Reversing a slow speed engine,

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For an engine to be reversed consideration must be given to the functioning of the fuel pump, air distributor and exhaust valves. That is, there commencement and completion of operation in respect to the crankshaft position.

Distributor
Due to the differing requirements for the change in angle between the distributor and the Fuel and exhaust cams, two camshafts are fitted although this adds to the cost of installation. The small distributor camshafts has seperate ahead and astern cams adjacent to each other. Reversing is by pulling and pushing the camshaft axially.

During normal engine operation the pistons of the distributor are held off the cams, this simplifies the changeover of the camshaft. When starting air is required the pistons are first forced onto the cams. Starting air is emitted during the low points and stop and the majority high points.
Reversing methods

    There are five solutions to reversal of the engine timing are
        Reversing servos on all camshaft-such as older Sulzers
        separate ahead and astern cams with axial movement to bring cams into align with rollers
        timing of fuel pumps, exhaust valve symmetrical
        fit air distributor as per doxford where reversal is performed internally by air flow
        fit fuel pumps as per B&W new design where the follower is repositioned relative to the camshaft and this retimes pump for new direction.The Exhaust timing is symetrical

Modern reversing method