DE VELO PMENT G A S OLINE ENGINES
THE NEW BOXER ENGINE FROM BMW MOTORRAD BMW Motorrad is using combined air and liquid cooling in its new opposed-twin “boxer” engine for the R 1200 GS, for the first time in this engine concept. The engine has been completely redesigned to accommodate the new system. The following report describes the key features and functional properties of the new engine.
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AUTHORS
DIPL.-ING. WOLFGANG MATTES is Manager Development Powertrain at BMW Motorrad in Munich (Germany).
DR. GEORG UNTERWEGER is Manager Thermodynamics and Application at BMW Motorrad in Munich (Germany).
DIPL.-ING. OLIVER HARTMANN is Manager Basic Engine Boxer, Urban Mobility, at BMW Motorrad in Munich (Germany).
REINTERPRETATION OF A TRADITIONAL DRIVE CONCEPT
The boxer engine lies at the heart of BMW Motorrad. 2013 it celebrates its 90th anniversary as a drive concept for motorcycles from BMW. The first type R32 boxer engine was produced in 1923. Its excellent suitability as a motorcycle engine played a significant role in the success of BMW motorbikes over the intervening decades. BMW boxer motorcycles gain an international reputation through the numerous world records set by Ernst Henne and the legendary Isle of Man triumph of “Schorsch” Meier on his Kompressor BMW in 1939. The boxer engine was continuously developed over the decades, from the air-cooled twin valve engine of the early years through numerous stages of evolution, including the oil and air-cooled four-valve engine of 2004 to the last configuration stage with radial valve cylinder head from 2009 [1]. The boxer engine is one of the most seasoned and sustainable drive concepts for motorcycles. The new engine received its market launch in spring 2013 with the new R 1200 GS. This was the first time that a combined air-water cooling system was used in conjunction with other deepseated conceptual changes. The engine concept has been completely redesigned and the engine has been upgraded for
current and future requirements. The following article describes the most important concept features and presents the functional properties of the new boxer engine. ENGINE CONCEPT
The greatest possible integration of functions and an uncompromising, future-safe thermodynamic design were the key objectives when it comes to the development of the new boxer engine. Thus, in comparison with the previous engine, the clutch, gearbox and generator were integrated in the engine and a compact unit was produced that enabled additional benefits when it came to the design of the bike, such as an extended rocker arm, ❶. The crankshaft uses a primary pinion with 1:1 ratio to drive the front oil bath clutch. A countershaft is used to transfer the torques to the integrated six-speed gearbox. The secondary pinion with the cardan shaft was switched from the right to the left side. The generator sits at the back end of the crankshaft and makes a key contribution to the increase in mass inertia on the primary side. A counterbalance shaft that rotates in the opposite direction at the same speed as the crankshaft reduces the inertial forces of the first degree. This has an internal countershaft in a so-called shaft-in-shaft con-
DIPL.-ING. ULI BLÜMELHUBER is Manager Peripherals at BMW Motorrad in Munich (Germany).
❶ Engine concept including shaft layout 09I2013
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CONSTRUCTION TYPE
TWO-CYLINDER FLAT ENGINE
Engine Power [kW] at [rpm]
92 at 7750
Torque [Nm] at [rpm]
125 at 6500
Displacement [cm³]
1173
Bore / stroke [mm]
101 / 73
Cylinder distance [mm]
44
Valve per cylinder [-]
4
Valve angle intake / outlet [°]
8 / 10
Diameter intake valve [mm]
40
Diameter exhaust valve [mm]
34
Valve lift intake / outlet [mm]
10.0 / 9.7
Main bearing diameter [mm]
55
Conrod bearing crank pin / small end [mm]
50 / 22
Conrod length [mm]
125
Max. engine speed [rpm]
9000
Compression [-]
12.5:1
Engine weigth (including gearbox) [kg]
77.6
CYLINDER CRANKCASE WITH LDS BORE
❷ Technical data
cept for compactness. The four-valve cylinder head was converted from horizontal to vertical flow direction. The use of an air-water cooling system meant that it was possible to reduce the temperatures in the engine significantly. The key engine data can be seen ❷.
CRANK MECHANISM
Optimised calculations and the prudent choice of suitable materials meant that it was possible to make the crank mechanism lighter and significantly more rigid than its predecessor, ❸. Lightweight
❸ Crankshaft assembly
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forged pistons instead of cast pistons help reduce the weight of the engine by 160 g. The resulting reduction in inertial forces enabled the design of the con-rod to be optimised, despite the increase in the maximum engine speed to 9000 rpm. The con-rods are now 4 mm narrower and also weigh 30 g less. On the one hand, the 42CrMoS4 crankshaft has a thicker internal plate and a larger crank pin diameter of 50 mm instead of 48 mm, making it significantly more rigid than its predecessor. On the other hand, the optimised weight and reduction in the friction coefficient would reduce the main bearing diameter from 60 mm to 55 mm and make all bearing points narrower. The generator is attached to the crankshaft cone by means of a force fit and also acts as a centrifugal mass.
The required reduction in height and length in comparison with the previous engine, while maintaining the engine width, as well as the smaller number of components, led to the integration of the oil pump, clutch, transmission and previously separate cylinders in the crankcase. The crankcase is produced in a die-casting process and consists of two vertically split halves, containing all the driving elements and the crankshaft and transmission bearings, ❹. The coolant and oil galleries are pre-cast insofar as possible, so as to avoid the need for additional work and external lines. As in the previous crankcase, the tried-and-tested AlSi9Cu3 alloy is used. The vacuum-supported die-casting process reduces the formation of gas occlusions. Heat treatment at approximately 250 °C significantly reduces internal stresses in an even way before the first mechanical change to the cylinder surfaces. Unlike the previous engine with cylinder tracks made from silicon carbide (Nikasil), in the new crankcase a track with LDS arc spray coating is used [2]. This technology was successfully introduced by BMW in automobile engines [3] and has now been adapted for the new boxer engine. The ferrousbased track features a low friction coefficient and high level of resistance to wear and tear. The LDS process involves shooting melted particles onto the roughened, activated aluminium sub-
The decompression mechanism integrated in the two outlet camshafts reduces drag and thus simplifies the starting process of the boxer engine. The mechanism operates on the basis of the engine speed by means of a flyweight with driving cam. At initial engine speed, because the driving cam interferes with the base circle of the cam when idle, one outlet valve per cylinder is always partly open. This reduces the compression pressure in both cylinders. After the start speed has been exceeded, the centrifugal force pushes the driving cam out of its base circle. The outlet valves now close completely. ❹ Crankcase with integrated liners
strate surface at high kinetic energy. The coating material flows into the cavities through capillary action and, after setting, forms a positive bond with the aluminium substrate actuated by adherence. Approximately 70 % of the relatively raw LDS spray coating is honed off, creating a compact running surface with good thermal conductivity whose lamellar character and porous nature mean that there a numerous lubrication pits in the surface, so that the oil film is well distributed and the wear and frictional coefficients are improved. CYLINDER HEAD WITH CONTROLLER
Unlike all previous boxer engines, air is now passed through the cylinder head with four valves. Until now the design of the intake channels was characterised by compromise, however identical intake lengths can now be implemented for both intake valves. The injection valves in the intake valves are arranged to ensure the best possible mix just before the intake valves. A turbulence system (air feed via a bypass) is integrated in an intake channel for optimising combustion. A significant improvement in the preparation of the mix and in combustion meant that the dual ignition and knock control could be dropped. The compression ratio was increased from 12.0 in the previous model to 12.5 : 1 09I2013
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by redesigning the combustion chamber and optimising the layout of the channels. Changing the direction of flow from horizontal to vertical means that there are now individual intake and outlet camshafts. These are made from tempered steel and are arranged horizontally. In addition, the free space behind the cylinder head now affords more space for the rider’s feet. The camshaft is driven by the sprocket wheel by means of a central countershaft on the two camshaft gears. For optimum acoustics, the automated measurement of the tooth flank backlash with subsequent pairing of the camshaft gears. The camshafts of the right cylinder are driven by the counterbalance shaft, while those of the left cylinder are driven by the crankshaft. A hydraulic chain tensioner is used in each camshaft drive chain. With 8° on the intake side and 10° on the outlet side, the four valves are positioned at narrow angles, enabling a compact combustion chamber. At 40 mm on the intake side and 34 mm on the outlet side, the diameters of the valves are 1.0 mm larger than in the previous model in each case. The valve springs are significantly shorter as a response to the speed level. The valves are actuated by means of very light, speed-resistant cam followers whose design was borrowed from the high-performance fourcylinder engine of the BMW S 1000 RR [4]. The play of the valves is adjusted by means of removable adjusting shims.
OIL AND WATER CIRCUIT
A two-stage oil pump that is integrated in the crankcase ensures the supply of oil to the new boxer engine, including transmission and clutch. The pressure level draws the cooled oil out of the rear section of the oil pump and supplies all bearings via the oil galleries. The arrangement of the shafts in the gearbox ensures the supply of oil to the clutch. The returned oil is drawn off in the front section of the oil sump by the coolant pump and is channelled towards the cylinder heads. Cast oil coolers are integrated in the front part of the cylinder heads, which are cooled by the airflow. The two return lines come together under the front cover and flow into an oil reservoir for slowing down and out-gassing from there, the cooled oil is returned to the rear part of the oil sump. The entire oil circuit was optimised and secured for extreme operating conditions, such as wheelies. Previously, boxer engines had a combined air and oil cooling system in which oil was passed through some parts of the cylinder head. In order to increase thermal stability and to meet future requirements in terms of emissions and fuel consumption, the coolant used in the new boxer engine has been changed to water-glycol, ❺, because of its higher thermal capacity and, as well as the most heavily loaded areas of the cylinder heads, the upper parts of the cylinders are also liquid cooled. The frontal water pump is directly driven by the crankshaft and its asymmetrical outlets ensure the liquid is distributed evenly between the two cylinders. The implementation of a precision cooling system means that only the areas with the highest thermal load
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DE VELO PMENT G A S OLINE ENGINES
are liquid cooled. The coolant is routed by the pump via corresponding channels directly to the cylinder casing in the upper area of the cylinder. From there it flows in to the cylinder head, where it flows around the outlet valves, spark plug area and then the inlet valves. The total coolant volume is a comparatively modest 1.3 l. The coolant passes through a thermostat into the left and then the right cooler, both of which are discreetly integrated above the cylinders. There is no need for a separate oil cooler. CLUTCH AND DRIVETRAIN
The clutch was changed from the classic dry clutch to a multi-disc oil bath clutch, ❻. With an external diameter of 145 mm, this is very compact for the torque class. The clutch is driven by the crankshaft by means of primary gearing with integrated damper and also includes servo support for low manual force and an anti-hopping function (torque limitation in thrust mode in order to reduce rear wheel stamping). These functions are implemented by means of sloping ramps in the traction and thrust directions. The clutch is actuated hydraulically by means of a push rod on the transmission side. Helical six-speed gearbox with socket coupling has been implemented as a cassette gearbox and, unlike the previous
❺ Water circulation
model, integrated in the basic engine. The three-shaft transmission is driven by a countershaft with idle vibration damper. Typically for a boxer engine, a drive shaft transfers the torque via a cardan shaft with elastomer damping to the rear axle gear. The rear axle gear has been changed to a braced bearing concept, familiar from automobiles, and the weight has also been reduced.
❼ Intake and exhaust system ❻ Clutch assembly
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THERMODYNAMICS
The vertical air flow means that the intake channels could be optimised without any geometric restriction due to the control assembly. In order to control transient behaviour, the butterfly valve is placed directly in front of the cylinder head, ❼. The volume between the butterfly valve and intake valves is thus
tion control, speed regulation and engine speed limiter. The E-Gas system comprises two identical throttle valve assemblies, each with its own electric servomotor. The engine controller has a special running-smoothness control that can use the rotational non-uniformity to compensate for differences in the two cylinders caused by tolerances, valve play and control times, thus contributing to a smoother ride at low load ranges. The introduction of the E-Gas system with different drive modes made it possible to reduce the gas control angle from 85° to 70°. Despite this, the operating forces are reduced and dosability is significantly improved. In the interest of achieving the best possible touring properties, great significance was placed on the ability to operate after sensor or actuator failure. This means that even if an E-Gas handle, or one or both E-Gas servomotors should become inoperative due to a fall, the motorcycle can still be ridden, although at a low speed. It is also possible to operate on one cylinder in an emergency, should one side of the ignition and injection system fail.
❽ Cylinder head with vertical gas flow
small. The positioning of the injector in the cylinder head led to a further reduction in the intake volume. In turndown this yields a higher intake pressure level after the intake has been closed and the return flow of combustion gases in the intake channel is reduced. The reduction of the residual gas leads to a significant improvement in smoothness and a reduction in fuel consumption. It was possible to further stabilise combustion in turndown by increasing the charge movement using so-called air feed, ❽. This is a bypass pipe and is positioned near the seat of the valve, connecting an intake channel with the clean-air area in front of the throttle-valve. At narrow throttle-valve angles, part of the intake air enters the intake channel at very high speeds and this additional impulse ensures excellent charge motion and preparation of the mix. Because of the narrow diameter of the pipe, filling is only regulated by the throttle-valve, even when almost idling. These measures meant that that there was no need for dual ignition or knock control in comparison with the previous system. The new engine is designed for fuel with 95 ROZ octane rating. This can be adjusted to 91 ROZ for special markets. 09I2013
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ENGINE CONTROLLER
The BMS-X engine controller, which is based on the Bosch ME 17, has already been introduced in the BMW K1600 GT and GTL [5] and has been further developed for use in the boxer engine. In order to be able to implement the motorcycle requirements as efficiently as possible, all ride-related software is developed at BMW. This is a load/engine speed controller with torque interface for trac-
FUNCTIONAL RESULTS ❾ shows the generous torque and performance curve in comparison with the previous model with the same cubic capacity. The maximum torque is 125 Nm at 6500 rpm. The nominal output is 92 kW at 7750 rpm. In order to improve the already excellent drive statistics of the previous model and to achieve dynamic driving performance even at low and
❾ Power and torque
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❿ Fuel consumption and performance
medium speeds. The gas dynamism was specially optimised for these operating ranges. Almost 10 Nm is already available at 2000 rpm. The new engine features excellent responsiveness and increased agility. As well as efficient combustion, this leads to very low fuel consumption in the new R 1200 GS of 4.8 l/100 km (corresponding to 108 g CO2/km) in the statutory WMTC (World Motorcycle Test Cycle) driving cycle. ❿ shows the 7 % reduction in fuel consumption in comparison with the previous model, as well as the improvement in driving performance. The combination of low fuel consumption and increased driving performance means that the new R 1200 GS assumes pole position in the competitive field. Exhaust emissions are significantly lower than the current Euro 3 exhaust limits as well as the Euro 4 limits that will apply after 2016. Five different riding modes are available to the rider at the press of a button for different purposes, such as off-road riding, touring or dynamic riding style. Among other things, the riding modes enable the rider to adjust the responsiveness of the throttle-valve to current requirements. Other new features include
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a shift point display that can be activated by the rider as required; this uses ridability to prompt the rider to shift up a gear as soon as a fuel consumption benefit can be achieved in the next highest gear and the optional speed regulator, which covers the speed range from 30 to 120 km/h. The rigid design of the structural components of the engine helps to reduce vibrations in comparison with the previous model. In order to reflect the character of the boxer engine, the sound of the exhaust system was rendered more powerful and emotional, while still complying with legal requirements. SUMMARY
The development of the new engine for the new R 1200 GS meant that the boxer engine was upgraded for current and future demands. The compactness of the engine and the high level of functional integration and uncompromising thermodynamic design make a significant contribution to reduced fuel consumption and excellent torque. In addition, the new engine was developed as an integral part of an optimised vehicle concept and has a positive influence on ridability.
REFERENCES [1] Landerl, C.; Miritsch, J.; Mittler, G.; Post, J.: Der BMW Boxermotor für den Sporteinsatz – Herausforderungen an die Technik bei traditionellen Konzeptmerkmalen. 16 th Aachen Colloquium Vehicle and Engine Technology, 2007 [2] Landerl, C.; Mattes, W.; Hartmann, O.; Gibisch, R.; Bauer, A.; Wagener, W.; Heyl, G.: 90 Jahre BMW Boxermotoren – Gussanwendungen gestern, heute und morgen. VDI Symposium Gießtechnik im Motorenbau, Magdeburg, 2013 [3] Steinparzer, F.; Klauer, N.; Kannenberg, D.; Unger, H.: The New 2.0-l Four-Cylinder Gasoline Engine with Turbocharger. In: MTZ 72 (2011), No. 12 [4] Landerl, C.; Hoehl, J.; Miritsch, J.; Post, J.; Unterweger, G.; Vogt, J.: Der Antrieb der neuen BMW S1000RR – Höchstleistung für Rennstrecke und Landstraße. 18 th Aachen Colloquium Vehicle and Engine Technology, 2009 [5] Mattes, W.; Hege, H.; Haimerl, M.; Unterweger, G.: Der neue Motorrad-Sechszylindermotor von BMW. In: MTZ 72 (2011), No. 6
THANKS The authors would like to thank Reinhard Jooß, Project Leader Powertrain, and Dr. Christian Landerl, Executive Vice President Development, both BMW Motorrad in Munich (Germany), for their effort on this article.
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