The Audi TDI – Tech Workshop 2014
Turbocharging, fuel injection and emissions control are the three main drivers for the development of TDI engines. The engineers are working continuously to further reduce fuel consumption, increase power and torque, and improve running smoothness.
Additional requirements have their roots in legislation and business, such as emissions standards, quality requirements and the composition of diesel fuel in the various markets throughout the world. In Europe, the focus is on biodiesel admixtures and the future RDE (real driving emissions) test cycles. The new 3.0 TDI is already equipped with a two-stage, cooled exhaust gas recirculation system for the ULEV-II standard in some states in the U.S.A. If China, a country in which fuel quality still varies widely today, becomes a diesel market, the criteria altitude and thin air will become more important.
The numbers used today to describe the Audi turbochargers are impressive. The turbocharger in the new 3.0 TDI develops up to 2.0 bar of relative boost pressure, and at full load can theoretically compress 1,200 cubic meters (1.2 metric tons) of air per hour. Its drive power is in the range of 35 kW with over 200,000 revolutions per minute.
Audi is continuously working to advance the development of turbocharger technology, and its engineers place great emphasis on efficiency, torque development, transition behavior, acoustics and lightweight construction. Progress is made in the form of countless individual steps and in the thousandths of a millimeter range. One example of this are the future compressor wheels. They are machined from a solid block and are even more precise than the castings used today.
The turbocharger in the new 3.0 TDI uses an electric VTG actuator that adjusts the position of the turbine wheel vanes in less than 200 milliseconds. It is installed in a newly developed cartridge, whose halves are riveted together. The narrow rivets disrupt the flow less than the cast connecting points of the previous-generation component. The exhaust gas temperatures, which can reach 830 degrees Celsius, place extreme demands on the moving parts in particular. Any further increase in temperature would require new materials.
With most engines, Audi uses peak pressures of 2,000 bar in the common rail injection system. The next target is 2,500 bar, and the engineers are already thinking beyond this mark. The TDI engine in the Audi R18 e-tron quattro race car shows what is possible. The four-liter V6 uses an injection pressure of over 2,800 bar to produce roughly 100 kW per liter of displacement. Steel pistons, another option for production models, absorb the ignition pressure, which at for more than 200 bar exceed the level of street TDI engines.
The piezo injectors that Audi uses in its V-engines have nozzle holes only 0.1 millimeter (0.004 in) in diameter so that the fuel can be finely atomized even at low load. The higher the pressure, the more precise the mixture, and that benefits not just power and torque, but also smoothness and emissions.
The common rail system in the new 3.0 TDI can make nine individual injections per work cycle. Pre-injection helps to ensure that the engine runs smoothly at low speeds; post-injection serves regeneration of the particulate filter and desulfatizing of the future NOx storage catalytic converter. Audi injection systems must retain their precision, which is measured in the milligram range, over many tens of thousands of kilometers. Even tiny deviations could adversely impact emissions testing results.
In the past, the developers had to design emissions-control components for early response. As the efficiency of the TDI engines increases, exhaust gas temperatures are steadily falling. In the ECE cycle, temperatures measured downstream of the oxidation catalytic converter take 2.5 minutes to reach 150 degrees Celsius. Conversion does not take place below this threshold.
With the new 3.0 TDI, both catalytic converters – the enlarged oxi-cat and the diesel particulate filter with SCR coating – have been moved extremely close to the engine. The water-cooled SCR pump injects the AdBlue solution into the short, bent connecting pipe between the two. With the 160 kW (218 hp) version of the new V6 diesel, the new V6 biturbo and the 4.2 TDI, the oxi-cats are also electrically heated.
Audi’s next step will come in 2015 with the 3.0 TDI. Instead of an oxidation catalytic converter, a new NOx storage catalytic converter will be used. The NOC (NOx Oxidation Catalyst) stores the oxides of nitrogen until it is completely full. Cleaning is by means of mixture enrichment in the engine. To keep fuel consumption as low as possible, the NOC is only active at low exhaust gas temperatures, i.e. following engine start and at low load. In all other situations, NOx conversion is handled by the diesel particulate filter with SCR coating. With the great potential harbored by these technologies, Audi is extremely well positioned to meet future emissions regulations.