Variable Displacement - Theory of Operation

Theory of Operation

Cylinder deactivation is used to reduce the fuel consumption and 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 means low fuel efficiency, because at lower pressures, a combustion reaction requires a higher activation energy, using more fuel. 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.

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 timing and throttle position (thanks 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, up to 6 cylinders can be disabled.

Two issues to overcome with all variable-displacement systems are the unbalanced cooling and vibration of variable-displacement engines.