Active Fuel Management (formerly known as Displacement on Demand) 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.
BACKGROUND
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 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 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 under 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.
SECOND GENERATION
In 2004, the electronics side was improved greatly with the introductions of Electronic Throttle Control, electronically controlled transmissions, and transient engine and transmission controls. In addition, computing power was vastly increased. A solenoid control valve assembly integrated into the engine valley cover contains solenoid valves that provide a pressurized oil signal to specially designed hydraulic roller lifters provided by Eaton Corp. and Delphi. These lifters disable and re-enable exhaust and intake valve operation to deactivate and reactivate engine cylinders [1]. Unlike the first generation system, only half of the cylinders can be deactivated. It is notable that the second generation system uses engine oil to hydraulically modulate engine valve function. As a result, the system is dependent upon the quality of the oil in the engine. As anti-foaming agents in engine oil are depleted, air may become entrained or dissolve in the oil, delaying the timing of hydraulic control signals. Similarly engine oil viscosity and cleanliness is a factor. Use of the incorrect oil type, i.e. SAE 20W40 instead of SAE 5W20, or the failure to change engine oil at factory recommended intervals can also significantly impair system performance.
In 2001, GM showcased the 2002 Cadillac Cien concept car, which featured Northstar XV12 engine with Displacement on Demand. Later that year, GM debuted Opel Signum concept car in Frankfurt Auto Show, which uses the global XV8 engine with displacement on demand. In 2003, GM unveiled the Cadillac Sixteen concept car at the Detroit Opera House, which featured an XV16 concept engine that can switch between 4, 8, and 16 cylinders.
On July 21, 2008, General Motors unveiled the production version of the 2010 Chevrolet Camaro. The Camaro SS with an automatic transmission features the GM L99 engine, a development of the LS3 with Active Fuel Management which allowed it to run on four cylinders during light load conditions.
Active Fuel Management | General Motors
Active Fuel Management™ is the proprietary technology for General Motors' variable displacement technology, The technology was designed and implemented to conserve fuel during driving situations that require low power demands. The process works by switching off half of the cylinders in the engine until higher demand performance (such as acceleration) reactivates the dormant cylinders. According to EPA test drives, the technology effectively improves fuel economy by 6 to 8 percent.
Active Fuel Management is typically reserved for larger [[GM] vehicles with V6 or V8 engines. In vehicles with V6 engines, the technology temporary turns the vehicle into an inline V3 engine. V8 engines are momentarily reduced to V4 performance. The technology made its debut in Cadillac's ill-received 1981 L62 V8-6-4 engine. Due to unpredictable functionality, the technology was panned until the 2005 model year, when advancements allowed for more reliable performance. Originally known as Displacement on Demand in concept cars such as the Cadillac Cien and Cadillac Sixteen, the technology was officially renamed Active Fuel Management prior to being made commercially available.
Active Fuel Management - How it Works
The concept behind the technology of Active Fuel Management is that high-powered engines are excessively inefficient when high-demand power is not necessary. In fact, it is estimated that the average V6 or V8 engine only requires 25 percent of the engine's total power settings during the majority of everyday driving. Rather than install a smaller, more efficient engine and compromise acceleration and towing capacity, GM's Active Fuel Management system effectively deactivates unnecessary cylinders as needed to improve engine efficiency.
Generally speaking, the deactivation of cylinders begins by turning off the intake and exhaust valves. This is done through a solenoid control valve assembly that is signaled via pressurized oil to activate and deactivate hydraulic roller lifters. These lifters are the mechanism that physically close and open the exhaust and intake valves. Once both valves are closed, exhaust gas remaining in the cylinders expands in one cylinder as it decompresses in another. This compression adequately maintains power during low-demand situations. To initiate more power as needed, the exhaust valve is reopened to discharge the old exhaust gas and allow in a new cycle. On V8 engines, cylinders 1, 4, 6 and 7 are shut off during this process.
Due to the extreme precision necessary to create seamless operation of an Active Fuel Management engine, considerable electronic control is required. Advancements in vehicle system computing power, engine emission controls, electronic transmissions and GM's Electronic Throttle Control all contributed to the successful integration of Active Fuel Management technology in 21st-century vehicles.
GM Vehicles with Active Fuel Management
Presently, GM manufactures four different engines with Active Fuel Management technology. These four engines are the Vortec 5.3-liter V8, Vortec MAX 6.0-liter V8, 3.9-liter V6 and 5.3-liter small-block V8. Vehicles that are available with Active Fuel Management include the Chevrolet Avalanche, Chevrolet Impala, Chevrolet Silverado, Chevrolet Suburban,Chevrolet Tahoe and Chevrolet Trailblazer.
Competitor Equivalents to Active Fuel Management
Other automotive manufacturers offer similar variable displacement technologies. Mitsubishi was the first to offer an alternative to GM's technology, with the integration of Modulated Displacement (MD) technology in their 1982 1.4-liter 4G12 straight-4 engine. Like GM's first attempt in 1981, Mitsubishi's technology was discontinued shortly thereafter. The Japanese automaker improved and reintroduced the technology in 1993 under the moniker MIVEC-MD, only to be re-shelved in 1996. Mercedes-Benzintroduced Active Cylinder Control™ for their 12-cylinder engines in 2001, but discontinued the technology in 2002.
Currently, there are two variable displacement technologies that rival GM's Active Fuel Management system. Chrysler's Multi-Displacement System™ (MDS) was introduced in 2004 and is available in vehicles that feature a 5.7-liter HEMI V8 engine. Honda's Variable Cylinder Management™ (VCM) system uses i-VTEC technology to achieve similar results.
