Everything about Variable Displacement totally explained
Variable displacement is an
automobile engine technology that allows the
engine displacement to change, by deactivating
cylinders, for improved
fuel economy. The technology is primarily used in large, multi-cylinder engines. Many automobile manufacturers have adopted this technology as of
2005, but it isn't a new concept. Most variable displacement systems work by turning off a bank of cylinders in a
V engine, but the initial systems worked differently.
Theory of operation
Cylinder deactivation is used to reduce the fuel consumption and emissions of an 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. 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. This reduces pumping losses and increases pressure in each operating cylinder. Fuel consumption can be reduced by around 20 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 gasses (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 gasses have an equalising effect – overall, there's 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.
History
The oldest engine technological predecessor for thevariable-displacement engine is the
hit and miss engine, developed in the late 19th century. These single cylinder
stationary engines had a
centrifugal governor that cut the cylinder out of operation so long as the engine was operating above a set speed, typically by holding the exhaust valve open.
Cadillac L62 V8-6-4
This technology was first applied to multiple cylinder engines during
WWII, was pioneered in 1981 on
Cadillac's ill-fated
L62 "V8-6-4" engine. The technology was made a standard feature on all Cadillac models except
Seville. Cadillac, in conjunction with
Eaton Corporation, developed the innovative V-8-6-4 system which used the industry's first ECU to switch the engine from 8- to 6- to 4-cylinder operation depending on the amount of power needed. Because Cadillac's system proved to be a failure and a four cylinder engine was used, Mitsubishi hailed their own as the world's first. The technology was later used in Mitsubishi's V6 engines. Mitsubishi's effort was also short-lived, mainly because of a lack of response from car buyers.
Variable displacement technologies
Further Information
Get more info on 'Variable Displacement'.
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