Cylinder deactivation systems selectively disable some of the cylinders in an internal combustion engine to improve fuel economy and reduce CO2 emissions when the full power of the engine is not required. When power requirements from the engine are low, the engine does not run at its peak performance level. The throttle air intake is minimal and the intake of air to the cylinders is more difficult. Not only is more force required to overcome the internal vacuum, but the cylinders do not completely fill with air. With less air in the cylinder, the combustion pressure is reduced. This situation is commonly referred to as pumping loss and can significantly reduce the efficiency of the engine.
Cylinder deactivation effectively decreases the displacement of the engine by closing the intake and exhaust valves and cutting fuel injection for a particular cylinder. The pistons in the deactivated cylinders compress the trapped gases and are pushed back down, thus expending zero net work. The remaining cylinders compensate for the loss in power due to the inactive cylinders by operating at a higher combustion pressure. As a result, for a given load on the engine, the throttle valve is more open allowing the cylinder mean effective pressure to be closer to the optimal level and increasing the efficiency of the engine.
Cylinder Deactivation History
Driven by U.S. Corporate Average Fuel Economy (CAFE) standards, manufacturers needed new ways to increase their vehicles' fuel economy. The first production cars to implement cylinder deactivation for increased fuel efficiency were from the 1981 Cadillac lineup. GM, in conjunction with Eaton Corporation, developed a new fuel management system they called "Modular Displacement" for an existing six liter pushrod V8 engine. Marketed as the "V-8-6-4," the system had the ability to change the number of cylinders operating from eight to six to four, depending on engine load. Controlling the deactivation of the cylinders was the engine control module, or ECM, which determined how many of the cylinders to shut off.
To deactivate cylinders, solenoids controlled by the ECM would move, allowing the desired cylinders' valve rocker arms to disengage from their respective pushrods. By separating from the pushrods, valves for those cylinders no longer received mechanical lift provided by the camshaft, and thus would remain closed. Fueling on the V-8-6-4 was achieved via an early means of electronic fuel injection known as "throttle body injection," where fuel is supplied to all cylinders from a single point in the throttle body. When cylinders were deactivated, the amount of fuel injected was reduced by the ECM, but cutting fuel to specific cylinders was not possible with this system. The system is generally said to have suffered from drivability problems attributed to the lack of computing power of the ECM and the lack of precise fuel control offered by throttle body injection.
Soon after, Mitsubishi implemented a version of cylinder deactivation, known as "Modulated Displacement." Based on the same principles as Cadillac's design, Mitsubishi designed a four cylinder engine that was capable of deactivating two of its cylinders. Neither design was well received by the public and cylinder deactivation remained unpopular for a number of years.
Cylinder Deactivation Today
Today, companies including Mercedes-Benz, Chrysler Group, General Motors (GM), Honda, and Volkswagen have brought back the idea of cylinder deactivation, along with new ideas that include variable valve timing and variable compression.
Engines today operate with one of two camshaft configurations: the pushrod design or the overhead cam design. For both designs, the cylinder deactivation system closes the intake and exhaust valves and stops injecting fuel into the deactivated cylinders. Control over all components of cylinder deactivation comes from the ECM. The ECM acquires information from several sensors to decide when to initiate cylinder deactivation. Deciding factors are usually vehicle speed, engine speed, engine load, and throttle position.
Chrysler and GM currently employ cylinder deactivation in select V6 and V8 pushrod engines. Both companies use a special type of hydraulic lifter to pause intake and exhaust valve actuation in the deactivated cylinders. This prevents the lifting motion generated by the rotating camshaft from being translated to selected pushrods and their respective valves.
Hydraulic lifters can be thought of as a plunger in a cylinder; the cylinder contacts and follows the up-and-down motion of the cam lobe. The plunger, which sits inside the top of the cylinder (or lifter body), transfers the lift from the camshaft to the pushrod and onwards to the valve. The plunger usually doesn't move within the lifter body because the body is filled with oil.
In the special hydraulic lifters used for cylinder deactivation, the plunger is allowed to collapse into the lifter body, thus preventing the lifting motion of the cam lobe from reaching the valves. This is achieved when high pressure oil is allowed to flow into the lifter body, which disengages a locking pin and allows the plunger to collapse. The high pressure oil supply is turned on and off via an electronic solenoid that is controlled by the ECM. Valve springs will keep the valves for a cylinder shut, so long as no force from the camshaft is pushing the valves open.
Overhead Cam (OHC) Engines
Mercedes, Honda, and Volkswagen have all recently used cylinder deactivation in a wide range of OHC engines, from four cylinders to V12s.
Mercedes and Honda use similar systems. Like those systems deployed in Chrysler and GM pushrod engines, Mercedes' and Honda's systems rely on solenoids that control high pressure oil flow according to an ECM signal. Unlike the pushrod systems, the OHC systems from these two manufacturers utilize special rocker arms to control the motion of valves in deactivated cylinders. For each valve in a cylinder that can be deactivated, there are two separate rocker arms that sit directly next to each other and share the same fulcrum. One of these rockers is in constant contact with the camshaft, just like a normal rocker. The second rocker is in constant contact with the valve.
Newer models of Honda's all-aluminum SOHC 60-degree V6 engine employ two intake valve lobes enabling Honda's i-VTEC technology at both low and high RPMs. A third zero-lift lobe attached only to the rear bank, allows these cylinders' intake valves to be closed, effectively deactivating them.
During normal operation, the movement of the two rockers is locked together by a pin, so that the lift from the camshaft influencing the first rocker is translated to the second rocker and thereby opens the valve. In cylinder-deactivation mode, the ECM energizes the electronic solenoids, which allows high pressure oil to disengage the pin locking the two rocker arms together. The result is that the two arms now move independently of each other: the first rocker continues following the cam, but this motion is not passed on to the second rocker, and thus the valve does not move, being held shut under the pressure of the valve spring.
Volkswagen utilizes a different system altogether. They use a special, multi-piece camshaft design that uses short sections which fit like sleeves over the main shaft. On these sleeves, there are two different lobe profiles to be used by one valve, as well as a spiral-shaped slot which is cut into the sleeve. When cylinders are to be deactivated, a pin lowers from an electromagnetic actuator that is mounted above the camshaft, which fits into the spiral groove on the rotating cam sleeve. The sleeve follows the pin along the path of the spiral groove, causing the sleeve to shift left or right.
During normal operation, the valve rocker will follow one of the lobe profiles on the sleeve, which is shaped like a normal cam lobe. When the sleeve is shifted axially upon entering cylinder deactivation mode, the valve rocker begins following the second lobe profile, which is not a cam-shape at all, but instead is completely round - a "zero-lift" lobe. The rocker stays in continuous contact with the cam sleeve, but is now following a lobe that provides no lift; thus, the respective valve remains closed.
The U.S. Dept. of Energy estimates that cylinder deactivation systems improve fuel efficiency by about 7.5%, but manufacturers currently offer systems claiming efficiency improvements as high as 20%. Cars with a cylinder deactivation option include the Honda Odyssey, Accord, and Pilot; Volkswagen Polo BlueGT; Lamborghini Aventador LP 700-4; Audi A1, A3 and A8 L; as well as the Chevrolet (GM) Impala, Suburban, Silverado, Tahoe, Caprice, Camaro, and Corvette Stingray.
Cylinder Deactivation Tomorrow
A company called Tula Technologies has developed a technology they call Dynamic Skip Fire. Instead of switching off a fixed set of cylinders, this technology constantly monitors the torque demands on the engine and determines whether or not to fire each cylinder at each firing opportunity. General Motors is a major investor in Tula Technologies and has indicated that this technology could appear in future models.