Background
A diesel engine’s rotational speed depends on the balance between the torque produced by combustion and the torque demanded by the load. With more fuel, the engine accelerates; with less fuel, it decelerates. Without active control, even small load changes would produce large speed variations: a propeller’s torque demand changes substantially with ship speed, weather, and trim, and the engine speed would oscillate uncomfortably.
The governor is the control system that automatically adjusts fuel quantity to maintain a target engine speed regardless of load. Governors have evolved from purely mechanical centrifugal devices in the 19th and early 20th centuries to today’s electronic systems integrated with engine control software.
Governor performance is measured by:
- Speed regulation: how much speed deviates from setpoint with changing load
- Stability: absence of oscillation around the setpoint
- Response time: how quickly the governor reacts to load changes
- Droop characteristic: the steady-state speed change with load
This article covers governor types, control characteristics, parallel operation considerations, and modern integrated control systems.
Governor types
Mechanical governors
Older slow-speed two-stroke engines used mechanical governors based on the principle of centrifugal force. Flyweights mounted on a rotating shaft moved outward as engine speed increased; the outward motion mechanically reduced fuel pump rack position, reducing fuel quantity. As engine speed decreased, flyweights moved inward, increasing fuel.
Common mechanical governor designs:
- Watt governor: simple two-flyball arrangement
- Hartung governor: more complex, better stability
- Woodward UG type: hydraulic-mechanical, widely used until the 1990s
Mechanical governors:
- Robust and reliable
- Fixed control characteristics (cam profile defines droop)
- Limited adjustment range
- No load-sharing capability without external aids
Electronic governors
Modern engines use electronic governors integrated with the engine control system:
- Speed sensor (typically the crankshaft sensor at high resolution)
- Electronic control unit running governor algorithm
- Fuel quantity command to fuel injection system
- Operator interface for setpoint and parameter adjustment
Electronic governors offer:
- Software-adjustable parameters
- PID control with multiple gain settings
- Isochronous or droop modes
- Load sharing for parallel operation
- Integration with broader engine control
Control characteristics
Speed regulation
Speed regulation is the variation in steady-state speed when load changes. Two principal characteristics exist:
Droop: speed decreases linearly as load increases. A typical droop value is 4-5%, meaning full-load speed is 4-5% lower than no-load speed. Droop provides stability and natural load sharing in parallel operation.
Isochronous: speed is constant at all loads (zero droop). Achieved by integrating the speed error in the controller, eliminating steady-state error. Used when speed must remain exactly at setpoint regardless of load.
PID control
Modern electronic governors implement Proportional-Integral-Derivative (PID) control:
- Proportional gain: produces a fuel correction proportional to instantaneous speed error
- Integral gain: produces a fuel correction proportional to accumulated error (eliminates steady-state error)
- Derivative gain: produces a fuel correction proportional to error rate of change (improves response)
The combination provides fast response, zero steady-state error (with integral term), and stability (with proper tuning).
Tuning
PID gains are tuned for the specific engine and operating conditions:
- Too high gains: oscillation, overshoot, instability
- Too low gains: slow response, lag, poor regulation
Tuning is usually done at sea trial and adjusted over the engine’s life as conditions change.
Load sharing in parallel operation
When two or more engines drive a common load (e.g. shaft generators feeding the same electrical bus, or twin-screw installations with synchronisation), load sharing requires coordination:
Droop-based sharing
Each engine has a droop characteristic. As load rises, an engine with droop slows, naturally yielding to the other engines. Engines automatically share load in proportion to their droop characteristics. This is simple and self-correcting but produces some speed variation with load.
Isochronous load-sharing
For isochronous operation, all engines must have identical droop. This is achieved by:
- Master-slave configuration: one engine is master (isochronous setpoint), others follow
- Centralised load sharing controller distributing fuel commands to all engines
- Communication network sharing load and speed data
Isochronous load-sharing provides constant speed but requires more complex control.
Application to twin-screw
For twin-screw ships, each engine drives its own propeller. Load sharing is set by the bridge: same engine telegraph orders to both engines produces approximately equal load. Minor differences are absorbed by the propeller match.
Application to shaft generators
When the main engine drives a shaft generator (PTO), the engine speed is fixed by the electrical grid frequency (60 Hz typically). The governor maintains constant speed isochronously, and load sharing with diesel generators is via electrical droop.
Operational considerations
Load changes
The governor responds to load changes:
- Slowly varying loads (sea-state changes): governor smoothly adjusts fuel
- Rapid load changes (large telegraph commands): governor follows but with some lag
- Step changes (sudden full-throttle from idle): governor outputs maximum fuel quickly
Speed setpoint changes
When the bridge commands a different engine speed, the governor:
- Updates the speed setpoint
- Calculates new fuel quantity needed
- Ramps the fuel command at controlled rate (avoiding shock loads)
- Settles to the new speed within seconds to minutes
Rate limiting
Most governors include rate limits on fuel command changes:
- Maximum acceleration rate: prevents excessive thermal load on combustion chamber
- Maximum deceleration rate: prevents engine stalling
These rate limits are software-configurable on modern engines.
Barred speed ranges
Some engines have barred speed ranges due to torsional resonance. The governor is programmed to:
- Accelerate quickly through the barred range during ramping
- Refuse to settle at speeds within the barred range
- Return to bridge if commanded to operate within the bar
Manoeuvring mode
During manoeuvring (entering port, etc.), governors may operate in different modes:
- More aggressive response to telegraph commands
- Lower minimum speed allowed
- Direct bridge speed control bypass possible
Integration with engine control
Engine control system
Modern governors are part of the integrated engine control system. The governor algorithm is one component within a larger control scheme that also includes:
- Cylinder balancing
- Variable injection timing
- Exhaust valve timing control
- Cylinder lubricator control
- Performance monitoring
- Safety shutdowns
Integration enables coordinated control: changes in one subsystem inform others.
Software updates
Governor parameters and even algorithm logic can be updated through software, allowing:
- Adjustment for changing operating conditions
- Implementation of improved control strategies
- Bug fixes and feature additions
Updates are managed through the engine manufacturer’s service programme.
Diagnostic capabilities
Modern governors continuously self-monitor:
- Sensor signal quality
- Control loop stability
- Fuel command saturation
- Communication network health
Faults are reported to the operator before they cause control problems.
Failure modes
Sensor failure
Speed sensor failure can cause loss of governor function. Backup speed sensors (typically two independent crankshaft sensors) maintain operation if one fails.
Control system failure
Failure of the electronic control unit (ECU) or its software may cause loss of governor function. Modern engines have:
- Redundant ECUs
- Fail-safe behaviour (e.g. fixed fuel position on failure)
- Backup mechanical or simpler electronic governors
Excessive gain (instability)
Aggressive gain settings can produce oscillation. The governor must be retuned to restore stability.
Slow response (low gain)
Insufficient gain produces sluggish response to load changes. Recalibration restores responsiveness.
Maintenance
Routine maintenance
Routine maintenance of governor systems includes:
- Speed sensor calibration verification
- ECU diagnostic checks
- Software audit and update
- Fuel command output verification
Scheduled overhaul
At major intervals (every few years), the governor undergoes scheduled overhaul:
- Complete diagnostics
- Sensor replacement if recommended
- ECU firmware update
- Tuning verification
Failure investigation
Reported faults are investigated:
- Fault history retrieval
- Sensor diagnostics
- Mechanical inspection if applicable
Related Calculators
- Engine Speed Regulation Calculator
- Governor Droop Calculator
- PID Tuning Calculator
- Load Sharing Calculator
See also
- MAN B&W ME-C Electronic Control Overview
- Engine Starting Air System on Marine Diesel Engines
- Engine Reversing System on Two-Stroke Marine Diesel Engines
- Engine Emergency Stop Circuits on Marine Engines
References
- MAN Energy Solutions. (2023). Engine Governor Manual. MAN Energy Solutions.
- WinGD. (2023). X-Series Engine Control System Specifications. Winterthur Gas & Diesel.
- Woodward Governor Company. (2022). Marine Engine Governor Selection Guide.
- IACS. (2018). UR M64: Engine Speed Control Requirements.
- Welbourn, D. B. & Smith, J. D. (1970). Marine Diesel Engine Governing. Elsevier.