Audi already has various hybrid models on the market. This year will see the rollout to dealers of the compact A3 Sportback e-tron with its plug-in hybrid technology. The next step will follow shortly – the new models with longitudinally mounted engines.
The second-generation modular longitudinal engine mounting system is designed to allow the electric motors to work together with the combustion engines – including the TDI units. Their interaction is tailored to the specific model. Audi has developed a technology matrix incorporating electrification stages through to the plug-in hybrid drive.
The interaction of the TDI with the electric motor offers the developers new freedom, permitting the load point to be shifted so as to benefit both fuel efficiency and emissions. In urban driving, where the TDI nowadays has to handle low loads, the electric motor provides zero-emission power on a local level.
The electric biturbo is a completely new technology developed by Audi. In it, the turbocharger works together with an additional electrically powered compressor. Instead of the turbine wheel, it has a small built-in electric motor which accelerates the compressor wheel rapidly to very high speeds.
The electric turbocharger is arranged downstream of the charge air cooler; in most operating states it is bypassed. If the exhaust gas has little energy at very low engine speeds, the bypass valve closes and the electric turbocharger is activated. The new technology enables previously unachieved levels of spontaneous power build-up when driving off and at low revs.
When the Audi 100 TDI was launched back in 1989, an oxidation catalyst was sufficient to clean the exhaust gas. In the early years of the new millennium, the technology made rapid progress as particulate filters became standard equipment. Exhaust gas cleaning has been at the core of diesel technology ever since. Today, the Euro 6 emissions standard necessitates the installation of high-tech systems such as SCR/DPF catalytic converters. They are designed to respond as early as possible – the increasing efficiency of Audi TDI engines is continually reducing exhaust gas temperatures.
A turbocharger comprises a turbine driven by the exhaust gas flow and a compressor for the intake air. The two components are mounted on a single shaft. The performance specifications of modern-day Audi turbochargers are impressive. The turbocharger of the new 3.0 TDI engine builds up to 2.0 bar relative charge pressure, and at full load can theoretically compress 1,200 cubic meters (1.44 metric tons) of air an hour. Its driving power is around 35 kW and it achieves a speed in excess of 200,000 rpm.
Audi's ongoing advancement of turbocharger technology is focused particularly on efficiency, torque development, transitional behavior, acoustics, and lightweight construction. Progress is being made through innumerable single steps, and in many cases by increments of thousandths of a millimeter. Exhaust gas temperatures, reaching peaks of 830 degrees Celsius (1526 degrees Fahrenheit), impose high demands on moving parts, and any further rise necessitates the use of new materials.
At high combustion temperatures, unwanted oxides of nitrogen are formed in all internal combustion engines. Most of them can be avoided by the exhaust gas recirculation process. The EGR process in Audi TDI engines feeds most of the exhaust gas back into the combustion chambers. This reduces the proportion of oxygen-rich fresh air, and the combustion temperatures fall.
EGR was introduced in Audi’s first 2.5 liter TDI during its 1994 evolution phase. To boost its effect, water-cooled EGR is nowadays used in all models, featuring a cooler governed by a mapped characteristic in the return exhaust line to the engine.
The new three- and four-cylinder TDI engines combine high and low pressure EGR. The uncooled high pressure EGR is activated after cold-starting and at very low load. The cooled low pressure EGR, covering most driving, is very compact and designed to ensure low flow losses.
In order to comply with the limits laid down by the Euro 6 emissions standard, necessitating a substantial reduction in nitrogen oxide emissions, Audi has converted its TDI engines to clean diesel technology. In most cases this requires measures in the engine and the exhaust tract; for the more compact engines and vehicle models a Nox storage-type catalytic converter is sufficient for the purpose.
The technology for the larger models and engines is more complex. The new 3.0 TDI, for example, has an enlarged oxidation catalyst, which in the 160 kW (218 hp) version is electrically heated. The diesel particulate filter is installed directly downstream of the oxidation catalyst. The filter walls have a coating which additionally converts the nitrogen oxides in the exhaust gas based on the SCR technique. To ensure rapid activation, the exhaust gas cleaning components are located as close to the engine as possible.
The next step by Audi will be in the 3.0 TDI scheduled for launch in 2015. It will feature a new NOx storage-type catalytic converter instead of the oxidation catalyst. This stores the nitrogen oxides until it is full; the cleaning is effected by enrichment of the mixture in the engine. So as to minimize any increase in fuel consumption, the new catalytic converter only activates at low exhaust gas temperatures, after starting the engine and under low load. In all other situations the SCR-coated diesel particulate filter converts the NOx.
When diesel is burned in an engine, soot particles are formed in some areas of the combustion chambers. To eliminate these particles, Audi installs what is known as a wall flow filter, a closed-circuit system 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 dependent on the way the vehicle has been driven. This burn-off process is initiated by deliberately retarding the post-injection of fuel into the engine, which causes the exhaust gas temperature to briefly rise dramatically.
Audi’s first TDI with a particulate filter was the 3.0 TDI, introduced in 2004. Since the spring of 2006 the brand with the four-ring emblem has installed this technology as standard equipment in all TDI models. The latest developments are particulate filter with a SCR coating, which additionally convert the NOx.
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 inside each injector, and the expansion of this cluster – less than a tenth of a millimeter – is transmitted directly to the injector needle.
The injectors close again after one or two thousandths of a second. In this way very small amounts of fuel weighing as little as 0.8 of a milligram can be injected. The fuel leaves the nozzle holes at a speed that can be as high as 2,000 km/h (1,242 mph).
A Common Rail fuel injection system is a tubular high-pressure accumulator which stores the fuel constantly under high pressure. It is replenished by a pump driven by the engine. The injectors are connected to the rail by short steel pipes, and opened and closed by electrical impulses. Common Rail technology separates the pressure generation from the fuel injection, so the developers can define all injections in the map at will.
Audi's Common Rail fuel injection system runs in most engines up to peak pressures of 2,000 bar; the next target is 2,500 bar. The nozzle holes in the piezo injectors Audi uses in its V-engines are only about 0.1 millimeters in diameter, enabling the fuel to be finely atomized even under low load. The higher the pressure, the more precise the mixture formation – and that boosts power output and torque, as well as making the engine run more smoothly and helping to cut emissions.
The Common Rail system of the new 3.0 TDI is able to deliver nine single injections per stroke. The pre-injections help the engine run smoothly at low speeds especially, while the post-injections regenerate the exhaust gas cleaning components. Audi fuel injection systems must assure milligram precision for tens of thousands of miles.
A factor of key importance on a diesel engine is the fuel injection pressure. High pressure can achieve an optimal spray pattern in the combustion chamber, so that ignition takes place more rapidly and smoothly. More efficient combustion means higher power output, lower fuel consumption and therefore reduced exhaust emissions. The pump-nozzle injection system had its Audi premiere in 1999, when the 1.4 TDI began to be supplied with fuel at a maximum pressure of 2,050 bar. Each of the engine’s three cylinders was provided with a separate unit in which the pump and nozzle were integrated. A separate cam on the camshaft actuated a roller cam follower that moved the piston in the pump. The pressure built up in this way inside the pump was boosted hydraulically to more than 2,000 bar before the fuel reached the injector nozzle. The actual injection of fuel into the combustion chamber was controlled by a solenoid valve, with pre-injection made possible by a complex additional control system. Audi used the pump-nozzle injection principle on its three- and four-cylinder engines until it was discontinued in 2008
Engines with four valves in the combustion chamber operate more efficiently than two-valve engines, because internal gas flow is speeded 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 principle with double 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 decisive advantage was obtained by giving the two inlet ports different patterns: The swirl-action port created turbulence in the intake airflow at low loads and engine speeds, which resulted in higher torque. The tangential port made the engine more dynamic at higher speeds by reducing friction. To modulate the swirl action as accurately as possible, Audi installed switchable flaps in the ports.
The first turbochargers used on Audi’s TDI engines had rigid blades, but in 1995 the 81 kW (110 hp) 1.9 TDI was given adjustable blades on the exhaust side. Known as variable turbine geometry (VTG), this principle allowed torque to build up smoothly and without delay at much lower engine speeds than before. If the driver presses the gas pedal down firmly at a low engine speed, the turbine blades move to a shallower angle. This reduces the cross-section of the inlet to the turbine housing and forces the exhaust gas flow to speed up and strike the outer face of the blades. 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 if less boost pressure is needed, the turbine blades return to a steeper angle. This increases the inlet cross-section, so that the exhaust gas flows less rapidly and the speed of the turbine rotor is reduced. Boost pressure and turbine output remain more or less constant in this operating situation.
The distributor-type fuel pump, perhaps more accurately described as an axial piston injection pump, was the technology that enabled Audi to start series production of the TDI engine. It was a compact unit, driven by toothed belt from the crankshaft. The central element was a slotted piston that delivered fuel at high pressure to the pipes that led to the injectors. Two-spring injector holders opened the injector needles in two stages: first with a smaller, then with a larger stroke. The axial piston pump could build up a maximum pressure of 900 bar at the injector nozzles; a further evolutionary stage in 1994 increased this to about 1,000 bar. For the 1997 V6 TDI, Audi switched to a radial piston injection pump. This operated according to a similar but more advanced principle, and was able to generate a pressure of 1,500 bar, later 1,850 bar at the injector nozzles. The amounts of fuel injected were between two cubic millimeters for the pre-injection stage and 50 cubic millimeters, equivalent to one droplet of fuel, at full load.
In 1892, when Rudolf Diesel applied for a patent on his first engine, he was already considering direct fuel injection, but for many years this proved impossible to achieve, since it was too difficult to manufacture high-performance injection pumps.
The first diesel engines in passenger cars were introduced in 1936. They were naturally aspirated, with fuel injected at a pressure of just over 100 bar into a pre-combustion chamber. This was located ahead of the main combustion chamber in the cylinder head. The fuel vaporized and ignited there, after which the combustion process spread through an aperture into the main chamber. This principle had the advantages of relatively smooth, gradual mixture ignition at an acceptable acoustical level, and improved dynamics. The principal disadvantages were flow losses, heat dissipated through the cylinder wall and the slow speed of the combustion process. The turbocharged diesel that gained some ground in the 1970s suffered from the same shortcomings.
These handicaps were eliminated by the direct injection principle, which has evident benefits in terms of efficiency. It was first adopted in the 1950s for commercial vehicles. As injection pump design progressed and electronic system management became practicable, Audi produced a version suitable for use in passenger cars in the late 1980s.
The first Audi TDI, the 2.5-liter five-cylinder engine introduced in 1989, had an electronically controlled distributor-type injection pump operating at a pressure of up to 900 bar. The engine incorporated a large number of high-end features: swirl-action inlet ports to impart turbulence to the incoming air, four-hole injector nozzles for an accurate fuel spray pattern and two-spring nozzle holders that permitted a pre-injection phase which made the combustion process less violent and reduced the noise level. The charge-air intercooler lowered the temperature of the air compressed in the turbocharger, so that the combustion chambers were supplied with a higher proportion of oxygen. With an output of 88 kW (120 hp), and the notably high peak torque of 265 Nm (195.45 lb-ft), even this first TDI engine was an impressive source of power.