Less cylinders in action, more fuel economy. It's an equation that's been applied by automakers such as General Motors (the Chevrolet Corvette is a recent example), and now, German automaker Audi is getting into the act. Audi's calling the feature "cylinder on demand" (COD) and says it can boost a car's fuel economy by as much as 20 percent relative to fully-active mode.

Audi will debut the deactivation system in three of its gasoline engines: the 1.4 TFSI COD four-banger, the 4.0 TFSI COD six-cylinder mill, and the W12 COD 12-cylinder motor that's in the new Audi A8 L. In each case, half the cylinders are deactivated by cutting off fuel injection when engine speeds and loads are low, then reactivated when the pedal is mashed. As it is, the four-cylinder engine gets about 50 miles per gallon, while the eight-cylinder gets about half that, per the more lenient European standards.

GM this summer released details of the new Corvette and said that LT1 V8 model's cylinder deactivation system boosted highway fuel economy by about 2 mpg to about 30 mpg. Such a system has also been used in GM's SUVs. Check out Audi's press release below for more details on its COD.
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Efficiency on the rise: COD technology already available for three engines

Cylinder deactivation cuts fuel consumption by up to 20 percent

Used in 1.4 TFSI COD, 4.0 TFSI COD and W12 COD

Audi RS 6 Avant with 412 kW (560 hp), but just 9.8 liters per 100 kilometers (24.00 US mpg)

Audi A8 L W12 6.3 FSI quattro

Audi demonstrates its expertise in engine construction with cylinder on demand (COD) technology, which reduces fuel consumption by up to 20 percent. Three gasoline engines already include the innovative cylinder deactivation technology: the 1.4 TFSI COD, the 4.0 TFSI COD and now also the W12 COD in the new Audi A8 L.

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 dramatically reduces fuel consumption. With the RS 6 Avant and RS 7 Sportback, for instance, the saving amounts to roughly five percent in the NEDC. The 4.0 TFSI COD, which in these models produces a powerful 412 kW (560 hp), consumes just 9.8 liters of gasoline per 100 kilometers (24.00 US mpg). The 1.4 TFSI COD is powerful, producing 103 kW (140 hp), yet in the Audi A3 consumes an average of just 4.7 liters of fuel per 100 kilometers (50.05 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 percent.