ME running rough at slow steaming — fuel injection timing within tolerance. MAN B&W experience?

The phenomenon of a MAN B&W main engine running "rough" during slow steaming, despite fuel injection timing being within specified tolerances, is primarily a consequence of combustion instability and thermodynamic imbalances inherent to low-load operation. When an internal combustion engine operates significantly below its Continuous Service Rating (CSR), typically below 30–40% load, the reduced mass flow of scavenge air and lower peak combustion pressures can lead to incomplete fuel atomization and irregular ignition delays. In MAN B&W two-stroke engines, the transition to slow steaming requires a critical evaluation of the fuel injection equipment and the scavenge air system. Even if the static timing is correct, the dynamic injection pressure may be insufficient for proper atomization if the fuel valves are not optimized for low-load operation. The use of "Slide Fuel Valves" is a standard industry recommendation for MAN B&W engines to mitigate this issue. Slide valves eliminate the "sac volume" found in conventional valves, preventing fuel dripping and subsequent carbon formation (carbon crowns) on the injector tips, which is a frequent cause of uneven firing and "rough" running at low RPMs. Furthermore, the operation of auxiliary blowers is a critical factor. If the engine load fluctuates near the cut-in/cut-out threshold of the auxiliary blowers, the resulting scavenge air pressure instability can cause the engine to hunt or vibrate. Under the ISM Code, specifically Element 10 (Maintenance of the Ship and Equipment), the Safety Management System (SMS) must provide clear procedures for the operation of the main engine in non-standard modes such as slow steaming. This includes the adjustment of Variable Injection Timing (VIT) or, in the case of ME-C (electronically controlled) engines, the optimization of the Fuel Injection Valve Actuation (FIVA) parameters to ensure stable combustion. From a regulatory perspective, MARPOL Annex VI regulations regarding NOx and SOx emissions must be strictly adhered to during slow steaming. While reducing speed helps in achieving better Carbon Intensity Indicator (CII) ratings and compliance with EEXI requirements, the engine must still operate within its NOx Technical File limits. Additionally, SOLAS Chapter II-1, Regulation 26, requires that the propulsion machinery is capable of maintaining maneuverability under all service conditions. A rough-running engine at slow speeds poses a risk to the vessel’s safe navigation, particularly in restricted waters or during heavy weather. To resolve these issues, Chief Engineers should consult Directorate General of Shipping (DGS) guidelines and the engine manufacturer’s service letters (SL). Key diagnostic steps include: 1. Performing a scavenge port inspection to check for "cold corrosion" and carbon buildup, which are exacerbated by low-load operation and low cylinder liner temperatures. 2. Adjusting the cylinder oil lubrication rate via the Alpha Lubricator system to prevent over-lubrication (which leads to deposits) or under-lubrication (which leads to liner wear). 3. Verifying the fuel viscosity at the engine inlet; if the fuel is too viscous due to inadequate heating, atomization at low loads will fail regardless of timing accuracy. 4. Utilizing electronic indicator tools to capture Pmax and Pcomp values, ensuring that the power distribution between cylinders is balanced within a 2-3 bar deviation. Adhering to these technical and regulatory standards ensures that slow steaming remains an economically viable and safe practice without compromising the structural integrity of the engine components.