Variable displacement

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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 is not 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.

1 Theory of operation
2 History
- 2.1 Cadillac L62 V8-6-4
- 2.2 Mitsubishi MD
3 Present
4 Variable displacement technologies
5 See also
6 References

[edit] 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.^[1]

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.^[1]

Two issues to overcome with all variable displacement systems is the unbalanced cooling and vibration of variable-displacement engines.^[2]

[edit] History

The oldest engine technological predecessor for the variable-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.

[edit] Cadillac L62 V8-6-4

This technology was first applied to multiple cylinder engines during WWII,^[3] 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.^[3] The original multi-displacement system turned off opposite pairs of cylinders, allowing the engine to have three different configurations and displacements. But the system was troublesome, a rash of unpredictable failures led to the technology being quickly retired.^[3]

[edit] Mitsubishi MD

One year later, in 1982 Mitsubishi developed its own variable displacement in the form of MD (Modulated Displacement) which proved that the technology, first used in Mitsubishi's 1.4 L 4G12 straight-4 engine, can function successfully. ^[4] 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.^[5] The technology was later used in Mitsubishi's V6 engines.^[6] Mitsubishi's effort was also short-lived, mainly because of a lack of response from car buyers.^[2]

In 1993, a year after Mitsubishi developed its own variable valve timing technology, the MIVEC-MD variant was introduced. The revived MD technology was now in its second generation with improved electronic engine controls enabling the switch from 4 to 2 cylinders to be made almost imperceptibly. In MD mode, the MIVEC engine utilizes only two of its four cylinders, which reduces significantly the energy wasted due to pumping losses. In addition, power loss due to engine friction is also reduced.^[5] Depending on conditions, the MIVEC-MD system can reduce fuel consumption by 10–20 percent.^[6] Modulated Displacement was dropped around 1996.^[6]

[edit] Present

No automaker attempted the same trick again until Mercedes-Benz experimented with their Multi-Displacement System V12 in the late 1990s. It was not widely deployed until the 2004 DaimlerChrysler Hemi. Other systems appeared in 2005 from GM (Active Fuel Management in the Generation IV small-block) and Honda (Variable Cylinder Management on the J family engines). Honda's system works by deactivating a bank of cylinders, while the Chrysler Hemi shuts off every other cylinder in the firing order.

The Hefley Engine Company is currently developing a unique variable displacement engine that has the ability to change compression ratios and can run on regular gasoline at low compression, then change to diesel where high compression is required.^[7]