COVER You STORY will find
V12 Engine theBMW figures mentioned in this article in the German issue of MTZ 7-8/2003 beginning on page 546.
Der neue 6,0-l-Zwölfzylindermotor von BMW
The New BMW 12-Cylinder Engine Following the 4-cylinder in-line and V8 engines, as a further step in the renewal strategy for its entire range of petrol engines, BMW has developed a completely new V12 engine for the 760i vehicle, representing the top engine of this new petrol engine generation. The 60° V design has a displacement of 6 litres, and for the first time combines a fully variable valve train with petrol direct injection. This combination makes the new V12 engine the most powerful and fuel-efficient, naturally aspirated engine in the luxury vehicle class.
1 Introduction
By Hubert Fischer, Klaus Fröhlich and Ernst Jägerbauer
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V12 engines represent the culmination of consistent development in engine engineering and technology for passenger vehicles. Like no other type of engine, they effectively combine the best engineering features and technical properties. The complete lack of free mass action of the crankshaft drive in the classic arrangement of two 6-cylinder banks at an angle of 60° makes these engines free revving and therefore top-performance engines. At the same time, this physically optimum crankshaft drive in conjunction with the short and constant firing intervals provides an unequalled degree of refinement. The overall sum of these properties therefore predestines V12 engines as fascinating top performance versions for the luxury vehicle class.
BMW fitted the first 12-cylinder engine in the BMW 7 Series in 1987 [1]. This engine was reengineered in various stages and about 96,000 units were produced up to 2001. Compared with all other manufacturers, this means that BMW has built the greatest number of vehicles fitted with 12cylinder engines worldwide. The new BMW 7 Series was launched in 2001 with the petrol engine models 735i/745i. In September 2002, this was followed by the production of the diesel models 730d/740d. Finally, series production of the top models 760i/760Li with the new 12cylinder engine began in January 2003. The following strategic objectives were defined for the new engine: ■ absolute superior driving performance in the new 7 Series ■ outstanding response, agility and a broad engine speed range
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■ exceptional comfort features ■ lower fuel consumption compared to the
predecessor ■ compliance with all legal requirements
and legislation worldwide ■ high further development potential. The engine concept was presented within the framework of the new BMW V-engine family and was developed within only a short space of time after the V8 engine. 2 Design Features
As early on as the concept phase of the new BMW V-engine generation, the V8 and V12 engines were considered together, in order to exploit the potential for synergy to the full. An optimum basic design of the components was sought for all engine versions, with the principle of uncompromising design applied systematically to all technical decisions. Following the 8-cylinder engine launched in 2001, the 12-cylinder engine now completes the new range of BMW Vengines. Figure 1 shows the cross-sectional and longitudinal views of the engine, and Table 1 the engine's main data. 2.1 Crankcase and Crankshaft Drive
At 98 mm, the bore spacing is identical to that of the 8-cylinder engine. Consequently, a 6.0-litre displacement was realised while creating sufficient potential for further development. The crankcase exhibits the classic V arrangement for 12-cylinder engines comprising two 6-cylinder banks at an angle of 60°. The offset of the two cylinder banks in relation to one another is 18 mm. The deep skirt crankcase is based on an open-deck design and is therefore cast in a metallic mould in a coreless low-pressure casting process. The material used is a hypereutectic aluminium alloy, Figure 2. The cylinder barrel is produced by exposing the hard silicon crystals. The ferrous-coated pistons run directly in this noncoated bore. The pistons are cast from high temperature-resistant aluminium alloy and cooled by oil spray nozzles. The cracked forged steel connecting rod is divided at an angle of 30° at the large eye, thus enabling a very compact crankshaft enclosure, while facilitating use in all newgeneration V-engine variants, with a uniform gauge of 140 mm. The crankshaft is forged from steel; it has 12 balance weights and is mounted in 7 bearings, Figure 3. 2.2 Cylinder Head
Cooling water flows transversely through the aluminium die cast 4-valve cylinder head. The valves with a stem diameter of 6 mm are arranged at an angle of 30.5°, re-
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sulting in a very compact combustion chamber with an optimum surface/volume ratio. The complete combustion chamber and sections of the intake and exhaust ports are machined in a metal-cutting process. The compression ratio was raised to 11.3:1. The nozzles for the petrol direct injection are arranged beneath the intake ports at an angle of 30°. The two high-pressure fuel pumps are arranged above the exhaust camshafts that drive the pumps by means of a triple cam between cylinders 4 and 5 and cylinders 10 and 11 respectively, Figure 4. The two chilled cast iron camshafts per cylinder bank are driven by the crankshaft each via a triangular sleeve-type chain drive with hydraulic tensioner. The exhaust camshaft of cylinder bank 1 drives a mechanical vacuum pump for brake power assistance mounted on the end face of the engine. An important integral part of the cylinder head is the Valvetronic system, which, based on the principle of "advanced intake valve closing", enables efficient partial load control of the engine with low throttle losses and reduced fuel consumption. The functional principle and the realised design are identical to those of the BMW 4-cylinder and 8-cylinder engines that have been in production for two years now and have been described in many publications [2], [3], [4]. With the exception of features specific to the number of cylinders, the individual components such as the hydraulic camshaft adjuster (VANOS) for the intake and exhaust camshafts, eccentric shafts, intermediate levers, roller rocker arm followers and hydraulic valve clearance compensating elements are of identical design and have the same arrangement as the 8-cylinder engines. Only the arrangement of the electric motors for adjustment of the eccentric shafts was relocated from the intake side to the exhaust side for package reasons. The cylinder head covers have integrated oil separators; they are made from magnesium and are produced in a pressure die casting process. 2.3 Engine-Internal Cooling Circuit
The water pump feeds the coolant through the centre of the V-space to the rear end of the engine. From here it is routed into the two cylinder banks, and on the outlet side of the crankcase it flows upward into the cylinder heads, which it transverses in the cross-flow system. It is then collected on the inlet side and fed forward to the mapcontrolled thermostat. As a result of this cross-flow cooling concept in the cylinder head, pressure losses
BMW V12 Engine
are substantially reduced, while exceptionally uniform component temperatures and low peak temperatures in the temperaturecritical areas in the cylinder head are achieved. The map-controlled coolant thermostat is integrated into the water pump housing. In conjunction with the temperature sensor and control unit connection, the result is a highly integrated module that is also used on the 8-cylinder engine as part of the common parts strategy. 2.4 Oil Circuit/Ancillary Components
The aluminium housing of the newly developed oil pump consists of two rotors running in separate chambers as well as a control valve and a burst pressure valve. The oil filter is accessible from below through an opening in the oil pan. The complete oil pump/filter unit is bolted directly to the crankshaft bearing caps, thus avoiding the need for chain tensioners or adjustment elements for the chain drive. The oil wiper is integrated into the upper section of the oil pan. The lower section of the oil pan, which is made of a double sheet metal panel, supports the newly developed oil quality sensor which, in addition to the oil level and temperature, also supplies a measurement signal that provides an indication of the oil condition. Together with the operating parameters of the engine, this makes it possible to provide individual, load-dependent information on the operating period remaining until the next oil change. For customers, this system means average oil change intervals of about 30,000 km. The ancillary components – coolant pump, power steering and dynamic drive pump as well as the liquid-cooled alternator – are driven by the crankshaft with a 6rib poly-V-belt. The A/C compressor drives a second 4-rib belt. 2.5 Petrol Direct Injection System
The new V12 engine is the first BMW petrol engine to be equipped with petrol direct injection. The system developed in cooperation with Bosch is designed for worldwide use. All components that come into contact with fuel are made of stainless steel and are therefore resistant to ethanol. This makes it possible to use the engines not only in Europe but also in all markets worldwide, particularly in the USA and in South America, without the possible risk of alcoholate forming. Each cylinder bank has a slot-in plunger high-pressure pump driven by a triple cam on each exhaust camshaft between cylin-
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BMW V12 Engine
ders 4 and 5, and 10 and 11, Figure 5. Each high-pressure pump achieves a geometric delivery rate of 250 mm3/stroke. For partial delivery there is a pump-internal fuel delivery control valve with end-of-delivery regulation. The fuel is compressed from a constant pre-delivery pressure of 6 bar up to 120 bar in the fuel distributor. The single-hole high-pressure injectors are located at the side of the cylinder head, beneath the intake ports. Their position was optimised with regard to injection geometry to ensure soot-free combustion. 2.6 Intake Manifold
The layout of the V12 engine and the available installation space in the vehicle necessitate a flat intake manifold concept. The principal configuration criterion was high torque in the middle speed range and a good power characteristic in the upper speed range. The form in which the intake manifold has been realised is an assembled magnesium-shell composite design. The collectors are each located above the cylinder head of the opposite bank, and the pipes approach the intake port with a slight curvature. The use of the material magnesium permits a weight-optimised design. The intake manifold is assembled from several shell elements, Figure 6. The adhesive used ensures its intactness and tightness, and the parts are additionally bolted together for reliable handling in the production process. In view of its corrosion behaviour, magnesium necessitates greater attention to design details; for example, the electric potential of connections and brackets must be examined. To be able to assemble the high-pressure injectors and the fuel distributor, it was necessary to separate the intake pipes and the collector unit geometrically. The intake manifold is vibrationally isolated from the basic engine by means of decoupling elements. In designing the individual intake pipes, considerable attention was paid to obtaining an even mixture distribution in the individual cylinders; the overall design was supported intensively by 1D and 3D calculations at each phase. This calculation method was also used to define the optimum introduction points for the tank and crankcase breather connections. Insulated lines prevent the crankcase breather from icing up even at temperatures significantly lower than -30 °C. The pressure-regulating valve for the crankcase breather is located immediately behind the throttle body. As the intake manifold is in the central visible zone of the engine compartment, it is lacquered as one of the final stages of the manufacturing process.
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The central cover trim on the centre of the engine reduces noise emissions from the 12 injectors and further enhances the engine's visual appeal. The transition from the radiator to the engine is concealed by an additional trim in which the oil filler is also integrated. 2.7 Cooling
The common part concept for 8- and 12cylinder petrol engines was the priority in the development of a new modular cooling system for the BMW 7 Series. The cooling module is identical for both engine versions of the 7 Series [5]. The only feature that is specific to the 12cylinder version is the fundamental use of the engine oil cooler, which is only needed on the 8-cylinder version in hot-climate markets where an extremely high cooling output is required. 2.8 Exhaust Manifold with Catalytic Converter
The exhaust system is characterised by newly developed air-gap-insulated exhaust manifolds with hydroformed pipes, and cascaded main catalytic converters with monoliths made from thin-wall ceramic. The manifold gallery of the 6-in-2-in-1 exhaust manifolds is so compact in design that identical versions can be used for both sides of the cylinder head and for both lefthand-drive and right-hand-drive vehicles. Three exhaust pipes per cylinder bank converge to form a D-pattern inner pipe. This double D pipe ends in a nozzle in which the oxygen sensor admits a constant load from all cylinders. The nozzle with the air-gap-insulated inlet funnel for the catalytic converter is designed for optimum flow to the catalytic converter. This exhaust manifold design permits the package-driven concept of a cascaded main catalytic converter in the engine compartment with a monolith volume of 1.76 litres (1st monolith 3.5mil/600cpsi and 2nd monolith 4.5mil/400cpsi) for each cylinder bank, Figure 7. The much more compact manifold design compared with the predecessor engine offers significantly reduced heat storage capacity, and the close-coupled arrangement of the main catalytic converters means that the optimum catalytic converter temperature is achieved as rapidly as possible. The monitor sensor is housed in the air-gap-insulated outlet funnel. This close-coupled overall system of exhaust manifold/catalytic converter has rendered underfloor catalytic converters entirely superfluous. For purposes of assembly in series production, as on the predecessor model it is bolted onto the cylinder head in such a way
that an automated bolting station can be used. This permits optimum bolting conditions thanks to the simultaneous tightening of all nuts to the specified tightening torques. For servicing purposes, the manifolds with catalytic converters can be removed from the vehicle without it being necessary to dismantle the engine. Likewise, all oxygen sensors are accessible in the vehicle with conventional tools. 3 Functional Features 3.1 Full Load
In the concept phase, petrol direct injection was selected as an effective method of enhancing performance. By avoiding the gaseous proportion of fuel in the intake air, this increases cylinder charging and achieves a higher compression ratio thanks to the cooling effect that fuel vaporisation has inside the cylinder. As a result of the avoidance of stratified operation, the ports have been configured for optimum performance. Direct injection has made it possible to boost power output by 3 % and torque by 5 %. Components that are of relevance for the charge cycle have moreover been geometrically optimised with regard to flow-through behaviour and acoustics. All these measures add up to the highest specific values for any naturally aspirated engine in this displacement class, at almost 55 kW/litre and 100 Nm/litre. Compared with the predecessor, the power output has been increased by 36 % to 327 kW and the torque by 22 % to 600 Nm, Figure 8, whereas the displacement has been increased by only 11 %. 3.2 Configuration of the Combustion Process
The fuel consumption and emissions requirements were likewise very ambitious. For example, while satisfying all worldwide emissions legislation, fuel consumption was to be cut by 10 % compared with conventional four-valve engines. These objectives could only be achieved with BMW Valvetronic, which proves to be particularly effective on large-displacement engines. A combustion process that combines Valvetronic with petrol direct injection consequently needed to be developed. The objectives in developing the combustion method were: ■ charge-optimised ports and compact combustion chamber shape to achieve high full-load values ■ high compression ratio and rapid combustion for low fuel consumption ■ low untreated emissions and stable com-
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bustion process for catalytic converter heating, to satisfy emission requirements ■ avoidance of accumulation of liquid fuel on walls, to maintain low untreated emissions and avoid dilution of the oil. The development focus was consequently placed on an in-depth understanding of the mixture formation process and of the process of interaction between the fuel spray, the occasionally very high flow rates as a result of the Valvetronic's variable valve lift, and the walls of the combustion chamber. Spray modelling, extensive 3D calculations and analyses of the optical engine were the methods used for this purpose. Figure 9 shows the comparison between the spray model with Mie measurements in the BMW spray chamber for various temperatures and pressures (influence of point of injection). In the 3D calculation, fuel preparation and mixture homogenisation have been optimised as a function of the control times and valve strokes as well as the flow through the port and the points of injection. The mathematical optimisation of a medium load point is shown in Figure 10 by way of an example. After being introduced, the fuel mass initially wets the piston crown. The air flowing in deflects the spray vertically at part-stroke, thus preventing wetting of the walls and improving the mixture preparation process. The flow field then encounters the piston crown, supports the evaporation of the fuel on the piston and is deflected centrally towards the spark plug. By the time the ignition point is reached, there is a fully homogenised mixture with no film on the wall. The combustion chamber geometry in the vicinity of the injector point likewise exhibited a clear impact on the engine's functioning, Figure 11. On the initial design, the afterflow of air to the injection hole and the locally low static pressure resulted in the spray striking the combustion chamber dome. This caused a high local lambda value and soot emissions. The spray was enabled to spread undisturbed by optimising the wall geometry. The optimisation measures regarding the injector position, spray geometry and combustion chamber shape were again evaluated by way of a summary on the optical engine, Figure 12. Thanks to reduced interaction with the combustion chamber dome and inlet valves, soot-free operation was even achieved close to full load. 3.3 Fuel Consumption and Emissions
A consumption advantage of 10 % compared with a conventional four-valve con-
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figuration was demonstrated over the European Driving Cycle. When compared with the predecessor engine, the reduction in fuel consumption was 4 % despite the larger, heavier vehicle, the significantly higher displacement and the outstanding full-load values. The new BMW 12-cylinder engine undercuts all statutory emission limits worldwide. Figure 13 shows the extent to which a catalytic converter in a new condition exploits the limits of the European EU4 and American LEV standards. 3.4 Noise, Vibration, Harshness
The NVH characteristics of the new engine in the 760i were developed within the entire engine/transmission/vehicle system and defined in the form of individual targets for all components. With compensation of free mass forces and moments of mass as well as short firing intervals, the V12 featuring a classic 60° design is the ideal basis for outstanding refinement in engine operation. Compared to the V12 predecessor, it was possible again to substantially reduce the transmission of mechanical noise components in the car body by way of specific structural measures on the basic engine. For instance, it was possible to increase the natural frequency of the engine/support arm structure from 420 Hz to over 800 Hz, thereby substantially reducing the emission level at the engine mounts as well as in the firing order and chain order. The highest possible natural frequency of the engine/transmission assembly is also an important factor for achieving good interior acoustics. Optimisation measures implemented on the oil pan and crankcase as well as the compact design of the new 6speed automatic gearbox contributed to increasing the natural frequency from 160 Hz to 180 Hz. Other important modules governing finely tuned drive acoustics are the exhaust system and intake system. A degree of tuning was selected that on the one hand ensures an adequate degree of idle and slow driving comfort in line with the vehicle class, while on the other hand enhancing the displacement and dynamic impression under high load through a discreetly noticeable, sonorous sound. The 760i is equipped with an exhaust system concept that features two parallel rear silencers for this purpose. The tailpipe of the inner silencer is equipped with a map-controlled exhaust flap. Depending on the load and drive status, with the flap closed, the silencer acts as a Helmholtz resonator and, with the flap open, as a through-flow reflection silencer. On the intake side, additional air openings are re-
MATERIALS
leased in the unfiltered air area via flaps in the upper engine speed and load range. 3.5 Road Performance/ Fuel Consumption
In conjunction with the 6-speed automatic transmission, the new 12-cylinder engine in the BMW 7 Series top-of-the-range saloon provides outstanding fuel consumption and road performance figures. Compared with the predecessor model, acceleration from a standing start has been cut by 1.3 s from 6.8 s to 5.5 s, while fuel consumption has at the same time been reduced by 0.5 l/100 km despite the vehicle's larger size. The top speed is limited electronically to 250 km/h.
4 Summary
The BMW 12-cylinder engine, an entirely new development, completes the range of V-engines that was first introduced in 2001 with the new 8-cylinder version. It is the first engine in the world to combine the BMW innovation Valvetronic with direct petrol injection, and is the first direct-injection petrol engine to be suitable for worldwide use irrespective of the fuel grades available. All in all, the BMW V12 engine opens up a new dimension of customer-relevant product characteristics in petrol engines. As the highest-performance, most economical 12-cylinder naturally aspirated engine in the luxury vehicle class, it is the topof-the-range engine for the 7 Series saloon and makes a substantial contribution towards systematically refining the idea of efficient dynamism, in other words enhanced performance coupled with reduced fuel consumption. References [1]
[2] [3]
[4] [5]
Fischer, A.; Gaede, G.; Göschel, B.; Schlott, H.; Tischer, J.: Der neue BMW-Zwölfzylindermotor mit 5 l Hubraum. In: MTZ 48 (1987), Nr. 9 Flierl, R.; Hofmann, R.; Landerl, Ch.; Melcher, T.; Steyer, H.: Der neue BMW-VierzylinderOttomotor. In: MTZ 62 (2001), Nr. 6 Hirschfelder, K.; Hofmann, R.; Jägerbauer, E.; Schausberger, Ch.; Schopp, J.: Der neue BMW-Achtzylinder-Ottomotor, Teil 1. In: MTZ 62 (2002), Nr. 9 Achilles, D.; Klüting, M.; Liebl, J.; Munk, F.: Der neue BMW-Achtzylinder-Ottomotor, Teil 2. In: MTZ 62 (2002), Nr. 10 Schmidt-Troje, D.; Eckerskorn, W.; Brielmair, M.; Jahns, A.; Kümmerlen, S.; Braun, O.: Das Kühlsystem des neuen BMW-7er. In: ATZ 104 (2002), Nr. 6
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