Active Fuel Management

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. EPA tests show a 6% to 8% improvement in fuel economy, but real-world highway use promises even larger gains.

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


Background

In the U.S., high-powered multi-cylinder internal combustion engines are perceived to be necessary to satisfy driver demands for quick acceleration, oversized vehicles 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. Thus, a high-powered, large-displacement engine is highly inefficient and wasteful when being used for normal driving conditions- the vast majority of the time.

In general a Naturally-aspirated engine provides maximum power when the engine throttle is held wide open. When less power is needed, the throttle is mostly closed. As such the engine has to work to simply draw air through the throttle. The work that's done is called a "pumping loss". If some of the cylinders could be switched off, however, less air would be required, and the throttle held further open, thereby reducing pumping losses and increasing overall engine thermal efficiency. This is the motivation for cylinder deactivation.

In order to deactivate a cylinder, the exhaust valve is prevented from opening after the power stroke and the exhaust gas charge is retained in the cylinder and compressed during the exhaust stroke. Following the exhaust stroke, the intake valve is prevented from opening. The exhaust gas in the cylinder is expanded and compressed over and over again and acts like a gas spring. As multiple cylinders are shut off at a time (cylinders 1, 4, 6 and 7 for a V8), the power required for compression of the exhaust gas in one cylinder is countered by the decompression of retained exhaust gas in another. When more power is called for, the exhaust valve is reactivated and the old exhaust gas expelled during the exhaust stroke. The intake valve is likewise reactivated and normal engine operation is resumed. The net effect of cylinder deactivation is an improvement in fuel economy and likewise a reduction in exhaust emissions. General Motors was the first to modify existing, production engines to enable cylinder deactivation.


Second generation

The electronics side was improved greatly with the introductions of Electronic Throttle Control, electronically controlled transmissions, transient engine and transmission controls, engine emissions controls, and vastly increased computing power. 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. 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 Signum² 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 April 8, 2003, General Motors announced this technology (now called Active Fuel Management) to be commercially available on 2005 GMC Envoy XL, Envoy XUV and Chevrolet TrailBlazer EXT using optional Vortec 5300 V8 engine. GM also planned to extend the technology on new High Value LZ8 V6 engine in some 2006 mid-size passenger cars. In both designs, half of the cylinders can be switched off under light loads.