Audi has been building diesel engines for over 30 years, the first of which was a five-cylinder unit with prechamber injection that made its debut in 1978. 11 years later, the brand made a technological breakthrough that revolutionized the diesel market. Under the abbreviation TDI, direct injection, turbocharging and totally electronic engine management were combined to create an entirely new dimension of powertrain technology.
Since then, the Audi brand's engineering advances have propelled diesel engines to an undreamed-of technological boom and overwhelming market success. The TDI from Audi had put an end once and for all to the old diesel image as "lame, loud, and dirty" and turned it into the opposite. Today nearly every automaker includes this technology in its program.
"20 years of TDI means 20 years of progress and dynamic change, sporty power and efficiency," says Michael Dick, Member of the Board of Management for Technical Development at AUDI AG. "The TDI has been a key factor contributing to the advance of our brand into the premium segment. It has become the world's most successful efficiency technology that is unsurpassed in the relation of power output to fuel efficiency."
The father of the Audi brand's TDI technology is Richard Bauder, who continues to head the company's diesel engine development to this day. The program was launched in 1976 with the 1973 oil crisis still fresh in mind, as Bauder reports. "What we set out to achieve was to develop an internal combustion engine with the lowest possible fuel consumption. We checked out all conceivable concepts all the way to a two-stroke diesel engine, and in the process analyzed and improved diverse injection and combustion methods. One of our big breakthroughs was the dual-spring injection-nozzle holder, which allowed the pre-injection of smaller fuel amounts. The result was smoother combustion and improved acoustics – the fundamental requirements for use in passenger cars."
The first TDI was a huge success right from the start. When installed in the third-generation Audi 100, the five-cylinder engine's 2,461 cc displacement achieved 88 kW (120 hp) and 265 Nm (195.45 lb-ft) of torque, the latter at 2,250 rpm. A distributor-type injection pump sprayed the fuel into the combustion chambers.
A pioneering achievement: Top speed of nearly 200 km/h (124 mph), fuel consumption of 5.7 l/100 km (41.27 US mpg)
As the engine went into production in the Audi 100 Avant late in 1989, its muscular power marked the dawning of a new era. At that time, diesel engines were considered economical and durable but rather dull performers. However, with a top speed of nearly 200 km/h (124 mph), the Audi 100 2.5 TDI propelled its way into the ranks of fast touring sedans, also featuring enormous acceleration from zero and amazing fuel economy of 5.7 liters of diesel fuel/100 km (41.27 US mpg) – established in line with the valid standards at the time. What's more, the car's travel range between refills was very generous.
In the midsize class too, TDI engines from Audi began to triumph. Starting in 1991, the Audi 80 was powered by a four-cylinder, 1.9-liter diesel unit that delivered 66 kW (90 hp) and 182 Nm (134.24 lb-ft). Four years later, an upgraded version with 81 kW (110 hp) was added. The increase in power was mainly due to the use of a new turbocharger with adjustable guide blades on the exhaust side: The so-called VTG charger with variable turbine geometry enabled torque to be boosted smoothly and promptly, even from very low engine speeds.
In 1993, the brand with the four rings converted its entire diesel program to TDI engines, and then in 1994 Audi took the next step: The five-cylinder unit was upgraded to 103 kW (140 hp). A six-speed transmission became the production model, and for the first time the TDI was combined with permanent all-wheel drive – in the first TDI quattro. With 290 Nm (213.89 lb-ft) of torque at 1,900 rpm, 208 km/h (129.25 mph) top speed and 9.9 seconds for the standard acceleration test from standstill, this car almost instantly became a sensation. The TV spot with the question "Where's the tank?" made it legendary: The A6 TDI covered a distance of up to 1,300 kilometers (808 miles) on a single tank of fuel.
1997: The first V6 TDI in a passenger car
In 1997 Audi yet again initiated a revolution – with the debut of the world's first V6 TDI in a passenger car. With its four-valve cylinder head, another innovation, this 2.5-liter power unit delivered up to 110 kW (150 hp) and 310 Nm (228.64 lb-ft) of torque. Two years later this was followed by the first V8 TDI from Audi. Installed in the A8, this too introduced a groundbreaking technology – common rail injection. The 3.3-liter unit delivered 165 kW (224 hp) and produced 480 Nm (354.03 lb-ft) of torque. Its 242 km/h (150.37 mph) top speed and quiet operation further contributed to the pleasure of driving a TDI car.
Another important new solution was developed for the four-cylinder engine. In 2000, a new high-pressure injection system, whose integrated pump/nozzle elements generated 2,050 bar of pressure, boosted the output to 85 kW (115 hp) and later to 96 kW (130 hp).
In the compact class, Audi set another milestone in 2001: The A2 1.2 TDI achieved an average fuel consumption of 2.99 liters/100 km (78.67 US mpg). The compact A2 with its lightweight aluminum body featured a three-cylinder diesel engine with 1.2 liters displacement. A derivation from its big brother, the 1.4 TDI, it generated 45 kW (61 hp) and 140 Nm (103.26 lb-ft) of torque. The A2 3L TDI was the world's first five-door car with fuel consumption of under three liters per 100 kilometers – and remains so to this day. In 2003, Audi also produced a more powerful cousin of its 1.9-liter TDI in the shape of the 2.0 TDI.
New generation: The 3.0 TDI
In large engines too, Audi vigorously advanced its technology. A 4.0-liter V8 with 202 kW (275 hp) was introduced in 2003 that anticipated some of the technical details of the new generation of V engines. The first full-fledged member of this family followed a year later in the shape of a new V6 TDI with 3.0 liters displacement. Its attributes – 90 degrees cylinder angle, 90 mm cylinder spacing, camshaft chain drive on the rear side – are standard features now in the new family of Audi V engines.
The three-liter engine with an output of 165 kW (224 hp) was equipped with common rail technology and innovative piezo injectors. These can inject very small doses of fuel, and by virtue of extremely fast opening and closing can deliver multiple separate pre-, main- and post-injections. When a voltage is applied to piezo crystals, they slightly expand in a fraction of a millisecond. In the injector, several hundred tiny piezo disks are stacked on top of each other, and the expansion of this stack is transferred directly to the injection needle.
Piezo injectors can thus produce precisely regulated increases in pressure and a uniform combustion process, which has brought engine acoustics into the region of gasoline engines. Another innovation was a particulate filter to clean the exhaust gas. In 2007/2008 the new four-cylinder 2.0 TDI too was equipped with a common rail system including piezo injectors. In 2009 this became the standard technology in all diesel engines.
Richard Bauder, who is in charge of diesel development at Audi, proudly takes stock of what has been achieved to date: "In 1989 we started out with 900 bar of injection pressure; today we're at 2,000 bar. During this period, our TDI units have increased by more than 100 percent in power output and 70 percent in torque relative to a given displacement. At the same time, emissions have been reduced by 98 percent." In the past 20 years, the TDI has experienced impressive growth: During this period, Audi has produced more than five million of these engines. Taking all makes into account, the TDI principle today powers every second passenger car built in Europe.
The TDI units marketed by Audi today are amazingly well-rounded performers – clean and efficient, dependable, refined, comfortable, and powerful. In motorsport, the world's toughest test lab, they are proving how closely efficiency and dynamics are related at Audi.
The R10 TDI racing car, whose V12 diesel engine delivers more than 480 kW (650 hp), has won the Le Mans 24 Hours three times in a row starting in 2006. Its successor, the R15 TDI with a V10 diesel engine, has been earning top rankings right from its debut in 2009.
In production cars too, TDI engines from Audi excel with their muscular power. The top version of the Audi Q7 is equipped with the world's strongest production diesel engine. The V12 TDI generates 368 kW (500 hp) and 1,000 Nm (737.56 lb-ft) of torque with its 6.0-liter displacement, endowing the big SUV with the performance of a sports car.
The 2.0 TDI with 125 kW (170 hp) and 350 Nm (258.15 lb-ft) of torque is another sporty engine. It delivers lively power and refinement, and thus lends brilliance to the dynamic TT and the TT Roadster. The lightweight, mainly aluminum bodies of this model series contribute to its superior driving performance: The TT Coupé, with quattro permanent all-wheel drive as standard equipment, accelerates in 7.5 seconds from zero to 100 km/h (62.14 mph) and attains a top speed of 226 km/h (140.43 mph). Yet on average it gets by on only 5.3 liters of diesel fuel per 100 km (44.38 US mpg).
Even better consumption: Technology from the modular efficiency platform
By their very nature, diesel engines utilize the energy contained in fuel especially well, and the technologies from the Audi brand's modular efficiency platform further amplify this effect. Intelligently managed subsystems such as a demand-controlled oil pump consume less energy. Sophisticated production methods reduce friction within the engine. In many TDI units for instance, laser beams are used to perfectly smooth down the cylinder paths. This makes them last even longer, further reduces oil consumption and reduces stresses at the piston rings, which decreases friction as well as fuel consumption.
In addition to the engines, start-stop and recuperation systems also contribute to high efficiency. So do many other engineering features in these vehicles, from lightweight aluminum body design to LED lights, and from the highly efficient air conditioning system to economical route guidance in the navigation system.
Owing to this abundance of cutting-edge technology, Audi now has 33 models in its program with CO2 emissions of less than 140 grams per kilometer (225.31 g/mile), nearly all of them with TDI engines. They achieve exemplary fuel economy. The 2.0 TDI in the A4 puts out 100 kW (136 hp) and gets by on 4.6 liters/100 km (51.13 US mpg) – a CO2 equivalent of 119 grams/km (191.51 g/mile).
The 1.6 TDI, which will join the A3 model series in 2010, delivers 77 kW (105 hp), but consumes only 3.8 liters of fuel per 100 kilometers (61.9 US mpg), corresponding to a mere 99 grams of CO2 per kilometer (159.33 g/mile).
During the very recent Audi Efficiency Challenge A to B, a tour of Europe covering more than 4,182 kilometers (2,599 miles), the two four-cylinder units have already proven their worth: The A3 1.6 TDI got by on 3.3 liters/100 km (71.28 US mpg), the A4 2.0 TDI on 4.4 l/100 km (53.46 US mpg). Such endurance tests are a tradition at Audi: Even the first Audi 100 TDI models traversed the continent, achieving sensationally low fuel consumption values as well as long distances traveled between refills.
The next step: Audi clean diesel
Even 20 years after the market launch of the TDI, Audi is continuing to expand its lead and is excellently positioned for the future. Its innovative clean diesel technology renders exhaust fumes still purer by largely converting nitrogen oxides into harmless nitrogen and water. The 3.0 TDI clean diesel engine already meets the strictest exhaust emission limits in the United States as well as the Euro 6 limits announced for 2014. It is already propelling both the Q7 and the A4.
In the medium term, Audi is relying on second-generation biofuels such as sunfuel. These fuels use the residues of energy plants – not merely their fruits, as is the case in current versions of biodiesel. During combustion, they merely emit as much carbon dioxide as they have consumed from the atmosphere during their growth. Independently of this, Audi will continue to further reduce fuel consumption of its fleet of vehicles – with a target of 20 percent below its 2007 level by 2012. The potential inherent in the TDI is far from exhausted. In the past 20 years, Audi has written a tremendous success story with this engine concept – and will continue to do so during the next decade.
A complete overview of the history of the TDI including all engines, technologies, data and facts is presented in a new multimedia tool – the TDI Chronicle from Audi. Designed especially for the media, this reference source is published concurrently on DVD and on the Internet. You'll find the link as customary at www.audi-mediaservices.com.