Okay so we have alot of yapping about VTECs and VVTis and what not. I feel iys appropriate to actually have a better understanding of what variable valve lift and timing ACTUALLY is!.
Like i mentioned earlier, the engine is in essence a glorified air pump. It sucks in the air, compressed it, ignites and expels it. All this movement in and out of the engine is cintrolled by valves. If you keep in kind the example i gave you regarding an engine at slow speed and then speeding it up considerably, we noticed that the real worl scenario changes drastcally. As the piston speeds up, so does the intake charge(Air/Fuel mixture). Now when valve suddenly closes according to a camshaft cycle, this mixture comes to a halt. factoring in the thermodynamics, temperature effects, expansion of gases as such, in the real world as the engine speeds up and the temperatures rise, the amount of air/fuel mixture that enters the engine drops. Ideally, we would like valves to stay open longer at higer rpm to rectify this phenomenon and to get a smoother power curve all the way to the redline.
To counter this, manufacturers came up with Variable Valve lift/cam timing. In essence, no ONE camshaft suits all the powerbands of the engine, that is Bottom end, midrange and topend. So by utilizing variable valve lift and timing what we are actually experiencing is a couple of camshafts stuffed into one. Different manufacturers have different oatented machanisims, and although the principal is the same, it is concequently achieved in different using different methods..
Following are some of the mechanisms used by manufacturers.
1. VTEC/VTEC-E/i-VTEC (Honda)
The Acronym stands for Variable Valve Timing & Lift Electronic Control. The "i" stands for Intelligent here, mneaning, the ECU can actually vary the amount of lift required rather than just and "on & "off" effect.
2.VVTi/VVTL-i (Toyota/Lexus)
Stands for Variable Valve Timing with Intelligence, and Variable Valve Timing & Lift with intelligence used in most Toyota and Lexus Engines.
3.VARIOCAM/PLUS: (Porsche, Audi)
Porsche's take on Varibale cam timing.
4.VVL/CVVT(Nissan, Hyundai)
Stands for Variable Valve Lift or Continuous Variable Valve Timing used u NIssan like the SE20VE and such
5.MIVEC(Mitsubishi)
You know what the "M" stands for. Mitsubishi Innovaative Valve timinmg & lift Electronic Control.. Comparaitivly commonplace on cars such as the Lancer 1.8 GDI 4G53 the FTO and such.
- VANOS/Double VANOS (BMW)
Variable Onckenwellen Steuerung, which is used in the BMW engines and also the Double VANOS system on the new 3 Series (DOHC)
Apart from this, many other manufacturers such as Lamborghini,Ford and even ferrari use variable cam timing.