|Cylinder deactivation (Zylinderabschaltung, ZAS) relies upon two different profiles on the cam pieces: a conventional full profile and a zero-lift cam that is activated (right) under cylinder shut-off. Click to enlarge.|
Volkswagen will debut cylinder deactivation (cylinder shut-off) technology in the new 1.4-liter TSI turbocharged, direct-injection engine beginning in 2012. Volkswagen is the first carmaker to implement this technology in a turbocharged four-cylinder engine in large-scale production.
The primary goal of the system is to reduce significantly fuel consumption by temporarily shutting off two of the four cylinders (cylinders 2 and 3) during low to mid loads; Volkswagen says that cylinder shut-off reduces fuel consumption of the 1.4 TSI by 0.4 liter per 100 km in the NEDC driving cycle. When the Stop/Start functionality is integrated, which deactivates the engine in neutral gear, the savings effect adds up to about 0.6 liter per 100 km.
|The new 1.4 TSI outputs 103 kW (140 PS, 138 hp), with 250 N·m (184 lb-ft) of torque between 1,500 and 4,000 rpm.|
The greatest benefits of the technology are realized while driving at constant moderate speeds. At 50 km/h (31 mph), in third or fourth gear, savings amount to nearly one liter per 100 km. Even when running on two cylinders, the 1.4 TSI—with its excellent engine balance—is still very quiet and low in vibration, Volkswagen says.
Cylinder shut-off is active whenever the engine speed of the 1.4 TSI is between 1,400 and 4,000 rpm and its torque is between 25 and 75 N·m (18–55 lb-ft). This applies to nearly 70% of the driving distance in the EU fuel economy driving cycle. As soon as the driver presses the accelerator pedal sufficiently hard, cylinders 2 and 3 are reactivated unnoticed.
Under the deactivation process, the combustion chambers are filled with air—this entrapped fresh air leads to minimal cylinder pressure and therefore to lower energy consumption. Afterwards, the system closes the intake and exhaust valves of cylinders 2 and 3; engine ignition only occurs once per crankshaft revolution. The pistons of the deactivated cylinder are now dragged by the crankshaft. On the other hand, efficiency increases in the two active cylinders, because their operating points are shifted to higher loads.
The valves are closed using a complex set of actuators: on both the intake camshaft and the exhaust camshaft, there are two adjustable sleeves (cam pieces) that are placed on special tooth systems. They are responsible for the eight valves of the second and third cylinders. At the ends of each cam piece, there are two different profiles adjacent to one another: a conventional full profile and a so-called zero-lift cam. The full profiles actuate the roller cam followers, which in turn actuate the valves in four-cylinder operation; that is, they behave like very conventional cams. However, the zero-lift cams rotate over the followers—i.e. they do not actuate them—and the valve springs hold the valves shut. Engine management simultaneously shuts off fuel injection.
Spiral-shaped slots are milled in the outer surfaces of the rotating cam pieces; these slots permit shifting the sleeves a few millimeters along the shafts very quickly; when electromagnetic actuators in the valve cover get a signal from the engine controller, two integrated metal pins engage the slots from outside and move them to their end positions. Finally, the cam pieces are locked in place by spring-loaded balls. As soon as the driver presses the accelerator pedal sufficiently, cylinders 2 and 3 are reactivated.
All mechanical switch-over processes are executed within one-half camshaft revolution; they last between 13 and 36 milliseconds, depending on engine speed. These processes are smoothed by accompanying interventions in ignition and throttle valve control.
Volkswagen utilizes information from the gas pedal sensor to detect the driver’s mode of driving. If the driving exhibits a non-uniform pattern—e.g. while driving in roundabout traffic or in a sporty style on a country road—shut-off functionality is suppressed.
The components for cylinder shut-off weigh a total of just over 3 kilograms. Their actuators, the camshafts and their bearing carriers are integrated in the valve cover. Two roller bearings reduce the friction of the shafts.