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Variable Displacement for Better MPG
If you think the days of the internal combustion engine are over, especially when it comes to high performance ones, you would be wrong. Every time it seems this familiar engine technology can’t run clean enough or deliver sufficient fuel economy to meet environmental and fuel economy standards, technology comes to the rescue. Examples abound. Computerized engine management systems and electronic fuel injection not only have allowed the internal combustion engine to meet increasingly tighter standards, but have brought us engines with longer lifetimes, less maintenance requirements, and often better performance. When is the last time your engine didn’t start because of a carburetor problem, pinged because of poor fuel quality, or required timing to be reset for high altitude driving?
The latest example that’s breathing new life into the internal combustion engine is the variable displacement technology being applied by several auto manufacturers. The concept is straightforward: The internal combustion engine is quite versatile, with the ability to supply just enough power for idling or cruising at 70 mph, and then in an instant provide large amounts of power for passing or climbing a steep hill. Unfortunately, this flexibility means that an engine must be designed to handle maximum power requirements, even though power demands are far less most of the time. This inefficient approach means that vehicles use more fuel than necessary under most driving conditions.
One answer is to continue using an engine large enough to handle all possible power needs, while allowing some of the cylinders to deactivate under specific operating conditions so a high output V-8 or V-6 operates like a four- or three-cylinder engine. Enter the modern cylinder deactivation systems that are now on American highways.
Chrysler’s version, dubbed the Multi-Displacement System (MDS), allows the 5.7-liter HEMI V-8 in the 2005 Chrysler 300C, Dodge Magnum RT, Dodge Ram, and Jeep Grand Cherokee to produce 340 horsepower and 390 lbs-ft of torque while still getting up to 17 mpg city/25 mpg highway fuel economy. Chrysler has an even more potent 6.1-liter V-8 HEMI on the way with MDS, rated at 425 horsepower and 420 lbs-ft of torque to power the 2005 Chrysler 300C SRT8. This car will have performance that surpasses musclecar-era Mopars with performance targets of 0-60 mph times in the low five-second range, blowing through the quarter-mile in just over 13 seconds. Chrysler says MDS reduces fuel use by about 20 percent.
General Motors calls its cylinder deactivation system Active Fuel Management. The first application of this technology in GM vehicles is found in the 5.3-liter Vortec V-8, available in the 2005 Chevrolet TrailBlazer EXT, GMC Envoy XL, and Envoy XUV, as well as the new Gen IV 5.3-liter V-8 (LS4) engine in the 2005 Pontiac Grand Prix GXP. According to GM, Active Fuel Management provides fuel savings of 8- to 25-percent, depending on driver and driving conditions, by switching seamlessly between V-8 and V-4 operation. Peak output for this engine is 300 horsepower and 330 lbs-ft of torque. Incidentally, this engine is the latest rendition of the legendary small block Chevy V-8 that debuted in 1955! That’s just another example of the great adaptability of internal combustion engines.
The all-new 2005 Honda Odyssey uses Variable Cylinder Management (VCM) that allows its 3.0-liter i-VTEC V-6 engine to run on either six or three cylinders. This engine, which is rated at 255 horsepower and 250 lbs-ft of torque, reportedly combines the performance of a 3.0-liter V-6 engine with the kind of fuel economy experienced with a 2.4-liter four-cylinder engine. This V-6 with VCM is also part of the Integrated Motor Assist (IMA) system in the Honda Accord Hybrid.
This not the first time cylinder deactivation has been used in passenger vehicles. General Motors offered it in the infamous V-8-6-4 engines used in 1981 Cadillacs. Depending on driving conditions, the V-8-6-4 engine ran on four, six, or eight cylinders. However, GM discontinued the trouble-prone V-8-6-4 after only one year, although it was used on Cadillac limousines through 1984. The technology was not quite ready for the market since the available computers and software of the time could not smoothly shut down the cylinders. The deactivation technology was also limited by a cable throttle and mechanically controlled transmission. We’ve come a long way, since today's engine computers are about 25 times faster, have 50 times the computing power, and 100 times the memory of the 1981 controller. Electronic throttles and electronic transmissions are now also available.
With Active Fuel Management, the powertrain control module determines load conditions from vehicle sensors and driver commands. Under light load this sophisticated, 32-bit controller automatically closes both intake and exhaust valves on alternate cylinders of each cylinder bank (for example, numbers 1, 7, 4 and 6 cylinders). The valves are reopened the instant the control module determines that vehicle speed or load requires more power. The module also controls fuel injectors, electronic spark advance, and electronic throttle for transition between V-4 and V-8 operation so quickly that engine output increases immediately. The switchover is seamless and virtually imperceptible. The engine is started on eight cylinders.
Four solenoids control the flow of engine oil to special hydraulic valve lifters on the intake and exhaust valves that are deactivated. One section of the lifter telescopes into the other section and the two sections can be either coupled or uncoupled by a locking pin. Oil pressure pulls out the pin so the lifter collapses and closes the valves. Removing pressure returns the locking pin, causing the lifter to transfer the lift of the camshaft to the rest of the valve train. When uncoupled, the lifter acts like a spring and the valve train doesn't move, stopping that cylinder from producing power. Deactivation and activation for all four cylinders occurs within one engine cycle, that is, two revolutions of the crankshaft.
Active Fuel Management was developed with assistance from the Eaton Corp., which developed the Cadillac V-8-6-4 engine. According to GM, Active Fuel Management is most easily adapted to overhead valve (OHV) engines with only two valves-per-cylinder. GM has stuck with more traditional OHV engines but has highly developed them to keep up with competition. With two valves-per-cylinder, only two actuators-per-cylinder are needed. Active Fuel Management will work with multi-valve OHC (overhead camshaft) engines, but with more complexity and at greater cost, requiring four actuators-per-cylinder and controls that must be packaged within the cylinder head assembly.
With its OHC and 32-valves, the HEMI V-8 presents a more complete application. While Active Fuel Management has been effectively added to an existing engine design, MDS was included by Chrysler in its engine design from the start. This allowed a cylinder deactivation system that is relatively simple with fewer parts, maximum reliability, and lower cost. MDS deactivates the valve lifters to keep the four valves in four cylinders closed. In addition to stopping combustion in these cylinders, energy is also saved by not pumping air through these cylinders. The four activation solenoids supplied by Saturn Electronics are located in the cylinder block. Advanced components like high-speed electronic controls with sophisticated algorithms and electronic throttle control enable the HEMI V-8 to transition from eight cylinders to four in a mere 40 milliseconds.
Variable Cylinder Management uses Honda’s i-VTEC (intelligent Variable valve Timing and lift Electronic Control) to stop the valves on three cylinders from opening. The i-VTEC engine has overhead camshafts with a pivoting cam follower riding on the camshafts. Two rocker arms on either side of the cam follower are interlocked with the cam follower, so as the follower moves the rocker arms open the valves. To deactivate valves, hydraulic oil pressure is supplied by a computer-controlled solenoid to move a pin that interlocks the rockers and cam follower. As this occurs, the cam follower is still free to move as the camshaft rotates, but the rocker arms are no longer connected to it. This pin moves back and forth, linking or unlinking the rocker arms to control valve operation. Even though parts are rotating at several hundred cycles per minute, they can be linked or unlinked in a fraction of a second to switch from six to three cylinders, or back to six cylinders again.
The VCM system stops and starts the opening of intake and exhaust valves of the three cylinders in the rear bank on this engine, based on computer analysis of throttle opening, vehicle and engine speed, and gearing. With zero valve lift, the cylinders are sealed, fuel is not injected, and pumping losses are thus reduced by as much as 65 percent.
Running a six-cylinder engine on only three cylinders represented a challenge to Honda engineers. VCM required several advanced technologies to mask the vibration inherent in three cylinder engines with their more widely-spaced power pulses. To deal with this, the "drive-by-wire" electronic throttle computer assures that power neither increases or decreases during the switchover. Also, an Active Noise Control system cancels out excessive engine noise using a microphone to detect the noise, and then generating a signal 180 degrees out of phase to cancel out the noise. These canceling sound waves are emitted from the front and rear speakers during three-cylinder operation, idling, and at-start running. The ANC system is not needed when running on all six cylinders. Finally, two active control engine mounts, one in front of the engine and another behind, are controlled by the engine computer, which uses solenoids to damp fluid movement in the mounts. During three-cylinder operation, the computer monitors changes in crankshaft rotation rpms and sends this information to the mounts, which then compress or extend an actuator to dampen the engine vibration.
Cylinder deactivation allows the internal combustion engine do what it does best – produce gobs of horsepower and torque when needed, while still providing decent fuel economy under most driving condition. It is a technology that could make a significant impact in the automotive world if implemented in a growing number of engine families in the future.
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