ENGINE / DRIVE
FOUR CYLINDER INLINE ENGINE 1300 CC.
Also the second four-cylinder generation in the eighty five year history of BMW Motorrad does full justice to the traditional BMW principle of providing sophisticated, independent solutions far superior to the standard. And this purebred high performance engine still continues its pioneering role in the world of motorcycle design.
The transverse four cylinder 1300 cc inline engine presents a beefy, harmonious torque curve and a high level of drive comfort for absolute mastery with the optimal ridability. The displacement of 1293 cc and a max speed of 11,000 rpm present a unique mix of power, torque, and performance.
Its rated power output is 129 kW (175 hp) at 9250 rpm, and its max torque of 140 Nm is available at 8250 rpm. Over 70% of this max torque is available at speeds as low as 3000 rpm. Weighing in at 82.8 kg, the drive is still one of the lightest engines in this displacement class on the market. Particular importance was attached to the elaborate overall concept and the space saving arrangement of the auxiliary units and the integrated gearbox. The result is a highly compact drive with the ideal concentration of mass at the centre of the vehicle.
In the typically BMW manner, the engineers eliminated the drawbacks associated with the engine's relatively high centre of gravity in the conventional four cylinder concepts. For instance, the cylinder axis was inclined to the front through 55°. This resulted in not only a low centre of gravity, but also a balanced distribution of weight – indispensable for a ride feel of precision and unambiguous feedback, particularly for sporty riders. The inclined engine also creates space for an intake system with optimal flow properties directly over the engine and facilitates the ideal arrangement of the frame sections with respect to the flow of forces (Knee contact! Ride feel “in the bike”, and not on a bucking bronco).
The crankshaft of the four cylinder engine is a single forged part of quenched and tempered steel and presents eight counterweights and the traditional 180° offset for equal ignition intervals. The stroke to bore ratio of 64.3:80 mm allows adequate overlap for the journal, in turn safeguarding high rigidity. The diameter of the main and connecting rod journals is 38 mm.
The camshafts in the cylinder head are driven over a chain that itself is powered over a timing sprocket pressed on the right end of the camshaft.
The connecting rods on friction bearings are designed as lightweight forged parts of quenched and tempered steel. Only 119 mm long, they minimise the forces on the sides of the pistons and ensure the engine's smooth running. Together with their friction bearings, they weigh 404 grams. A BMW characteristic, the connecting rod's upper eye carries a bearing bush. The connecting rods are split horizontally by means of so called cracking, allowing high precision assemblies without the need for additional centring.
The flat crown of the combustion chamber, the contour of the piston head, and the valve recesses promote good thermodynamic properties during the combustion process. The contours of the piston head also serve to optimise weight. The weight of the piston, complete with pin and rings, is only 287 g. As a measure to eliminate the free second order inertial forces that are unavoidable in four cylinder inline engines, the crankshaft (as in the predecessor model) drives a toothed belt drive to power two balancing shafts arranged below and symmetrically in front of and behind the crankshaft. The degree of balancing is 86%. The balancing shafts rotate twice as fast as the crankshaft.
As a measure to minimise noise levels, the balancing weights are connected via elastomer elements to the balancing shafts.
The cylinder crankcase split horizontally at the height of the crankshaft axis is made of high strength aluminium alloy. The compact top section of gravity diecast forms a highly rigid construction together with the four cylinders and the top bearing block for the crankshaft.
The cylinder block with the water jacket is designed as a so called open deck structure, and the bores are lined with a nickel-silica dispersion for high antiwear and low friction properties. The bottom, pressure diecast section forms the counterpart to the crankshaft's main bearing and houses the gearbox.
This affects decisively the performance, characteristics, combustion quality, and therefore also the fuel consumption of the cylinder head and VTC. The four valve cylinder head in the K 1300 S was designed for the optimal duct geometry, compactness, the optimal thermodynamics, and efficient heat balance. Owing to the small valve angle, the inlet duct presents the ideal linear design and the combustion chamber compact dimensions for high compression and the optimal efficiency.
Featuring a cam follower controller with double overhead camshafts, the four cylinder 1300 cc inline engine delivers the maximum possible power and a stable torque, at the same time fulfilling the criteria for rigidity, min moving masses, and optimal timing cross sections at the valves. It presents the perfect combination of max rigidity and min weight of moving VTC components with a highly compact cylinder head design at the same time.
The valve angles are 10° on the inlet side and 11° on the exhaust side, values that are setting the trend on this market segment. Of the two overhead camshafts, only the exhaust shaft is powered over a timing chain by the crankshaft. The inlet camshaft is powered over a toothed belt drive by the exhaust shaft. So only one sprocket is needed in the cylinder head. The advantages of this design lie in the greater precision of the timing and a leaner cylinder head. The exhaust valve timing was accommodated in the revised exhaust valve springs and an optimised tightening mechanism for the timing chain.
The camshafts are arranged directly over the valves. The overall geometry of the cylinder head allows the ideal ratio of 1:1 for the cam followers. They are therefore exposed only to the minimum of bending forces. The followers could therefore be given an extremely filigree and lightweight design. The speed threshold set down for the series is 11,000 rpm, yet the purely mechanical speed capacity is far higher.
The engine relies on the tried and tested dry sump lubrication as it is used primarily in racing motorcycle design. Besides a high level of operating reliability even under extreme conditions, this principle also permits a flat design for the crankcase and therefore a deeper position for the installed engine, for a low centre of gravity. The absence of the oil sump means that the engine can be installed 60 mm lower than on a conventional design.
The oil reservoir takes the form of a tank positioned in the frame triangle behind the engine. Operating in the rear section of the crankcase and powered over a chain connected to the clutch shaft, a double oil pump draws the lubricating oil out of the oil tank and first supplies it as hydraulic oil to the oil filter (full flow filter). This is located in the bottom left side of the crankcase, easily accessible from outside.
The small installed width was obtained when the electrical auxiliary units and their actuators were relocated to the space above the gearbox behind the crankshaft. The three phase alternator is driven by the primary gears of the clutch. The rated alternator power output is 580 W, the max current 50 A. The intermediate starter motor is coupled to a freewheel that acts on the alternator drive gear.
The four cylinder 1300 cc inline engine represents sporting character and riding dynamics with high comfort, playfully easy handling with absolute riding stability, majestic high performance with perfect everyday practicality, an even leaner and sportier look with optimised wind and weather protection at the same time, and perfected ergonomics.