U.S. customers haven't had quite the range of diesel vehicles their European counterparts enjoy, but it's still hard to believe that one of the biggest badges in diesel technology--TDI--is now a quarter-century old.

Audi launched its first TDI diesel model at the 1989 Frankfurt Auto Show.

That car was the Audi 100 TDI, a 2.5-liter, five-cylinder turbodiesel producing a modest 120 horsepower and 195 lb-ft of torque. Since then, the cars and the engines that power them have changed almost beyond recognition.

TDI stands for Turbocharged Direct Injection. While neither direct injection nor turbocharging, nor even a combination of the two were unique to Audi in 1989, the German automaker and its parent company Volkswagen certainly popularized the concept.

By combining direct injection of atomized fuel into the cylinder and turbocharging to boost the engine's power output, TDI engines could develop more power and torque yet use less fuel than their predecessors.

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Developments in the technology were such that European buyers bought TDI-badged models in their droves--throughout the 1990s, economy from turbodiesel Audis and Volkswagens was significantly better than their gasoline counterparts.

As Europe began to adopt a CO2-based taxation system, TDIs became even more popular--the diesels' high economy figures resulted in lower CO2 outputs.

At the same time, improvements in software systems, fueling, engine design and emissions technology saw performance increase, refinement improve and emissions fall significantly.

This has reached a head with the latest generation of urea-injection technology. This reduces pollution by combining ammonia and carbon dioxide with nitrogen oxides in the exhaust fumes, turning them into water and nitrogen.

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The advent of particulate filters has also cut down on diesel pollution over that time. Harmful particulates in diesel exhaust are trapped in the filter and burned off when speeds rise.

Fuel efficiency benefits are clear too--most Volkswagens using the current 2.0-liter TDI engine are capable of over 40 mpg highway in EPA testing--potentially more in the real world--while even Audi's range of 3.0-liter V-6 TDIs promise highway figures in the high 30s.

The 2015 Audi A6 TDI and 2015 Audi A7 TDI, for example, promise 24 mpg city, 38 mpg highway and 29 mpg combined figures with the standard automatic gearbox.

Figures for the 2015 Audi A3 TDI sedan--with its 150hp, 236 lb-ft 2.0 TDI--have not yet been revealed, but the $32,600 sedan should be one of the most efficient cars in its class when it hits the streets later this summer.

Throw in a quarter-century of diesel concepts and multiple wins for TDI engines at the famous Le Mans 24 Hours race in France, and it's easy to see why Audi is celebrating TDI's 25th year.


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