blackfoxsteam
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Why are crankshaft throws on twin cylinder model marine engines such as Rudy Kouhoupt's spaced at 90 degrees instead of 180 degrees?
Model Marine Engine
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terrywerm
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You have to remember that an engine cylinder has two dead centers, one at the top of the stroke, and one at the bottom. By placing the crankpins at 90* to one another, one cylinder is in the middle of its power stroke when the other is on a dead center. In other words, there are no places where the engine cannot start due to being on a dead center. The engine will always be self starting.
The answer is really very simple , most steam engines are double acting , ie. steam is admitted to either side of the piston in turn so the piston is powered in each direction of travel.
It follows then that a twin cylinder engine has four power strokes per revolution , these power strokes are applied evenly to the crankshaft by setting the cranks at 90° to each other.
This arrangement also results in an engine that is self starting as at any position of the crankshaft there will be a piston in a power stroke position , whereas at 180° if one piston was at TDC then the other would be at BDC and the engine would not start without help turning the crankshaft.
I am currently building a three cylinder locomotive which uses 120° setting for the cranks , in effect a 6 cylinder engine , perhaps you can now work out the settings for a four cylinder locomotive such as the GWR King class.
Dan.
It follows then that a twin cylinder engine has four power strokes per revolution , these power strokes are applied evenly to the crankshaft by setting the cranks at 90° to each other.
This arrangement also results in an engine that is self starting as at any position of the crankshaft there will be a piston in a power stroke position , whereas at 180° if one piston was at TDC then the other would be at BDC and the engine would not start without help turning the crankshaft.
I am currently building a three cylinder locomotive which uses 120° setting for the cranks , in effect a 6 cylinder engine , perhaps you can now work out the settings for a four cylinder locomotive such as the GWR King class.
Dan.
blackfoxsteam
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That makes sense. Thanks.
blackfoxsteam
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Is it therefore necessary to modify the flywheel to balance the asymmetrical crankshaft throws?
Steam engines are heavily built robust pieces of machinery and the type used in marine applications would not be run at very high speed so generally speaking balancing is not necessary, I guess a heavy flywheel helps.
Railway engines have balance weights on the driving wheels which counter the side to side thrust of the pistons , but I wouldn't know where to start with a marine engine.
Dan.
Railway engines have balance weights on the driving wheels which counter the side to side thrust of the pistons , but I wouldn't know where to start with a marine engine.
Dan.
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