Displacement on demand

Active Fuel Management (formerly known as displacement on demand (DoD)) is a trademarked name for the automobile variable displacement technology from General Motors. It allows a V6 or V8 engine to "turn off" half of the cylinders under light-load conditions to improve fuel economy. Estimated performance on EPA tests show a 5.5%-7.5% improvement in fuel economy.

GM's current Active Fuel Management technology uses a solenoid to deactivate the lifters on selected cylinders of a pushrod V-layout engine.

High-powered multi-cylinder internal combustion engines may be necessary to satisfy driver demands for quick acceleration and/or heavy towing capacity, but during daily use they are generally operated at power settings of less than 25%. For example, at freeway (motorway) speeds, less than 40 hp (30 kW) are required to overcome aerodynamic drag, rolling friction and to operate accessories such as air conditioning.

However, when a gasoline (petrol) internal combustion engine is operating under less than full load, the effective compression ratio is much less than the measured compression ratio. Under light load, the throttle is not fully open, and the cylinders receive less than a full charge of air on each intake stroke. The pressure and temperature generated at combustion are therefore less than full load and the thermodynamic laws which apply to all heat engines dictate that the engine will then be operating at less than its maximum possible thermal efficiency.

Thus, a high-powered, large-displacement engine is highly inefficient and wasteful when being used for normal driving conditions. This is the motivation for cylinder deactivation, to effectively spread the work load of the engine over fewer active cylinders which then operate under higher individual loads and therefore at higher efficiency.

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