A New Innovative Automobile Concept in the Compact Car Class

Seoul 2000 FISITA World Automotive CongressJune 12-15, 2000, Seoul, KoreaF2000G362The Technology of the Audi A2 – A New Innovative Automobile Concept inthe Compact Car ClassDipl.-Ing. (FH) Dietrich Engelhart 1) * Dr.-Ing. Ernst Lindner 2)1) Audi AG, D-85045 Ingolstadt, Germany2) Audi AG, D-85045 Ingolstadt, GermanyThe Audi A2 signals the advent of a new model line in the compact car class. The vehicle concept is based on the AudiSpace Frame (ASF). The use of a body with a basic structure made entirely of aluminium, in this particular case for acompact car, is unique in volume production. Production methods which permit a high degree of automation had to bedeveloped for this model. As part of an overall technical concept, the Audi Space Frame brings about a significantreduction in the vehicle's weight. The Audi Space Frame moreover provides the ideal basis for an outstanding packageand an appropriate safety concept. The car's low aerodynamic drag is made possible by the low c D value. The lowweight of the A2 combined with the high efficient spark-ignition and Diesel engines is resulting in an very agile and fueleconomiccar.Keywords: Audi Space Frame, aluminium car body, weight reduction, fuel economy, CO 2 emission reductionINTRODUCTIONAt the start of the 21st century, the car remains the mostimportant and most popular means of transport. Inaddition to its primary function as a form of transport,the car also embodies the individual's rising expectationsin respect of freedom and mobility. These values arebecoming ever more pronounced, above all in societiesthat are becoming increasingly leisure-oriented and wheregreater emphasis is being placed on the individual.The car is therefore increasingly having to satisfy highexpectations in respect of comfort and equipment, whichgo hand in hand with rising standards of passive safetyand higher overall operating distances. This ultimatelyleads to a higher gross vehicle weight, with the result thatthe engine, suspension and body rigidity need to bemodified accordingly, thus adding further to the vehicle'sweight. This mechanism, known as the upward weightspiral [1], ultimately means that despite the significantimprovement in engine efficiency over recent years, therehas been no appreciable reduction in fuel consumption. Inthe light of repeated international calls to reduce the CO 2emissions in order to protect the environment andclimate [2], the desired degree of progress with regard tothe contribution of the road traffic is therefore notachieved sufficiently so far.The overall energy consumption of a vehicle is inproportion to its overall resistance to motion, which isdefined as the sum of rolling, accelerating, aerodynamicresistance and resistance of grade. As all these forms ofresistance to motion apart from aerodynamic resistanceare influenced by the vehicle's mass, a significantreduction in resistance to motion and therefore energyconsumption can be achieved by reducing the weight.* dietrich.engelhart@audi.de1The upward weight spiral can only be broken by firstreducing the weight of the vehicle's principal components,namely the body and the suspension. A reduced bodyweight as part of an overall technical concept then makesit possible to exploit secondary scope for weightreduction, through measures affecting the engine,transmission, package and tank, ultimately reversing theweight spiral.The use of aluminium for suspension components, e.g.subframe, axle support and control arms, is already thestate of the art. On the other hand, the Audi A2, Fig. 1,is the first high volume-production car whose body is ofaluminium.Fig. 1: The Audi A2 is the first high volume productioncar with the Audi Space Frame.The Audi A2 is not merely the first high volumealuminium car; more importantly, it signals theestablishment of an entirely new vehicle segment. A verylightweight but rigid car, it offers outstanding interiordimensions inside a body that is impressively compact.

Its standards of soundproofing, seating comfort andequipment moreover satisfy the high expectations ofpresent-day and future customers [3].AUDI SPACE FRAME ASF®The Audi Space Frame of the A2, Fig. 2, is a light alloybody, which consists of 22% aluminium castings, 18%aluminium profiles and 60% aluminium panels.The whole body is constructed of curved and straightaluminium extruded sections. Within this frame structure,every element performs an integral, supporting role. Thecorners and nodes, which have to bear high loads,incorporate complex diecast elements that are joined tothe extruded sections. As seamless, enclosed hollowprofiles, the extruded sections are very rigid. Because theflanges that are commonplace in steel bodies are largelyunnecessary here, the space that is freed up can be put tooptimum use in varying the cross-section of the profile.By means of ribbed panels and tailored blanks, the rigidityof the diecast elements is precisely matched to the forcesintroduced from the engine and suspension mounts. Inresponse to the need for high buckling strength, thealuminium walls are thicker than their steel equivalents bya factor of approximately 1.4. In conjunction with thethicker aluminium panels, the aluminium body isextremely strong and exhibits a high energy absorptioncapacity when deformed in a collision.In the front structure, the longitudinal member comprisesa vacuum diecast element consisting of two shell halves,and an extruded section bolted into it. This easy-to-repairdesign is the basis of the car's low insuranceclassification. With integral engine mounts, subframeconnections and locating points for the car jack, this is amultifunctional large-size component.Fig. 2: The A2 Audi Space Frame ASF.Thanks to the ASF, the A2 represents a high standard oflightweight design. This means that its static torsionalrigidity in relation to tare weight is high. Through thefunctionally optimized use of material, the A2's ASFachieves torsional rigidity values that are significantlyabove the figures for competitor models in this class. Thedynamic strength of the ASF, again high, guaranteesoutstanding vibrational comfort.On the A2, the use of the ASF body means that theweight is over 40 % lower than that of a comparable steelbody. The reduction of the body weight as part of anoverall technical concept also makes it possible to exploitsecondary scope for reducing the weight of the engine,transmission, suspension and tank. The A2 1.4 l TDI, forexample, is around 230 kg lighter than the average ofvehicles with comparable interior dimensions, Fig. 3.Tare weight [kg]14001300120011001000900800700Fig. 3: A2 1.4l TDI vehicle tare weight comparison.PRODUCTIONComparison Vehiclesaverage value- 230 kgA2 1.4l TDIWith the Audi A8 Aluminium technology was firstimplemented on a production vehicle by Audi in 1994 [4].The challenge for the A2 was above all to manufacturealuminium vehicle bodies on a large scale. In contrast tothe large A8, where manual processes are still used tosome extent, automated processes had to be adoptedthroughout on the A2. Its production facilities are gearedup to an annual output of around 60,000 units. The A2'sASF has undergone further refinement compared with thedesign used on the A8. In particular, the A2 has a largenumber of straight profiles and fewer cast nodes, which,however, entail a high degree of integration.The A2's side section, for example, consists of a singlesection of deep-drawn sheet aluminium extending fromthe lower edge of the windscreen to the tail lights. The A-post, on which the hinges of the front doors are mounted,is made from a single section of diecast aluminium.Whereas the B-post on the A8 still comprises six separatesections, much larger diecast components are used on theA2. This brings the obvious advantage that fewer partsneed to be handled and joined together in the productionprocess. Further advantages are the greater dimensionalaccuracy and the absence of joints. A large number offunctions can moreover be integrated into the largecastings, such as reinforcing panels or ribs, which had tobe attached in a separate process on the A8.In order to reduce production time still further, allprincipal extruded sections on the A2 such as the sideroof frame are calibrated in a hydroforming plant beforeassembly, significantly reducing the need for bending andmilling operations. In the hydroforming plant, thepreshaped components are inserted in a two-piece die,where perfect dimensional accuracy is achieved underhigh hydraulic pressure. The very low tolerances that canbe obtained by means of this technology permit the use of2

laser welding, which is much faster and erodes thematerial less than electric -arc welding. Less auxiliarymaterial is needed in laser welding than in conventionalwelding processes, but the resulting welds are strongerand the vehicle body exhibits greater rigidity. In totalaround 30 metres of weld seams are produced by thismethod on every vehicle body. MIG welding is also usedto a considerable degree in the production process, inparticular for the T-joints on the welded platform. Spotwelding, on the other hand, is no longer used. Robots jointogether other body sections, such as the rear panel, bymeans of about 1800 punch rivets.strength and specific deformation behaviour with a verylow weight.In an front offset crash, the Audi Space Frame's frontlongitudinal members absorb the bulk of the deformationenergy, whereas the beam structures behind them formthe strong passenger cell and barely sustain deformation.Deformation of the longitudinal member follows a definedpattern, from the front extruded section to the rearaluminium casting. Energy absorption via the longitudinalmembers ensures that the passenger cell remains largelyfree from deformation, Fig. 5.PACKAGE AND VARIABILITYThe objective of the package's design was to create aninnovative spatial concept allowing passengers to find ancomfortable seating position, in conjunction withcompact exterior dimensions, Fig. 4.Its basis was created by the transverse engine principleand space-saving axle concepts.Inside, the ergonomics of the A2 are optimized; whichmeans that the position of the front seats conveys afeeling of spaciousness. At the rear, the contour of thecar's floor incorporates a recess, the so-called "SpaceFloor", permitting rear passengers to sit upright in ancomfortable seating position. In general the slightly raisedposition of the front and rear seats also enhancesoccupant comfort.Fig. 5: Euro-NCAP 64km/h front offset crash,simulation of structural deformation.Full-size airbags for the driver and front passenger andside airbags in the seat backs, Fig. 6, in conjunction withseat belts that are fitted with belt tensioners and forcelimiting devices, provide added occupant protection. Headairbags (Sideguard system) are available as an optionalfeature.Fig. 4: A2 Package.When the concept was elaborated, considerableimportance was also attached to interior variability. Theavailability of 4- and 5-seater versions reflects thediversity of customer requirements. With the rear seats'multiple folded constellations, including scope for theirremoval entirely from the car, the A2 offers a wide rangeof different layouts, with a luggage capacity ranging from390 to 1140 litres.THE SAFETY CONCEPTThe safety concept is based substantially on the AudiSpace Frame. The Audi Space Frame is a highly effectivethree-dimensional beam structure that combines highFig. 6: Safety concept.3

Within the vehicle concept the sills are relatively high,with the result that the position of the seats in a sideimpact collision is above the area most commonlyaffected by intrusion, providing a very high standard ofsafety. The strong door impact members, in conjunctionwith the B-post taking the form of a large-size ribbedcasting and side airbags fitted as standard, reduce theload on the car's occupants. The structure with a roofframe designed for maximum rigidity in addition affordsvery good protection in a rollover.The Audi Space Frame is likewise beneficial in a rear-endcrash. The sturdy structure formed by the longitudinaland cross-members at floor level permits effective energyabsorption and braces the impact forces in a forwarddirection. Energy is absorbed in a highly effective mannerthanks to the folding-dent behaviour of the extrudedsections of the rear longitudinal members. The casting infront of these is designed such that it sustains minimumdeformation, to keep the fuel tank protected. Thedeformation of specifically only the rear section meansthat the passenger cell protects the occupants veryeffectively in a rear-end collision, too.AERODYNAMICSAudi’s new aero-acoustic wind tunnel was used for theaerodynamic testing of this model. With the road surfacemoving at the wind speed being simulated, and thanks tothe rotation of the vehicle wheels, the boundaryconditions that prevail in reality can also be simulated, sothat realistic measurements are obtained.The good aerodynamic performance has been achievedby defining the vehicle's fundamental characteristics at anearly stage:The range of engines, Table 1, currently includes a 1.4 lspark-ignition engine and two Diesel engines withdisplacements of 1.2 l and 1.4 l. These engines are sharedwith other cars built by the VW Group, and have beenadopted on the A2 with slight modifications.1.4L 55 KW SPARK-IGNITION ENGINEThe 1.4 l engine has an output of 55 kW at 5000 rpm anddevelops a torque of 126 Nm at 3800 rpm.This engine, designed for low weight and fuelconsumption, has an aluminium diecast crankcase withcast-in grey cast iron liners. The engine is fitted with ashell-type plastic intake manifold and an exhaust systemmade entirely from stainless steel.The two camshafts, which each operate two valves viafrictionally-optimized timing gear with roller cam follower,are mounted in the two-piece cylinder head. The enginecharge cycle has been designed for an ample torquecharacteristic.The frictionally optimized engine is an ideal basis for lowfuel consumption. The A2 1.4 l recorded fuelconsumption of 6.0 l/100 km over the MVEG II cycle.The low fuel consumption is partly attributable to the lowidle speed of 680 min -1 . CO2 emissions are only 144g/km. The EU 4 emission standard is fulfilled in particularthanks to the inclusion of a primary catalytic converter inthe front pipe.• Short rounded front structure• Flat, inclined windscreen that flows continuously intothe arched roof• Relatively high, aerodynamically effective fastback• Optimized exterior rear mirrorsThe aerodynamic fine-tuning, such as the measures to theunderbody, has produced low c D values. A dragcoefficient of c D = 0.28 has been measured for the basicversions with the two 1.4 litre engines, and of c D = 0.25for the 1.2 l TDI.The aerodynamic drag, which is proportionally c D * A, isrelatively low, particularly on the A2 1.2 l TDI, at 0.544.Unlike other vehicle concepts, Fig 7, the A2's lowaerodynamic drag is achieved by reducing the dragcoefficient, and not the cross-sectional area. Theergonomics of the A2 are consequently much better thanfor comparison vehicles.ENGINES AND PERFORMANCEFig. 7: Cross-sectional area and c D value comparison.1.4 l 55 kW DIESEL ENGINEThe following features are making the 1.4l 55 kW TDIDiesel engine particularly suitable for use on the A2:• Inline 3-cylinder engine as a spacesaving, weightoptimizedpower unit for transverse installation,4

• it fulfils the emission standards currently in force,while maintaining the advantageous fuel consumptionof the direct-injection principle,• Low-cost, durable engine concept.cylinder engine blocks can be manufactured on the sameproduction line.For a high standard of refinement, a balancing shaft isdriven in conjunction with the oil pump via a chain drivewith a hydraulic tensioning element. The balancing shaftis mounted in a diecast aluminium ladder frame bolted tothe bearing pedestals in the engine block.Table 1: Engine and vehicle dataThe rated output of 55 kW laid down in the specificationsis achieved at an engine speed of 4000 rpm. The highpressurefuel injection assures efficient mixturepreparation, with the result that peak torque of 195 Nm isachieved as low down as 2200 rpm.A key feature of this engine is the new high-pressure fuelinjection system, which permits injection pressures of upto 2050 bar. A precisely metered, separate pilot injectionprocess, which reduces combustion noise and nitrogenoxide emissions, is achieved with this injection system.To reach the objective, the pump-injector units wereincorporated directly into the cylinder head. The pumpinjectorelements are driven by the camshaft via the rollerrocker arm by means of additional lifting cams.In view of the high cylinder pressures, the cylindercrankcase is made from grey cast iron. As the engineblock has the same design features (distance betweencylinder axes 88 mm; bore 79.5 mm) as the 4-cylinderengine blocks of the new Golf DI engines, the 3- and 4-1.2l 45 kW DIESEL ENGINEThe Diesel engine with two-valve technology has adisplacement of 1.2 l (bore: 86.4 mm, stroke: 76.5 mm)and a compression ratio of 19.5 : 1. An overheadcamshaft driven by a toothed belt controls the valves viabucket-type tappets. To reduce free inertial forces, thethree-cylinder engine has a base-mounted balancing shaftrotating in the oil sump. Both the cylinder head and theengine block of this Diesel engine are made fromaluminium. This measure helps to keep the engine's dryweight down to approximately 100 kg.The key to this engine's high level of efficiency is onceagain a pump-injector direct fuel injection system. Theturbine geometry of the exhaust turbocharger withcharge-air cooling is variable, thus further reducingemissions and fuel consumption.The engine achieves its maximum power output of 45kW at 4000 rpm. Maximum torque of 140 Nm is availableacross an engine speed range of 1800 to 2400 rpm.5

The A2 1.2l TDI has a stop-start drivetrain managementsystem, which switches off the engine when the car isstanding at traffic lights or in a tailback and the footbrakeis held down for more than three seconds. As soon as thebrake is released, the engine starts again.TRANSMISSIONSThe transmissions were developed for the specificrequirements of the engines:• A lightweight 5-speed manual gearbox withmagnesium casing has been chosen for the 1.4l55 kW spark-ignition engine.• The higher-torque 1.4l 55 kW Diesel engine has areinforced 5-speed manual gearbox with aconventional aluminium casing.• In view of the engine's high efficiency, the A2 1.2lTDI is supplied with an automised, five-speedmechanical gearbox with aluminium casing. Inconjunction with its Economy program, whichconsistently strives to establish the lowest possibleshifting speeds, and a high overall ratio for fueleconomy, it is able to operate at the best specific fuelconsumption in any given situation.The 1.4l TDI accelerates from 0 to 100 km/h in 12.3 s,and reaches the same top speed of 173 km/h as with theA2 1.4l Thanks to its pump-injector technology, the 1.4lTDI achieves fuel consumption of only 4.3 litres ofDiesel fuel per 100 km. Compared to similar vehicles theA2 1.4l TDI has approximately 13% up to 53% less fuelconsumption, Fig. 9. Its CO 2 emissions are 116 g/kmand it satisfies the EU 3 emission standard.The 1.2l TDI achieves fuel consumption of only 2.99l/100 km for the MVEG cycle, accelerating from 0 to 100km/h in 14.8 seconds and is reaching a top speed of 168km/h [5].Relative fuel consumption for MVEG-cycle [%]160%150%140%130%120%110%100%90%80%4,3 l/100 km3 4 5 6Comparison Vehicles10A2 1,4l TDI1 2 7 8 9Fig. 9: Fuel consumption for the A2 1.4l TDI.VEHICLE PERFORMANCE / FUEL CONSUMP-TIONAfter 12.0 seconds the A2 1.4l is accelerated from 0 to100 km/h. This is an outstanding low value in comparisonto similar cars, Fig 8.1817161514comparison vehiclesSERVICE MODULEThe A2 has a new functional feature that also has thedouble role of a design element: the service module, Fig.10. The conventional radiator grille has been replaced bya folding cover behind which the oil dipstick and the filleropenings for engine oil and washer fluid are to be found.It is now no longer necessary to open the enginecompartment lid to check the engine oil level and to topup oil or washer fluid.If more extensive servicing work involving access to theengine compartment is required, the engine cover –known as the "front top" – can easily be removed byreleasing the quick-acting couplings.1312A2 1,4l TDIA2 1,4l1140 45 50 55 60 65 70 75 80Engine Power [kW]Fig. 8: Acceleration from 0 to 100 km/h.The driver gets an agile feeling up to a top speed of 173km/h. The fuel consumption nevertheless is 6,0 litres per100 km. The emission standard EU IV is satisfied and theCO 2 emissions are 144g/km.Fig. 10: A2 Service Module.6

ENVIRONMENTIn order to achieve also environmental objectivesespecially such as the reduction of CO 2 emissions, it isessential to view the broader context. Therefore it has tobe focused on the entire life-cycle of the car, from itsproduction, through its operation, to its ultimate recycling.As the bulk of the energy required throughout the lifetimeof a vehicle is consumed during its operation,consumption-cutting measures focusing on resistance tomotion, such as the reduction of the vehicle's weight, areparticularly significant. On the Audi A2, measures aimingin this direction were implemented systematically throughthe use of the Audi Space Frame body and the lowconsumptionengines. The bigger amount of energyrequired for the initial Aluminium Space Frameproduction in comparison to a conventional steel body isalready compensated after a driving distance ofapproximately 85.000 km.The tare weight of the Audi A2 (with 1.4 l, 55 kW sparkignitionengine) is only 895 kg, thanks to the use ofmodified weightsaving materials such as aluminium,magnesium and plastics. According to the materialsclassification of VDA guideline 231, the overall proportionof the weightsaving materials aluminium/ magnesium/polymers is 52%. The proportion of steel and ferrousmaterials is about 34% by weight.With regard to the more and more eminent air pollutionproblem, the A2 vehicle concept, Fig. 11, represents anadequate technical solution, because the amount of CO 2emissions for this vehicle concept is on a very low level.The Audi A2 is a sophisticated, progressive andresponsible vehicle, which satisfies the rising expectationsof the individual customer, while meeting the increasinglychallenging requirements of society and the legislative.ACKNOWLEDGMENTWe thank the colleagues of the Audi A2 Team, who allsupported the success of this sophisticated project.The collegues of the plant and of the AluminiumTechnical Centre in Neckarsulm also merit our thanks fortheir expertise which provided the very basis for volumeproduction of the Audi A2.The ASF body's large castings as well as the extrudedsections and alloy panels, made from the same alloy, canbe cut away completely without undue effort, allowingthese special aluminium alloys to be recycled separately.Following their removal, the castings can be used in theproduction of new cast-alloy components.Along with the rise in the number of vehicles with a highaluminium content such as the Audi A2, the world's firstvolume-production car with an all-aluminium body, thesupply of secondary aluminium from scrap vehicles willoutstrip demand for primary aluminium. In anticipation offuture developments in the field of scrap-vehiclerecycling, Audi has established the basis for effectiverecycling of aluminium by providing scope for thesegregation of cast and wrought alloys through suchtechniques as filleting and new cut detection methods.CONCLUSIONThe expertise in aluminium technology that Audi AG hasacquired over the past decade was an essential basis forbuilding the world's first aluminium car, the Audi A2, ona large scale.The use of the ASF aluminium body and the exploiting ofsecondary weight reduction measures is resulting in alower vehicle weight that is used to the customer'sbenefit. The low weight of the A2, combined with highlyefficient engines, results in a particularly agile, low fuelconsumptionvehicle concept. The A2's interiordimensions are nevertheless generous, and its variabilityclearly above-average.Fig. 11: The very low CO 2 emissions of the A2 are oneof the important technical highlights.REFERENCES[1] Kaiser, M.: Aluminium as a body in white material, 6.Aachener Kolloquium Fahrzeug- und Motorentechnik,1997[2] Radiative Forcing of Climate Change. The 1994Report of the Scientific Assessment Working Group ofIPCC, 1995.[3] Der neue Audi A2, Special edition ATZ/MTZ, ISSN0001-2785, March 2000.[4] Paefgen,F.-J.; Leitermann, W.: Audi Space FrameASF. Ein neues Pkw Rohbaukonzept in Aluminium, VDIReport 1134, 1994.[5] Engelhart, D.; Mödl, C.: The Development of the AudiA2, a new Vehicle Concept in the Compact Car Class,VDI Report Nr. 1505, 19997