When the drivetrain in the Audi RS5 TDI Concept goes into production – and it is confirmed to do so – the diesel engine will finally be fit for sports car usage.
For the most part on paper, the Audi RS5 TDI Concept should be no different to any other sweet, but hardly sporting diesel on the market.
On the outside it looks just like the $156,000 production petrol-powered RS5 coupe that has been selling in Australia for the past four years. Under the bonnet, however, the production RS5’s 4.2-litre naturally-aspirated V8 engine has been replaced by a 3.0-litre twin-turbocharged V6 diesel.
It all seems humble enough, especially considering the V6 diesel is just like the one found in the Audi A6. It isn’t, though – it has been completely changed.
Like a barnacle clinging to a wharf, this V6 diesel has an electric-powered compressor attached to its side. The RS5 TDI Concept uses a 48-volt power system on board, four times the voltage of the production RS5 but around nine times less than a proper hybrid model needs. In its boot, the RS5 TDI Concept gets a small lithium-ion battery pack and DC/DC bi-directional converter that respectively give and feed power to the electric compressor up front.
The electric compressor mimics a turbocharger to give the diesel engine a boost. In hybrids or proper electric cars, the electric motor can provide instant torque to an engine without any hesitation. As the name suggests, however, the Audi system isn’t an electric motor but rather a generator that builds instant boost pressure for the engine to maximise performance. The aim is the same, though – to provide instant response.
Instant response is something even the newest and best turbochargers struggle with. They need to wait for the engine to send out exhaust gases before re-routing them into a turbocharger that then needs to spin up its turbine wheel like a windmill. Turbo lag, as it's known, makes the throttle pedal go doughy and dilutes engine response, if only for a moment in the best performance petrol and diesel engines. Making the windmill smaller makes it turn faster; making it bigger can deliver higher boost pressure but it takes longer to spin up and therefore adds boost.
Enter the RS5 TDI Concept drivetrain. It uses the electric compressor to instantly build boost pressure for the engine, and is powered by batteries recharged by regenerative braking so there are no economy downsides. Up to 3800rpm a bypass valve is open to feed air into the electric compressor, which allows the two remaining mechanical turbos to take a break.
Those two turbos can also be bigger, because the electric compressor is nullifying what would be turbo lag with its instant response. Above 3800rpm the electric compressor’s work is done. The bigger turbos start huffing and puffing away to generate proper performance car outputs.
The RS5 TDI Concept produces 283kW of power at 4200rpm, and 750Nm of torque between 1250rpm and 2000rpm. A ‘normal’ twin-turbo V6 diesel in the Audi A6 makes 230kW and 650Nm, while the petrol V8 in the production RS5 makes a bit more power (331kW) but a massive 320Nm less torque.
But it’s really the immediate response of the RS5 TDI Concept more than the outright numbers that blow you away.
At a test circuit in Sweden, Audi have lined up the RS5 TDI Concept for a drag race not against its RS5 twin, but rather an RS6 Avant that has a 4.0-litre twin-turbo petrol V8 engine that makes 412kW and 700Nm, and claims a 3.9-second 0-100km/h.
Heading out onto the main straight, there’s little giveaway that you’re in something properly quick – there’s a distant growl from the engine that still sounds more diesel than petrol, and it’s dominated by exhaust blurt to make a curious mix.
Even the RS5’s seven-speed dual-clutch automatic gearbox has been replaced by an eight-speed automatic, despite torque-converter autos not being as crisp to shift as dual-clutchers that are now largely the sporting benchmark.
Left foot on the brake, right foot momentarily hard on the accelerator for the countdown, then the other one lifted. The RS5 TDI Concept leaps off the line like a supercar, pinning you into its fixed seat bucket. The growl becomes louder, but as it grabs second gear, maximum revs are still only 5700rpm, high for a diesel but very low for a petrol engine, so there’s not the same sensation of sound.
There is the sensation of speed, however, particularly as the RS5 TDI Concept gets a half-car jump on the RS6 Avant off the line, which then briefly extends to almost a full length. Only above 80km/h does the big wagon reel the lithe coupe in, which means Audi has to claim a near-identical 4.0sec 0-100km/h overall time for its new diesel sports car.
We tuck behind the RS6 Avant for a couple of flying laps of the circuit, only to discover a level of throttle response from the RS5 TDI Concept that’s scarcely believable for a diesel. Turning into corners, you can balance the car on mere millimetres of throttle and get response from the engine – either from the electric motor for the lower two-thirds of the rev range, or already fast-spinning and heavily boosted turbochargers in the top third.
With so much grunt and response everywhere, the low 5700rpm redline becomes the least of a driver’s concerns, especially with paddleshifters and eight gears on hand that are shifted between dual-clutch-quickly. Even the engine note becomes sweet to the ear under load, though it stops well short of sounding as arousing or theatrical as the petrol V8.
The prototype runs the full chassis gear from the RS5, which includes all-wheel drive and a crown centre differential that can instantly send more torque to the rear wheels, to then divert more to the loaded inside wheel with the most grip.
In longer-radius hairpins, you can get on the throttle early and slide the RS5 TDI Concept around in small or big drift angles depending on your throttle position.
Audi won’t confirm that the V6 diesel under the bonnet is heavier or lighter than the V8 petrol except to say that the batteries/converter/electric compressor adds a total 23kg to the kerb weight. Experiencing the RS5 TDI Concept’s tight turn-in and lithe changes of direction indicates there’s little penalty over the front wheels.
Where the petrol V8 claims to slurp 10.5 litres of premium unleaded per 100 kilometres, the diesel V6 “mild hybrid” in the RS5 TDI Concept should still post much less, adding to its advantages.
Audi has confirmed this drivetrain will be rolled out on a future production model soon, but has declined to nominate which model. With a brand-new Audi A4 due within a year, it could be a candidate. Not only is ‘diesel sports car’ no longer oxymoronic, but with the help of electric trickery the diesel engine can now properly challenge and even eclipse performance petrols.