Editing Variable valve timing (section)

==Background theory==
The valves within an [[internal combustion engine]] are used to control the flow of the intake and [[exhaust gas]]es into and out of the [[combustion chamber]]. The timing, duration and lift of these valve events has a significant impact on engine performance. Without variable valve timing ([[variable valve lift]]), the valve timing is the same for all engine speeds and conditions, therefore compromises are necessary to achieve the desired result in intake and exhaust efficiency .  This has been described in simulations. Practical results will vary based on available ambient combustion cycle gases in a naturally aspirated system, or forced air geometry as well as fuel pulse width timing and other factors which may or may not be available on vehicles equipped with variable valve timing.<ref name="wu2007">Wu, B. (2007). A simulation-based approach for developing optimal calibrations for engines with variable valve actuation. Oil and Gas Science and Technology, 62(4), 539–553.</ref> An engine equipped with a variable valve timing actuation system is freed from this constraint, allowing performance to be improved over the engine operating range.

[[Piston engine]]s normally use [[valve]]s which are driven by [[camshaft]]s.  The cams open (''lift'') the valves for a certain amount of time (''duration'') during each intake and exhaust cycle.  The ''timing'' of the valve opening and closing, relative to the position of the crankshaft, is important.  The camshaft is driven by the crankshaft through [[timing belt (camshaft)|timing belts]], [[gear]]s or [[roller chain|chain]]s.

An engine requires large amounts of air when operating at high speeds.  However, the intake valves may close before enough air has entered each combustion chamber, reducing performance.  On the other hand, if the camshaft keeps the valves open for longer periods of time, as with a racing cam, problems start to occur at the lower engine speeds.  Opening the intake valve while the exhaust valve is still open may cause unburnt fuel to exit the engine, leading to lower engine performance and increased emissions. According to engineer David Vizard's book "Building Horsepower", when both intake & exhaust are open simultaneously, the much-higher-pressure exhaust pushes the intake-charge back, out from the cylinder, polluting the intake manifold with exhaust.

===Continuous versus discrete===
Early variable valve timing systems used discrete (stepped) adjustment. For example, one timing would be used below 3500 rpm and another used above 3500 rpm.

More advanced variable valve timing systems offer continuous (infinite) adjustment of the valve timing. Therefore, the timing can be optimized to suit all engine speeds and conditions.<ref name="wu2007"/><ref name="hong2004">Hong, H. (2004). Review and analysis of variable valve timing strategies - eight ways to approach. Proceedings of the Institution of Mechanical Engineers, Part D: Journal of Automobile Engineering, 218(10), 1179–1200. Available at [https://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.873.5451&rep=rep1&type=pdf citeseerx.ist.psu.edu] (accessed on 2022-04-29)</ref>

===Cam phasing versus variable duration===
The simplest form of VVT is ''cam-phasing'', whereby the phase angle of the [[camshaft]] is rotated forwards or backwards relative to the crankshaft. Thus the valves open and close earlier or later; however, the camshaft lift and duration cannot be altered solely with a cam-phasing system.

Achieving variable duration on a VVT system requires a complex system, such as multiple cam profiles or oscillating cams.