Modern ships equipped with Fixed Pitch Propellers (FPP) depend entirely on the main engine reversing system to move the vessel astern. Unlike ships fitted with Controllable Pitch Propellers (CPP), where the blade angle changes to reverse thrust, an FPP always produces thrust according to its direction of rotation. Therefore, to move the ship backward, the main engine itself must rotate in the opposite direction.
Among the most widely used low-speed two-stroke marine diesel engines are the Sulzer RTA and Sulzer RT-flex engine series. Although both engines perform the same task of reversing the crankshaft direction, their methods are entirely different.
The Sulzer RTA engine uses a hydraulic cam-sleeve mechanism to change fuel injection timing mechanically, whereas the Sulzer RT-flex engine employs an advanced electronically controlled common rail system that eliminates the need for mechanical camshaft movement.
In our blog, we’ll explore the complete working principle, reversing procedure, common failures, maintenance practices, and frequently asked questions related to Sulzer engine reversing.
Main engine reversing is the process of changing the rotational direction of a marine diesel engine so that the propeller rotates in the opposite direction, producing astern thrust.
This operation is required during:
The Sulzer RTA engine is a mechanically controlled low-speed two-stroke diesel engine fitted with a camshaft.
Instead of moving the entire camshaft, Sulzer engineers designed a hydraulic cam-sleeve arrangement.
Each fuel pump cam is mounted on a rotatable sleeve around the camshaft.
When reversing is ordered:
An important feature of the Sulzer RTA design is that the exhaust valve cam is symmetrical about Bottom Dead Centre (BDC). Because of this symmetrical profile, it generally does not require repositioning during reversing, simplifying the mechanism compared with some other engine designs.
The hydraulic reversing system consists of several important components:
Each component must operate correctly for successful reversing.
Step 1 – Receive Astern Command
The bridge telegraph sends an astern command to the engine control room.
The engineer on watch confirms the manoeuvring order and ensures that the propulsion system is ready.
Step 2 – Reduce Engine Load
Engine speed is gradually reduced while maintaining safe operating conditions.
Load is removed carefully to avoid thermal and mechanical stress.
Step 3 – Stop the Engine
The engine is brought to a complete stop.
The control system confirms:
No cam movement is permitted while the crankshaft is rotating.
Step 4 – Activate the Hydraulic Reversing System
Hydraulic oil is supplied to the reversing servomotor.
The servomotor rotates the cam sleeves through the required lost-motion angle.
Fuel injection timing changes from ahead timing to astern timing.
Step 5 – Verify Cam Sleeve Position
Position sensors and microswitches verify:
Both mechanical and electrical confirmations must agree.
Step 6 – Reverse Starting-Air Distributor
The starting-air distributor changes the firing order.
Compressed air will now enter cylinders in the correct astern sequence.
Step 7 – Verify Safety Interlocks
Before fuel admission, the engine control system checks:
Any abnormality prevents engine starting.
Step 8 – Admit Starting Air
Starting air rotates the crankshaft in the astern direction.
As the engine reaches firing speed, fuel injection begins using the new timing.
Step 9 – Combustion Begins
The engine fires successfully in reverse rotation.
The governor gradually increases speed.
The vessel begins moving astern.
The Sulzer RT-flex engine represents the next generation of marine propulsion technology.
Unlike the RTA engine, RT-flex engines eliminate the mechanical camshaft completely.
Instead, they use:
When an astern command is received, the ECU automatically changes:
Since there are no mechanical cams to reposition, reversing becomes faster, smoother, and more precise.
The RT-flex engine simplifies reversing considerably.
The sequence is:
No mechanical cam movement occurs.
Problem | Possible Cause | Recommended Action |
Engine refuses to reverse | Cam sleeves not moving | Check hydraulic pressure and servomotor |
No astern indication | Faulty microswitch | Inspect and calibrate position sensors |
Starting air ineffective | Air distributor fault | Test distributor timing |
Engine trips immediately | Safety interlock active | Identify and rectify alarm condition |
RT-flex engine will not start astern | ECU fault | Run diagnostics and inspect sensors |
The Sulzer RTA engine uses a hydraulic cam-sleeve mechanism to mechanically change fuel injection timing, whereas the RT-flex engine uses an electronic common rail system and ECU to control fuel injection, exhaust valve timing, and starting-air sequencing without moving mechanical cams.
Reversing while the crankshaft is still rotating can result in incorrect fuel injection timing and severe mechanical damage. Both mechanical and electronic control systems require the engine to be completely stationary before changing operating direction.
Hydraulic cam sleeves allow the fuel injection timing to shift between ahead and astern operation without moving the complete camshaft, providing a reliable and efficient reversing mechanism.
The most common causes include low hydraulic oil pressure, stuck cam sleeves, faulty position sensors, malfunctioning starting-air valves, hydraulic leaks, ECU faults (RT-flex), and active safety interlocks.
Engineers should maintain clean hydraulic oil, replace filters regularly, inspect and exercise cam sleeves, calibrate position sensors, test starting-air valves, verify hydraulic pressure, and perform routine ECU diagnostics on RT-flex engines to ensure dependable reversing performance.
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