Engine Reversing System on Two-Stroke Marine Diesel Engines

Engine reversing is the operational procedure for changing a slow-speed two-stroke marine engine’s rotation from ahead to astern (or vice versa). Three architectures exist: direct-reversible engines that physically rotate the crankshaft in either direction, reverse gearboxes that transmit power in both directions through a fixed-rotation engine, and controllable pitch propellers (CPP) that reverse thrust by reversing propeller blade pitch. Direct-reversible engines are common on slow-speed two-strokes; CPP and gearboxes are more common on four-stroke installations. This article covers reversal mechanisms, the air distributor reversal, fuel and exhaust valve retiming, and operational procedures. Visit the home page or browse the calculator catalogue for related propulsion engineering tools.

Contents

Background

A ship’s propeller produces thrust ahead by rotating in one direction and thrust astern by rotating in the opposite direction (for a fixed-pitch propeller) or by reversing pitch (for a CPP). To manoeuvre the ship into port, alongside, or away from a berth, the propeller must be reversible.

Three principal architectures provide propeller reversal:

Direct-reversible engine: the engine itself rotates in either direction. The fuel injection, exhaust valve timing, and starting air admission must all reverse with the rotation. This is the standard for large slow-speed two-stroke engines.
Reverse gearbox: a fixed-rotation engine drives a gearbox that transmits power either ahead or astern through clutches. Common on smaller engines and on twin-screw installations.
Controllable pitch propeller (CPP): a fixed-rotation engine drives a propeller whose blade pitch can be varied by hydraulic actuators. The pitch is controllable from full ahead through neutral to full astern, allowing thrust reversal without changing engine rotation.

Direct-reversible engines have historically dominated slow-speed two-stroke applications because of:

Low total propulsion mass (no gearbox, no CPP hub)
Simpler shaft system
Direct connection of engine to propeller

CPP installations are more common on four-stroke medium-speed engines and on container ships and tankers requiring frequent manoeuvring.

This article focuses on direct-reversible slow-speed two-stroke engines: the mechanics of reversal, the operational procedure, and the limitations.

What reverses on a direct-reversible engine

When an engine rotates in the opposite direction, several systems must adapt:

Crankshaft

The crankshaft itself can rotate either direction; no special arrangement is needed. The connecting rods and pistons follow normally.

Fuel injection timing

In ahead operation, fuel is injected at TDC; in astern operation, the same TDC is approached from the opposite direction. The fuel injection control system must:

Recognise the rotation direction
Adjust the timing of injection to align with the new TDC approach
Maintain proper injection profile

Exhaust valve timing

Similarly, the exhaust valve must open and close at the correct crank angles for the new rotation direction. With electronic actuation, this is straightforward: the engine control system applies a reversed timing map.

Air distributor

The starting air distributor must admit air to cylinders in reverse firing order during astern starting. The distributor either:

Has a separate reverse-rotation cam profile
Uses solenoid-based control with reversed software map

Cylinder valves

Cylinder starting valves function the same in both directions; air is admitted to push the piston downward regardless of rotation direction.

Mechanical implementation

Mechanical-camshaft engines

Older slow-speed engines with mechanical camshafts had a separate astern cam profile on the camshaft, displaced from the ahead profile. To reverse:

Engine stopped
Camshaft shifted axially using a hydraulic mechanism, engaging the astern profile
Air distributor similarly shifted to astern position
Engine started in reverse

The shift mechanism was a relatively complex arrangement requiring careful sequencing.

Electronic-control engines

Modern ME-C and X-DF engines use software-defined timing maps. To reverse:

Engine stopped
Operator commands “Astern”
Engine control system loads astern timing maps
Air distributor (solenoid-controlled) prepared for astern admission
Engine started in reverse

Electronic reversal is faster and simpler than mechanical, with no moving parts to shift.

Reversal sequence

A typical bridge-to-bridge reversal procedure (from full ahead to full astern):

1. Telegraph “Stop”

The bridge orders engine stop. The engine room confirms.

2. Fuel cut-off

Fuel injection is cut. The engine continues to rotate ahead under propeller momentum and ship’s forward speed dragging the propeller.

3. Compression braking

Some engines use compression braking: keeping the exhaust valves closed during compression to absorb rotational energy. This slows the engine faster than freewheeling alone.

4. Engine stops

Engine rotation stops. This may take 30 seconds to 3 minutes depending on initial speed, ship momentum, use of compression braking, and propeller resistance.

5. Reverse preparation

The engine control system reverses to the astern timing maps. Air distributor switches to astern.

6. Telegraph “Astern”

The bridge orders astern movement.

7. Astern starting air

Compressed air is admitted to cylinders in astern firing order. The engine rotates astern.

8. Astern fuel injection

Once rotational speed reaches starting threshold, fuel injection begins in astern timing.

9. Engine running astern

The engine continues running astern at the commanded speed.

Total time

The complete sequence typically takes 1-3 minutes, depending on engine size and starting air supply. For very large engines, 3 minutes may be required.

Limitations

Air consumption

Each reversal consumes substantial starting air. After 2-3 reversals, air bottles may be depleted enough to require recharging before further manoeuvres. Frequent reversals (e.g. during tight pilotage) can deplete air faster than compressors can recharge.

Time penalty

Direct reversal takes 1-3 minutes. For comparison:

CPP reversal: seconds (just change pitch)
Reverse gearbox: 10-30 seconds (clutch operation)

This time penalty is significant during emergency manoeuvres.

Wear

Frequent reversal cycles wear engine components: bearings see load reversals, cylinder rings see opposite-direction sliding, starting valves see frequent operations. Direct-reversible engines are designed for these cycles, but excessive reversal frequency does shorten component life.

Crankshaft loads

During reversal, the engine experiences unusual load patterns: mass deceleration during stopping, impulsive loads during astern starting, possible mechanical reversal stresses. These are within design limits but contribute to long-term fatigue.

CPP installations

For ships with CPP, engine reversing is not normally needed. The propeller pitch reverses, providing thrust reversal:

Engine continues running in the same direction (typically the engine’s design rotation)
Fuel and timing maps remain ahead
Pitch actuator controls thrust direction
Engine speed may be adjusted independently of pitch

CPP advantages:

Faster thrust reversal (pitch change is faster than engine reversal)
Less starting air consumption
More consistent engine wear (same direction continuously)
Better manoeuvrability

CPP disadvantages:

More complex propeller hub
Higher capex
Lower propeller efficiency at part-load (variable pitch is less efficient than fixed pitch at the design point)
More maintenance on the propeller

CPP is common on container ships, ferries, naval vessels, and other ships requiring frequent manoeuvring.

Reverse gearbox installations

Some engines drive through a reverse gearbox, which transmits power in either direction through clutches:

Forward clutch: engages forward gear train
Reverse clutch: engages reverse gear train (with idler gear to reverse rotation)
Hydraulic actuators: control clutch engagement

Reverse gearbox advantages: shorter reversal time than direct engine reversal, engine continues at idle during reversal, simpler engine control.

Reverse gearbox disadvantages: mechanical complexity and weight, power loss through gearbox (typically 1-3%), capex of gearbox itself, maintenance of clutch system.

Reverse gearboxes are common on smaller engines (below ~10,000 kW) and on twin-screw installations.

Bridge control

Modern ships have integrated bridge control of the engine:

Conventional telegraph

The traditional engine-room telegraph: a lever the bridge operator moves to indicate desired engine state (Full Ahead, Half Ahead, Slow Ahead, Stop, Slow Astern, Half Astern, Full Astern). Engine room responds via similar telegraph.

Combined throttle-pitch

CPP installations often use a single combined lever that controls both engine speed and propeller pitch through software, simplifying the bridge interface.

Direct manoeuvring

Direct bridge-to-engine control bypasses the engine room for routine manoeuvring, with the engine room standing by for emergency intervention.

Operational considerations

Pilotage operations

During pilotage (entering/leaving port), reversals may be frequent. The pilot accounts for engine reversal time when planning manoeuvres. Starting air consumption is monitored.

Emergency stop

In emergency situations, immediate engine reversal may be required. The procedure is: bridge orders “Emergency Astern”, standard procedure followed but expedited, engine room may use engine emergency stop systems, compression braking employed if available.

Anchor handling

Specialty operations like anchor dropping require precise reversal timing. The pilot or master coordinates with engine room to time reversals against ship motion.

Modern engine reversal

Modern electronic engines have made reversal substantially smoother:

Software-defined reversal eliminates mechanical shift mechanisms
Faster response than older mechanical engines
Better control during the reversal sequence
Reduced wear on reversal components

These improvements have not eliminated the fundamental physics: bringing a 2,000-tonne propulsion mass from full ahead to full astern still takes minutes.

References

IACS. (2018). UR M61: Engine Reversing Capability.
MAN Energy Solutions. (2023). Engine Reversing Manual. MAN Energy Solutions.
WinGD. (2023). X-Series Engine Reversal Specifications. Winterthur Gas & Diesel.
Carlton, J. S. (2018). Marine Propellers and Propulsion (4th ed.). Butterworth-Heinemann.
Woodyard, D. (2009). Pounder’s Marine Diesel Engines and Gas Turbines (9th ed.). Butterworth-Heinemann.