Cylinder Deactivation or Variable Displacement explained

Explanation

Cylinder Deactivation also called variable displacement is a feature on some internal combustion engines to improve fuel economy and reduce exhaust emissions during light usage of the engine like during cruising as only about 30% of the engine's power is being used.
Cylinder deactivation usually switches off half of the cylinders while the engine is running or an adequate amount to maintain a balance in the engine especially V engines (it may be more suitable to switch off two cylinders in a V6 engine as opposed to three).
This essentially reduces the displacement of the engine. For instance when a four liter V8 engine has half of its cylinders deactivated it operates as a two liter V4 engine.
During the deactivation the engine will not be as fuel efficient as a genuine two liter 4 cylinder engine nor will it gain twice the efficiency of the four liter V8 but there will be a noticeable improvement.
Fuel economy is improved by only using some of the engines cylinders when all of them are not required, whenever there is no stressful demands on the engine. Rapid acceleration or hill climbing will cause the engine to automatically use all the cylinders.

Similar technology to variable valve timing may be used to alter the normal operation inside the engine head to switch off the cylinders.
One alteration is by using lobes of various shapes or sizes for the desired cylinders and hydraulically or electronically adjusting the camshaft on the fly so the lobes for the deactivated cylinders miss the valves as they rotate but the other lobes continue to open the valves of the functioning cylinders.
Another way of altering the process in the engine head is to use a solenoid to hydraulically lock the rocker arms over the deactivated cylinders thereby preventing the valves from opening.

The engine management system or electronic throttle control in more advanced engines may also restrict the air/fuel supply to these cylinders if and where possible depending on the type of fuel injection and other parameters of the engine.
All of the cylinders cannot be shut down at exactly the same time but more like one or two strokes after the other (still within milliseconds). The deactivation of the cylinders must be timed to ensure that the exhaust gases remain inside.
The trapped gases creates a spring like effect on the deactivated pistons as these pistons compress the gas on every up stroke which adds some force to the piston on every down stroke as the compressed gas expands.
The cylinders to be deactivated must be carefully selected because of balancing of the engine as stated earlier and the deactivated cylinders should be moving in opposite directions to balance out each other. The force used to compress the exhaust gas in one cylinder is countered by the downward force of the other deactivated cylinder.

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