Testing Audi’s new e-booster reveals turbocharging's future

  • 04-Aug-2014 11:58 EDT
Audi07-14TDI RS5 concept.jpg

Now installed as a technology demonstrator in its RS5 TDI Concept, Audi's e-booster is set to be fitted to a range of production models. (For more images and data, click on the small arrow on the upper right corner of this box.)

The car: Audi RS5 TDI V6 3.0-L 283-kW (379-hp) bi-turbo Concept.

The new technology: e-booster electric compressor.

The test: From a standing start, to keep pace to 100 km/h (62 mph) with an Audi RS6 4.0-L V8 412-kW (552-hp) gasoline bi-turbo. Both cars have 8-speed automatic transmissions.

The venue: Sturup Raceway, Malmö, Sweden.

The instructions to this Automotive Engineering editor: “On a countdown of three to zero — GO!”

The purpose of our comparison test was to prove Audi Technical Development chief Dr. Ulrich Hackenberg's claim that adding an electric compressor to a turbocharged engine makes step-off from traffic lights decidedly more brisk.

I mashed the RS5 Concept's throttle pedal to the metal, racing the far more powerful RS6 to a narrow gap between cones ahead. The two cars ran perfectly parallel until the brute force of the RS6's V8 pulled it ahead just as 100 km/h approached. We passed through the cones virtually bumper-to-bumper. My time: 4.0 s—a mere 0.1 s behind the RS6.

The RS Concept uses the company’s latest V6 diesel (http://articles.sae.org/13223/) which, as well as its 283-kW power output in this application, delivers 750 N·m (553 lb·ft) from 1250 to 2000 rpm. It is aurally characterful for a diesel.

As Dr. Hackenberg said, an electric compressor would indeed help step-off from traffic lights: “With electric turbocharging, we already achieve very dynamic torque development at low engine speeds and strengthen both sprinting and pulling power.” His point was convincingly proved. Indeed, in my drag race boost pressure was available immediately after each shift, thanks to the e-booster.

Zero turbo lag and other benefits

Now in its advanced development phase, the compressor is placed between induction system and intercooler. It uses a separate 48V system from the car’s regular electrics to spin-up the smaller of the RS5 Concept’s turbos. Developed with partner Valeo, the e-booster takes only 250 ms to reach its maximum revs, and provide 7 kW (9 hp) of power. From very low revs, when exhaust gas energy to drive the turbocharger is in short supply, a bypass valve shuts to re-route the air to the compressor.

As well as the bi-turbo RS5 Concept, Audi also has a single-turbo concept of this set-up installed in an A6 demonstrator that also uses the new V6 engine. Although described as being used for “technology studies,” production of the e-booster is assured.

The A6 version produces 240 kW (322 hp) and 650 N·m (479 lb·ft) from 1500 to 3500 rpm. Audi quotes acceleration from 60 to 120 km/h (37 to 75 mph) for the A6 in sixth gear as being cut from a relatively leisurely 13.7 s to a quick 8.3 s.

The RS5 TDI Concept delivers what turbocharged cars have needed for years: a thoroughly seamless power delivery with no detectable lag. Recuperated energy provides most of the energy needed by the compressor, which has a lithium-ion battery and power electronics in the trunk. A DC/DC converter is used.

The decision to use a 48-V sub-system, which will be available across the production model range soon, means it can also support other applications including electromechanical rear brakes, and oil and water pumps. It could also be used for starting the engine, which would make stop-start operation smoother. There is also a weight saving via smaller cable cross-section.

Dr. Hackenberg said that with the second generation of the VW Group’s modular longitudinal matrix architecture (MLB: Modularer Längsbaukasten), different types of electrification will be applied in each model family including the plug-in hybrid.

We will make use of all possibilities," he noted. "And the 48-V onboard supply will play an important part. It will allow us to transfer large amounts of energy and is a precondition for the electrically powered compressor.”

Reaching the technology threshold

Main strategic technology development areas at Audi now include “rightsizing.” It is a term the company prefers to “downsizing” as it is regarded by Audi as being more “appropriate” for what it is aiming to achieve in terms of efficiency improvements. So V6 and V8 diesels (the latter running at very low revs in the cruise) are seen as apposite for particular models.

"We define the right size of an engine for each size of car. In combination with the MLB, we are examining all forms of electrification,” Dr. Hackenberg asserted.

He confirmed that a TDI e-tron is entering series production: “We are combining the 3.0-L V6 TDI with a powerful electric motor. This results in a power output of 275 kW and torque of 700 N·m. With a proper electric driving experience, the electric range of the TDI e-tron will be more than 50 km.”

Production models using the e-tron technology will be available at the turn of this year.

Audi’s name is closely linked with motorsport and Dr. Hackenberg underlined its importance in relation to the company’s overall R&D programs. “For three decades it has been our toughest test bench for all kinds of technology for the series models," he said, noting that the TDI is a focus area.

Audi has used diesel engines in its Le Mans racecars since 2006, notching eight overall victories out of nine starts. Now, said Dr. Hackenberg, the racing priority is firmly placed on energy efficiency.

“In this context, we have developed what we see as the optimal drive system with the optimal degree of electrification. This year [at Le Mans] we used a newly developed 4.0-L V6 TDI for the first time. It consumes 22% less fuel than its predecessor and it is optimized for full-load driving during the Le Mans race—which is 73% of the time.”

But he added a word of caution to those who may regard such improvements to be almost exponential: “With these engines, we are at the limits of what is technically possible.”

The emphasis appears to be on the words “these engines,” which indicates the application of fresh thinking for both track and road. In the coming years, far more radical solutions are set to emerge from the VW Group/Audi and electrification will play a central role.

Evolving TDI technology

So will the diesel, in the medium term at least and probably far longer. Audi’s TDI technology is based on decades of steady development. The original I5 was a direct-injection turbo with full electronic management. It came some 15 years after Audi started TDI development in the wake of the mid-1970s oil crisis.

A team of only 10 members started that program and decided multi-jet technology was needed. Bosch came up with an electronically controlled axial piston injection pump. Two spring holders were used to open the nozzle needles in two stages with varying amounts of lift, facilitating pre-injection of smaller amounts of fuel. This was a significant advance to make car diesel engines more acoustically acceptable. Pressure was 900 bar (13,000 psi), which compares with current diesel pressures of around 2500 bar (36,200 psi). But 2700 bar (39,000 psi) systems are in the works, and race engines are already at 3000 bar (43,500 psi).

The Audi 100 delivered 88 kW and 265 N·m (118 hp and 188 lb·ft, respectively) but at a relatively high 2250 rpm. By 2001, Audi had developed a 1.2-L diesel for the aluminum-bodied A2 with a CO2 figure of 81 g/km. Power output was 45 kW (60 hp) with 140 N·m (103 lb·ft). The car was capable of delivering 3.0 L/100 km fuel consumption.

Generally today, 1.4-L is regarded as the smallest swept volume for most passenger car diesel power units (except for some minicars), and Audi is to introduce the VW Group’s three cylinder 1.4-L TDI in its compact models with transverse engines. The engine has an aluminum-silicon alloy crankcase manufactured using gravity chill casting. The entire engine with balance shaft weighs 132 kg. The common rail injection system operates at 2000 bar (29,000 psi). Claimed output is 66 kW (88 hp) and 230 N·m (170 lb·ft) is available from 1500 to 2500 rpm.

And will it eventually have an e-booster? Audi bosses just reply with enigmatic smiles.

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