C OVER STORY High - Performance Engines
The 16-cylinder High-performance Engine in the Bugatti Chiron From a technical viewpoint, the highlight of the Bugatti Chiron is again the power plant. The sixteen-cylinder engine with 1103 kW and a maximum torque of 1600 Nm is the heart of the newly developed production super sports car. It was used for the Veyron 16.4 and has been enhanced in an evolutionary manner for the Chiron.
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A U T H ORS
Dipl.-Ing. Dipl.-Wirt.-Ing. Wolfgang Dürheimer is President of Bugatti Automobiles S.A.S. in Molsheim (France).
Dipl.-Ing. Willi Netuschil was Member of the Board and Head of Development at Bugatti Automobiles S.A.S. in Molsheim (France) until end of 2016.
THE ROUTE TO FURTHER IMPROVE PERFORMANCE
The W16 engine in the Chiron reaches its maximum output of 1103 kW at a nominal engine speed of 6700 rpm. Generating a maximum torque of 1600 Nm, which is constantly available in the wide engine speed range between 2000 and 6000 rpm, the engine underlines its exceptional position. Compared with the 883 kW engine in the Veyron 16.4 Super Sport, the engineers were able to compensate the dynamic deficits in producing the maximum torque by using sequential turbocharging. Furthermore the maximum low-end torque could be reached 1000 rpm earlier despite the torque increase from 1500 to 1600 Nm. In particular, the broad torque plateau that stretches across the engine speed range 1900 to 6850 rpm at 95 % of the maximum torque should be highlighted. DEVELOPMENT GOALS IN THE ENGINE COMPARTMENT AREA
Dipl.-Ing. Andreas Kurowski is Head of Engine Design at Bugatti Automobiles S.A.S. in Molsheim (France).
Dipl.-Ing. Gregor Gries is Head of Powertrain at Bugatti Automobiles S.A.S. in Molsheim (France).
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The demanding task of enhancing the drive train of a super sports car holding several speed records represented a special challenge. The expectation associated with the performance of the Chiron to make the best significantly better repeatedly demanded readiness to test the boundaries of what is technically possible in many areas. The development goals for the new engine were demanding and multi-faceted: –– achieving outstanding output characteristics with new top figures for competition-beating performance –– a compact overall concept with a high displacement density due to the traditional compact vehicle dimensions –– engine response despite a considerable increase in performance from the very good level of the previous Bugatti Veyron model delivering 883 kW –– perfect function of the engine even in extreme driving dynamic conditions that are reached with sports cars in this performance class –– optimised comfort thanks to improved engine acoustics and low idle speed –– far-reaching compensation of the increased weight resulting from the higher output –– high level of reliability and long life –– safe fulfilment of the applicable emissions regulations.
DRIVE TRAIN LAYOUT IN THE COMPACT ENGINE COMPARTMENT PACK AGE
In order to integrate an engine with this considerably higher output into the vehicle package the basic design of the sixteen-cylinder engine has been configured in the short-length W layout. The W16 engine is incorporated with the seven-speed dual clutch gearbox placed in front of it in the direction of travel as a longitudinally mounted mid-engine. The rear axle is driven via a compact side shaft running next to the engine and a differential with a controlled axle differential lock that is bolted directly onto the beltdrive side of the crankcase, FIGURE 1. The positioning of the engine along the vehicle longitudinal axis directly behind the monocoque-style passenger cell allows optimum weight distribution for driving dynamics. In conjunction with the dry-sump lubricating oil system for the engine and gearbox, a low centre of gravity on the drive train could also be created. NEWLY DEVELOPED TWO -STAGE TURBOCHARGING SYSTEM
The output increase required a fundamental new concept for the four turbochargers. To achieve this Bugatti took inspiration from the two-stage regulated forced induction (sequential turbocharging) established in the field of diesel engines which uses two turbochargers connected in series, FIGURE 2. The W16 engine for the Bugatti Chiron offered the opportunity to implement sequential turbocharging without adding further turbochargers thanks to the existing arrangement of two turbochargers per cylinder bank. The solution implemented for fast build-up of charge pressure uses the existing bi-turbo concept for each cylinder bank in which one turbocharger is separated from the exhaust gas mass flow by means of a suitable form of decoupling and consequentially all of the exhaust gas available from eight cylinders is applied only to one turbocharger, FIGURE 3 (top). As a result, this constantly running turbocharger reaches the speed required to build up charge pressure consider ably faster and, thanks to the air mass flow being available at an early stage, generates a high low-end torque with
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C OVER STORY High - Performance Engines
FIGURE 1 Drive train arrangement of the Bugatti Chiron (© Bugatti)
the resulting good starting behaviour without “turbo lag”. From a certain engine speed, the air mass flow delivered by this turbocharger is no longer sufficient to generate the required engine torque on its own. Further operation in two-turbocharger mode would result in the torque development breaking off above this switching speed. For this reason, regulated activation of the second turbocharger, which is deactivated on both cylinder banks in this phase, occurs by continuously opening the exhaust gas regulating flap that was closed up to this point. The engine only reaches its maximum output in the subsequent four-turbocharger mode, F IGURE 3 (bottom).
Both, the permanently running as well as the on-demand turbocharger purposefully have identical dimensions. The decisive advantage over a “large/small” combination selected in favour of even faster spool-up lies in a constant torque development that does not have a noticeable drop at the switch-over point. EXHAUST GAS REGULATING FLAP AS REGULATED BYPASS VALVE
To create the switch-off system that needs to be placed in front of the turbocharger being deactivated for the described function, Bugatti developed an exhaust gas regulating flap with a shaft located in the exhaust
gas mass flow and thus similar to the throttle valve familiar from other the intake systems, FIGURE 4. The exhaust gas regulating flap regulates the distribution of the exhaust gas mass flow continuously or also closes the flow of exhaust gas to one turbocharger completely on either bank. During full load operation, the flap gradually opens the flow to the deactivated turbocharger from 2000 rpm and is used as a regulated bypass valve in the subsequent engine speed range as the flap opening angle increases. Above an engine speed of approximately 3800 rpm, the Chiron engine switches back completely to a four-turbocharger mode without the driver hardly noticing. In addition, the turbochargers on both banks are activated not synchronously, but in sequential order. Due to the slight delay, the torque development of the whole engine is smoothed and thus negative effects on the driving performance are avoided. The exhaust gas regulating flap needs to withstand exhaust gas temperatures of 980 °C in inertia with full movement. Therefore for its individual assembly parts like flap, shaft and flap housing high-temperature alloy Inconel 713C is used. The blanks produced using an investment-casting technique need to be processed mechanically in some areas with considerable work and high precision. The flap shaft runs in two cageless ceramic needle bearings on both sides of the housing and is permanently joined to the moving flap with the aid of a honed bore. SWITCHING STRATEGY
FIGURE 2 The Bugatti two-stage turbocharging system uses the existing arrangement of two turbochargers per cylinder bank (© Bugatti)
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The engine load is in the lower map range during restrained driving or low vehicle longitudinal dynamics. The exhaust gas regulating flaps in front of the regulated turbochargers are open when the engine runs in regular four-turbocharger mode. If, however, the moment requirement exceeds a value that can no longer be depicted at engine speeds below 3800 rpm in four-turbocharger mode, the exhaust gas regulating flaps on both cylinder banks will be quickly moved to their closed position. The resulting decoupling of one turbocharger on each cylinder bank has the consequence that the exhaust gas flow for the now eight cylinders is guided to
FIGURE 3 Bugatti two-stage turbocharging system – two-turbocharger mode (top) and four-turbocharger mode (bottom) (© Bugatti)
the remaining continuously running turbocharger. As a result, this turbocharger accelerates on demand at a high rate to the necessary speed in order to spontaneously provide the required charge pressure. A return flow via the compressor of the deactivated turbocharger is prevented by the non-return valve integrated into this charge air duct closing automatically. Operation in two-turbocharger mode is set in this map range if one looks at the complete engine. If the air mass requirement of the engine continues to rise, the two-turbocharger mode will no longer be sufficient. The turbocharger deactivated for each cylinder bank is therefore returned to its speed by controlled opening and regulating of the shut-off unit. The air mass flow supported during spool up, which is still low in this phase, is first guided via an electrically operated spool-up blocking valve that is integrated into the charge pressure system in front of the compressor in the continuously running turbocharger. As a result of the slight vacuum that is present here, firstly one of the turbochargers to be accelerated reaches its speed faster while secondly the continuously running turbocharger is supported in the transition phase of the exhaust gas mass, FIGURE 5. Once the turbocharger speeds have come close to each other, the spool-up blocking valve closes the bypass air route so that the previously described non-return valve is opened again automatically by the rising charge pressure. As a result, the previously decoupled turbocharger delivers the air mass required by the engine. All four turbochargers are thus in operation and deliver the air mass flow required to reach maximum power. MEASURES TO INCREASE OUTPUT
FIGURE 4 Exhaust gas regulating flap for exhaust gas temperatures up to 980 °C (© Bugatti) MTZ worldwide 03|2017
Apart from the new Bugatti two-stage turbocharging system and the larger turbochargers the new exhaust system Bugatti introduced further developments to implement the output increased as defined in the product brief. The following section highlights the most important, FIGURE 6. The largest part of the output increase is contributed without a doubt by the fundamentally redesigned exhaust side of the W16 engine in the Chiron. In addition to the new Bugatti two-stage tur-
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C OVER STORY High - Performance Engines
The gaiter ensures gas tightness from the surrounding atmosphere and absorbs the movements of the manifold halves against each other that have been reduced to a minimum through a defined position of the flange fixings. The application of high-temperature integrated insulation reduces the heat losses from the exhaust gases during the warming-up phase and, at high engine output, the transfer of heat from the exhaust gases, which can reach up to 980 °C, into the engine compartment. As a result, the surface temperature of the exhaust manifold can be limited to a maximum of 350 °C even at full load. DE-THROTTLING OF EXHAUST SYSTEM AND CATALY TIC CONVERTER DIMENSIONING
FIGURE 5 Turbocharger operating strategy (© Bugatti)
bocharging system, the de-throttling of the exhaust system together with the newly developed catalytic converters plays an important role here. Redesigning the exhaust manifold was a basic requirement to achieve the previously described sequential turbocharging function, FIGURE 7. While on the Veyron engine separate individual manifolds on each cylinder bank each combine the exhaust gas mass flow of four neighbouring cylinders and guide it to one turbocharger, the exhaust manifolds on each bank had to be combined into an eight-channel version for the new sequential turbocharging system.
Each of the two cast steel manifolds covering the respective cylinder bank has two flanges on the front of which in the direction of travel a turbocharger activated by the exhaust gas regulating flap is bolted as is another continuously running turbocharger on a flange furher back. The manifolds are split in the middle in order to limit the temperature related expansion during engine operation with the extreme load cases of full load or overrun operation as well as the tensions induced in the fixed component as a result to a controll able level. A compensator in the form of a metal gaiter between the two halves created in this way serves as a connecting and decoupling element.
After leaving the turbochargers, the exhaust gases from combustion are guided through a total of six catalytic converters in the double-wall rear silencer. In order to keep the weight down, titanium was chosen as a material for the silencer that is exposed to an exhaust gas mass stream that reaches temperatures of up to 825 °C. This allowed Bugatti to keep its weight below 25 kg despite a volume of over 32 l including attachments. The exhaust gas then continues via six 80 mm tailpipes to the outside. Four of the tailpipes are visible at the rear of the car while the two others run into the rear diffuser, FIGURE 8. The challenge inside the vehicle was to arrange the pipe and catalytic converter cross-sec-
FIGURE 6 Output-related modifications to the basic engine (© Bugatti)
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FIGURE 7 Redesigned combined exhaust manifold (© Bugatti)
tions required to reduce the exhaust back pressure (reduction of knock tendency) while observing the essential distances from neighbouring components for thermal reasons. This was complicated by the fact that all elements conducting exhaust gases needed to be shielded with high-temperature insulation in order to keep the heat transfer to the engine compartment as low as possible. The total of four close-coupled starter catalytic converters bolted to the turbocharger turbine housing each have a diameter of 118 mm. The subsequent connection of the dual front exhaust pipe free of tension with fl exible joints on each cylinder bank is formed above the rear axle drive shafts for reasons of space. The continuing single-flow
inlet pipes each with 150 mm diameter are connected to the main catalytic converters measuring 200 × 90 mm in diameter that are placed in front of the neighbouring rear silencers. Thanks to these extreme dimensions, it was possible to limit the exhaust back pressure at maximum mass throughput to 600 mbar. AIR SYSTEM AND FUEL SUPPLY
The engine characteristics of output and response that have been focused on are greatly influenced by the configuration of the intake side. Consequently the objective was optimum air intake with reduced flow resistance and highly efficient charge air cooling. The maximum air mass flow through the
whole engine is 4400 kg/h, the maximum absolute charge pressure 2.85 bar. INCREASED OUTPUT CHARGE AIR CIRCUIT
The charge air compressed by the two compressors on one engine side is sent in two channels via charge air pipes, which have improved flow and, thanks to the 65 mm diameter, are de-throttled, to a large dimensioned and water-cooled charge air cooler, which has been positioned above the cylinder head cover, FIGURE 9. In order to prevent a backflow of charge air while the engine is running in two-turbocharger mode, the charge air cooler uses the non-return valve integrated into the charge air system of the on-demand turbocharger.
FIGURE 8 Exhaust system with reduced exhaust gas back pressure (© Bugatti)
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C OVER STORY High - Performance Engines
FIGURE 9 Charge air cooler fixed to engine (© Bugatti)
The two water-to-air charge air coolers are incorporated into a separate low-temperature coolant circuit via which the arising heat quantity is dissipated to the heat exchanger arranged centrally and diagonally at the front with a core area of 3060 cm 2. The coolant volume flow of up to 220 l/min required for this is supplied by a separate mechanical auxiliary coolant pump that forms an integral component as a tandem with the hydraulic pump and is driven by the ancillary drive. It circulates the 14.6 l coolant volume completely within 3 s. The cooled, highly compressed charge air is guided to the intake side of the cylinder heads separately for each cylinder bank via a throttle valve positioner with a diameter of 82 mm bolted onto the front and a separate air collector on the single-piece CFRP intake manifold to
FIGURE 10 Duplex fuel injection – four fuel rails with eight injectors each (© Bugatti)
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the intake side of the cylinder heads. When the inlet valves are open, it flows via the intake manifold channels that have been optimised by means of CFD investigations into the corresponding combustion chambers. DUPLEX INJECTION AND FUEL PRESSURE REGULATION
The multi-point injection system for the engine in the Bugatti Chiron has been fundamentally revised. To ensure the extreme spread between the idle and the full load fuel quantity, the MPI injection system has been expanded with an additional injector per cylinder and now uses a total of 32 injectors with 16 close to the combustion chamber and 16 others in the intake manifold channels, FIGURE 10. This double arrangement, which is known as duplex injection, allows two
injectors with different specifications to be used for each cylinder. To improve the metering of the injection quantity in the partial load area and in order to stabilise the idling speed of 650 rpm, which has been lowered for reasons of comfort, a small valve with a static flow rate of 211 g/min has been placed immediately in front of the inlet valves in the cylinder head. A considerably larger valve with a static flow rate of 385 g/min is activated for the high-load range. This has been accommodated upstream in the flow direction in the area of the intake manifold intake port. The pressure in the fuel rails is set in four accompanying, closed regulating circuits that work independently and adaptively. Depending on the charge pressure, fuel pressures between 5.5 and 8.5 bar (absolute) are regulated. The fuel injection quantity is divided dynamically between the two injectors depending on the engine load and speed and leads to shortened injection times. The arrangement of the second injector for each cylinder required a new design for the intake manifold. The previous three-piece design made with aluminium investment casting has been changed to a single-piece concept. In addition to a considerable weight saving of 3.8 kg (reduction of 45 %), the innovative design using carbon provides the necessary space for the duplex injection.
SUMMARY
Due to the far-reaching enhancement of the highly complex sixteen-cylinder engine and its peripherals Bugatti was able to build the most powerful pro duction engine in the super sports car segment. Delivering a maximum output of 1103 kW and 1600 Nm, the engine with four turbochargers and compact design thanks to its W configuration, underlines its uniqueness. By implementing the two-stage t urbocharging system as a guarantee for high low-end torque, a spontaneous engine response could be created despite this considerable increase in output. The new W16 engine in the Bugatti Chiron thus marks ultimate top figures for car engine production in terms of output and agility. By consistently implementing ambitious goals, the requirements in the production super sports car segment have been met and superior performance has been attained in comparison with the competition.
THANKS A big thanks for the work carried out and his participation as co-author goes to Dipl.-Ing. Tilo Fürstenberg, Head of Engine Testing at Bugatti in Molsheim (France).
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