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    By admin


    September 27, 2006


    Source: Audi AG

    Audi is continuing to extend its range of V6 petrol engines. The new 2.8-litre engine, which will be celebrating its debut in the Audi A6 towards the end of the year, delivers an output of 154 kW (210 bhp) and a peak torque of 280 Nm, available from engine speeds of 3,000 to 5,000 rpm. The new V6 operates with ultra-efficient FSI direct injection and furthermore introduces an entirely new valve control technology – the Audi valvelift system. This, together with the further reduced friction of all components, cuts fuel consumption by 10 percent.

    Audi views the new valvelift system as a solution offering considerable future potential. Its simple, compact design allows a high degree of compatibility and makes it efficient to build, and the modular principle permits substantial synergy within petrol engine model ranges. Audi manufactures a large proportion of the components itself, at its plant at Györ in north-western Hungary, where the V engines are likewise built.

    High peak output, beefy pulling power at mid-range engine speeds, low fuel consumption and refined running – these strengths typify all Audi V-engines. The new engine family, consisting of V6, V8 and V10 power units, was first unveiled in 2004. Its distinguishing technical features are a standard cylinder angle of 90 degrees, an internal calibre of 90 millimetres and timing-chain drive of the camshafts and auxiliaries mounted compactly on the rear of the engines. The new 2.8 FSI fits into the range between the 3.2 FSI and the 2.4.

    The 2.8 FSI has the same bore as its big brother at 84.5 mm. Meanwhile its stroke is only 82.4 mm instead of 92.8 mm – this slightly oversquare configuration produces a swept volume of 2,773 cm3 and excellent responsiveness.

    The alloy crankcase, derived from the version used for the 3.2 FSI, is very compact at just 360 mm long, 430 mm wide and 228 mm high. It weighs a mere 33 kilograms – with the entire engine tipping the scales at 165 kg. Its two banks of cylinders are offset by 18.5 mm.

    The cylinder crankcase is produced by low-pressure die-casting, from a hypereutectic aluminium alloy containing 17 percent silicon and 4 percent copper. Its benefits include high static and dynamic strength, minimal distortion and good thermal conductivity. A bedplate – an intermediate frame into which the bearing bridges are cast – further improves the torsional rigidity and therefore the vibrational behaviour, which the driver experiences as mechanically highly refined. The grey cast iron bearing bridges absorb most of the forces in the bedplate, and simultaneously keep the amount of play at the main crankshaft bearings within close tolerances.

    The diameters of the crank pins and main bearings have been reduced from 56 mm to 54 mm and from 65 mm to 58 mm respectively, to reduce friction. The cracked trapezoidal forged conrods have a very low weight – each one weighs just 0.52 kg. On the balancing shaft positioned in the cylinder block’s “vee”, which eliminates the free inertial forces of the first degree, the weights have been adapted to the changed circumstances by the engineers.

    Narrow webs: 5.5 mm between the cylinders

    The alloy cylinder block requires no separate cylinder liners. The liners are honed from the material by a purely mechanical three-stage process, which is particularly eco-friendly compared with conventional techniques. The webs between the cylinders, only 5.5 millimetres thick, incorporate cooling holes. A wear-resistant running surface made from Ferrostan – an iron coating applied by electroplating – coats the shafts of the cast aluminium pistons, each of which weighs only 420 grams including pins and rings.

    World debut: Audi valvelift system

    The two four-valve cylinder heads of the 2.8 FSI are related to those of the 3.2 FSI. The two exhaust camshafts and both intake camshafts can be adjusted continuously by 42 degrees crankshaft angle by means of phase adjusters to optimise filling of the combustion chambers. Whereas only minor details changed at the exhaust end, the inlet end saw an innovative technology put in its first appearance – the fundamentally new development of the Audi valvelift system, for variable control of valve lift.

    Conventional technologies in this domain all involve additional mechanical elements such as components that are engaged or slid into place between the camshafts and valves. They consequently introduce disadvantages in several other areas – friction rises, the moving masses are higher and the rigidity of the valve gear falls.

    Audi has radically embraced a different approach that is equally effective, but brilliantly simple. The two-stage Audi valvelift system manages without these interfering additional components between camshaft and valve – it simply transfers the actuating components directly to the camshafts themselves.

    Splines are rolled onto the two inlet basic camshafts of the new 2.8 FSI, each of them bearing three cam pieces. These are cylindrical sleeves that carry two cam contours for small and large valve lift side by side, in other words for part-load and full-load operation.

    Internal teeth allow the cam pieces to be adjusted longitudinally by a little less than 7 mm. This task is performed by two metal pins on each camshaft, located vertically in the cylinder head above the shaft and lowered by 4 mm by lightning-fast electromagnetic actuators. They operate in unison with sliding grooves on both ends of the cam pieces.

    Simply brilliant: how the cam piece migrates

    The recessed pin engages in the groove, with its spiral contour. This action, in conjunction with the cam piece’s own rotation, causes the cam piece to move longitudinally; the now de-energised metal pin is then pushed back mechanically again. This leaves the cam piece positioned precisely in line with an axial bearing. A spring-loaded pin integrated into the basic camshaft guarantees locks it in position by extending its ball head into an internal groove. If the cam piece subsequently needs to be returned to its home position, it is moved back by the second pin in conjunction with the displacing groove on the opposite side.

    The higher of the two cam profiles – the full-load profile – opens the valves through 11 mm by means of new, extra-narrow roller cam followers, whereas the part-load profile opens them through only 5.7 and 2 mm (at part load, the two inlet valves on each cylinder are deliberately opened asymmetrically). In conjunction with a special design for the inlet port and combustion chamber, this effect produces a combined swirl and tumble flow. Thanks to this so-called “dumble”, the 2.8 FSI requires no charge movement flaps in the intake admission tract, an entirely new departure for an FSI engine.

    The changeover processes take place very rapidly within a combustion cycle –equivalent to two engine revolutions – within a speed range of 700 to 4,000 rpm.

    Combined temporary manipulation such as a changeover to retarded ignition, adjustment of the camshafts and closing of the throttle valve prevents the torque from rising abruptly when the valve lift is adjusted – the transition takes place gently and imperceptibly. The driver notices nothing more than a smooth, turbo-like buildup of power while they accelerate, a characteristic that they will in any case readily associate with all of Audi’s V6 engines.

    The engine is controlled without the involvement of an airflow meter; it uses primarily the intake manifold pressure, camshaft position and engine speed. In those ranges in which the 2.8 FSI is operated fully dethrottled, the intake manifold is constantly under pressure, as a result of which it is unable to gain any positive control information for the engine management system.

    This task is performed by optimised camshaft sensing technology that precisely monitors the position of the adjustable inlet camshafts. The engine management is very comprehensive: it comprises two complete operating programs for part and full load, as well as a changeover manager for the transition between these operating modes.

    The Audi valvelift system realises its full potential for savings of up to seven percent at constant speeds mid-way up the part-load range. It is there, when the driver selects a moderately fast speed in a higher gear, that the fuel saving compared with a conventional engine is particularly noticeable. In the Audi A6, the engine operates with the partial valve lift at up to 140 km/h in fifth gear, and at up to 150 km/h in sixth gear.

    Even when the V6 is running at full lift, the driver will notice one particular strength of the new technology: thanks to its straightforward structure, it makes high engine speeds of up to 6,800 rpm and the output associated with this responsiveness. And it exploits yet another advantage for cold starts – emissions are lower, because the catalytic converter reaches its operating temperature faster.

    FSI: the technology out in front, with a compression ratio of 12.0:1

    The 2.8 FSI achieves particularly effective combustion thanks to its high compression ratio of 12.0:1. This is all thanks to FSI petrol direct injection, which, incidentally, operates in homogeneous mode, i.e. with a lambda value of 1 – the evaporation of the fuel draws heat out of the mixture in the combustion chamber. Audi’s FSI technology first supplied compelling evidence of its superior potential in June 2001, when an FSI engine took the Audi R8 sports prototype to overall victory in the Le Mans 24 Hours. Over the following years, 64 more wins out of 80 starts followed.

    A high-pressure pump driven by the right-hand inlet camshaft delivers the petrol to two interconnected reservoirs (“rails”). The common rail injection system injects the fuel directly into the combustion chambers in precisely metered amounts, at a pressure of up to 100 bar. In the two-stage plastic intake manifold, a vacuum-controlled flap switches between long intake paths for high torque and short paths for high power output.

    Other technical advances on the 2.8 FSI concern the three timing chains that drive the camshafts. The intermediate gears and sprockets on the camshafts now have more teeth, to make them run more quietly and at reduced chain force. The tri-oval, in other words virtually triangular, design of the sprockets on the camshafts has a similar effect. This geometry reduces the torsional vibrations of the camshaft and the influences on the chain. The three Simplex roller chains, too, have been reengineered and optimised for smooth running and maximum wear resistance; no maintenance or even changing is needed throughout the entire lifetime of the engine. Thanks to the lower forces in the chain drive, the four hydraulic tensioners operate with less damping force, and less preload also means less friction.

    A fourth chain drives the oil pump – this component, too, has 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, when the spraying nozzles for the piston crowns also cut in to prevent temperature peaks from occurring. There is a separate oil-to-water type oil cooler located next to the pump.

    Lower friction, lower consumption

    With the aid of a downsized water pump and other refined details, the engine developers were able to cut the total friction losses quite considerably. The frictional mean effective pressure at 2,000 rpm has been cut by 0.22 bar, equivalent to 25 percent. This effect produces a fuel saving of around 5 percent.

    The 2.8 FSI, which develops 154 kW (210 bhp) at 5,500 rpm and generates a constant torque of 280 Nm between 3,000 and 5,000 rpm, will be able to demonstrate both its refinement and its potential for economy in the A6. Audi achieves consumption of 8.7 litres of premium-grade petrol per 100 km under standard conditions in the MVEG cycle – a substantial improvement on the 2.8 litre V6, which was in use up until 2000 in the A6 generation of the day. In that model, it achieved combined-cycle consumption of 9.9 litres of premium-grade petrol per 100 km with a five-speed gearbox, in other words 1.2 litres or almost 14 percent more; it also produced eight percent less power, at 142 kW
    (193 bhp). What better illustration of Vorsprung durch Technik could there be?


    Resources:

  • AudiWorld’s Complete Paris Motor Show 2006 Coverage



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