2.0L Help understanding VTC variability of phasing

Charosenz

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I understand that VTC varies cam degrees (phasing) by way of varying the oil pressure that goes to the cam gear by movement of the spool valve. In other words, I understand that when the VTC spool valve is energized it moves so the oil is routed to the oil passage that puts pressure on the cam gear to rotate and advance the timing of the gear.

The part that I don't understand is how it varies the oil pressure to the cam gear, since the spool valve is either "on" or "off". (I would think that the cam gear would simply go from zero degrees when the spool valve is not energized to be fully advanced when the oil control valve receives power, but I don't believe this is true, as I know the cam gear can be advanced anywhere in between zero to the limit (25-30 degrees).

Is the spool valve is pulsed quickly (on and off several times a second) to cause a variation in the pressure of the oil that feeds the cam gear?

Or is there some other mechanism that varies the oil pressure that the cam gear receives which causes the incremental rotation that the ECU desires?
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I think it works with pressure diferential between the 2 sides of the oil chamber inside the VTC gear.
 
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Charosenz

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I think it works with pressure diferential between the 2 sides of the oil chamber inside the VTC gear.
That makes sense to me too, but with the system, partial movement of the spool valve is not possible.

The only way that would be possible if the spool valve could be controlled to move partially from the resting position where spring pressure holds it in place which opens the path to retard the gear. But it can't. Because once the valve is energized the electromagnet thrusts it forward completely, instantly, which of course opens up the passage for the oil to go to the path that pressurises the vane in the cam gear to rotate in to the advanced position. When this happens spool plunger also opens up the oil passage which allows the oil on the other side of the vane inside the cam gear to drain.

I wish there was a way to actually monitor the oil pressure in both the advance oil path and the retard oil path to see what is really happening, but that would be quite the undertaking. I have actually thought about it though. You would just have to put a 1/2" plate sandwich between the oil control valve and tap a port in to those passages with 1/4npt threads and connect a mechanical line to a guage or a pressure transducer....

Still a mystery...

Charlie
 

whymi

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Pulse width modulation, duty cycle. VERY fast "on"- "off" signals. More advance, longer "on" time. Similar to fuel pump regulation.
 
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Charosenz

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Pulse width modulation, duty cycle. VERY fast "on"- "off" signals. More advance, longer "on" time. Similar to fuel pump regulation.
I suspected that is the case, as I mentioned in my first post. I just hesitated on this because normally a pulse width duty cycle is just perceived as a reduction in voltage and usually comes from a logic level at 5v. But nonetheless, I agree with your comment - even given the spool valve is a 12 volt device.

I do not know if this is true, but just thinking, if it was cycled at say 10hz with 75% on the advance position and 25% on the retard side, then you would basically see the cam gear advanced 75% of its max potential of travel. It really is, (in my thought) the only way you could get a partial travel of the gear. Just sort of a novel way of varying the pressure....

Curious if anyone knows if there is any white paper that documents this?

Thanks!!!

Charlie
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