Variable Valve Timing In I.C. Engines
VALVE TIMING (VT) is one of the most important aspects of consideration in the design of an automobile engine. Simply defined, it is the timing, or regulation of the opening and closing of the valves. In simpler terms, it is the way an engine 'breathes'.
In an I.C.engine, usually the inlet valves open a few degrees (of crank angle) prior to TDC, and close after BDC. Similarly, the exhaust valves open a few degrees before BDC and close a few degrees after TDC. This is done to maximise:
" Intake of air/air-fuel mixture; and
" Scavenging, i.e. the exhaust of burnt gases.
Until recently, most engines around the world utilised ordinary or static VT, where the parameters of valve opening, lift, and closing (VO, VL and VC) were fixed. This was satisfactory at normal engine speeds, but posed problems at high and low speeds. Since the VT did not vary with speed, the additional requirements that arose at the extreme speeds could not be met with static VT. For example, at high speeds, the engine requires greater amounts of air. This implies that the IV should remain open for a longer period of time. This, though beneficial at high speeds, would be a menace at low speeds as it may lead to exhaust of unburnt fuel, which results in fuel wastage, increased emissions and lower performance.
This is where variable valve timing (VVT) comes into play. As the name suggests, the timing of the valves is not fixed, but varies, as per the demands of the situations. Therefore, the extra demands of the engine can be met, which in turn, results in improved engine performance.
1.1 Valve Timing
Valve timing is the regulation of the points in the combustion cycle, at which the valves are set to open and close. Since the valves require a finite period of time in which to open or close without abruptness, a slight lead-time is always necessary for proper operation. The design of the valve-operating cam provides for the smooth transition from one position to the other, while the cam setting determines the timing of the valve.
In a typical four-stroke engine, the inlet valve is set to open before TDC (top dead centre), towards the end of the exhaust stroke and close after BDC (bottom dead centre), at the start of the compression stroke.
1.2 Inlet Valve Timing
While the intake valve should open, theoretically at TDC, most engines utilise an intake valve opening, which is timed to occur a few degrees prior to the arrival of the piston at TDC on the exhaust stroke. This is because by the time the valve becomes fully open, the piston would have travelled considerably down the bore, and since the valve would have to be fully closed before BDC, the actual time the valve would be fully open would be minimal.