Manual reversing — MAN B&W MC-type engine

Unlike the RTA-type engine (which carries two separate sets of cams — ahead and astern — on a camshaft that shifts axially), the MAN B&W MC engine uses a single set of cams per cylinder with a special "conjugate" cam profile. Reversing is achieved by rotating the camshaft angularly relative to the crankshaft, not shifting it sideways.

The camshaft is chain-driven from the crankshaft through an intermediate chain wheel. Fitted at this drive is a hydraulic reversing servomotor — typically a vane-type or rack-and-piston type actuator — connected between the chain wheel (driven by the crankshaft) and the camshaft itself. By admitting hydraulic oil to one side or the other of this servomotor, the camshaft can be rotated through the required angle (commonly on the order of 90°–100°, depending on the number of cylinders and firing order) relative to the chain wheel, repositioning the fuel and exhaust cams so that injection and exhaust valve timing now correspond to astern running.

Manual (local) reversing procedure:

1. Bring the engine to rest. Fuel is cut off and the engine allowed to run down to zero rpm, checked on the local tachometer.
2. Operate the local reversing lever/handle, which directs control (pilot) oil to a reversing control valve. This valve routes high-pressure hydraulic oil to the appropriate side of the reversing servomotor.
3. The servomotor rotates the camshaft relative to the crankshaft-driven chain wheel until it reaches the astern (or ahead) stop.
4. A mechanical/visual position indicator on the servomotor housing shows "ahead" or "astern" — the engineer confirms this before proceeding, since on manual control there is no automatic lockout.
5. Starting air is admitted manually to turn the engine over in the new direction until firing speed is reached.
6. Starting air is cut off and the fuel lever opened progressively to bring the engine away in the ordered direction.

Remote (bridge) control — why extra safety measures are needed

When the reversing lever is operated from the bridge, the engineer is not present to visually confirm shaft speed, camshaft position, turning gear status or air pressure before each step. All of these checks must therefore be done automatically, in a fixed sequence, with the sequence unable to proceed until each condition is satisfied — otherwise a bridge order given at the wrong moment could try to reverse a rotating engine, admit starting air with the turning gear engaged, or shift the camshaft only partially.

Safety precautions required for remote (bridge) control

• Zero-speed lock: The single most important interlock: the sequence controller must sense shaft rpm has fallen to zero (or a very low set value) before the reversing servo is allowed to operate. Attempting to shift the camshaft while the engine is still turning ahead can damage the reversing gear, throw the fuel pump timing badly out, or cause the engine to fire against its rotation.
• Turning gear interlock: A limit switch on the turning gear pinion prevents both the starting-air valve and the reversing servo from operating if the turning gear is engaged — otherwise starting air would attempt to drive the engine through the turning gear, wrecking it.
• Starting air pressure interlock/alarm: Low air pressure is checked before a manoeuvre is permitted; repeated manoeuvring can rapidly deplete the air receivers, and an engine that fails to start on air after several attempts should be locked out with an alarm rather than allowed to keep draining the bottles (also protects against wet starting-air line blow-back and overheating of the air start valves).
• Position feedback, not assumption: Unlike manual control, the system does not proceed to admit air/fuel until a limit switch physically confirms the camshaft has reached the full ahead or full astern position — this replaces the engineer's visual check with an electrical one.
• Indicator cocks/turning gear cross-checks, and load/acceleration limiting: The governor typically also incorporates a fuel limiter linked to scavenge air pressure, so that fuel cannot be increased faster than the turbocharger can supply air during rapid manoeuvring — protecting against overload and excessive exhaust temperatures.
• Control transfer interlock: Only one control position (bridge or engine room) can have command at any time, with a clear indicator showing which station is in control, and the engineer must always be able to take local control instantly.
• Failure fallback: Loss of the remote control signal, air supply, or electrical power triggers an audible/visual alarm on the bridge and in the engine control room, and the system reverts to a safe, defined state (commonly holding the last order or requiring the engine room to take over) rather than failing in an unpredictable way.
• Independent emergency stop: A hard-wired stop, bypassing the sequence logic entirely, is provided at both the bridge and the local stand.
• Movement recording: All telegraph orders and engine responses are automatically logged (course/engine movement recorder), partly for safety review and partly so engineers are aware manoeuvring is taking place