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When the drive select system is in efficiency mode, the new start-stop system turns off the engine before the car comes to a complete stop.

The Volkswagen Group sells more diesel-engine passenger vehicles than any other manufacturer – 7.5 million in the past 25 years. During that period many of the 40,000 engineers employed by VW have reduced harmful emissions from the inherently "dirty" diesel engine by 98 per cent while increasing power by more than 100 per cent relative to displacement.

However, the industry is being challenged to cope with increasingly stringent regulations amidst concern about the environmental impact of vehicles. The internal combustion engine in general – and diesels in particular – are under ever-increasing scrutiny in Europe. The pollutant nitrogen dioxide results from diesel combustion, as does the dangerous particle, PM2.5.

At a technical forum here, 25 years after the company introduced the diesel passenger car, Audi engineers demonstrated how the TDI diesel engine has kept pace with regulations, and outlined how it will continue to keep up with everything from electric turbos to bio fuels. The engineers have made a number of changes to the engine. It is lighter, cast from a different alloy. As part of a new integrated thermal management system, the pistons are oil-cooled and the block and head have separate cooling circuits. The turbocharger and exhaust gas treatment systems are new, the camshafts are hollow, the oil pump variable. When the drive select system is in efficiency mode, the new start-stop system turns off the engine before the car comes to a complete stop, as soon as the driver brakes at speeds below seven kilometres an hour. It responds again as soon as the throttle is touched.

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On public roads from Copenhagen in a 2015 A7 Sportback 3.0 TDI Quattro, the 428 foot-pounds of torque produced by its V6 diesel provided plenty of poke. It accelerated from rest to 100 km/h in 5.7 seconds – a number many high-end sports and performance cars would envy. Perhaps more impressively, this big, luxury car can achieve 5.3 litres/100 km on the highway.

Later, I drove the same automobile on the track at Sturup Raceway with a prototype TDI engine. Turbocharged and supercharged , the 3.0-litre V6 produced 326 horsepower and a whopping 480 foot-pounds of torque. I also spent some track time at the wheel of yet another prototype TDI – an RS5 coupe with a 3.0-litre V6, this one with a turbo and an electric compressor powered by a 48-volt secondary power source. It belted out 385 horsepower and 553 foot-pounds of torque.

The first TDI appeared in the 1989 Audi 100. It was an industry first – a 2.5-litre, electronically-controlled diesel engine with direct injection. Today, there are 156 vehicles within the VW Group (Audi, Bentley, Bugatti, Ducati, Lamborghini, Porsche, Scania, Seat, Skoda and Volkswagen) available with a TDI engine. They range in displacement from 1.4-4.2 litres in three-, four-, six- and eight-cylinder guise producing between 90 and 385 horsepower.

The first signs of future development will be a pair of newcomers to be introduced over the next year – a completely redesigned, low-emissions 3.0 V6 TDI and a new 1.4-litre three-cylinder TDI. There will also be a range of TDI models wearing the "Ultra" label. Of the 23 models in the A3, A4, A5, A6 and A7 model series, 15 will have TDI engines with fuel consumption ranging from 3.2 to 4.9 litres per 100 kilometres.

The electric bi-turbo mentioned above will be the first step in Audi's electrification of the TDI. New hybridization components follow including a TDI with plug-in hybrid technology. The power to drive that new small turbo will be captured during deceleration and coasting and stored in a separate, small 48-volt lithium-ion battery. That 48-volt system will also power some other systems and thanks to a converter connect to the 12-volt system.

The writer was a guest of the auto maker.

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