The Audi TDI – Tech Workshop 2014
From A for the Audi 100 TDI to V for Vermicular graphite cast iron, Audi has perfectly mastered the alphabet of diesel technology.
Audi 100 TDI (1989)
At the 1989 IAA Frankfurt Motor Show, Audi exhibited a milestone in automotive technology: The five-cylinder in the Audi 100, which displaced 2,461 cc, was the first direct-injection turbo diesel with fully electronic management – the first TDI. With 88 kW (120 hp) and 265 Nm (195.5 lb-ft) of torque, the latter at 2,250 rpm, the two-valve-per-cylinder engine produced ample power with groundbreakingly low fuel consumption.
Swirler channels in the inlet ports produced turbulence in the air. The distributor-type injection pump developed up to 900 bar of pressure, and the five-hole nozzles in the injectors ensured a precise spray pattern. The two-spring nozzle holders – one of the major breakthroughs in the development of the TDI – made possible a pre-injection that took the harshness out of the combustion process and reduced the noise level. The charge air cooler reduced the temperature of the compressed induction air.
Audi is also continuously improving the efficiency of the ancillaries. The latest oil pumps, for example, are hydraulically regulated by the volumetric flow and only consume the amount of energy they actually need. Audi is working on electrified units in the medium term.
Audi e-diesel is a synthetic, CO2-neutral fuel of the future. To make it, special microorganism that live in water produce long-chain alkanes – important components of diesel fuel, by photosynthesis. All they require for this is sunlight and CO2. The new fuel offer stands out for its chemical purity and high cetane number.
Audi has partnered with the American biotechnology company Joule to build a demonstration plant in New Mexico, which in addition to Audi e-diesel also produces Audi e-ethanol. Cars that use these fuels are similarly eco-friendly as pureLY electric cars operated on green electricity.
The 3.0 TDI biturbo is Audi’s most powerful V6 diesel. A changeover valve connects its two in-series turbochargers. At low revs it is closed. The small charger with its variable turbine geometry does most of the work, and the large charger is responsible for the pre-compression. From about 2,500 rpm, the valve starts to open and the small charger increasingly transfers the major share of work to its counterpart. In the range between 3,500 and 4,000 rpm, the valve opens completely, and only the large charger still operates.
Given its high-performance concept, Audi has refined numerous details of the engine and its periphery. A sound actuator in the exhaust system gives this diesel engine a rich, resonant sound reminiscent of an eight-cylinder unit. The 3.0 TDI biturbo is used in such models as the Audi SQ5 TDI, the first Audi S model with a diesel engine.
To drastically reduce oxides of nitrogen emissions and thus meet the limits of the new Euro 6 emissions standard, Audi is converting its TDI engines to clean diesel technology. In most cases, this requires changes to the engine and the exhaust system. A DeNOx catalytic converter suffices for the more compact engines and models.
The technology is a bit more complex for the large models and engines. The new 3.0 TDI has a larger oxidation catalytic converter, which in the version with 160 kW (218 hp) is electrically heated. A larger catalytic converter with oxygen sensor is arranged coaxially downstream of the turbocharger’s turbine outlet. Audi is the first carmaker to combine a NOx storage catalytic converter with a diesel particulate filter and SCR injection (selective catalytic reduction) in one assembly. A metering module injects the additive AdBlue.
A common rail injection system is a pipe-like high-pressure accumulator that maintains the fuel at a constant high pressure, which in Audi production engines is up to 2,000 bar. It is filled by a pump powered by the engine. The injectors are connected to the common rail by short steel pipes, and opened and closed by electrical impulses.
Common rail technology separates pressure generation from injection, which enables the developers to freely configure all injections in the characteristic. This gives them great freedom; up to nine individual injections are possible per work cycle. The pre-injection stages allow fuel pressure to be built up gradually, so that combustion is quieter; post-injection reduces pollutant emissions and is also used to regenerate the particulate filter.
The electric biturbo is a brand new technology from Audi. The exhaust turbocharger works together here with a supplemental, electric-powered compressor. Instead of a turbine wheel, it uses a small electric motor that accelerates the compressor wheel to very high speeds in an extremely short time.
The electric turbocharger is downstream of the intercooler. In most operating states, it is bypassed. When the energy of the exhaust gas is low at very low engine speeds, the bypass valve closes and the new component is activated. The new technology enables a spontaneous development of power never before seen when starting off and at low rpm.
Exhaust gas recirculation (EGR)
At high combustion chamber temperatures, oxides of nitrogen are produced in combustion engines. These can be largely avoided through the use of exhaust gas recirculation. With the TDI engines, EGR returns a large portion of the exhaust gas to the combustion chamber. This reduces the fraction of fresh, oxygen-rich air and combustion chamber temperatures decrease.
Audi introduced EGR with its very first TDI engine: The first generation of the 2.5-liter five-cylinder was equipped with it in 1994. To increase efficiency, a water-cooled system is used on nearly all engines today. On its way back to the engine, the exhaust gas flows through a water cooler. The new 2.0 TDI and the future 1.4 TDI combine a cooled and an uncooled EGR.
Engines with four valves operate more efficiently than two-valve engines, because internal gas flow is sped up and the cylinders filled more effectively. Since they burn their fuel with greater efficiency, they generate more power and torque, with reduced consumption and exhaust emissions.
Audi introduced the four-valve technology with dual overhead camshafts for the 2.5-liter V6 TDI diesel in 1997. The injector nozzle could then be in the ideal position at the precise center of the combustion chamber. Another major advantage was obtained by giving the two inlet ports different patterns: In the swirl duct, the inflowing air is turbulent at low load and rpm, which increases torque. The tangential channel enables high dynamics by reducing resistances at higher rpm.
Audi already has a variety of hybrid models on the market, and the compact Audi A3 Sportback e-tron with its plug-in hybrid technology is coming to dealerships this year. The next step will follow shortly: the new models with longitudinal engines.
The second-generation modular longitudinal platform is designed for combining electric motors with combustion engines, including TDI units. This will be done specifically for each model. Audi has developed a technology matrix with electrification stages up to the plug-in hybrid drive.
When a turbocharger compresses the intake air, it heats it to as much as 200 degrees Celsius. Hot air has a lower density, however, and thus contains less oxygen for combustion. An intercooler is therefore placed downstream of the turbocharger to greatly cool the compressed air before it enters the combustion chamber.
Intercoolers are standard equipment at Audi. Depending on their design, they use air and/or water from the coolant circuit as a cooling medium. The Audi engineers have also taken measures to maximize efficiency in the intercooler – in terms of weight, efficacy and lower flow resistance.
Audi has drastically reduced internal friction in many of its TDI engines. Among the means used to do this are high-end processing technologies in manufacturing, such as laser exposure and plate honing of the cylinder sleeves. The more durable and precise cylinder sleeves make it possible to minimize the tension of the piston rings so that they slide more easily. Smaller bearings at the crankshaft, the connecting rods and the camshafts also contribute greatly to the reduction in friction.
Another field of innovation are the materials in the engines. In the new 3.0 TDI, for example, the first piston ring has a coating produced using an innovative method. The piston pins have a diamond-like carbon (DLC) coating.
The common rail systems from Audi are extremely high-precision components. They inject tiny amounts of fuel into the combustion chambers at pressures of up to 2,000 bar. The fuel exits the nozzles at several times the speed of sound.
In some engines, Audi uses piezo injectors with eight-hole nozzles, with each hole only around 0.1 millimeters (0.0039 in) in diameter. The extremely fine atomization produces a spray pattern in the combustion chamber that provides for ignition and combustion that is fast, homogenous, acoustically comfortable and above all efficient.
When diesel oil is burned in an engine, soot particles are formed in the combustion chamber in certain operating ranges. To eliminate these particles, Audi uses diesel particulate filters with an efficiency of more than 95 percent.
As the particles flow into the filter, they adhere to its porous wall. They are burned off at intervals that depend on the way the vehicle has been driven. This burn-off process is initiated by deliberately retarding the post-injection of fuel, which causes the exhaust temperature to rise sharply for a brief time.
The piezo principle is an ideal complement for common rail fuel injection. Piezo crystals change their structure in a few thousandths of a second by expanding slightly when an electrical voltage is applied to them. Several hundred piezo wafers are stacked one above the other in the injector. As this stack expands, linear movement takes place and is transmitted directly to the injector needle, with no mechanical linkage in between.
The injectors close again after mere thousandths of a second. In this way, very small amounts of fuel weighing as little as 0.8 of a milligram can be injected.
SCR catalytic converter
SCR stands for selective catalytic reduction for the conversion of oxides of nitrogen in the exhaust gas. A solution known as AdBlue is injected into the SCR catalytic converter from a storage tank. In the hot exhaust gas flow, this waterborne additive breaks down and forms ammonia, with the help of which the oxides of nitrogen are converted into harmless nitrogen and water. Audi has combined the SCR catalytic converter and the diesel particulate filter in the new 3.0 TDI.
The thermal management system reduces the fuel consumption of TDI engines by several percent. The details vary depending on the engine. In the new 3.0 TDI, for example, the cylinder crankcase housing and the cylinder heads have separate cooling circuits. To reduce the pressure losses, the water jackets for the heads are divided into two sections.
During the warmup phase, the coolant is not circulated and the oil cooler is bypassed. The motor oil quickly comes up its operating temperature and the phase of elevated friction losses due to cold, viscous oil in the crank and valve train are greatly reduced. The head circuit supplies the cabin heating and exhaust gas recirculation system. The coolant in the crankcase is often not circulated, even at low load when the engine is warm. This saves drive energy for the water pump.
A turbocharger comprises a turbine driven by the exhaust gas flow and a compressor for the intake air. The two components are opposite one another on a common shaft, and their maximum speed can be more than 200,000 rpm. The Audi turbochargers develop up to 2.2 bar of relative boost pressure. The 3.0 TDI biturbo theoretically compresses 1,200 cubic meters of air per hour at full load.
The monoturbos and the biturbo in the Audi lineup will be joined in the future by the electric biturbo. Audi is working very hard to further advance all aspects of turbo technology to make throttle response, efficiency, weight and acoustic even better.
Variable Turbine Geometry
Variable Turbine Geometry (VTG) is standard with the TDI engines from Audi. It provides for a spontaneous and smooth buildup of torque at low rpm. If the driver presses the gas pedal down firmly, the turbine vanes move to a shallow angle. This reduces the inlet cross-section into the turbine casing and forces the exhaust gas to flow in at a higher speed. The turbine wheel rotates faster, the volume of fresh air delivered by the turbocharger increases and boost pressure builds up instantly.
As the volume of exhaust gas increases or at low load, the turbine vanes return to a steeper angle. The inlet cross-section increases, and the exhaust gas flows more slowly. The turbine wheel also spins slower, while boost pressure and turbine output remain virtually constant. Audi uses electric VTG actuators in the large TDI engines, pneumatic ones in the four-cylinder diesel engines.
Vermicular graphite cast iron
The crankcase of the Audi V6 TDI engines and the eight-cylinder engine are made of vermicular graphite cast iron (GJV-450). The material with the wormlike distribution of graphite produced in a high-tech casting process is characterized by extreme strength, even at high temperatures. Compared to gray cast iron (GJL), it permits lower wall thicknesses, which reduce the weight.