BMW Versus Audi - The Multi-Turbo Diesel Engine Battle

Audi's V8 TDI of the SQ7 (left) and BMW's Quad-Turbo Diesel Engine for the 750d 6 photos
Photo: Collage from Audi and BMW press photos
BMW 750dAudi SQ7BMW's quad-turbo six-cylinderAudi's V8 TDI engineAudi's V8 TDI engine versus BMW's quad-turbo six-cylinder
German automakers have had their horsepower wars, and it slowly led to a battle of multi-turbo diesel engines. In 2016, BMW and Audi have become leaders when we talk about power figures for diesel passenger cars.
Both BMW and Audi launched phenomenal top-of-the-line diesel engines this year, but the two units are completely different in conception.

So we decided to look at each one to see how the two German carmakers managed to squeeze 400 HP and loads of torque from diesel engines meant for passenger vehicles.

First of all, let’s start with the main differences between the two units. Audi uses a 4.0-liter V8 unit, while BMW has a 3.0-liter inline-six engine.

What starts out as a similar conception goes on two separate paths when we look at the solutions employed by each carmaker to generate ridiculous levels of horsepower from oil burners.

BMW fitted its top-of-the-line diesel engine with four turbochargers, making it the first of its kind. The same automaker is the first and only company to launch a diesel engine with three turbochargers. All the turbochargers mentioned above and employed by BMW are conventional ones.

Meanwhile, Audi has a mixed supercharging solution. The V8 unit comes with two conventional turbochargers operated sequentially and an electric turbocharger. The latter is run with the aid of a dedicated 48-volt electrical system. This configuration is also unique in the automotive world, but we expect Audi to replicate it in other models of its lineup.

Audi’s V8 TDI engine in detail

Audi's V8 TDI engine
Photo: Audi
The most powerful production diesel available in Audi’s portfolio is only fitted to the new SQ7 for now. It also comes with the first implementation of the Vavelift system from Audi, which brings continuous variable valve timing to a diesel engine.

Concerning specs, the new powertrain has a peak output of 435 PS (320 kW), which is available between 3,750 and 5,000 rpm. The torque figure is also impressive, as the unit can provide 900 Nm (663.8 lb-ft). Audi’s most powerful production V8 TDI engine provides its peak torque between 1,000 and 3,250 rpm. These figures mean that the unit will be extremely elastic, and that turbo lag is almost entirely eliminated.

With this new engine and a matching drivetrain, Audi’s SQ7 has an NEDC average fuel consumption of 7.4 liters for 100 kilometers, the equivalent of 31.8 US MPGs. Its acceleration time from 0 to 62 mph (100 km/h) is of 4.8 seconds, while the top speed is electronically restricted to 250 km/h (155 mph).

Naturally, the new electric compressor is the secret behind the incredible performance developed by the new 4.0-liter V8 TDI unit. However, the same technology is under fire from traditionalists, as it requires a complicated electric system to operate it. Audi’s SQ7 has two separate electrical circuits on board, and one of them is just for the new electric supercharger.

Because of this new electric compressor, Audi will have to train technicians for specialized servicing, as well as equip specialized tooling in all of its authorized workshops. These will imply costs to all service departments involved, as well as bring complexity.

As modern engines have to withstand ever-increasing demand from consumers and governments alike, one can only hope the new electric compressor from Audi will maintain its performance for as long as the vehicle lives. Otherwise, Audi SQ7 owners might be the unfortunate early adopters of a technology that might not live to receive a second generation.

BMW’s Quad-Turbo Inline-Six Diesel Engine

BMW's quad\-turbo six\-cylinder
Photo: BMW
While Audi has gone big on capacity and cylinder count, BMW has maintained its signature inline-six engine of less than 3.0 liters and fitted four turbochargers. Just like Audi, BMW operates the turbochargers of the 750d’s 400 HP engine in a sequential manner. The turbos are grouped in pairs of two for the two sets of three cylinders that form the new powerplant.

Concerning specs, the 2017 BMW 750d provides 400 HP and 760 Nm (560 lb-ft) of peak torque. The maximum output is reached at 4,400 rpm, while the top torque is available between 2,000 and 3,000 rpm. BMW notes that the torque of this unit is 450 Nm (332 lb-ft) at 1,000 rpm. That’s half of the Audi SQ7’s torque at the same level of rpm.

The horsepower figure is close to Audi SQ7’s peak output, but a full 140 Nm (103 lb-ft) below in terms of torque. However, its capacity is smaller by one liter, and it has two cylinders less, which counts when you consider the output per liter, where the BMW unit shines.

BMW’s quad-turbo setup works with two small turbos with variable geometry placed in a single housing, and two bigger turbos. The latter housing is smaller than the larger turbocharger used on the previous three turbo diesel engine from BMW, thus bringing improved throttle response. At the same time, four turbochargers on a single engine bring a level of complexity only matched by the Bugatti Veyron and Chiron.

At normal operation, BMW’s quad-turbo engine will permanently operate three of the four turbos. The fourth unit is either bypassed or put to work using a flap system. The latter is also employed to let the engine spin the smaller turbos on hard accelerations from low rpms. While accelerating hard from low revs with a forced-induction engine that has exhaust gas activated supercharging is an unfortunate idea, it happens more than often.

BMW’s engineering team tried to solve the lag issue with the aid of a second flap system, which will focus exhaust gas pressure to the smaller turbos. These will spool more quickly than the larger units, which both come into play at around 2,500 rpm.

Thanks to the new powerplant, BMW’s 750d can sprint from 0 to 100 km/h (62 mph) in just 4.6 seconds. The extended wheelbase version will be slower by 0.1 seconds. When comparing the latter to its predecessor, it is 0.3 seconds faster than the three-turbo unit.

Furthermore, from a fuel economy point of view, the new 750d has an NEDC average rating of 5.7 to 5.9 liters/100 kilometers, which is the equivalent of 39.8 and 41.2 mpg. What’s impressive is that BMW made this new quad-turbo diesel just 0.2 seconds slower than its twin-turbo V8 gasoline equivalent on the same 7 Series, the 750i.

Critics of BMW’s latest diesel engine will say that the company has added another turbo for just a five-percent increase in output and an 11-percent decrease in fuel consumption. Instead of installing a 48-volt electric system to handle an electric compressor, BMW just had to figure out how to keep its four turbos cool, so they included an electrically operated coolant pump and extra heat exchangers, as well as two cooling circuits.

Audi Versus BMW - Conclusions of the diesel battle

Audi's V8 TDI engine versus BMW's quad\-turbo six\-cylinder
Photo: Collage with manufacturer's pictures
For the moment, Audi and BMW do not fit their top-of-the-line diesel engines in comparable models. We expect this to happen sometime next year or in a maximum of three years. This assumption is based on previous strategies employed by the two automakers, and consumers are those who benefit from this diesel horsepower battle.

Until then, all that’s left for us to do is to compare the two units on technology alone, but we cannot say which is the definitive best. Audi’s solution does not have a position as good as BMW’s when comparing output/displacement, but the Ingolstadt brand has twice as much torque at 1,000 rpm, as well as a generous band on which the engine provides its peak moment.

BMW’s work is no less impressive, as they managed to make a four-turbo engine work in a model made for clients with high expectations, the 7 Series. Furthermore, the Munich-based brand provides 400 HP out of a 3.0-liter unit with only six cylinders, while Audi had to go for a larger displacement and two more pistons to achieve a slightly bigger figure.

One can only wonder what the engineering teams of these two companies could achieve if they worked together. A five-turbo diesel engine with electric turbocharger, anyone? Mercedes-Benz, the ball is in your court, and the clock is ticking.
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About the author: Sebastian Toma
Sebastian Toma profile photo

Sebastian's love for cars began at a young age. Little did he know that a career would emerge from this passion (and that it would not, sadly, involve being a professional racecar driver). In over fourteen years, he got behind the wheel of several hundred vehicles and in the offices of the most important car publications in his homeland.
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