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Offset cylinder engines

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  • #26875
    • Town/City: Tauranga
    • Bike: Suzuki DR650 ADV, BMW R100GS
    • Rank: Round the World Adventure Globetrotter

    We’ve discussed a number of unique engine designs on the site, most of them were concepts that are not yet actually in main stream use.

    This one is more a design variation of the standard engine and has been in use by Yamaha in outboard motors since 2002, and more recently in the Yamaha R1 and several models of Yamaha motocross bikes, but no Adventure bikes that I can find.

    Usually an engines crank is centered directly underneath the center of the cylinder so that the piston moves up and down directly inline with the crank. In an offset engine the crank and cylinder don’t line up.

    Yamaha outline why they do this, and it makes sense.

    In general, the piston in an engine is pushed down by the explosive force of combustion in the combustion chamber and this is the origin of the drive force created by an engine. Strictly speaking, however, the piston is not actually pushed straight downward. Although it is small, there is in fact a diagonal component to the downward force that causes the piston to be pushed against the wall of the cylinder. This results in friction and a corresponding amount of horsepower loss. This friction-induced power loss can be reduced by lengthening the connecting rod, but lengthening the rod makes the engine heavier and larger overall.

    The offset cylinder is a design that solves both of these inherently conflicting problems. This is accomplished by “offsetting” the cylinder from the crankshaft so that the axis of the piston and connecting rod are aligned vertically at the moment of combustion—when the pressure in the cylinder is at its peak. Doing this greatly reduces the friction from the diagonal force pushing the piston against the cylinder wall. At the same time, this enables more effective use of the energy produced by combustion. The engine in the F2.5A has a bore × stroke of 54 mm × 31.5 mm. The cylinder offset was set at a large 10 mm, increasing combustion efficiency and enabling a more compact design at the same time.

    In the offset cylinder design, the amount of offset affects the fuel efficiency, power output, engine response and more. Yamaha’s waza lies in determining the optimal amount of offset during development in line with the model’s characteristics.

    You can red more about this on the Yamaha website: https://global.yamaha-motor.com/about/technology/power_source/004/

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