temperature
Coolant temperature sensor (B11/4)
- Engine load
Pressure and temperature sensor downstream of throttle valve (B28/27)
- Engine speed
Crankshaft Hall sensor
- Engine oil level and temperature
Engine oil fill level sensor (B40/6)
The function sequence is described in the following steps:
- Release of camshaft adjustment
- Oil pressure
- Adjustment
- Adjustment range
- Start position
- Valve overlap
- Monitoring the camshaft positions
- Diagnosis
- Intake camshaft valve lift adjustment
Release of camshaft adjustment
Camshaft adjustment is enabled by the ME-SFI [ME] 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 Engine oil temperature is important for ensuring that there is sufficient oil pressure (> 1.5 bar) for adjusting the camshafts, even with hot Engine oil.
Enable of adjustment of the exhaust camshaft does not occur until a higher rotational speed compared to the intake camshaft. In this way for exhaust, also at a low oil pressure level, reaching the locking position against the "retard" operating reaction torques of the camshaft are secured. There is a return spring located for support in each camshaft positioner.
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:
- At 80 °C Engine oil temperature as from approx. 600 RPM
- At 120 °C Engine oil temperature (intake camshaft) as from approx. 800 RPM
- At 120 °C Engine oil temperature (exhaust camshaft) as from approx. 1, 050 RPM
Oil pressure
The Engine oil pressure is regulated via the Engine oil pump valve (Y130) in order to ensure an adequate oil supply and to lower the oil pressure if necessary (a saving on fuel). The ME-SFI [ME] control unit actuates the Engine oil pump valve directly.
Adjustment
The intake camshaft adjustment solenoid (Y49/1) and the exhaust camshaft adjustment solenoid (Y49/2) are actuated by the ME-SFI [ME] control unit by means of a pulse width modulated signal. The control plungers are adjusted via the characteristics map-dependent duty cycle. The oil filling quantities for the camshaft positioner is controlled according to its position. Thus, in the camshaft positioners, the vane pistons that are firmly connected to the camshafts are turned.
Adjustment range
- Intake camshaft: a 4° CKA before TDC (Top Dead Center) up to a 36° CKA after TDC (intake open)
- Exhaust camshaft: 25° CKA BTDC up to 15° CKA after TDC (exhaust closes)
Start position
- Intake camshaft: 36° CKA after TDC (intake opens)
- Exhaust camshaft: 25° CKA BTDC (exhaust closes)
The camshafts are locked in a fixed position for starting by catch bolts (locked). This start position is hydraulically unlocked when the first actuation of the adjustment solenoids takes place.
Shown is oil flows in the intake camshaft camshaft positioner
Valve overlap
At low Engine speed and load, the ME-SFI [ME] 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. This lowers the combustion temperature and reduces the formation of nitrogen oxides (NGX).
The intake air mass is reduced by the amount of exhaust gases present. The ME-SFI [ME] control unit shortens the injection period accordingly.
The smallest valve overlap during gas exchange arises if the exhaust camshaft is adjusted to the maximum BTDC (advanced) and the intake camshaft to the maximum ATDC (retarded). A higher Engine torque and a higher Engine output are achieved due to the higher fresh air content.
Monitoring the camshaft positions
The positions of the camshafts are recorded by the intake camshaft Hall sensor and by the exhaust camshaft Hall sensor. The ME-SFI [ME] control unit directly reads in the signals of the intake camshaft Hall sensor and the exhaust camshaft Hall sensor. The positions recorded through detection of the positions of the pulse wheels, which are located at the front on the camshafts.
Shown is the intake camshaft adjustment
Shown: intake camshaft camshaft positioner
Diagnosis
During camshaft adjustment diagnosis, the ME-SFI [ME] control unit checks whether the camshafts are in the start position at Engine start and whether the requested adjustment is achieved 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.
Intake camshaft valve lift adjustment
With the valve lift switchover, a two-stage switchover of the cams on the intake camshaft occurs depending on the characteristics map.
The intake valves close earlier due to the switchover of the camshaft to a shorter stroke. This leads to an optimization of the gas exchange in partial-load range. The valve lift switchover to the smaller cam stroke is performed in an RPM range of 1000 to 4000 RPM.
Shown on intake camshaft
The intake camshaft valve lift switchover actuator is actuated by the ME-SFI [ME] control unit by means of a pulse width modulated signal.
During this process, a coil in the intake camshaft valve lift switchover actuator is energized and a tappet (S) moves into a corresponding curved track (K) on the camshaft. The turning of the camshaft and the shape of the curved track (K) cause the camshaft to be moved axially and the flatter half of cam (Hf) affects the intake valves.
An elevation in the curved track (K) causes the tappet (S) to be moved back to its default position.
In order to reset the camshaft to the steeper half of cam (Hs), a second tappet (S) moves into a corresponding curved track (K) and the reset occurs in the same manner.
The position of the tappets (S) is determined via the intake camshaft valve lift adjustment Hall sensor (Y49/8b1), which transmits a signal to the ME-SFI [ME] control unit for the purpose of adjusting the valve lift.
| Electrical function schematic for camshaft adjustment | PE07.10-P-2716-97FBI | ||
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