"Twenty-five years ago, Audi launched the first TDI on the market, writing the first chapter of an enduring success story," says Prof. Dr. Ulrich Hackenberg, Audi board member for technical development. "Our latest innovation is the electric turbocharger, which further improves not just sprint times and pulling power, but also efficiency. This technology illustrates the possibilities harbored by 48-volt electrical systems, which we are currently developing for use in production vehicles."
Electric turbocharging is a bit of a misnomer since turbos rely on exhaust gas-powered turbines by definition, but Audi's been in the habit of calling it that since revealing it several years ago. Since then, it's been applied to the R18 e-tron quattro LMP1 car and now the new RS 5 TDI concept, which shows what it could do in a more road-friendly application.
The RS 5 TDI concept loses the 4.2-liter V8 engine of the production RS 5 Coupe for a 3.0-liter V6 TDI with triple turbos – the e-turbocharger supplements the two traditional turbos. Replacing the typical exhaust-powered turbine wheel, the electric turbo's motor spins the compressor wheel to over 70,000 rpm in a few hundredths of a second. This action essentially eliminates turbo lag and provides a powerful boost right off the line.
"What all turbocharged engines have in common is that the turbocharger is driven by energy from the exhaust," Audi explained when it discussed its electric turbo technology in 2012. "This means that starting from very low revs, the rise in boost pressure and therefore torque becomes gradually greater only as the exhaust energy increases.
A new development stage is the electric biturbo. This makes it possible – independently of the exhaust energy available – to build up charge pressure quickly and achieve high levels of torque even at very low revs."
The RS 5 TDI Concept has a dedicated 48-volt power system providing the large quantities of energy needed to feed the electric turbo. The system is energized by brake recuperation, storing electricity in a lithium-ion battery.
The tri-turbo set-up gives the RS 5 TDI a bank of 385 hp (287 kW) and 553 lb-ft (750 Nm) of torque, firing it to 62 mph (100 km/h) in four seconds (more than half a second faster than the 450-hp production RS 5) and 124 mph (200 km/h) in under 16 seconds. The car's top speed is listed at 174 mph (280 km/h). Meanwhile, it rolls for more than 44 miles for every gallon of diesel (5.3cL/100 km), another large improvement over its production sibling, which combines for 18 mpg (13 L/100 km).
Audi doesn't make any hints at RS 5 TDI production, but the e-turbo seems likely to find its way into dealerships. A report in Drive this week suggests that the Q7 SUV will be the first production model with the technology when it debuts at this year's Paris Motor Show.