Variable displacement engine

Variable displacement is an automobile engine technology that allows the engine displacement to change, usually by deactivating cylinders, for improved fuel economy. The technology is primarily used in large, multi-cylinder engines. Many automobile manufacturers have adopted this technology as of 2005, although the concept has existed for some time prior.
Cylinder deactivation is used to reduce the fuel consumption and emissions of an internal combustion engine during light-load operation. In typical light-load driving the driver uses only around 30 percent of an engine’s maximum power. In these conditions, the throttle valve is nearly closed, and the engine needs to work to draw air. This causes an inefficiency known as pumping loss. Some large capacity engines need to be throttled so much at light load that the cylinder pressure at top dead centre is approximately half that of a small 4-cylinder engine. Low cylinder pressure means low fuel efficiency. The use of cylinder deactivation at light load means there are fewer cylinders drawing air from the intake manifold, which works to increase its fluid (air) pressure. Operation without variable displacement is wasteful because fuel is continuously pumped into each cylinder and combusted even though maximum performance is not required. By shutting down half of an engine's cylinders, the amount of fuel being consumed is much less. Between reducing the pumping losses, which increases pressure in each operating cylinder, and decreasing the amount of fuel being pumped into the cylinders, fuel consumption can be reduced by 8 to 25 percent in highway conditions. Cylinder deactivation is achieved by keeping the intake and exhaust valves closed for a particular cylinder. By keeping the intake and exhaust valves closed, it creates an "air spring" in the combustion chamber – the trapped exhaust gases (kept from the previous charge burn) are compressed during the piston’s upstroke and push down on the piston during its downstroke. The compression and decompression of the trapped exhaust gases have an equalising effect – overall, there is virtually no extra load on the engine. In the latest breed of cylinder deactivation systems, the engine management system is also used to cut fuel delivery to the disabled cylinders. The transition between normal engine operation and cylinder deactivation is also smoothed, using changes in ignition timing, cam timing and throttle position (thanks to electronic throttle control). In most instances, cylinder deactivation is applied to relatively large displacement engines that are particularly inefficient at light load. In the case of a V12, up to 6 cylinders can be disabled.

Two issues to overcome with all variable-displacement systems are the unbalanced cooling and vibration of variable-displacement engines.

No automaker attempted the same trick again until Mercedes-Benz experimented with their Multi-Displacement System V12 in the late 1990s. It was not widely deployed until the 2004 DaimlerChrysler Hemi. Other systems appeared in 2005 from GM (Active Fuel Management in the Generation IV small-block) and Honda (Variable Cylinder Management on the J family engines). Honda's system works by deactivating a bank of cylinders, while the Chrysler Hemi shuts off every other cylinder in the firing order.
There are currently two main types of cylinder deactivation used today, depending on the type of engine. The first is for the pushrod design which uses solenoids to alter the oil pressure delivered to the lifters. In their collapsed state, the lifters are unable to elevate their companion pushrods under the valve rocker arms, resulting in valves that cannot be actuated and remain closed. The second is used for overhead cam engines, and uses a pair of locked-together rocker arms that are employed for each valve. One rocker follows the cam profile, while the other actuates the valve. When a cylinder is deactivated, solenoid-controlled oil pressure releases a locking pin between the two rocker arms. While one arm still follows the camshaft, the unlocked arm remains motionless and unable to activate the valve. .

Although the attempts to use variable-displacement technology failed in the past, automakers have been able to overcome the problems that occurred using new advancements in computers. With computers this fast cylinder deactivation and reactivation occur almost instantly.
After the price of oil surged in 2008, consumers were looking for a more fuel efficient car without sacrificing peak power. This has led many manufacturers to put variable-displacement controls into their cars, especially those with V8s installed.

It is also possible to alter the engine's displacement by shortening or lengthening the stroke of the pistons, thereby changing the actual cylinder displacement, rather than simply deactivating and sealing off cylinders. There are no production vehicles that use this design, however.

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.

The solenoid control valve in Chrysler’s Multi-Displacement System allows high-pressure oil to reach the switching lifter, which stops the valves from opening.

GM’s 5.3-liter V-8 is available with Active Fuel Management in specific applications.

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.

Special lifter used in the GM Active Fuel Management System.

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.

In Active Fuel Management’s activation mode (right), hydraulic pressure dislodges the locking pin in the lifter and collapses the lifter, closing the valve. Removing pressure (left) results in the locking pin returning to its latched position so the lifter functions as normal.

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 System installed on Honda’s V-6 i-VTEC engine.

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.