Science Fair Project Encyclopedia
Displacement on Demand
Displacement on Demand is an automobile variable displacement technology from General Motors. It allows a V6 or V8 engine to "turn off" one bank of 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 Displacement on Demand technology uses a solenoid to deactivate the lifters on one bank of a pushrod vee engine.
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Background
High power multi-cylinder gasoline engines are typically 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.
In general a naturally aspirated gasoline 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 magically switched off, however, less air would be required, and the throttle held wide open, thereby reducing pumping losses and increasing overall engine thermal efficiency. This is the motivation for cylinder deactivation.
There is in fact a way to switch off cylinders, and it's not magic. Following the power stroke, the exhaust valve is prevented from opening 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. 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.
First Generation
GM's first use of Displacement on Demand was in 1981 on the Cadillac V8-6-4 engine. GM's engine controller, based on engine load, signalled electro-mechanical actuators designed, developed, and manufactured by Eaton Corp. to engage or disengage rocker arms to permit or prevent engine valve operation, thereby deactivating cylinders. The system was capable of operation with 4, 6, or 8 cylinders.
GM's design used the existing manual, cable throttle control, a new throttle-body fuel injection system, a conventional automatic transmission, and a new engine controller. Upon deactivation of engine cylinders, the system was dependent upon the driver to depress the accelerator to maintain vehicle speed. Perhaps the most troubling issue with this system was that it employed a throttle-body fuel injection system, i.e. electronic carburetor. It continuously supplied fuel to all cylinders. When a cylinder was deactivated, fuel could accumulate in the intake port until the intake valve was reactivated. At this point the accumulated fuel was dumped into the cylinder causing an over-rich mixture, poor combustion, and erratic engine output torque, ultimately resulting in drive train jerk and poor driveability. The automatic transmission used a conventional torque converter and was not equipped to dampen engine output torque variations which occurred during cylinder deactivation and reactivation. The engine controller was barely capable of processing sensor signals and valve activation/deactivation signals in real-time in a system that demands micro-second accuracy. Deactivation of a single cylinder must begin during the power stroke, i.e. during the exhaust cam's base circle. The controller must signal the rocker arm pedestal-mounted actuator to mechanically disengage the rocker arm that opens the exhaust valve. Similarly the deactivated cylinder's intake rocker arm must also be disengaged. Upon reactivation, the controller must signal the appropriate exhaust and intake valve rocker arm actuators at the correct times. Unfortunately, the engine controller was unable to properly control the timing of deactivation and reactivation signals at high engine speeds, and as a result the stored exhaust gas charge could backflow into the intake manifold causing engine backfire. All of these multiple issues, in addition to innovative, but unreliably implemented technologies applied to carburetion and engine control contributed to erratic engine behavior when deactivating and reactivating engine cylinders. Because of its unreliability and poor performance, many of these vehicles had been converted to use 8 cylinders all the time. Cadillac V8-6-4 only lasted one year, but continued on Cadillac limousines until 1984.
Cars using the Cadillac 4-6-8 engine include:
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 hydraulicly 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 International Motor 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 displacement on demand technology 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.
See also
Links
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