In simple terms, Audi engineers have added an electrically powered blower to the engine’s induction system, which, at very low engine speeds, forces air into the induction system, spinning the smaller turbo into life.
This ‘e-booster’ is connected to the engine’s intercooler on one side and to the induction system on the other, pushing air through the smaller turbocharger impeller between start-up and 3000rpm. At higher speeds, the e-booster is bypassed entirely.
The e-booster is also intended to keep the engine on boil on, say, twisty back roads. Typically with diesel engines, braking for a corner also slows the engine and bleeds off boost. This results in slower acceleration out of the same corner while the engine spins up enough to get the turbos back on boost.
But fitted with this new e-booster, the engine’s turbochargers can be primed while the driver is still braking, so that full torque is available virtually immediately the driver gets back on the gas.
Although Audi engineers experimented with turbocharger units that were driven directly by an electric motor, they decided not to pursue the design because it resulted in extra inertia, which, ironically, slowed the turbocharger response times.
The e-booster is powered by its own 48v electrical system, while the rest of the car uses a conventional 12v electrical system.
What's it like?
As you might expect with 553lb ft of torque on tap from just 1250rpm, this Audi RS5 diesel is bombastically rapid.
We drove the car at Audi’s new short driver training circuit near Munich. While this was a long way from the open road, the track’s very tight curves, which demand a great deal of braking and re-acceleration, were ideal for testing out the ‘e-boost’ promise.
We followed a hot RS6 pace car – driven by a professional Audi driver – around the track in order to give us some idea of how this RS5 prototype can deal with Audi’s fastest RS road car.
From a standing start, the RS5 had the measure of the RS6 for the first couple of seconds, before the RS6 pulled away. There’s no doubt that that this engine gets away from rest very quickly indeed. But after that the wall of seamless torque never seems to let up.
Left in the automatic gearbox's ‘Sports’ mode, the engine never had a chance of revving out, not just because the transmission’s brain wouldn’t let it – there’s little point in trying with torque peaking at 2000rpm and power at 4200rpm.
Another part of the reason that the engine responds less like a diesel is that the crankshaft, conrods and pistons have all been redesigned to reduce weight. These reciprocating parts are a claimed 20kg lighter than normal. That’s a lot less mass to speed up and slow down.
The promise of massive pull being instantly on tap as you pull out of slow corners is absolutely upheld. On six quick-ish laps, the engine was never left floundering for instant pace. Indeed, this engine drives hard enough to have the torque-vectoring rear differential working right up to the point it has to let the rear wheels slide a little.
Though I’m sure purists will say the car’s dynamic performance is ‘artificial’, this Audi RS5 doesn’t understeer. The steering weight remains constant even under the hardest cornering and – importantly – it is relatively easy to drive very hard.
Should I buy one?
You can’t – yet. While the Audi engineers on hand during our test were tight-lipped, I’d expect this engine to be offered some time next year.
It is also likely to appear in RS form thanks to the fact that the ‘diesel-ness’ of this unit has been almost entirely eliminated, thanks to a combination of e-boosting, its pretty free-revving nature and the artificial sound generation of the exhaust system.
The question that hasn’t yet been answered is why RS customers – who presumably are not too sensitive to petrol prices – would want to buy a diesel-powered car. The Audi engineers I spoke to are still very bullish on the future of diesel and suggest that oil-burning is still the supreme motive power for long distance, high-speed, motorway journeys and cross-continental drives.
Moreover, after winning Le Mans with diesel powered sports cars, Audi finally has a diesel engine good enough to put in a future R8.
Audi RS5 V6 TDI-e prototype
Price na; 0-62mph ‘under 4secs’; Top speed 174mph (limited); Economy n/a; CO2 n/a g/km; Kerb weight n/a; Engine V6, 2967cc, twin turbocharged with electric booster; Power 380bhp at 4200rpm; Torque 553lb ft at 1250-2000rpm; Gearbox 8-spd torque converter automatic