Answer
To understand how engine derating is done, you have to look at it from two perspectives: Software-based/Mechanical Power Limitations (used for standard compliance) and Permanent Thermodynamic Derating (where the physical matching of the engine is altered).
As a Chief Engineer, you must clearly distinguish between EPL (Engine Power Limitation), SHaPoLi (Shaft Power Limitation), and Thermodynamic Re-matching.
1. Electronic Engines: Software Power Limitation (EPL / SHaPoLi)
On modern electronically controlled engines (such as MAN B&W ME-C or WinGD RT-flex/X-series), derating is highly precise and requires no physical modifications to the fuel infrastructure.
EPL (Engine Power Limitation): The Engine Control System (ECS) software is modified by the maker. A code lock or firmware update sets a maximum ceiling on the Fuel Index or Exhaust Valve Timing, preventing the engine from calling for more fuel than the new derated power limit (PEPL) allows.
SHaPoLi (Shaft Power Limitation): A class-approved strain gauge torque meter is fitted on the intermediate shaft. This system continuously calculates power:
Power (kW)=Torque (kNm)×Angular Velocity (ω)
If the shaft power begins to exceed the certified limit, the SHaPoLi control unit sends a command to the governor to automatically reduce the fuel command, regardless of the bridge throttle position.
Class Requirement: Both systems must feature a tamper-evident physical or electronic seal on the control panel. Breaking this seal to activate the emergency override (under SOLAS safety conditions) immediately triggers a logging event that must be reported to the Flag State Administration.
2. Mechanical Engines: Physical Fuel Modification
For older, mechanically governed engines (such as MAN B&W MC-C or Wärtsilä RTA), physical restrictions must be placed on the fuel linkages.
A. The Mechanical Fuel Rack Limiter
A physical, calibrated steel collar or stopper device is bolted onto the main fuel regulating guide rail or rack linkage. This mechanical stop physically blocks the governor from moving the fuel pump racks beyond a specific index position, effectively capping the maximum fuel injection volume per stroke.
B. Adjusting Injection and Valve Timing
Maker may adjust:
Fuel injection timing
Variable injection timing setting
Start of injection
Exhaust valve timing in electronically controlled engines
Cylinder pressure balancing
Purpose is to keep:
Peak pressure within limit
Exhaust temperature normal
SFOC optimum
NOx compliance intact
3. Thermodynamic Re-Matching (The Turbocharger Challenge)
Derating an engine fundamentally alters its air-to-fuel ratio. When you burn less fuel, you generate less exhaust gas energy. If you leave the air system untouched, the existing large turbocharger will operate outside its design map, leading to Turbocharger Surging and a severe lack of scavenge air pressure.
To properly complete a physical derating, engineers must execute one of the following retrofits:
Turbocharger nozzle ring change
Turbine area change
Compressor matching
Wastegate adjustment, if fitted
Auxiliary blower cut-in/cut-out adjustment
Scavenge pressure optimization
.
C. Auxiliary Blower Upgrades
Because the engine spends its entire operational life at lower loads, the main turbochargers cannot provide sufficient scavenge pressure during low-speed maneuvering. The electrical control circuit of the Auxiliary Blowers is reconfigured so they cut in at a slightly higher scavenge air pressure threshold than before, ensuring the engine never starves for oxygen.
The "Emergency Override" Clause: Even though an engine is legally derated for environmental compliance, maritime safety regulations (SOLAS) dictate that the captain and Chief Engineer must have the ability to override the power limitation instantly. If the vessel is facing severe adverse weather (e.g., a typhoon) or dodging pirates and needs immediate maximum power for the safety of the crew and cargo, the seal can be broken to restore full original engine power. This event must be logged and reported to the administration immediately.