Audi Tech: 1.8 TFSI and 2.8 FSI with Audi Valvelift System

[Bartek Sikorski]

Audi has been building highly efficient combustion engines for many years. That applies not only to the TDI, which Audi pioneered, putting the first engine onto the market in 1989, but also and in particular to the spark-ignition engines. The abbreviation FSI stands for petrol direct injection. Together with the additional letter T it means there is also a turbocharger. Audi's TFSI engines play a pioneering role in the field of compact, lightweight and highly efficient 4-cylinder engines, and the current V6 FSI enjoys top ratings among its competitors thanks to a whole package of technological innovations. A turbocharged 4-cylinder direct injection petrol engine achieves fuel savings of 10 – 15 percent compared to a conventional V6 naturally aspirated engine.

FSI technology, where the fuel is injected directly into the combustion chambers, is thermodynamically superior to conventional intake manifold injection systems. Preparing the mixture in the combustion chamber removes heat from the fresh air drawn in. The result is a higher mixture density that increases the charge and at the same time reduces the tendency to knock. This allows ignition timing that is superior from the point of view of efficiency. The upshot is greater compression, which in turn improves effectiveness and thus efficiency. This enables the FSI engines to develop more power and dynamism with lower fuel consumption.

Combined with a turbocharger, FSI technology displays even greater potential. This was demonstrated for the first time in June 2001, when a TFSI engine powered the Audi R8 sports prototype to overall victory in the Le Mans 24 hour race. In the years that followed it won a further 64 races in 80 starts all around the world. In particular it was the extra range resulting from the lower fuel consumption that gave the direct injection engines the edge over the competition.

Supreme and efficient: the new V6 FSI – 2.8 FSI in the A6 and 3.2 FSI in the A5
When it comes to spark-ignition engines with six or eight cylinders, Audi still opts for naturally aspirated concepts. On the new Audi A5, the 3.2 FSI assumes the role of the most powerful spark-ignition engine, with longer stroke and slightly larger bore it is the big brother of the 2.8-litre unit found in the A6 and the Audi A8. From the autumn on, it will make the sportiest luxury saloon the most economical vehicle in its class. Compared to the predecessor model, the stately, highly refined V6 has been greatly modified. It now combines a whole package of high-tech solutions: FSI petrol direct injection, the innovative valve control system "Audi valvelift system" and various measures for reducing internal friction and thus further increasing efficiency.

As members of the Audi's family of V engines, the 2.8 FSI and the 3.2 FSI are characterised by a cylinder angle of 90 degrees, compact dimensions and a light weight of 165 kg and 171 kg respectively. The crankcase is made of an aluminium/ silicon alloy. The overall vehicle weight and the axle load distribution also benefit from this.

The new 2.8-litre engine, which debuted in the Audi A6 at the end of 2006, develops a power output of 154 kW (210 bhp) and a peak torque of 280 Nm in the engine speed range 3,000 to 5,000 rpm. Its big brother in the new Audi A5 3.2 FSI develops 195 kW (265 bhp) from a 3197 cm3 capacity. Its peak torque of fully 330 Nm is consistently available across the range 3,000 to 5.000 rpm. It accelerates the Audi A5 3.2 FSI quattro from 0 to 100 km/h in just 6.2 seconds. This impressive power can be enjoyed with remarkably low fuel consumption – just 8.7 litres/100 km for the 3.2 FSI multitronic continuously variable automatic transmission. The Audi A6 2.8 FSI with manual gearbox also only needs 8.7 litres/100 km.

About half of this progress in fuel consumption stems from the Audi valvelift system. The new valve control technology saves a good five percent of fuel. With the Audi valvelift system, the valve lift characteristics are changed between full load and part load.
This effects a dethrottling of the intake process in the part load range. At the same time, the charging movement in the combustion chamber can be specially tuned through the different valve lift patterns of the two inlet valves. Load is controlled by the variable valve lift and the variable camshaft system, only to a moderate degree by the throttle valve. The engine consumes less energy during load changes and is therefore more economical.

While conventional technologies in this field work with complex and bulky elements between the camshafts and the valves, Audi has shifted the control mechanism directly onto the camshafts – a solution that is both highly efficient and brilliantly simple. The intake camshafts on the V6 are made up of a basic shaft with splines, each of these bearing cylindrical sleeves. These so-called cam pieces carry two different cam contours for small and large valve lift. Driven by lightning-fast electromechanical actuators, metal pins engage in the spiral grooves on the sides of the rotating cam pieces, moving these seven millimetres along the basic shaft.

In the part-load range, the small cam contours actuate the roller cam followers of the valve control system. Here, the valves open 2.0 and 5.7 millimetres respectively. The different opening allows the mixture to swirl in a defined manner, making for particularly clean combustion. In the full-load range, the larger contour opens both valves by 11.0 mm. Depending on the engine's operating situation in the speed range 700 to 4000 rpm, the changeover process is completed within two turns of the crankshaft. The engine management system uses numerous complementary measures to ensure that the transition is gentle and imperceptible. All the driver feels is a smooth, turbo-like build-up of power and a spontaneous response.

Its straightforward layout makes the Audi valvelift system stable at engine speeds up to 7,200 rpm. This allows high power output. Its greatest potential for savings is at constant speeds mid-way up the part-load range. At a gentle motorway cruise at about 150 km/h in sixth gear, i.e. at about 4,000 rpm, the engine is still working with short valve lift.

The cooling effect of the FSI petrol direct injection allows the 3.2 FSI to achieve a high compression ratio of 12.5:1, which makes a substantial contribution to the highly efficient combustion. The intake paths are variable – an electronically controlled flap in the intake manifold changes between two path lengths depending on load and engine speed.

The new engines achieve further progress through the timing chains that drive the camshafts. Typically for Audi's V engines, these are compactly mounted at the rear of the engine. The intermediate gears and sprockets on the shafts now have more teeth – making them quieter and reducing the forces on the chains. The tri-oval, i.e. virtually triangular, design of the sprockets achieves a similar effect. The three newly developed Simplex roller chains have been optimised for smooth running and maximum wear resistance. No maintenance, let alone a change, is needed throughout the entire service life.

The oil pump has also been considerably modified. With a 30 percent lower delivery rate, the pump is now controlled by volumetric flow and therefore demand-responsive. At an engine speed of 4,600 rpm, it changes from the low to the high pressure stage to meet the increased need for oil and oil pressure. The spray jets for cooling the piston crowns then also cut in.
The sum of all these measures, which also include a smaller water pump and reduced piston ring tension, is a significant drop in losses due to friction within the engine. The so-called friction mean effective pressure at 2000 rpm has been reduced by 0.22 bar, equivalent to 25 percent. This effect produces a fuel saving of around 5 percent.

Efficient and compact: the new TFSI engines
FSI technology is complemented so well by a turbocharger because the fuel evaporates directly in the combustion chamber, absorbing heat. This resolves an old problem with all turbocharged engines, namely the great heat development and the resulting tendency to knock. This phenomenon used to make it necessary to reduce the basic compression ratio, meaning a drop in overall efficiency.

The 2.0 TFSI has a compression ratio of 10.3:1, higher than some naturally aspirated engines. For the new 1.8 TFSI, which celebrated its debut in early 2007 on the Audi A3 and A3 Sportback, Audi opted for a compression ratio of 9.6:1 to make operation with premium and regular grade petrol (RON 95/91) permissible.

On the 1.8 TFSI, Audi's engineers have intensively developed the concept that they presented three years ago. Since 2005, the two-litre engine has been voted "Engine of the Year" three times in succession by a jury of international journalists. Its little brother is one of a new engine series with the project designation EA 888, which systematically pursues the principle of greater power density, i.e. of developing the same or even more output from a smaller displacement. The new engine family was developed by Audi for worldwide use in all brands of the VW Group. With its compact displacement and turbocharger, the 1.8-litre engine develops as much output and torque as a V6 naturally aspirated engine did a few years ago. And with an average fuel consumption of 7.2 l/100 km, it needs far less fuel.

The four-cylinder engine, with its 1798 cm3 of displacement, develops an output of 118 kW (160 bhp) and a high peak torque of 250 Nm, which is constantly available across a wide range from 1,500 to 4,200 rpm. It propels the Audi A3 powerfully forward: on the version with the six-speed manual gearbox it gives a top speed of 220 km/h and accelerates the vehicle from standstill to 100 km/h in 8.0 seconds.

The crankcase: lightweight construction in grey cast iron
The EA 888 series engines have been newly developed from their very roots. Just about all that remains of the crankcase from the predecessor engine from the 827 series is the 88 millimetre cylinder spacing. It is made of grey cast iron, which provides good noise insulation but weights only 33 kilograms – the whole engine weighs a mere 135 kg. Inside the crankcase, two counter-rotating balancing shafts compensate for second degree free inertial forces. The rigid basic structure of the new engine also helps to eliminate humming noises and vibrations.

The toothed chain that drives the balancing shafts has also been designed for this quiet running. Another one drives the oil pump. With its volumetric flow control and two pressure stages, it saves 0.2 litres of fuel for 100 km. A third toothed chain drives the two camshafts. The newly developed variable camshaft system, which smoothly adjusts the intake camshaft through 60 degrees on the crankshaft, has a particularly spontaneous response.

In the interest of compact overall dimensions, the ancillary equipment bracket holds the oil cooler, oil filter and the two oil pressure switches – the upright oil filter is easily accessible for servicing, saving both time and money. Water pump, thermostat and temperature sensor are located in a single housing.

On the 1.8 TFSI, Audi's engineers have increased the injection pressure in the common rail system to 150 bar. The exhaust camshaft drives the newly developed high-pressure pump via a four-section cam. The six-hole injectors, which are also new, distribute the fuel precisely in the combustion chamber to achieve more efficient combustion. A two-stage injection is executed after a cold start, the first during the intake stroke, the second during the compression stroke to rapidly bring the ceramic catalytic converters, which are located close to the engine, to their operating temperature.

A water-cooled type K03 turbocharger manufactured by Borg Warner charges the cylinders. Its turbine housing is integrated together with the elbow in a single module made of high-alloy grey cast iron. An optimised turbine wheel improves the response characteristics at lower engine speeds. At an engine speed of 2000 rpm, the 1.8 TFSI needs just 1.2 seconds to build up its peak torque of 250 Nm – the predecessor engine, a 1.8-litre unit with manifold injection, needed 1.7 seconds to develop its 225 Nm.
Within the intake system on the 1.8 TFSI, a newly designed charge movement flap provides the necessary turbulence for a high-quality, homogeneous mixture. The charge air cooler is also a new development that is characterised by its very high efficiency, low weight and compact dimensions.

For the more powerful sibling of the 1.8 TFSI, the 2.0 TFSI, Audi is already working on an important further development. The two-litre engine is to be equipped with the Audi valvelift system. This new technology for variable valve lift control, which Audi introduced just a few months ago on its six-cylinder engines, will boost performance and efficiency even further.
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[Bartek Sikorski]