Elbow engine

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An elbow engine is a piston-based engine typically fed by steam or compressed air to drive a flywheel and/or mechanical load. It is based on a mechanism known as a Hobson's joint. Although not commonly used today for practical purposes, it is still built by hobbyists for its uniqueness.

Principle of operation[edit]

Elbow engines have two rotating, circular, cylinder blocks. Each block contains a ring of parallel cylinders and can itself rotate on a central axis, similar to a revolver cylinder. The two blocks are placed at 90° to each other. Each piston is L-shaped, and circular in cross section with one end fitted into each cylinder block. The two cylinder blocks rotate together, coupled only by the pistons. Engine output is taken from the rotation of one cylinder block.

Pressure is supplied to each cylinder at the point where the pistons are at a stage of upward travel, and an opening to exhaust is provided where the pistons are at their descent. The rotation of the cylinders provides the valving effect necessary for operation; the pressure feeds and exhaust remain stationary. This compounded with the fact that each shaft features two pistons results in a low number of moving parts.


Despite their apparent simplicity, elbow engines can be difficult to troubleshoot. They are also known to splatter oil when operating.

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