K1200S Engine Description

[Trevor Rossack]

The following is the transcription of the 'sidebar' from the recent AMCN K12000S test ride detailing the engine. There are a some very interesting details in the below description:-


F1 INSPIRATION
With a history of performance engines on four wheels it was only a matter of time before BMW got 'serious' about horsepower on two wheels - and the K1200S is proof.

Fitted cross-wise in the aluminium frame, the 1157cc four-cylinder power unit pumps out a claimed 167ps at 10,250rpm and develops 13.2kg-m at 8250rpm, with BMW claiming more than 70 per cent of the engine's maximum torque is available from 3000rpm.

The 55-degree slant-block engine weighs 81.3kg (including the clutch and gearbox).

The forged crankshaft comes with eight counterweights and a crank angle of 180-degrees, with oil supplied to the crankshaft and bearings in a radial flow inspired by F1 practice. This makes the usual ring groove superfluous and keeps the bearing slimmer and more compact allowing for a short and compact crankshaft and minimum distances between cylinders, which in turn keeps the overall engine slender and compact.

The connecting rods are forged steel components with the upper conrod opening carrying a bearing bush for a running life of more than 100,000km.

The pistons are lightweight box-type items with an extra-short skirt and three rings ( two compression and one oil removal ring). Piston weight (complete with the pins and rings) is 299grams, with the piston surfaces cooled by oil injection nozzles in the crankcase.

The four-valve cylinder-head features a narrow valve angle with 10-degrees on the intake and 11-degrees on the exhaust. Only the outlet camshaft is driven by a chain from the crankshaft, the intake camshaft being driven by gear drive from the outlet shaft. BMW claims the advantage of this method is that only one sprocket is required on the cylinder-head, thus keeping the width of the cylinder-head to a minimum.

The overall layout of the cylinder-head allows a 1:1 ratio on the follower arms, thus keeping valve-train flex to a minimum and allowing the use of lightweight arms. Redline is 11,000rpm.

Valve diameter is 32mm on the intake and 27.5mm on the exhaust, with a ratio of 13:1.

An oil reservoir for the dry-sump engine is located in the frame triangle right behind the engine with an oil cooler located in the fairing beneath the headlight.

The K1200S features a multi-plate oil bath clutch - a first for a BMW motorcycle with shaft drive.

The gearbox is a compact cassette unit, and to reduce weight the hollow gearshift cylinder is made of aluminium. The shift forks are also made of aluminium with the two transmission shafts running above one another to aid compactness. The gears themselves are straight-toothed.

The K12's BMS-K (BMW Engine Management with Anti-Knock Control) engine management system is an in house development, first featured on the R1200GS and now upgraded for four-cylinder power units. BMW claims the advantages of the new technology are fully sequential cylinder-specific fuel injection, integrated anti-knock control, raid processing of comprhensive sensor signals, compact layout, low weight and self diagnonsis.


I hope this has been somewhat enlightening.


Cheers
TREVOR