Cylinder deactivation or variable displacement (Courtesy: Bosch)

What Is Cylinder Deactivation Technology In Engines?


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What is Cylinder Deactivation?

Manufacturers also term cylinder deactivation as an engine with variable displacement. Some internal combustion engines use this feature to improve fuel economy and reduce exhaust emissions during light usages. The engine uses only about 30% of the power while cruising. Generally, cylinder deactivation turns off half or an adequate amount of the cylinders to maintain a balance in the engine while the engine is running. It is more convenient to switch off two cylinders in a V6 engine as opposed to three. Engine runs by deactivating half of the cylinders, making them dormant in slow driving, low-load conditions.

So, basically, this action reduces the displacement of the engine. For instance, when the system deactivates half of the four cylinders, it works as a two-cylinder engine. During the deactivation, the engine won't be as fuel-efficient as a two-cylinder engine. It will also not gain twice the efficiency of the two-cylinder engine. However, there will be a noticeable improvement. Fuel economy improves by using only some of the engine’s cylinders. During part-throttle, when there is no demand for power, all of the cylinders are not required. However, the driver’s actions such as rapid acceleration or hill climbing will automatically restore all the cylinders.

Cylinder Deactivation Methods:

Furthermore, manufacturers use technology similar to variable valve timing to change the normal operation inside the cylinder head for cylinder deactivation or variable displacement. In one method, manufacturers use lobes of different shapes or sizes for the desired cylinders. The system then adjusts the camshaft hydraulically or electronically while it is in motion so that the lobes for the deactivated cylinders miss the valves as they rotate. However, the other lobes continue to open the valves of the active cylinders. In other methods, manufacturers use a solenoid to hydraulically lock the rocker arms of the deactivated cylinders, thereby preventing the valves from opening.

Cylinder deactivation or variable displacement
Cylinder deactivation or variable displacement

In some cases, manufacturers also use engine management systems or electronic throttle control to restrict the air/fuel supply for cylinder deactivation or variable displacement as and when possible. However, it depends on many parameters such as engine speed, the torque provided by the engine, vehicle speed, and the gear engaged. When the driving condition allows the cylinders to be deactivated, the ECU readies the engine to switch off. Thus, it saves fuel and reduces CO2 emissions.

Limitations:

Technically, all the cylinders cannot be shut down at the same time. However, they could be one or two strokes after each other and happens within milliseconds. Manufacturers program the cylinder deactivation in such a way that it ensures the exhaust gases remain inside. The trapped gases produce a springing effect on the deactivated pistons. The pistons compress the gas on each upward stroke. As the compressed gas expands, it adds some pressure on the piston in each downward stroke.

Manufacturers carefully choose the cylinder deactivation to maintain the balancing of the engine. Besides, the deactivated cylinders should be working on opposite sides to balance out each other. The pressure used to compress the exhaust gas in one cylinder is countered by the downward force of the other deactivated cylinder.

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