|Cylinder on demand V8. Click to enlarge.|
Audi has three gasoline engines equipped with its cylinder-on-demand (COD) technology (cylinder deactivation): the 1.4 TFSI COD, the 4.0 TFSI COD and now also the 12-cylinder W12 COD in the new Audi A8 L. (Earlier post.) COD can deliver reductions in fuel consumption of up to 20%, Audi says.
The Audi COD technology is versatile, equally suitable for use in the four-cylinder direct-injection unit, as well as the V8 TFSI and the 12-cylinder naturally aspirated unit. In all engines, it deactivates half of the cylinders at low to intermediate loads and engine speeds in the upper gears until the driver once again accelerates strongly. Efficiency in the active cylinders increases because the operating points are displaced toward higher loads.
Activation and deactivation are so smooth that they are barely noticeable. In the eight-cylinder models, active engine bearings largely compensate for vibrations and Audi Noise Cancellation (ANC) uses precise antiphase sound to reduce intrusive frequencies in the deactivated phase. These solutions are used alone or in combination, depending on the model.
In the 1.4 TFSI COD, the system accesses cylinders two and three; in the 4.0 TFSI, cylinders two, three, five and eight.
The cylinders are deactivated by switching off the ignition and fuel injection. The valves are closed by means of cam elements that are shifted on the camshaft by means of electromagnetically actuated pins. Prior to this, the combustion chambers are filled with fresh air to reduce the internal pressure in the cylinders.
The 368 kW (500 hp) W12 engine uses a special variant of the COD technology in which the valves remain open. The system first switches off the fuel injection and ignition for all cylinders of the left bank so that the engine is operating on six cylinders. Before the temperature of the catalytic converter drops too much, the left bank is fired again and the right bank deactivated. During extended deactivation phases, the two cylinder banks are alternately active and inactive.
The COD technology reduces fuel consumption. With the RS 6 Avant and RS 7 Sportback, for instance, the saving amounts to roughly 5% in the NEDC. The 4.0 TFSI COD, which in these models produces a 412 kW (560 hp), consumes 9.8 liters of gasoline per 100 kilometers (24 mpgUS).
The 1.4 TFSI COD produces 103 kW (140 hp), yet in the Audi A3 consumes an average of just 4.7 liters of fuel per 100 kilometers (50 US mpg). In the NEDC, cylinder deactivation saves roughly 0.4 liters per 100 kilometers. With a moderate driving style, this can be as high as 20%.