ENGINE / DRIVE
TWO CYLINDER FLAT TWIN ENGINE 1200 CC.
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.
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.
Owing to the four valves' radial arrangement, the combustion chamber could be realised 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 optimised 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 millimetres and on the exhaust side from 31 to 33 millimetres. 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 101:73 for a displacement of 1170 cc. Also adopted unchanged were the crankshaft, its connecting rod, and its bearings, whereas the two pistons of diecast aluminium 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 throughflow. 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.
(R Series + HP Series)