ENGINE / DRIVE
TWO-CYLINDER BOXER ENGINE 1200 CCM (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engine used for the first time in the new R 1200 GS continues to use air/liquid cooling, however, the coolant oil has been replaced by a glycol-water mixture for the first time in a BMW Motorrad standard boxer.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 1170 cm3 capacity and the ratio of bore to stroke of 101 to 73 millimetres 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.
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.
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 optimisation 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 millimetres to 55 millimetres to reduce the drag forces. The crankshaft also has crankpins which are narrower but with a diameter enlarged from 48 to 50 millimetres 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 millimetres larger, with a size of 40 millimetres on the intake side and 34 millimetres on the exhaust side. The valve stem diameter is still 5.5 millimetres as before. Significantly shorter valve springs do justice to the increased engine speed level, while also ensuring an optimally defined drag effect.