ENGINE / DRIVE
TWO CYLINDER PARALLEL TWIN 800 CC.
Common to all F models is the high torque two cylinder inline engine (parallel twin) with a displacement of 798 cc. Its beefy torque characteristics reminiscent of a flat twin, its unique sound, and the spontaneous response at all speeds combine the liveliness of a single cylinder with the unbridled power of two hearts.
The parallel twin therefore manages not only to bridge high pulling power and dynamic sprint capacity. The new engine also proves that good performance does not necessarily have to involve a high fuel consumption.
The crankshaft of the two cylinder parallel twin features a crank pin offset of 0°. As a consequence, the transverse engine runs with a uniform ignition sequence that executes a cycle with each rotation of the crankshaft. This gives rise to an (intentional) sound pattern reminiscent of the flat twin engines, which also run with an ignition displacement of 360°.
Specifically, though, the uniform ignition sequence promotes a balanced charge change with harmonious torque curves and high torque utilisation. The basic requirements for a dynamic drive are therefore fulfilled.
Unavoidable on two cylinder engines, the first and second order inertial forces are neutralised by a balancing mechanism that is unique in our times. Instead of the conventional solution involving so called balancing or counterweight shafts, the oscillating inertial forces are eliminated by a system of articulated joints, positioned centrally on the crankshaft, comprising a predefined arrangement of counterweight masses. Arranged opposite to the crank pin, an eccentric
on the crankshaft carries a so called balance rod. This rod is joined to a balance pivot.
The kinematics are such that the balance rod moves up and down counter to the movements of the two piston rods. The movements over the relatively long swing arm gives rise to a relatively linear swinging motion for the rod end. The distribution of mass over the rod end and swing arm is such that the inertial forces generated by the swinging motion counteract the inertial forces generated by the crankshaft (piston and rod section) at all positions. This virtually eliminates the first and second order inertial forces, and the engine runs with low vibration levels. One other major advantage of this elegant design is its low noise levels because there are no gears or chains with their characteristic drive sounds.
Also the technology behind the oil circuit presents special features that go far beyond the standard usual in this class. The F 800 engine series is fitted with semidry sump lubrication that effectively minimises mixture losses. The oil leaking from the crankshaft's main bearings runs into the isolated duct of the balancing mechanism. Here, the lubricant is drawn off through an oil pump that distributes it through oil nozzles into the gearbox. Draining out of the gearbox through apertures in the crankcase, the oil mixes with the cylinder head oil draining through the chain duct and collects in the remaining space in the crankcase around the rod duct. This space is therefore utilised as an integrated oil tank. From this reservoir, a second pump supplies all of the bearing sites.
The two cylinder parallel twin engine of the BMW Motorrad F Series features two overhead camshafts that are driven by a timing chain and whose cam followers actuate the four valves on each cylinder. As a lightweight, low friction, and high revving cam follower design, the whole VTC is extraordinarily insensitive to wear despite the large valve lift and so needs maintenance at far more generous intervals.
Other features typical of BMW are apparent in the preparation of the fuel mixture, which is the responsibility of an intake injection with BMS KP engine management and two 46 mm throttle flaps. The engine management system can therefore determine the injection volume as a function not only of the injection duration, but also of the pressure that the fuel pump delivers according to the power requirements.
The fuel system operates without a return line and delivers only that amount the engine actually needs. This power saving regulation of fuel quantity means that the fuel pressure can be modified over a wide range for the optimal fuel mixture. This principle is protected by patents and therefore unique. Measuring the supplied fuel quantity is based not only on the known parameters like load, speed, and temperature, but also on the residual oxygen content in the exhaust gas. The required data are provided by an oxygen sensor that is positioned behind the exhaust manifold and is indispensable for the effective conversion of exhaust gases in the three way catalytic converter fitted as standard.
Not only the ultra modern fuel injection, also the low inertia contributes to the extremely agile performance. The relatively long bore/stroke ratio of 82 x 75.6 millimetres ensures a good compromise between high rev power and torque, while the power distribution figures of the models fitted with this engine are as impressive as the acceleration. Ninety percent of the maximum torque is available at 5000 rpm, and as the engine speed moves up to the nominal level of 8000 rpm, the curve continues to rise.
The compression ratio of 12.0:1 also means that fuel consumption is highly efficient. Depending on the model, a mean consumption of 4.4 litres per 100 kilometres is possible at 120 km/h, and can be as low as 3.4 litres per 100 kilometres at 90 km/h. And together with the regulated three way catalytic converter, it fulfils the EU3 standard with plenty of room to spare.
Fitted with a secondary drive and O ring chain, the engine can perform its whole range of activities from sporty dynamic to offroad.
When designing the two cylinder parallel twin, the BMW development engineers quite deliberately focused more on flexibility and majestic power delivery in the mid speed range than on high peak performance. The sporty rider will therefore thrill at the engine's ready acceleration, whereas the tourer can enjoy the high torque twin with the minimum of gearshifts.
(BMW F Series)