Editing Variable displacement (section)

==Theory of operation==
Cylinder deactivation is used to reduce the [[fuel consumption]] and [[Motor vehicle emissions|emissions]] of an [[internal combustion engine]] during light-load operation. In typical light-load driving the driver uses only around 30 percent of an engine’s maximum power. In these conditions, the [[throttle]] valve is nearly closed, and the engine needs to work to draw air. This causes an inefficiency known as pumping loss. Some large capacity engines need to be throttled so much at light load that the cylinder pressure at [[top dead centre]] is approximately half that of a small [[4-cylinder engine]]. Low cylinder pressure results in lower [[fuel efficiency]]. The use of cylinder deactivation at light load means there are fewer cylinders drawing air from the intake [[manifold]], which works to increase its fluid (air) pressure.  Operation without variable displacement is wasteful because fuel is continuously pumped into each cylinder and combusted even though maximum performance is not required. By shutting down half of an engine's cylinders, the amount of fuel being consumed is much less.  Between reducing the pumping losses, which increases pressure in each operating cylinder, and decreasing the amount of fuel being pumped into the cylinders, [[fuel consumption]] can be reduced by 8 to 25 percent in highway conditions.<ref name="reborn">{{cite web | url=http://autospeed.drive.com.au/cms/A_2618/article.html | archive-url= https://web.archive.org/web/20110728015843/http://autospeed.com.au/cms/article.html?&A=2618 | url-status=dead | archive-date=2011-07-28 | title=Cylinder Deactivation Reborn - Part 1, Autospeed, Issue 342, Michael Knowling |date=2005-08-03}}<br />
{{Cite web 
|date=2005-08-10
|title=Cylinder Deactivation Reborn - Part 2, Autospeed, Issue 343, Michael Knowling 
|url=http://www.autospeed.com.au/cms/A_2623/article.html
|archive-url=https://web.archive.org/web/20120331050613/http://www.autospeed.com.au/cms/A_2623/article.html
|url-status=dead
|archive-date=2012-03-31
|website=Autospeed AU}}</ref><ref name="greencar">{{cite web|url=http://www.greencar.com/articles/save-gas-cylinders-smart.php|archive-url=https://web.archive.org/web/20090703050625/http://www.greencar.com/articles/save-gas-cylinders-smart.php|url-status=dead|archive-date=2009-07-03|title=Save Gas: Cylinders get Smart|website= www.greencar.com|first=Bill|last=Siuru}}</ref>

Cylinder deactivation is achieved by keeping the intake and exhaust valves closed for a particular cylinder. By keeping the intake and exhaust valves closed, it creates an "air spring" in the [[combustion chamber]] – the trapped [[exhaust gases]] (kept from the previous charge burn) are compressed during the piston’s upstroke and push down on the piston during its downstroke. The compression and decompression of the trapped exhaust gases have an equalising effect – overall, there is virtually no extra load on the engine. In the latest breed of cylinder deactivation systems, the [[engine management system]] is also used to cut fuel delivery to the disabled cylinders. The transition between normal engine operation and cylinder deactivation is also smoothed, using changes in [[ignition timing]], [[Cam (mechanism)|cam]] timing and throttle position (due to [[electronic throttle control]]). In most instances, cylinder deactivation is applied to relatively large displacement engines that are particularly inefficient at light load. In the case of a [[V12 engine|V12]], up to 6 cylinders can be disabled.<ref name="reborn" />

Two issues to overcome with all variable-displacement engines are unbalanced cooling and vibration.{{Citation needed|date=October 2013}}