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Cylinder Deactivation

The Magic of Variable Displacement Engines

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What is cylinder deactivation? It is a method used to create a variable displacement engine that is able to supply the full power of a large engine under high load conditions as well as the fuel economy of a small engine for cruising.

The Case for Cylinder Deactivation

In typical light load driving with large displacement engines (e.g. highway cruising), only about 30 percent of an engine’s potential power is utilized. Under these circumstances, the throttle valve is only slightly open and the engine has to work hard to draw air through it. The result is an inefficient condition known as pumping loss. In this situation, a partial vacuum occurs between the throttle valve and the combustion chamber—and some of the power that the engine makes is used not to propel the vehicle forward, but to overcome the drag on the pistons and crank from fighting to draw air through the small opening and the accompanying vacuum resistance at the throttle valve. By the time one piston cycle is complete, up to half of the potential volume of the cylinder has not received a full charge of air.

Cylinder Deactivation to the Rescue

Deactivating cylinders at light load forces the throttle valve be opened more fully to create constant power, and allows the engine to breathe easier. Better airflow reduces drag on the pistons and the associated pumping losses. The result is improved combustion chamber pressure as the piston approaches top dead center (TDC) and the spark plug is about to fire. Better combustion chamber pressure means a more potent and efficient charge of power is unleashed on the pistons as they thrust downward and rotate the crankshaft. The net result? Improved highway and cruising fuel mileage.

How Does it All Work?

In a nutshell, cylinder deactivation is simply keeping the intake and exhaust valves closed through all cycles for a particular set of cylinders in the engine. Depending on the design of the engine, valve actuation is controlled by one of two common methods:

  • For pushrod designs—when cylinder deactivation is called for—the hydraulic valve lifters are collapsed by using 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.

  • For overhead cam designs, generally a pair of locked-together rocker arms is 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.

By forcing the engine valves to remain closed, an effective “spring” of air is created inside the deactivated cylinders. Trapped exhaust gasses (from previous cycles before the cylinders were deactivated) are compressed as the pistons travel on their upstroke and then decompressed and push back on the pistons as they return on their down stroke. Because the deactivated cylinders are out of phase, (some pistons traveling up while others are traveling down), the overall effect is equalized. The pistons are actually just going along for the ride.

To complete the process, fuel delivery for each deactivated cylinder is cut-off by electronically disabling the appropriate fuel injection nozzles. The transition between normal operation and deactivation is smoothed by subtle changes in ignition and camshaft timing as well as throttle position all managed by sophisticated electronic control systems. In a well-designed and executed system, the switching back-and-forth between both modes is seamless—you really don’t feel any difference and have to consult the dash gauges to know that it's happened.

Read more about cylinder deactivation at work in our review of the GMC Sierra SLT flex-fuel, and see the instant fuel economy it generates in the GMC Sierra test drive photo gallery.

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