VVT stands for variable valve timing, VVA for variable valve actuation.
If looked into an internal combustion engine the importance of the valve will be easily understood, as it facilitates the gas exchange process. the pistons drives the crankshaft and the crankshaft drives the camshaft, which in turn determines the opening and closing of valves. The gas exchange process is of paramount importance because it is the fuel that gets burned to give out energy.
At low RPMs the fuel is wasted and at higher loads it would be desirable if some extra charge can be provided. So it needs no genius to figure it out that if the gas exchange process can be effectively controlled a lot of improvement in fuel economy and peak power can be achieved.
For example, valve overlap is desired for race engines so that the burnt gasses maybe flushed out and the cylinder is filled with fresh charge, but valve overlap is not advisable for low loads because the engine can use internal EGR to save fuel at sub-optimal loads.
If delved deeper into the CAM of the camshaft, it comes out that there are 3 parameters which define a cam operation namely
- lift: how much the valve open (ranges between 5 to 8 mm for passenger cars)
- Phase: when the valve opens (ranges between 5 to 30 degrees before TDC)
- Duration: for how many degrees does the valve is kept open (200 to 270 degrees of crankshaft rotation.
After NISSAN rolled out its first car equipped with variable valve timing, all the companies have jumped in the bandwagon and currently all the automobile companies have developed one for their themselves namely VTEC, Vario-CAM, i-VVT.
All the famous valve timing systems would be discussed in detail in subsequent posts.