Background
A marine diesel engine, even when fully prepared, requires external rotation to bring it to firing speed. Various starting methods exist (electric, hydraulic, pneumatic), but for large slow-speed two-stroke engines, compressed air starting is universal. The reasons are practical: the engines are too large for electric starting (megawatts of starting torque required), hydraulic starting is mechanically complex at this scale, and compressed air systems can deliver high power for short durations from compact air bottles.
The starting air system on a typical large slow-speed engine consists of:
- Starting air receivers (also called air bottles): high-pressure tanks storing compressed air
- Starting air compressors: typically electric-driven, charging the receivers
- Distributor: a rotary or pneumatic valve timing air admission to cylinders
- Cylinder starting valves: one per cylinder, in the cylinder cover
- Pilot air: a separate small-bore air supply controlling the cylinder valves
- Piping and isolation valves
The complete system must:
- Provide enough air for typically 12 consecutive starts (per IACS requirements)
- Operate reliably at any ambient temperature in service
- Avoid mishaps such as backfires or air-line explosions
- Allow emergency starting independent of the main electrical system
This article describes system architecture, starting sequence, classification requirements, and operational maintenance.
System architecture
Starting air receivers
Starting air is stored in steel pressure vessels (receivers/bottles):
- Pressure: 25 to 30 bar working pressure (some smaller engines use 12-15 bar)
- Volume: typically 5 to 15 m³ total, in 2 or 3 separate vessels
- Material: low-alloy steel with internal corrosion protection
- Inspection: class society inspection at typically 5-year intervals
Multiple vessels are used for redundancy and to allow individual vessels to be isolated for maintenance.
Starting air compressors
Compressors charge the receivers from atmospheric pressure to working pressure:
- Type: usually multi-stage reciprocating compressors with intercooling
- Capacity: each compressor typically able to charge a receiver from 0 to working pressure in 30-60 minutes
- Number: at least 2 compressors required (redundancy)
- Power: typically 50-150 kW per compressor
- Drive: electric motor (main supply) plus emergency power option
Air distributor
The distributor times air admission to each cylinder. Two main types:
Rotary distributor: a small rotating valve geared to the crankshaft, with internal porting that opens and closes pilot air to each cylinder valve in firing order. As the crankshaft rotates, the distributor passes through the firing sequence, admitting air to each cylinder at the correct crank angle.
Solenoid distributor: a set of solenoid valves controlled electronically by the engine control system. Each solenoid corresponds to one cylinder and is opened at the correct crank angle (read from the crankshaft sensor). Solenoid distributors are increasingly common on modern electronic engines.
Cylinder starting valves
Each cylinder has its own starting valve mounted in the cylinder cover. The valve admits high-pressure air directly into the cylinder when commanded. Each valve is:
- Pilot-operated by the small pilot air signal from the distributor
- Spring-loaded closed when no pilot signal
- Opens admitting starting air at typically 25-30 bar
- Closes when pilot signal stops
Piping
Two air systems run parallel:
- Main starting air: large-bore piping (typically 50-100 mm) carrying high-flow air from receivers to cylinder valves
- Pilot air: small-bore piping (typically 10-20 mm) carrying control air from distributor to cylinder valve pilots
Both are protected by safety devices: bursting discs, relief valves, and isolation valves.
Safety devices
- Bursting disc: provides emergency pressure relief if the system overpressurises
- Pressure relief valve: bleeds excess pressure to atmosphere
- Flame arrestors: prevent flame propagation back into the system
- Non-return valves: prevent reverse flow that could pressurise compressors
Starting sequence
A typical starting sequence:
1. Pre-start checks
- Engine prepared for starting (lubrication, cooling, fuel)
- Turning gear disengaged
- Indicator cocks closed
- Starting air receiver pressure verified (>20 bar typical minimum)
2. Open main air valve
The main starting air isolation valve is opened, admitting starting air pressure to the cylinder valves.
3. Operator initiates start
Either via manual telegraph or remote control, the start command is given.
4. Air admission to cylinders
The distributor begins routing pilot air to each cylinder valve in firing order. As each cylinder reaches the firing-window crank angle, its starting valve opens and admits air. The air pressure pushes the piston downward.
5. Engine rotation
Successive cylinders receive air, each contributing a power impulse. The engine rotates, gathering speed.
6. Fuel injection begins
When the engine has reached approximately 30-40% of normal idle speed, fuel injection begins. The first few combustion events may be weak; combustion strengthens as the engine speeds up.
7. Starting air cuts off
When the engine reaches stable firing (typically at 60-80% idle speed), starting air admission is automatically shut off. The engine continues on its own combustion power.
8. Idle speed
The engine reaches idle speed (typically 30-50 rpm for slow-speed engines), confirming successful start. The bridge is informed.
Starting air consumption
A typical successful start consumes 0.5 to 2 m³ of starting air at working pressure, depending on engine size, ambient temperature, and operator technique. Cold starts use more air than warm restarts. The receiver must contain enough air for at least 6 cold starts and 12 warm starts (per IACS requirements).
Classification society requirements
IACS UR M62
International Association of Classification Societies (IACS) Unified Requirement M62 sets minimum requirements for starting air systems:
- Capacity: sufficient air for 12 consecutive starts of a reversible engine (6 starts of a non-reversible engine)
- Compressor capacity: charge to working pressure within 1 hour from empty
- Number of compressors: at least 2, with one able to charge alone within reasonable time
- Number of receivers: at least 2, each isolatable
- Quality: starting air must be cool and dry to prevent corrosion in piping and valves
Working pressure
Working pressures vary by engine class:
- Modern slow-speed two-stroke: 25-30 bar
- Older slow-speed two-stroke: 25-30 bar (similar)
- Medium-speed four-stroke: 25-30 bar typically
- Smaller engines: may use 7-15 bar
The working pressure is set by engine design; receivers and compressors are sized to match.
Pressure rating
Receivers are designed and tested to typically 1.5× working pressure. Pipes and valves are similarly rated. Periodic inspection and pressure testing verifies integrity.
Air quality
Starting air must be dry and clean to prevent:
- Rust in piping and valves from condensed water
- Oil contamination from compressor oil leaks (creates fire hazard in piping)
- Solid particulates clogging pilot air passages
The system includes:
- Aftercoolers at compressor discharge
- Moisture separators removing condensate
- Air dryers (some installations) for very dry air
- Filters removing oil and particles
Drained moisture is collected and removed regularly.
Maintenance
Compressor maintenance
Starting air compressors require regular service:
- Lubrication: oil changes at typical intervals
- Filter replacement: air intake filters at intervals
- Valve overhaul: compressor valves need periodic inspection
- Bearing service: at major intervals
Typical compressor overhaul interval: 10,000 hours. Major service: 30,000 hours.
Cylinder starting valve overhaul
Cylinder starting valves are overhauled with the engine’s piston overhauls:
- Inspect spring tension and condition
- Check seating surfaces for erosion
- Verify pilot operation
- Replace seals and gaskets
Typical interval: 16,000-24,000 hours.
Distributor service
The distributor is serviced periodically:
- Rotary distributor: bearing service, internal cleaning
- Solenoid distributor: solenoid valve inspection, electrical contact checks
Receiver inspection
Air receivers undergo class society inspection at intervals typically 5 years:
- Internal inspection through manholes
- External coating inspection
- Pressure test verification
Routine drainage
Daily drainage of moisture from receivers and condensate traps prevents corrosion and water carryover into the engine.
Operational issues
Failed starts
If a start attempt fails:
- The starting air supply is automatically interrupted to prevent starting valve overheating
- The engine is rotated by hand or air motor to a different crank position
- The fault is investigated (fuel, compression, electrical)
- A second attempt is made
Repeated failed starts deplete the starting air supply, requiring compressor charging.
Backfires through starting valves
If a starting valve malfunctions and fails to close, hot combustion gas can flow back through the valve into the starting air piping. The combustion gas can ignite oil residues in the pipe, causing an explosion. Modern engines include:
- Bursting discs at piping junctions
- Flame arrestors in critical locations
- Quick-closing isolation valves
These protect the system from backfire damage.
Starting air leaks
Leaks in the starting air system reduce available capacity. Routine leak testing identifies leaks before they affect starting reliability.
Cold weather starting
Very cold ambient temperatures complicate starting:
- Engine oil viscosity rises, increasing rotational resistance
- Combustion is slower, requiring more air-driven cycles before firing
- Starting air consumption is higher
- May require pre-heating of engine oil and cooling water
Some ships in cold-climate trade are equipped with engine pre-heaters specifically to support reliable starting.
Modern developments
Electronic distributors
Solenoid-based distributors integrate with the engine control system, providing:
- More precise timing
- Better diagnostics
- Easier integration with modern engine controls
- Lower mechanical maintenance
Air motor starting
Some smaller engines (and some medium-speed installations) use air motor starting instead of direct cylinder air admission:
- Air motor turns the engine through a starting pinion
- No cylinder starting valves required
- Smaller air consumption per start
- Simpler engine architecture
Used for smaller engines or specialised applications.
Hybrid systems
Some recent engines include both compressed air starting and battery-powered electric starting capability, providing redundancy and emergency starting independent of the main electrical system.
Related Calculators
- Starting Air Capacity Calculator
- Air Bottle Sizing Calculator
- Compressor Charging Time Calculator
- Starting Air Consumption Calculator
See also
- Two-Stroke Marine Diesel Engine Fundamentals
- Cylinder Cover Design and Cooling for Two-Stroke Engines
- MAN B&W ME-C Electronic Control Overview
- Crosshead Diesel Engine Architecture Overview
References
- IACS. (2018). UR M62: Starting Air System Requirements.
- MAN Energy Solutions. (2023). Starting Air System Manual. MAN Energy Solutions.
- WinGD. (2023). X-Series Starting System Specifications. Winterthur Gas & Diesel.
- Lloyd’s Register. (2022). Marine Engine Starting Systems Approval.
- Woodyard, D. (2009). Pounder’s Marine Diesel Engines and Gas Turbines (9th ed.). Butterworth-Heinemann.