1200 ccm two-cylinder opposed-twin engine (air/water cooled).
1200 ccm two-cylinder opposed-twin engine (air/water cooled) in the BMW R 1200 GS.
The previous engine displacement of 1,170 cm3 has been retained, as has the ratio between bore hole and stroke of 101 to 73 millimetres. As with the predecessor model, the camshafts are arranged horizontally. Whereas in the past combined intake/exhaust camshafts were required, the change in flow direction from horizontal to vertical has led to dedicated intake and exhaust camshafts.
Both of these camshafts are driven, as before, via the chain running in the shaft behind the cylinders (on the right engine side via the balancer shaft and on the left via the crankshaft). The timing chain drives an intermediate shaft located between intake and exhaust camshaft, from which power is transmitted to the camshafts via pairs of spur gears. A decompression device controlled by centrifugal force is located at each of the exhaust camshafts. This makes starting easier. As a result the weights of the starter and the battery could be reduced.
The highly efficient overall design of the power train meant the radial valve arrangement used previously could be dispensed with. The four valves are arranged at sharp angles to one another - at 8 degrees on the intake side, 8 and 10 degrees on the exhaust side, enabling a compact combustion chamber shape, which is a key factor for optimal combustion processes. The recalculated combustion chamber and much improved guidance and design has enabled the compression ratio to be increased from 12.0:1 to 12.5:1 compared to the predecessor model. Optimising the combustion processes and the ignition position means knock control is no longer necessary while at the same time the RON 95 quality fuel design has been retained and torque potential is nevertheless fully utilised.
The use of a turbulence system (air in-feed via a bypass) ensures optimal combustion, rendering complex twin-spark ignition superfluous. The design of the crankshaft has also been fully revised. To reduce drag effect, the main bearing diameter has been reduced from the previous 60 mm to 55 millimetres. In addition the crankshaft has more slender crankpins, despite these having increased in diameter from 48 to 50 millimetres, and more slender main bearings and guide bearings. This makes it lighter and more compact overall yet still far more rigid.
