Audi's Cylinder On Demand Explained

In these days of high fuel costs, the clean air dictate and the need to be socially conscious, car makers are trying to find the next best alternative to the IC engine.  While hybrids, hydrogen and full electrics are being pursued as are many other out-of-this-world concepts, the same set of techno-heads within the OEMs are taking a re-look to make the internal combustion engine ever more smarter, cleaner, more powerful and more frugal, all at the same time.

The more obvious approach these days is all about engine downsizing which is being tackled across the board. Using a smaller turbocharged six-cylinder engine instead of using a large V8 and a smaller blown four instead of a large six are already instances one sees from very many car makers product offerings these days. However, in the large premium car segments, the debate is how small can one get and also whether that would detract from the premium-ness which a prestige saloon or a sports coupe needs to pack in its make-up? So are the V12 engines made by the Prancing Horse living on borrowed times? No not necessarily because there is a V12 hybrid in the works for the interim but the writing on the wall is clear: Ferrari will want to stick to what is in its DNA and V12 engines surely are a major component in its genetic make-up. No wonder then that the Scuderia fought tooth and nail with the FIA when they were making the new F1 rules to lobby against a four-cylinder engine for Grand Prix racing. However, even Ferrari is not dismissing downsizing entirely, CEO Amedeo Felisa stating quite matter of fact “You’ll have to wait 50 years for a four-cylinder Ferrari, but a six is more affordable.”

Ferrari may have a unique and exalted place in the automotive world but then there are others who are just as focused on performance and prestige while serving an even larger customer base who are seized of how low should one go on the engine downsizing front. Being smart is therefore the key operative here and German car maker Audi has already taken an old idea and made it perform in the real world to deliver high fuel efficiency with lower CO2 emissions without sacrificing on the power and torque appeal. Cylinder deactivation is what it is all about and as I have said so above, it is not as if this hasn’t been tried before. In fact, GM’s Cadillac division was the very first to attempt this as long back as the late 1980s when it put a production car on the market with a V8 engine which cut off two or four cylinders when there was no need for the thrust from all eight pots in the driving cycle. Sadly while brilliant in theory, the electronics in use then were not up to scratch as they are today and the fluid control needed for cylinders to light up and close left a lot to be desired.

Audi, part of the VW Group, has been pushing the pedal on technology and its engine boffins decided to reassess the deactivation theme as part of their design brief for the new 4.0-litre TFSI direct injection petrol engine. This engine which has already appeared in the new S8 sports saloon will also be used in a range of new Bentley Continentals due out early next year is a classic 90-degree V8with a long stroke motor sporting cylinder dimensions of 84.5mm x 89.0mm giving it a swept volume of 3993cc. A mention must be made here of other larger engines in Audi’s model ine-up which includes the 4.2-litre V8 FSI (372 PS and 445 Nm), the 5.2-litre V10 FSI (450PS and 540Nm), 6.0-litre W12 (450PS and 580Nm) and 6.3-litre W12 (500PS and 625Nm) and then place the new 4.0-litre V8 TFSI and one clearly sees Audi’s attempts at engine downsizing. The new V8 mill develops520PS at 5800rpm and a steady torque stream of 650Nm between an ultra-wide rev band starting from 1700 rpm and stretching all the way to 5500rpm, making it a most flexible engine for on-road performance.

Key to the flexibility I refer to is the adoption of two twin-scroll turbochargers, each working one bank of four cylinders and helping to build the torque from very low down the rev range. Audi claims that even with the tacho needle hovering at 1000rpm, the 4.0 TFSI produces around 400Nm of torque! A completely new line of thought in packaging can be seen from the twin turbochargers being located in the inside of the V of the cylinder banks along with a common intercooler. This has been made possible thanks to a new design of the cylinder head with the exhaust ports on the inside of the V and the inlets on the outside. This helps in not just making the engine a very compact unit but also helping with short gas paths which help cut down on flow losses.

And this is where the most important aspect of all large engines is addressed – pumping losses. Audi went on the same premise as had Cadillac a couple of decades earlier in that there was no need for the engine to continue making large amounts of power and torque when motoring sedately at low speeds. The idea to close down a number of cylinders, two or four helps reduce pumping losses which are clearly huge when the engine is being used on part throttle. To give an idea of what actually one means by the term pumping loss would mean going back to physics school but then it isn’t so difficult as is made out to be. The internal combustion engine, is in effect, a large pump which draws in air, combusts and then produces power, this process generating exhaust gases which have to be extracted. At part throttle, when the engine is not asked to deliver much, the intake of air with the air intakes virtually throttled is a recipe for wasted energy. The Audi boffins, decided to do away with reducing the number of cylinders to keep on combusting by the simple expedient of shutting the inlet and exhaust valves while also at the same time switching off the fuel supply and the ignition spark.

What happens now is akin to what you get when you try to operate a bicycle pump with your finger over an open end while pulling the plunger – there is a sucking effect for sure and while there is a reasonable amount of work involved this is completely wiped out thanks to the creation of a giant air spring effect which pushes the piston on its upward stroke.

Audi’s cylinder on demand system is effected only at low speeds for this is where the actual gains would accrue and it happens when in third gear or higher with the engine spinning at anywhere from 960rpm to 3500rpm and also of course with the engine having attained a temperature of just around 30 degrees Celsius. Once such a situation is reached, the system shuts off the intake and exhaust valves of cylinders 2, 3, 5 and 8 on both banks and now the engine continues to run as a V4 with a regular firing order (1-4-6-7). What happens is that at part throttle the work is now done by half the cylinders who have a larger work load to contend with and thereby the pumping losses get minimised. And with no fuel being mixed in this influx of fresh air and no ignition, tail pipe emissions are also cut down massively in this phase.

The beauty of the system is that the switchover from eight to four pots and vice-versa is slick and seamless, taking just 200 to 500 milliseconds depending on the operating point. The AVS helps the top end of the engine close the valve operation thanks to electromagnetically operated sleeves working on special “zero lift” profiles machined on the camshafts. Once the engine is in four-cylinder mode, a display on the instrument panel conveys this to the driver and when the man behind the wheel demands more power the ignition system gets reactivated for all eight cylinders to get into action thanks to clever electronics built into the EMS.
What all this does is to reduce fuel consumption by almost 10 per cent when pottering around at 100km/h and with corresponding gains when using the start-stop function this figure is hiked to 12 per cent. There is also a significant drop in CO2 emissions, almost to the tune of 10g/km to 12g/km.

There is a flip side though to cylinder deactivation and the shift to a V4 operating configuration. As is a well known trait, four-cylinder engines have an inherent issue of torsional vibrations as a function of load and engine speed and there is a tendency for these to be transmitted into the passenger cabin. This is even more so from a V8 shifting to a V4 on the fly so to speak with the smoothness of the eight being roughed up. Audi has employed clever vibe cancelling technology courtesy of active engine mounts. These generate out-of-phase counter oscillations to kill the four cylinder engine’s secondary vibes. Using electromagnets to tweak the hydraulics in the engine mounts to react and cancel the engine’s natural vibration frequencies is a pretty neat touch.

Adding further to the clever techno-geek stuff to try and suppress vibes from making themselves audible in the cabin, Audi has packed in four microphones to monitor the engine’s natural vibration frequency via sensors connected to the ECU. Using noise cancellation technology based on the principle of destructive interference, these special microphones emit a precisely modulated cancellation sound that damps out the harsh drone effect in the cabin.

The internal combustion engine today needs every help it can get to do its job efficiently and cleanly without losing out on its inherent pleasures. However it also needs technologies away from its core to deliver the benefits of low fuel consumption and reduced tail pipe emissions with no loss in refinement or pleasure. It is a complex art but the boffins are hard at work eking out whatever gains they can liberate so as to keep the internal combustion motor pumping strongly, in the face of ever stricter legislation. Audi has just shown its hand, and I am sure many other OEMs already have their own similar systems ready for introduction.