The valve train is made up of valves, rocker arms, pushrods, lifters, and the cam shaft. The valve train’s only duty is likened to that of a traffic cop. It allows air and fuel in and out of the engine at the right time. The timing is regulated by the camshaft which is synchronized to the crankshaft by a chain or belt
If your vehicle only sends out grayish/white smoke at start-up, it is possible you have a leaking valve seals. Valve seals prevent oil from above the valve from leaking into the combustion chamber. However this wear out over time, they can allow oil to seep into the combustion chamber and settle there until your start the engine again. Your oil does not leak past the valve seals while the engine is running since the seals expand with the heat of the engine and plug the leak.
A second common problem is the timing chain or belt that could slip or even break causing the camshaft to stop rotation. Bear in mind that the camshaft tells the valves when to open and if it stops spinning then the valves stop opening and closing. Thus no valve moving, no engine running.
A familiar term used when talking about timing chains and belts is interference engine. When an engine is a state of “interference engine” the pistons and valves are so close together that if the valves were to stop moving (caused by broken belt or chain) and the crankshaft kept spinning they would crash into the piston. (this is the interference) This crash affects the engine negatively, breaking valve, bending pushrods, and even cracking pistons. This is why most manufacturers recommend changing the timing chain or belt every 60,000 miles. Timing belts dry out, stretch and deteriorate over time so even if you do not have 60,000 miles on the car think about changing the belt from its 6th year.
SEE ALSO:HOW THE PISTONS WORK