Cylinder deactivation technology is to play a major part in the Volkswagen Group’s campaign for lower fuel consumption and emissions, without detracting from performance, throttle response, and driveability.
Also known as cylinder-on-demand (COD), the technology is incorporated into a new 4.0-L turbocharged direct fuel injection (TFSI) V8 that will power 2012 Audi S6, S7, and S8 models, as well as Bentley’s GT and GTC. And COD technology also will be used by VW on some 2012 models, in what the company claims to be a “world first” application in a 4-cylinder turbocharged, direct-injected, volume-production engine.
A low-load solution
The new V8 incorporates or is linked to a raft of other new technologies, including active engine mounts and active noise control. It will be built in two versions for the Audi cars, with outputs of 309 kW (414 hp) and 382 kW (512 hp). Their respective maximum torque ratings are 550 N·m (405 lb·ft) and 650 N·m (479 lb·ft). Figures for its Bentley installation have not yet been released.
Bentley’s powertrain engineering team was involved in the development of the new COD system. Bentley has used similar COD technology in the 6.75-L V8 Mulsanne—which delivers 1000 N·m (737 lb·ft)—since 2009. Bentley and Audi have very close technology cross-linking.
The engine will be available from early next year. It uses two, twin-scroll turbochargers positioned in the engine’s V—the exhaust ports are positioned on the inside of the cylinder heads, facing the V, and the intake ports are on the outside, similar to the setup used by BMW. Packaging has been a design priority for the engine which is 497-mm (19.5-in) long.
In the Audi application, the COD is claimed to reduce fuel consumption by about 5% and emissions by about 10-12 g/km. Combined with engine stop-start, the system potentially provides an approximate 24 g/km reduction in emissions per vehicle.
The COD system is triggered at low loads, low-to-moderate engine speeds, and when the transmission is in third gear or higher (it will be linked to both 7- and 8-speed transmissions). It closes the inlet and exhaust valves (via sliding sleeves on the camshafts) of cylinders 2, 3, 5, and 8. Immediately before this, the combustion chambers fill with fresh air; fuel injection and ignition cease. Efficiency is then higher in the remaining active cylinders as operating points are displaced toward higher loads, according to Audi engineers.
At road speeds of 80 kph (50 mph), the claimed efficiency gain is 12%. It increases to about 7% at 130 kph (81 mph). Reduction in fuel consumption is achieved after three seconds or more.
COD may be inhibited if control logic monitoring the vehicle's accelerator, brake pedal, and/or steering input detects an irregular pattern of operation. In such situations fuel consumption may increase.
The driver is kept informed of engine status; a firm push on the accelerator pedal reactivates combustion in all cylinders in 300 ms.
Upper limit for COD initiation is from 25% to 40% of maximum torque. Coolant temperature needs to be 30ºC or above with at least third gear engaged and engine speed at 960–3500 rpm. V4 mode firing order is 1-4-6-7.
In its application on VW's 1.4-L inline four, the system deactivates cylinders 2 and 3, which reduces fuel consumption by 0.4 L/100 km in the NEDC driving cycle. When cylinder deactivation is coupled with a stop-start system, fuel consumption is reduced by as much as 0.6 L/100 km—sufficient for meeting EU6 emissions requirements. Under some conditions, fuel consumption is reduced by 1.0 L/100 km or more.
Cylinder shutoff, as VW terms it, is available to the car’s management systems from 1400 to 4000 rpm and torque levels of 25 to 75N·m (18-55 lb·ft), which translates to around 70% of the driving distance in the EU fuel economy driving cycle.
Information from an accelerator-pedal sensor is used to detect the manner in which the VW is being driven. A non-uniform pattern such as hard driving on a rural road would see all cylinders firing. The system is similar in essence to that of the V8 COD and involves complex technology. All mechanical switchover processes on the small VW are executed within a half revolution of the camshaft and within 13-36 ms, depending on engine speed.
Weight of the COD components for the VW engine is slightly more than 3 kg (6.6 lb). The 1.4-L TSI produces a claimed 103 kW (138 hp) from 1500 to 4000 rpm and a maximum 240 N·m (177 lb·ft).
With regard to the Audi and Bentley applications, high levels of transitional and operational smoothness are essential for COD acceptance. Engineers focused on attenuating the higher torsional vibrations generated by the crankshaft and reciprocating components when the engine switches to V4 mode.
Likewise, engineers noted a drumming-like exhaust noise which they admitted is difficult to suppress completely, despite use of an intelligent flap system.
An active noise control (ANC) system using destructive interference was implemented to cancel out these unwanted effects of deactivating engine cylinders. The Audi models use four small microphones in the roof lining linked to the ANC’s control unit, which are able to register the complete noise spectrum in their immediate area. The ANC system also receives information on engine speed via a crankshaft sensor. The system computes a different spatial sound image and emits a targeted cancellation sound.
A particular challenge for the COD application is the need for the ANC to react both rapidly and accurately during the transitional phase of cylinder shutoff or turn-on. It operates through the car’s sound system, regardless of the ICE mode, including switch-off.
The ANC is complemented by active engine mounts (AEMs) which build on the switchable electromagnetic mount technology already used on the A8. The mounts cancel out engine vibrations with out-of-phase counter oscillations via an electromagnetic oscillating-coil actuator. A rapid stroke is transmitted via a flexible diaphragm to the hydraulic fluid in the mounting; this absorbs oscillating movements from the engine. In the fluid, these are overlaid by the actuator movements and canceled out, company engineers explained.
Control units for the AEMs receive signals from a crankshaft sensor that are used to compute phase and frequency of the actuator signal, while accelerometers on the engine mounts supply data to determine the amplitude needed to cancel out the vibration. Both second- and fourth-order problems are addressed, according to engineers.