Audi’s quattro system explained

It’s hard to watch one of Audi’s catchy TV adverts without hearing the mention of the word ‘quattro’. The name has been synonymous with the German brand since 1980, when it first appeared on their revolutionary and hugely successful rally car.

The term quattro refers to the all-wheel drive system employed to devastating effect at a time when the rallying competition used only two-wheel drive and, as a result, the sport was turned on its head. The car that bore this first system was dubbed the Quattro (capital Q) and the all-wheel-drive system was, confusingly, also called quattro (lower-case q).

To this day, the name still has links to motorsport, only now in the shape of the firm’s Le Mans 24-hour winning prototype racers. It’s also common to find the name on the back of many of the firm’s high-end road cars too, but what makes the Audi system so special?

The technical stuff

For quattro, Audi’s engineers aimed to build an all-wheel drive sports car without the need for a traditional transfer box, as used in many traditional off roaders. They’re bulky and add significant weight – hardly ideal in a motorsport application. Audi’s system makes use of a centre differential.

A differential is a component which allows the engine’s torque to be divided between driven wheels even if they are rotating at different speeds. Ordinarily, one will be mounted on each driven axle, but quattro makes use of one to separate torque between front and rear wheels too. Mounted within the gearbox, it connects the two axles when slip is detected, allowing an infinitely variable split of torque depending on road conditions. Compared to a traditional transfer box, this system is much lighter and more efficient.

It’s great for road cars, too. In most applications, torque is split 50:50 from front to rear in order to provide greater stability, though sportier models like the RS4 and RS6 have a more rearward bias.

But the Audi R8’s engine is mid-mounted. How does it work there?

It’s beautifully simple, really. As the engine of the R8 is behind the driver, rather than ahead of it, the whole system is essentially flipped backwards. The engine sits at the back with a gearbox ahead of it, sending torque through the Torsen differential towards each of the four wheels. Due to the high performance nature of the R8, a much greater percentage of torque is diverted to the rear wheels: it’s approximately a 15:85 split.

How does it work for transverse-engined Audis?

Due to their engine layout, some Audis employ a different quattro system. The likes of the A3 and TT use the MQB platform, shared by the likes of the Volkswagen Golf, Seat Leon and Skoda Octavia. MQB cars – as with most cars in this class – feature a transversely (sideways) mounted engine. This means the Torsen system would not physically fit under the bonnet. Instead, these models employ a Haldex four-wheel drive system.

In contrast to the Torsen system, Haldex sends nearly all of the engine’s torque to the front wheels in normal conditions. If slip is detected in the front axle, the Haldex clutch can completely lock, diverting up to 100 per cent of torque rearwards. Then, with the use of electronic differentials integrated into the traction and stability control systems, torque can be diverted to individual wheels on each axle if necessary.

Due to the difference between the Haldex and Torsen systems, die-hard Audi fans will often refer to cars equipped with the Torsen system as ‘true quattros’, because the two systems have different characteristics under extreme driving conditions.

Any disadvantages?

In quattro systems, the need to package all the components means the gearbox is much longer than normal. This forces the engine to be positioned further forward, which means it’s forced out ahead of the front wheels. All this extra weight over the nose of the car results in an eventual loss of grip at the front during hard cornering.

When compared to the Haldex quattro models, the traditional Torsen versions are less economical, too. Under normal driving conditions, all-wheel drive isn’t necessary but, because torque is permanently sent to all four wheels, the driveline losses make the system slightly more wasteful, resulting in higher fuel consumption.

Audi quattro with ultra technology

Carrying the ultra name means this latest development of the company’s famous four-wheel-drive system is all about saving fuel and cutting CO2 emissions. Audi claims it will set benchmarks in both, with an average 0.3 l/100km boost to fuel economy when compared to the old quattro system. Theoretically, this system could take the 215hp, 3.0-litre diesel quattro Audi A4 model from 60.1mpg to an even more impressive 64mpg.

It works by allowing the car to run in front-wheel-drive mode – on dry motorways, say – when the extra grip of four-wheel drive is surplus to requirements and serves only to put extra stress on the engine.

If it’s needed, power is sent to the back wheels via two clutches – one mounted to the car’s gearbox, the other to the rear axle – which can re-engage the engine following a three-step strategy, named: proactive, predictive and reactive.

The proactive stage refers to the car’s sensors, which relay data to the four-wheel drive system’s control unit, allowing the car to predict when four-wheel drive might be needed and to prime the system accordingly – the rear wheels being engaged before the front end’s limits of grip are reached. The real-world result is a seamless transition to four-wheel drive.

The predictive element takes account of the user’s driving style, the settings chosen in the car’s Direct Drive system and data form the stability control. In other words, if you drive aggressively the car will respond by switching on the four-wheel drive system to maximise performance and safety.

Think of the car’s reactive phase as a clever safety net and you won’t be far wrong. It’s likely to be used the least in the real world and responds to sudden changes in conditions – encountering a patch of sheet ice would do it – calling on the quattro system’s grip in an emergency situation.

What about the future?

With the rise of the hybrid car, there is a chance Audi’s original system might be used less frequently. The current R18 e-tron quattro makes use of a traditional diesel engine to drive the rear wheels, while an electrical system – charged by energy recovered under braking – powers the fronts.

It isn’t impossible to think that a similar system will be used in high performance road-going Audis in the future – the A3 e-tron already uses a system similar to the R18’s. It’s much more likely, however, to use batteries in place of the R18’s rotating flywheel to store power. Torque from an electric motor can be deployed much more quickly and efficiently than in a regular combustion engine so, in a performance application, it can be used to great effect.

Sounds gripping. Now I want to look at some Audis…

If all this talk of quattro has you keen to find out more, then take a look at our reviews page for more info on Audi’s model range. If you’ve found the one for you, why not take a look at our new car deals page to find the best price on a new car?

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