2-CYLINDER BOXER ENGINES 1200CC.

ALL THE VIRTUES OF A BOXER, BROUGHT TO THE PEAK OF PERFORMANCE.

The flat twin engine from BMW is the best "boxer" ever. More powerful and more dynamic, it continues to arouse emotions a driver can't fail to resist - just as it has been doing for eighty years.

K2x (air/oil-cooled) K50 (air/water-cooled)

K2x (air/oil-cooled)

The charismatic two cylinder flat twin engines embody the typical BMW as its underlying concept has been seen and appreciated for over eighty years: majestic, high torque, and robust. The present generation is even stronger, lighter, more refined, and more economical in fuel consumption. With its air/oil cooling and four valve technology, the new DOHC two cylinder flat twin engine is identical in its design principle to the traditional BMW Motorrad two cylinder power units, but is nonetheless an all new engine development.
   
Engine type Air/oil-cooled flat twin ('Boxer') 4-stroke engine
(two camshafts and four radially aligned valves per cylinder central balancer shaft)
Displacement [ccm] 1.170
Bore x stroke [mm] 101 x 73
Compression ratio 12.0 : 1
Power [kW] @ [1/min] 81 @ 7.550
Torque [Nm] @ [1/min] 119 @ 6.000
Emission standard EU3
Mixture control/ engine management Electronic intake pipe injection
Power/Torque diagramm
   
Engine type Air/oil-cooled flat twin ('Boxer') 4-stroke engine
(two camshafts and four radially aligned valves per cylinder central balancer shaft)
Displacement [ccm] 1.170
Bore x stroke [mm] 101 x 73
Compression ratio 12.0 : 1
Power [kW] @ [1/min] 81 @ 7.550
Torque [Nm] @ [1/min] 119 @ 6.000
Emission standard EU3
Mixture control/ engine management Electronic intake pipe injection
Power/Torque diagramm

More information.

Designed for higher speeds, the new flat twin engine features chain driven double overhead camshafts (DOHC) for each cylinder. The valves are actuated with cam followers of a very lightweight design ideal for high revving.
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Owing to the four valves' radial arrangement, the combustion chamber could be realized as a highly compact design. As with the predecessor model, the mixture is ignited with two spark plugs. Also adopted unchanged was the compression ratio of 12.0:1. Thanks to its knock control, the vehicle can be fuelled with "Super (plus)" (95–98 RON). The rated power was determined with 98 RON.

In the new flat twin engine, the horizontal arrangement of the camshafts in the direction of travel gives rise to two technical peculiarities: Each camshaft actuates an intake and exhaust valve. Owing to the valves' radial arrangement, the cams are ground to a conical shape. The inlet and exhaust timing for the two camshafts was optimized specifically for a particularly punchy power delivery in the low and mid speed ranges together with a boost to high rev response.

Compared with the predecessor model, the valve head diameter on the inlet side was increased from 36 to 39 mm and on the exhaust side from 31 to 33 mm. Valve clearance compensation takes the form of lightweight hemispherical shims. A large free valve cross section was achieved when the valve lifts on the intake and exhaust sides were increased to 10.8 mm from their previous 10.54 and 9.26 mm respectively. The result is supreme character which convincingly demonstrates its superiority from the very lowest engine speeds – a perfect example of outstanding dynamic performance and free revving.

A key development goal in the new flat twin engine was also the further improvement of running smoothness as provided by a balancing shaft. So as to save space, it runs inside the hollow auxiliary shaft for the camshaft drive and thus ensures optimum running smoothness and silken soft pulling power across the entire engine speed range, but without diluting the essential character of the twin cylinder engine.

No changes were made to the previous bore-stroke ratio of 101x73 for a displacement of 1.170cm³. Also adopted unchanged were the crankshaft, its connecting rod, and its bearings, whereas the two pistons of die cast aluminum were redesigned and adapted to the new combustion chamber geometry.

The revised intake system now features throttle flap nozzles with 50 mm passage instead of the 47 mm in the predecessor model. In addition, the new engine features a new intake air snorkel design and an air filter element with a greater through flow. A new oil cooler design provides for thermal stability even under extreme riding conditions. Also the oil cooler mask was redesigned for better approach flow properties and a look of greater dynamics.

Actuated with an electric servomotor and opening and closing cables, the exhaust system generates a particularly sonorous flat twin sound that also conforms to the applicable laws.

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K50 (air/water-cooled)

The boxer engine used for the first time in the new R 1200 GS continues to use air/liquid cooling, however, the coolant oil has been replaced by a glycol-water mixture for the first time in a BMW Motorrad standard boxer. This ensures a high level of heat absorption capacity of the cooling liquid for more efficient heat dissipation. So-called precision cooling means that the cooling fluid flows through the thermally more heavily exposed engine elements – the two cylinder heads and parts of the cylinders. Heat dissipation is via two radiators positioned at the left and right of the front section. These are inconspicuously integrated and well protected by the radiator covers. An electric fan behind the right-hand radiator is automatically activated as needed, controlled by the thermostat – for example at high outdoor temperatures in city traffic. Thanks to the sophisticated cool-air ducting on both sides, the warm air is streamlined past the rider.

The 1.170cm³ capacity and the ratio of bore to stroke of 101 to 73 mm has also been preserved As in the predecessor model, the camshafts are arranged horizontally. By changing the through-flow direction from horizontal to vertical, however, it is now possible to have purebred intake and exhaust camshafts – in the predecessor model these were combined intake/exhaust camshafts.
   
Engine type Air/liquid-cooled four stroke flat twin engine
(double overhead camshaft, one balance shaft)
Displacement [ccm] 1.170
Bore x stroke [mm] 101 x 73
Compression ratio 12.5 : 1
Power [kW] @ [1/min] 92 @ 7.750
Torque [Nm] @ [1/min] 125 @ 6.500
Emission standard EU3
Mixture control/ engine management Electronic intake pipe injection
BMS-K+ digital engine management
electromotive throttle actuator
Power/Torque diagramm
   
Engine type Air/liquid-cooled four stroke flat twin engine
(double overhead camshaft, one balance shaft)
Displacement [ccm] 1.170
Bore x stroke [mm] 101 x 73
Compression ratio 12.5 : 1
Power [kW] @ [1/min] 92 @ 7.750
Torque [Nm] @ [1/min] 125 @ 6.500
Emission standard EU3
Mixture control/ engine management Electronic intake pipe injection
BMS-K+ digital engine management
electromotive throttle actuator
Power/Torque diagramm

More information.

As before, the two respective camshafts are driven by a chain running in the shaft behind the cylinders (on the right-hand side of the engine from the counterbalance shaft and to the left of the crankshaft). The timing chain drives an intermediate shaft between the intake and exhaust camshaft and it is from here that power is transmitted to the camshafts via spur gear pairs. At each exhaust camshaft there is an centrifugal-force-driven decompression facility which facilitates the start-up process. This makes it possible to save weight in the starter motor and battery.
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Due to the highly efficient overall design of the power unit, the previous radial valve arrangement is no longer required. At 8 degrees on the intake side and 10 degrees on the exhaust side, the four valves are at close angles to one another, making for a compact combustion chamber shape – a feature which is crucial to optimum combustion. Due to the new calculation of the combustion chamber and the much improved channel ducting and design, it has been possible to improve the compression ratio as compared to the predecessor model from 12.0:1 to 12.5:1. What is more, due to the optimization of the combustion process and ignition timing, no knock control is required, despite retaining the RON 95 set-up, while still making the most of torque potential.

The use of a turbulence system (air feed via a bypass) ensures optimum combustion, obviating the need for an elaborate dual ignition.

The crankcase has also been completely re-engineered. The main bearing diameter was reduced from the previous 60 mm to 55 mm to reduce the drag forces. The crankshaft also has crankpins which are narrower but with a diameter enlarged from 48 to 50 mm as well as narrower main and guide bearings. As a result, it is lighter overall yet much stiffer and more compact.

As in the previous boxer engine, the new engine also has a counterbalance shaft which runs at crankshaft rotation speed with newly defined imbalance masses so as to eliminate unwanted vibrations. The counterbalance shaft is designed as a hollow intermediate shaft within which the clutch shaft runs. This ensures that the new engine, with its higher engine speed level, runs more comfortably and with perceptibly less vibration across the entire engine speed range and at high engine speeds in particular. Nonetheless, the essential earthy boxer characteristics are still preserved.

The plate diameters of the valves have also been redefined to obtain higher output and torque efficiency. They are 1.0 mm larger, with a size of 40 mm on the intake side and 34 mm on the exhaust side. The valve stem diameter is still 5.5 mm as before. Significantly shorter valve springs do justice to the increased engine speed level, while also ensuring an optimally defined drag effect.

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