25 years of TDI: the Audi 100 2.5 TDI from 1989 and the latest study Audi RS 5 TDI concept

25 years of TDI: the Audi 100 2.5 TDI from 1989 and the latest study Audi RS 5 TDI concept

High-tech TDI engines that provide economy and performance sit at the heart of the Audi brand's values and technological DNA. As Audi marks a quarter century since it debuted TDI, Calum MacRae takes a look at Audi's strategy to develop TDI powertrain technology further to meet tighter future CO2 emission targets and the need for still higher efficiency from internal combustion engines.

Twenty-five years of TDI at Audi is a significant landmark. Think about it. The advent of TDI at Audi in 1989 has pretty much coincided with Audi's rise from a near-premium German brand to achieving a firm place in Germany's premium brand club. Redefining and positioning diesel as TDI - standing for turbo direct injection - was a way for Audi, together with its 1990s Bauhausian design essence, to be considered a technological leader along with the premium establishment at BMW and Mercedes-Benz.

However as Prof. Dr. Ulrich Hackenberg, head of technical development at Audi, recalled at a technical workshop to mark the anniversary, in the 1980s diesel was not an easy sell within senior management at Audi. The technology was pushed by Ferdinand Piech, then at Audi, and met with resistance from within (today it's hard to imagine anybody saying no to Dr Piech) and only began to be accepted when Audi senior managers realised the technology enabled them to complete their regular 600km trips to Wolfsburg without the need to stop for refuelling.

Fast forward 25 years and Audi has produced some 7.5 million cars with TDI engines and nearly 600,000 in 2013 alone. Diesel is tremendously important to Audi as a brand - according to figures from ICCT in 2012 while the EU27 markets were 53% diesel, Audi's sales in the EU27 were 72% diesel - and borne out by 66% of Audi's sales in the German market in 2013 being for diesel models.

Such high levels of diesel penetration at Audi have been key in reducing the brand's CO2 emissions an average of 3% per annum these past few years. What's more, despite Prof. Dr. Hackenburg's admission that there were "no more easy 10% savings in CO2, savings can just be 0.2g/km these days", Audi wants to maintain that 3% rate of improvement and is targeting its fleet CO2 emissions to be 95g CO2 per km by 2020. That's the same level as is targeted for the whole market and beyond the level Audi would have to reach defined by its limit value curve (a regression line set to determine CO2 targets by brand based on the average mass of their vehicles). Already, Audi (and eight other manufacturers including BMW, Peugeot, Citroen, Fiat, Ford, GM, Toyota, Seat and Volvo) achieved their 2015 fleet CO2 target in 2012, which in the case of Audi meant recording 122g per km for the fleet in 2012. Incidentally, the introduction of WLTC (worldwide harmonised light vehicle test cycle) in 2017/18 will effectively rebase all prior CO2 ratings for fleets and vehicles as measured under the EUDC, with some positing that due to WLTC's sustained higher speeds that 95g will be made more achievable.

Nevertheless, if there are no further easy savings how is Audi going to maintain its rate of improvement and achieve its 95g target? Electrification of the powertrain will be one area, but continued development of the internal combustion engine has to occur too. For 2030, Audi estimates that 40% of all cars will have some degree of electrification, but as two-thirds of these electrified vehicles will be hybrids it means that over 80% will still feature ICE - making its continued development an imperative.

So what is in Audi's plans for diesel to help the brand towards the 95g target? First, there are three new major engine programmes.

Entering production for Audi shortly is the EA288 1.4L three-cylinder diesel. The 1.4L, part of the MDB family, features an aluminium engine block that allows a weight saving of between 11kg and 12kg depending on whether VAG's 1.2L I3 or 1.6L I4 is considered the predecessor engine. In the case of Audi the new three could replace the current 1.6L I4 when it's in 77kW tune, although as initially Audi's taking only the 66kW version it looks like it could be a supplementary engine for Audi. Second is the new EA288 2.0L (again part of MDB) for installation in Audi's MLB component set. The 2.0L is Audi's bread and butter diesel engine but now features variable valve timing and a 2,000 bar common rail system with injections via solenoid injectors featuring 8-hole nozzles (compared with 7-hole in the 1.4L). In contrast to its 1.4L MDB bedfellow, the 2.0L still features an iron block suggesting that there's room for a switch to aluminium in the future to eke out a few more grams of CO2. Finally, there's a new 3.0L V6 TDI debuting in the revised Audi A7. The new V6 is not only Euro 6 compliant, but also offers improved CO2 emissions in engines ranging in power from 160 to 240kW. The cylinder head and block (again cast from compact graphite iron) have separate cooling circuits, allowing quicker warming by switching off at start-up and the facility to switch off at low load to save energy. Audi have done a remarkable job in close-coupling the exhaust after treatment of the new V6. Both the oxi-cat and SCR coated DPF are located in close proximity to the block, demonstrating the foresight and planning that has gone into the package protection for VAG's modular toolkits. From 2015, the V6 TDI's oxi-cat will be replaced by a NOC (NOx oxidation catalyst).

While the three new engines are the foundations for Audi reaching its 95g target, the brand is building on these with other CO2 measures.

Not least is its roster of 23 Ultra models each featuring detuned engines, taller gear ratios, aerodynamic modifications and a lower ride height. Presently, for example, the A4 Ultra TDI emits 104g/km (109g for the UK market version). This figure will reduce further in the future - competitors are already targeting 99g for 2015 - with Audi remarking that a next development stage for 99g would be advanced start-stop. Such belt-driven systems have already been shown by suppliers and could mean that Audi is ready to implement coast down - shutting the engine off when the accelerator is lifted - as part of its advanced stop-start strategy.

That start-stop is still set to feature as part of an automakers' CO2 reduction toolkit flies against some opinion that the penetration of stop-start will fall as its benefit will be negated by the introduction of the WLTC from 2017/2018 in Europe.

WLTC is a behind the scenes reason for Audi's latest diesel test bed - the electric biturbo as demonstrated in the RS5 TDI concept. In the concept the smaller of the two turbos works with an electrically driven compressor. Instead of the turbine wheel, it uses a small electric motor that applies seven kW to accelerate the compressor wheel to maximum speed within 250 milliseconds. WLTC has more accelerative phases than NEDC and the electric biturbo helps reduce emissions in such transient operations. Of course, this is not how Audi is positioning the technology. It's all about fun and reducing turbo lag as amply demonstrated at Sturup Test Track in Malmo, Sweden where RS5 TDIs concepts were drag raced off the line with RS6s. Despite giving away 129kW (173bhp), the RS5s more than held their own over the first fifty metres.

The electric biturbo is powered by a separate 48V circuit, which in itself opens up further future possibilities for CO2 reduction, with engine ancillaries and HVAC units potentially diverting less power from the ICE. Another opportunity being explored with 48V is the development of electric chassis control systems. A certainty is using the 48V circuit to produce a 48V mild hybrid that will take the advances mentioned already for future stop-start systems even further.

Audi is also investigating the possibility of using steel pistons in its TDI engines. Their greater thermodynamic properties give advantages in combustion that lead to a reduction in fuel consumption and emissions levels. Already Renault has announced that its 1.5L K9 diesels will switch to steel pistons shortly and deliver a 2% fuel economy benefit.

Injection pressures for common rail diesel systems have risen inexorably since their introduction. Higher injection pressures allow greater control of combustion to reduce emissions, more power and fuel economy benefits. In 2004, Audi's Bosch supplied common rail system was at 1,600 bar, today it stands at 2,000 bar and for the near future it is working on introducing 2,700 bar that would take it close to the 3,000 bar of its TDI race engines. Whether the higher system pressure, combined with future emission regulations would mean the end of solenoid injectors for some of its diesel programmes Audi would not divulge, although it's safe to say that solenoid injectors have been written off before and found a way back.

Similarly, turbo boost pressures are another area of development for Audi. It points out that boost was at 2.3 bar in 2004 and had increased to 3.2 bar in 2014, but with their race engines at 4.0 bar there is a clear opportunity to increase turbo boost further.

All this potential future development of the diesel engine at Audi seems very far removed from the original 88kW Audi 100 2.5L TDI of 25 years ago when only the convenience of stop-free 600km road trips to Wolfsburg could convince Audi management as to the future viability of diesel.  

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