Some internal combustion engines include engine control systems that deactivate cylinders under low load situations. For example, an eight cylinder engine can be operated using four cylinders to improve fuel economy by reducing pumping losses. This process is generally referred to as displacement on demand or DOD. Operation using all of the engine cylinders is referred to as an activated mode. A deactivated mode refers to operation using less than all of the cylinders of the engine (one or more cylinders not active).
In the deactivated mode, there are less cylinders operating. As a result, there is less drive torque available to drive the vehicle driveline and accessories (e.g., alternator, coolant pump, A/C compressor). Engine efficiency, however, is increased as a result of decreased fuel consumption (i.e., no fuel supplied to the deactivated cylinders) and decreased engine pumping. Because the deactivated cylinders do not take in and compress fresh intake air, pumping losses are reduced.
A disadvantage of engine operation in the deactivated mode appears during deceleration. When decelerating with the engine operating in the activated mode, engine pumping assists vehicle deceleration. However, because engine pumping is decreased during the deactivated mode, there is less engine pumping to assist deceleration. As a result, an operator experiences a sail-on feel and must use increased braking force to decelerate the vehicle during engine operation in the deactivated mode.