Editing Variable valve timing (section)

==Typical effect of timing adjustments==
{{one source|section|date=May 2013}}
{{anchor |Late intake valve closing |LIVC}}
'''Late intake valve closing (LIVC)'''
The first variation of continuous variable valve timing involves holding the intake valve open slightly longer than a traditional engine.  This results in the piston actually pushing air out of the cylinder and back into the intake manifold during the compression stroke.  The air which is expelled fills the manifold with higher pressure, and on subsequent intake strokes the air which is taken in is at a higher pressure.  Late intake valve closing has been shown to reduce pumping losses by 40% during partial load conditions, and to decrease nitric oxide ([[NOx]]) emissions by 24%.  Peak engine torque showed only a 1% decline, and hydrocarbon emissions were unchanged.<ref name="hong2004" />

{{anchor |Early intake valve closing |EIVC}}
'''Early intake valve closing (EIVC)'''
Another way to decrease the pumping losses associated with low engine speed, high vacuum conditions is by closing the intake valve earlier than normal.  This involves closing the intake valve midway through the intake stroke.  Air/fuel demands are so low at low-load conditions and the work required to fill the cylinder is relatively high, so Early intake valve closing greatly reduces pumping losses.<ref name="hong2004" /> Studies have shown early intake valve closing reduces pumping losses by 40%, and increases fuel economy by 7%.  It also reduced nitric oxide emissions by 24% at partial load conditions.  A possible downside to early intake valve closing is that it significantly lowers the temperature of the combustion chamber, which can increase hydrocarbon emissions.<ref name="hong2004" />

{{anchor |Early intake valve opening |EIVO}}
'''Early intake valve opening'''
Early intake valve opening is another variation that has significant potential to reduce emissions. In a traditional engine, a process called valve overlap is used to aid in controlling the cylinder temperature.  By opening the intake valve early, some of the inert/combusted exhaust gas will back flow out of the cylinder via the intake valve, where it cools momentarily in the intake manifold.  This inert gas then fills the cylinder in the subsequent intake stroke, which aids in controlling the temperature of the cylinder and nitric oxide emissions.  It also improves volumetric efficiency, because there is less exhaust gas to be expelled on the exhaust stroke.<ref name="hong2004" />

{{anchor |Early exhaust valve closing |EEVC|LEVC}}
'''Early/late exhaust valve closing'''
Early and late exhaust valve closing timing can be manipulated to reduce emissions.  Traditionally, the exhaust valve opens, and exhaust gas is pushed out of the cylinder and into the exhaust manifold by the piston as it travels upward.  By manipulating the timing of the exhaust valve, engineers can control how much exhaust gas is left in the cylinder. By holding the exhaust valve open slightly longer, the cylinder is emptied more and ready to be filled with a bigger air/fuel charge on the intake stroke.  By closing the valve slightly early, more exhaust gas remains in the cylinder which increases fuel efficiency.  This allows for more efficient operation under all conditions.

{{update section|date=July 2019}}