Prototype Drive
Are diesel engines and sports cars mutually exclusive? Most enthusiasts might say so. Compression ignition works well for long-distance cruisers, pack-mule pickups, and SUVs, and in Europe, there are even a few diesel-powered coupes and convertibles for the patient and the parsimonious. Sports cars, on the other hand, are all about aggressive response, charging to the redline, and emotional acoustics. Like oil and water, the two just don’t mix.
But a new sort of homogenization is underway. The Audi RS5 TDI prototype that we just flogged at the brand’s handling track at Neuburg an der Donau has managed to reset our notions of diesel engines. This 385-hp 3.0-liter V-6 TDI—still a prototype, but close to series production—has closed gaps with gasoline engines in areas where diesels used to be hopelessly outclassed.
As you might expect with 553 lb-ft arriving at just 1250 rpm, response at tip-in is brisk. From rest, the RS5 TDI charges forward without delay. In a drag race with a gasoline-fueled 553-hp RS6 Avant turbo V-8, the RS5 V-6 TDI held off the more powerful V-8 car for several car lengths. The TDI’s midrange response is also immediate, feeling like a high-performance, naturally aspirated gasoline engine, with none of the laziness and delay typical of turbo-diesels. This RS5 is surprisingly rev-happy, twisting the tach needle all the way to a relatively un-diesel-like 5500 rpm. We estimate the RS5 TDI’s sprint from zero to 60 mph will take about four seconds, with 100 mph arriving in around ten. Top speed is 174 mph, Audi claims. Despite these impressive numbers, the RS5’s 385-hp oil-burner is projected to use less fuel than the 313-hp version of the V-6 TDI it is based on.
Electric Supercharger
The key to the RS5 TDI’s performance and efficiency gains is a Valeo-supplied electric supercharger. It is positioned at the "cold side" of the intercooler and, thanks to a low-inertia rotor, capable of reacting within 100 milliseconds and revving to 70,000 rpm. It can blow at full blast independent of engine load and revs. This allows the engine’s twin turbos to be calibrated for maximum boost—up to 49.3 psi—and power. The leap to 385 horsepower at 4200 rpm is solely achieved by the turbos, not by the supercharger. And the TDI’s ample 553 lb-ft of maximum torque is available from 1250 to 2000 rpm. These figures mirror those of BMW's 381-hp, tri-turbo inline-six diesel, which is an impressive engine, but falls far short of this Audi mill in terms of responsiveness.
The electric supercharger requires a separate 48-volt power supply, and the conversion from the 12-volt system adds cost and complexity. Down the road, cost is expected to diminish as more electrical functions are integrated into a common 48-volt system.
Audi's own, seven-speed dual-clutch transmission would be the perfect companion for this ultrasporty diesel engine, but it lacks sufficient strength to handle the RS5 TDI’s torque. And so the RS5 TDI is fitted with the ZF 8HP eight-speed automatic, a quick-shifting and efficient unit that has become a benchmark among these types of gearboxes.
Speakers With Benefits
While we are no fans of artificial sound engineering, Audi is forgiven for squashing this diesel’s clatter with—we’re not making this up—a pair of speakers situated near the dual tailpipes. The auditory benefits are considerable. They emit a dark rumble at idle and deliver a subdued roar as you race up the rev scale.
The first series-production application for this engine will be in the next-gen Q7 crossover, after which it will migrate into the A6 and A7. But the RS5 is the ideal vessel in which to demonstrate its sporting capabilities. Here, the enhanced sound and the explosive power delivery are backed up by extremely agile handling. The car turns in with precision, and thanks to a Quattro sport rear differential, it can be coaxed into an easily controllable slide. Audi says the RS5 TDI prototype weighs no more than the conventional RS5 with its naturally aspirated V-8.
Two years ago, Audi launched an SQ5 with a 313-hp TDI engine for the European market, and it fostered discussion about the compatibility of "S" badging with a diesel engine. The question has been settled by the sales success of that version. Is it time to do an RS diesel? "We are considering the options," is all that Audi powertrain chief Stefan Knirsch will reveal. Our take: The technical barriers have disappeared and electric supercharging has the potential to transform the diesel. While the fuel efficiency of diesels is well established, the newfound power and responsiveness of the RS5 TDI are on par with that of high-performance gasoline engines. In our book, the “RS” and “TDI” look pretty good together.
