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The blacktops seem to work like this, as described on Club4AG -
1. Starting. When you crank the starter there will be VVT operation until the engine fires up, obviously to allow more air into the engine to allow an easier fire up.
2. Coolant temp. There is absolutely NO VVT operation when the coolant tempt is below 50°C except for that brief moment when you operate the starter. Reason obvious, who want to stress a cold engine.
3. Engine rpm. VVT will operate in any rpm between the range of 1500 and 7200 when the inlet manifold pressure is right. The min and max range can be a little out because I was reading from the car tacho. Trust me they are very close.
4. Engine load/inlet manifold pressure. This seems to be the single most important parameter controling the system. The VVT will NOT operate if the inlet manifold has more than about 5 inches of vacuum (can't get the exact reading because everything happen so fast. It's very close.). This is very close to zero vacuum which is atmospheric and that is about the maximum load the map sensor will read to tell the engine in an NA car. As you can figure out the throttle will usually be in the more than 3/4 position for this to happen.
5. VVT will work without the speed sensor.
Now, back to the above schematic of the VVT. It shows the second evolution of the VVT system - called VVT-i - where instead of the simple 'on' or 'off' positions of the earlier VVT system, this version can make the inlet cam retard/advance to any angle between the maximum limits, and to do this the camshaft has a position sensor on the back of the head. This means that the engine is even more flexible in it's power output than before. The latest version, VVTL-i is described on this page. It's completely different to the original VVT system, and is more like the V-TEC in operation.
There are two engines that commonly use the VVTL-i system, the 1ZZ-FE/2ZZ-GE series and the latest (in 1999 & onwards) 3SGE, as used in the sporty Altezza. The early generation 'redtop' four 3SGE's have a single inlet VVT-i and the later 'blacktop' generation four 3SGE's have dual VVT-i controllers, one on the inlet and the other on the exhaust cam, and makes 200hp from 2 litres.
So, using VVT technology, it's pretty easy to get around 100hp per litre.
Toyota has now gone to the third evolution of the VVT, and it not only alters the cam timing, but it also alters the valve lift as well. The 'old' VVT system simply can't do this, so Toyota has gone to a system much like the ....
ที่มา :
http://www.billzilla.org/vvtvtec.htm
