First, a few assumptions:
Default or "failsafe" position, with the solenoids de-energized, is caused by oil pressure supplied to the small side of the intake piston (retard) and the big side of the exhaust piston (advance). This is diagrammed in yellow in the MS43 PDF.
With the solenoids constantly energized, "full advance of intake/retard of exhaust (Full A/R)" is caused by oil pressure supplied to the big side of the intake piston (advance) and the small side of the exhaust piston (retard). This is diagrammed in red in the MS43 PDF.
But that whole red/yellow diagram is mislabeled; if you take the labeling on the left side of it to be totally correct, the VANOS does nothing.
Whether moving from default position or optimum position at a given RPM due to changing conditions/RPM, the basic actions of the solenoids are the same. Intake camshaft advance is via intake solenoid energization, which diverts oil pressure to the big side of the intake piston, and moves the piston towards the engine. Retard is achieved by de-energization of the intake solenoid, which diverts oil pressure to the small side of the intake piston, and moves the piston away from the engine.
The exhaust piston moves in the exact opposite way: energization of the exhaust solenoid diverts oil pressure to the small side, which retards the exhaust cam timing. De-energization of the exhaust solenoid diverts oil pressure to the big side, which advances the exhaust cam timing.
The tricky part then is how the pistons are held in positions that do not correspond to the two extremes (default/failsafe, or Full A/R).
For the intake piston, it's simpler: F "big side" = F "small side" + F "axial", forces cancel so there is no movement.
This is maintained by modulating the solenoid (switching the solenoid on and off really fast). It can (apparently) span a range of frequencies from approx. 100 to 220 Hz. I surmise that this is due to the change in oil pressure (read: change in force on each side of the piston from the oil pressure), and also the change in axial force on the piston due to changes in engine RPM.
Earlier, when oil pressure was only diverted to the big side, there was no F "small side" (no oil pressure to that side). The piston moves towards the engine because F "big side" > F "axial". Conversely, the piston moves away from the engine when oil pressure was diverted only to the small side, because there was no F "big side" pressure -- that is, F "small side" + F "axial" > 0.
If input pressure is constant at a given RPM, the side with the larger area experiences the larger force. Therefore, F "big side" is always greater than F "small side," when the same oil pressure is applied to each side.
Pressure = Force / Area, so Force = Pressure * Area. This is best illustrated in Bluebee's post showing a simplified hydraulic lift. This allows a simple "on-off-on-off-on-off" switching pattern for the intake solenoid, to maintain piston position. The small side is always "helped out" by the axial force at a given RPM.
For the exhaust piston, it's made more complicated by the spring. The signal to maintain exhaust camshaft position may be more like "on-on-off-on-on-off" etc. Or perhaps there are so few RPMs where the exhaust camshaft has to be anything but fully advanced, and are also very transient, that it always just returns to its default fully-advanced position. I have no idea.
I concede that this is very simplified, and doesn't even consider the total sum of forces on the pistons (like the teflon ring sliding on the walls of the VANOS cylinder, although the coefficient of friction of teflon is remarkably small).
EDIT: I included two images showing what I mean by the differences in surface area between the two sides of the piston. Please excuse their crudity. The black part is the surface area.
- Input oil pressure to the VANOS is constant at a given engine RPM.
- Input oil pressure increases with increasing engine rpm.
- The two sides of the VANOS pistons have different surface areas.
Area, "big" side = pi*(radius, "big" side)^2 --> this is a disk.
Area, "small" side = pi*(radius, "big" side)^2 - pi*(radius, "narrow part")^2 --> this is a "donut," or a disk with a hole in it. Oil does not leak "backwards" past the "narrow part" into the top of the cylinder head because of a second set of seals.
- The VANOS solenoid has two positions (de-energized/energized). This corresponds to two different valve positions. For the intake solenoid valve:
Oil outputs to the small side AND the exhaust solenoid valve (de-energized) --> retard.
OR
Oil outputs to the big side AND the exhaust solenoid valve (energized) --> advance.
- For the exhaust solenoid valve:
Oil outputs to the big side AND the "vent" (de-energized) --> advance
OR
Oil outputs to the small side AND the "vent" (energized) --> retard
- There is a constant force on the exhaust piston towards the engine from a spring. Iff all other forces on the piston were removed, the exhaust camshaft would be held in the fully-advanced position. This force increases with greater displacement (compression) of the spring.
- There is a constant axial force (thrust) on both pistons away from the engine, at a given RPM. Iff all other forces on the piston were removed, both camshafts would be held in the fully-retarded position. This force increases with engine RPM.
Default or "failsafe" position, with the solenoids de-energized, is caused by oil pressure supplied to the small side of the intake piston (retard) and the big side of the exhaust piston (advance). This is diagrammed in yellow in the MS43 PDF.
With the solenoids constantly energized, "full advance of intake/retard of exhaust (Full A/R)" is caused by oil pressure supplied to the big side of the intake piston (advance) and the small side of the exhaust piston (retard). This is diagrammed in red in the MS43 PDF.
But that whole red/yellow diagram is mislabeled; if you take the labeling on the left side of it to be totally correct, the VANOS does nothing.
Whether moving from default position or optimum position at a given RPM due to changing conditions/RPM, the basic actions of the solenoids are the same. Intake camshaft advance is via intake solenoid energization, which diverts oil pressure to the big side of the intake piston, and moves the piston towards the engine. Retard is achieved by de-energization of the intake solenoid, which diverts oil pressure to the small side of the intake piston, and moves the piston away from the engine.
The exhaust piston moves in the exact opposite way: energization of the exhaust solenoid diverts oil pressure to the small side, which retards the exhaust cam timing. De-energization of the exhaust solenoid diverts oil pressure to the big side, which advances the exhaust cam timing.
The tricky part then is how the pistons are held in positions that do not correspond to the two extremes (default/failsafe, or Full A/R).
For the intake piston, it's simpler: F "big side" = F "small side" + F "axial", forces cancel so there is no movement.
This is maintained by modulating the solenoid (switching the solenoid on and off really fast). It can (apparently) span a range of frequencies from approx. 100 to 220 Hz. I surmise that this is due to the change in oil pressure (read: change in force on each side of the piston from the oil pressure), and also the change in axial force on the piston due to changes in engine RPM.
Earlier, when oil pressure was only diverted to the big side, there was no F "small side" (no oil pressure to that side). The piston moves towards the engine because F "big side" > F "axial". Conversely, the piston moves away from the engine when oil pressure was diverted only to the small side, because there was no F "big side" pressure -- that is, F "small side" + F "axial" > 0.
If input pressure is constant at a given RPM, the side with the larger area experiences the larger force. Therefore, F "big side" is always greater than F "small side," when the same oil pressure is applied to each side.
Pressure = Force / Area, so Force = Pressure * Area. This is best illustrated in Bluebee's post showing a simplified hydraulic lift. This allows a simple "on-off-on-off-on-off" switching pattern for the intake solenoid, to maintain piston position. The small side is always "helped out" by the axial force at a given RPM.
For the exhaust piston, it's made more complicated by the spring. The signal to maintain exhaust camshaft position may be more like "on-on-off-on-on-off" etc. Or perhaps there are so few RPMs where the exhaust camshaft has to be anything but fully advanced, and are also very transient, that it always just returns to its default fully-advanced position. I have no idea.
I concede that this is very simplified, and doesn't even consider the total sum of forces on the pistons (like the teflon ring sliding on the walls of the VANOS cylinder, although the coefficient of friction of teflon is remarkably small).
EDIT: I included two images showing what I mean by the differences in surface area between the two sides of the piston. Please excuse their crudity. The black part is the surface area.
