10.3 Engine Support Systems: Fuel Oil, Lubricating Oil & Starting Air

10.3 Engine Support Systems: Fuel Oil, Lubricating Oil & Starting Air

A marine diesel engine cannot run in isolation. It relies continuously on three vital auxiliary fluid and pneumatic support systems: the Fuel Oil System for conditioning and injecting fuel, the Lubricating Oil System for reducing friction, cooling bearings, and neutralizing acids, and the Starting Air System for cranking and reversing the engine. Onboard merchant vessels, Trainee General Purpose (GP) Ratings work under the direct supervision of marine engineer officers to operate, monitor, and maintain these essential auxiliary support systems.

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1. Fuel Oil Conditioning and Injection System

Marine engines use residual, blended or distillate fuels approved for the installation. The circuit below illustrates high-viscosity residual HFO, commonly $380\text{--}700\text{ cSt at }50^\circ\text{C}$, which requires heating and cleaning before injection. VLSFO describes sulphur content, not a fixed viscosity or heating requirement. Follow fuel-analysis and maker instructions for each fuel. In an ECA, use an approved method of meeting the sulphur limit; a change to MGO is one common method.

+-------------------------------------------------------------------------+
|                    HEAVY FUEL OIL TREATMENT & SUPPLY CIRCUIT            |
+-------------------------------------------------------------------------+

 [ DOUBLE BOTTOM BUNKER TANKS ] (Heated to ~40-45°C by steam coils)
               |
               v (Fuel Oil Transfer Pump)
 [ HFO SETTLING TANK ] (Heated to 60-70°C; gravity water & sludge separation)
               |
               v (Purifier Feed Pump via Preheater @ 95-98°C)
 [ CENTRIFUGAL PURIFIER / CLARIFIER ] (Separates water & cat fines)
               |
               v Clean Purified Fuel
 [ HFO DAILY SERVICE TANK ] (Heated to 85-90°C; 8-12 hours operating capacity)
               |
   +-----------+---------------------------------------------------------+
   | PRESSURIZED BOOSTER INJECTION CIRCUIT                               |
   |                                                                     |
   |  [ Supply Pump ] (4-6 bar) ---> [ Circulating / Booster Pump ]      |
   |                                                |                    |
   |  [ Injectors ] <--- [ Jerk Pumps ] <--- [ Viscometer ] <--- [ Heater|
   |  (Pcrk: 250-380 bar)  (1000 bar)       (10-15 cSt)     (130-150°C)  |
   +---------------------------------------------------------------------+

The Fuel Journey: Storage to Injection

1. Bunker Storage Tanks: Fuel is stored in shipboard double bottom and deep tanks, fitted with internal steam heating coils to keep the heavy fuel pumpable at $40\text{--}45^\circ\text{C}$.
2. Settling Tanks: A positive-displacement fuel transfer pump pumps fuel from double bottoms into the HFO Settling Tank. Here, fuel is heated to $60\text{--}70^\circ\text{C}$ and held static for 12 to 24 hours. The elevated temperature reduces viscosity, allowing free water droplets and heavy sediment to settle to the conical tank bottom by gravity, where they are drained via spring-loaded, self-closing drain cocks.
3. Centrifugal Purifiers (Separators): Conventional HFO from the settling tank is commonly heated to $95\text{--}98^\circ\text{C}$ for efficient separation; the actual setting follows the fuel and separator instructions. Spinning at $7,000\text{--}10,000\text{ RPM}$, centrifugal force ($G\text{-force} > 5,000\text{ g}$) separates microscopic water droplets, heavy asphaltic sludge, and lethal catalytic fines (abrasive particles of aluminum oxide $Al_2O_3$ and silicon dioxide $SiO_2$ left over from refinery cracking). Cat fines are harder than engine steel and cause catastrophic abrasive wear on fuel pump barrels and cylinder liners if not removed.
4. Daily Service Tank: Pure, clean fuel discharges from the purifier into the Daily Service Tank, which is maintained heated at $85\text{--}90^\circ\text{C}$ and holds sufficient fuel for 8 to 12 hours of continuous full-power engine operation.
5. Pressurized Booster Circuit:
   • Supply & Booster Pumps: A supply pump delivers fuel at $4\text{--}6\text{ bar}$, and a booster pump circulates it at $8\text{--}10\text{ bar}$. Pressurization prevents the heated fuel from boiling or flashing into vapor (gassing off) in the pipelines.
   • Final Fuel Heater: Steam or thermal oil heat exchangers heat the fuel to $130\text{--}150^\circ\text{C}$.
   • Automated Viscometer (Viscotherm): Continuously measures fuel viscosity and automatically modulates the steam regulating valve to the fuel heater, maintaining an optimal injection viscosity of $10\text{--}15\text{ cSt}$ regardless of fuel batch variations.
   • Auto-Backflushing Filter: A duplex 10-micron fine filter captures any remaining microscopic debris.

High-Pressure Injection Equipment

• High-Pressure Fuel Injection Pump (Jerk Pump): In a conventional system, a cam drives a precision-lapped plunger inside a barrel with clearances of only $2\text{--}3\text{ microns}$. A fuel rack rotates the plunger's helix to control delivery. In a common-rail system, pumps maintain pressure in a shared rail; electronic injection-control units determine the timing and quantity delivered to each cylinder.
• Fuel Injector (Fuel Valve): A spring-loaded needle valve lifts when fuel pressure reaches the calibrated cracking pressure ($250\text{--}380\text{ bar}$). The multi-hole nozzle tip discharges fuel into the combustion chamber, achieving three vital criteria:
  • Atomization: Breaking the liquid fuel jet into microscopic droplets ($20\text{--}50\text{ microns}$) for rapid vaporization.
  • Penetration: Imparting sufficient kinetic momentum so droplets travel across the combustion space without hitting the relatively cooler cylinder walls (preventing bore wash and flame impingement).
  • Spray Pattern: Evenly distributing fuel droplets throughout the hot compressed air mass for rapid, smokeless combustion.

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2. Lubricating Oil Systems: Bearing Protection & Cooling

Marine diesel engines require extensive lubricating oil systems to eliminate metal-to-metal friction, remove heat from high-load bearings, cool the underside of piston crowns, and flush wear particles into filters.

+-------------------------------------------------------------------------+
|               MAIN ENGINE SYSTEM LUBRICATING OIL CIRCUIT                |
+-------------------------------------------------------------------------+

   [ DRAIN TANK / SUMP ] <=========================================+
            |                                                      |
            v (Magnetic Suction Strainer)                          | Gravity Drain
   [ MAIN LUBE OIL PUMP ] (Positive displacement / screw, 4-6 bar) | from Bedplate
            |                                                      |
            v                                                      |
   [ AUTOMATIC BACKFLUSHING FILTER ] (25-50 micron mesh)           |
            |                                                      |
            v                                                      |
   [ LUBE OIL COOLER ] (Plate heat exchanger, cooled by LT FW)     |
   (Maintains supply temp strictly at 45°C)                        |
            |                                                      |
            +--------------------------+---------------------------+
            |                          |                           |
            v                          v                           v
     [ MAIN BEARINGS ]        [ CROSSHEAD SLIPPERS ]      [ PISTON COOLING ]
     (Crankshaft journals)    & Top-End Bearings          (Cocktail shaker)

Dry Sump versus Wet Sump Arrangements

• Dry Sump (Slow-Speed 2-Stroke Propulsion Diesels): Lubricating oil draining from the main bearings, crosshead guides, and bedplate saddles does not accumulate in the engine bedplate. It drains immediately through wide discharge pipes into an independent Lube Oil Drain Tank (sump tank) built into the ship's double bottom hull directly beneath the engine. This keeps the bedplate empty ("dry"), preventing oil churning and frothing caused by the rotating crankshaft.
• Wet Sump (Medium- and High-Speed 4-Stroke Auxiliary Diesels): Oil drains into and is stored directly in the deep oil pan or bedplate trough of the engine itself. A dipstick measures oil level directly from the engine casing.

System Oil Circulation Loop

1. Main Lube Oil Pumps: Electrically driven screw or gear pumps draw oil from the drain tank through magnetic suction strainers and discharge it at $4\text{--}6\text{ bar}$.
2. Filtration: Oil passes through an automatic self-cleaning backflushing filter ($25\text{--}50\text{ microns}$) and a secondary manual bypass duplex filter to catch metallic particles and carbon.
3. Lube Oil Cooler: A titanium or stainless steel plate heat exchanger, cooled by Low Temperature (LT) fresh water, regulates the oil supply temperature strictly to $45^\circ\text{C}$.
   • If oil is too cold ($<40^\circ\text{C}$): High viscosity increases frictional drag, restricts oil flow through narrow bearing clearances, and delays circulation.
   • If oil is too hot ($>55^\circ\text{C}$): Oil film viscosity thins dangerously, reducing hydrodynamic load-carrying capacity and accelerating thermal oxidation and acid formation.
4. Distribution: Oil feeds the main bearings through fixed branches. In the crosshead arrangement described here, a telescopic pipe supplies the moving crosshead and guide shoes. Oil passes down through the connecting rod to the crankpin bearing and through the piston rod cooling passages to the piston crown; it then drains back to the tank.
5. Continuous Centrifugal Purification: A separate pump draws oil from the drain tank and sends it through a preheater to the purifier at the specified separation temperature. The purifier removes water and solids and returns clean oil to the tank; the main engine supply remains at its separate cooling-system setpoint.

Crosshead Dual Lubrication Regimes

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3. Starting Air System & Engine Reversing

Large slow-speed propulsion diesels commonly start by admitting compressed air, nominally 30 bar, directly into the cylinders in a timed sequence. Marine diesel installations can also use electric starting; the starting arrangement depends on engine design.

+-------------------------------------------------------------------------+
|                    STARTING COMPRESSED AIR SYSTEM                       |
+-------------------------------------------------------------------------+

   [ MAIN AIR COMPRESSOR 1 ]      [ MAIN AIR COMPRESSOR 2 ]
               |                              |
               +--------------+---------------+   Charged to 30 bar
                              |
            +-----------------+-----------------+
            |                                   |
            v                                   v
   [ AIR RECEIVER 1 (30 bar) ]         [ AIR RECEIVER 2 (30 bar) ]
   (Two independently usable receivers: combined reserve >=12 starts
    for the reversible-engine example, without compressor replenishment)
            |                                   |
            +-----------------+-----------------+
                              |
                              v
                 [ MAIN STARTING AIR VALVE ]
                 (Pneumatic automatic valve)
                              |
         +--------------------+--------------------+
         |                                         |
         v (30 bar Main Air)                       v (Control Pilot Air)
   [ ISOLATION NON-RETURN VALVE ]         [ STARTING AIR DISTRIBUTOR ]
         |                               (Cam-driven pilot valve rotor)
         v                                         |
   [ STARTING AIR MANIFOLD ]                       v Sequential Pilot Signal
         |                                         |
         +--------------------+--------------------+
                              |
            [ BRANCH FLAME TRAP OR BURSTING DISC ]
                              v
                 [ CYLINDER STARTING AIR VALVE ]
                 (Opens at 5°-10° ATDC on expansion stroke;
                  drives piston down until firing speed attained)

Compressed Air Regulations (SOLAS & Classification Rules)

For air-started main engines, IACS UR M61 sets compressor and starting-reserve requirements. A 30-bar system is a common design example:

• Fit two or more starting-air compressors with capacity approximately equally divided. At least 50% of the required capacity is driven independently of the main propulsion unit. Their total capacity supplies the required starting reserve from atmospheric pressure within one hour.
• Fit at least two starting-air receivers of approximately equal capacity, usable independently. Working pressure follows the approved design; 30 bar (3.0 MPa) is the example here.
• Receiver Capacity: The total stored air capacity must be sufficient to provide at least 12 consecutive starts for a direct-reversing main engine (alternating ahead and astern), or at least 6 starts for a non-reversing engine starting without opposite torque, without replenishing from air compressors.

Starting Air Circuit Operation

1. Main Starting Air Valve: An automatic, air-operated globe/piston valve located on the main air line. When the maneuvering lever on the Engine Control Room (ECR) console or bridge is moved to "START", control air opens this valve, admitting 30-bar air into the engine starting air manifold.
2. Starting Air Distributor: A rotational distributor driven by the engine camshaft (or timed electronic solenoid block). It sends synchronized pilot air signals to individual cylinders sequentially according to engine firing order.
3. Cylinder Starting Air Valve: Mounted on each cylinder cover. When the pilot air signal arrives from the distributor, it depresses the valve operating piston, snapping the valve open. 30-bar starting air rushes into the cylinder just as the piston passes TDC ($5^\circ\text{--}10^\circ$ ATDC) on its power stroke.
4. Engine Cranking & Firing: The high-pressure air drives the piston downward, rotating the crankshaft. Once the engine reaches firing speed ($15\text{--}25\text{ RPM}$), the starting air is shut off, the fuel injection pumps are activated, and the engine accelerates under fuel combustion.

Engine Reversing Mechanism

To reverse the direction of rotation (from Ahead to Astern) on a direct-reversing two-stroke engine:

• Fuel is cut off, allowing engine speed to drop.
• Reversing controls select starting-air distribution and fuel-injection timing for the requested direction. Mechanical engines may move fuel-pump cams or followers; electronic engines change control timing. Follow the engine's design: reversing does not necessarily involve shifting an exhaust cam.
• Starting air is admitted in the astern firing order sequence, checking the forward momentum of the engine, braking the rotation, and cranking the crankshaft in the reverse (Astern) direction. Fuel is then injected to run astern.

Safety Devices & Prevention of Starting Air Explosions

To protect personnel and machinery, starting air systems incorporate multiple redundant safety devices:

• Starting Air Line Relief Valves: Spring-loaded safety relief valves fitted along the air manifold to release sudden overpressure.
• Bursting Discs (Rupture Discs): Thin sacrificial brass or copper diaphragms backed by deflectors, engineered to rupture instantly and vent explosive pressure waves into the engine room atmosphere.
• Flame Traps (Flame Arrestors): Metal elements absorb heat and quench traveling flames. In the direct-reversing arrangement shown, a flame arrester or bursting disc protects each cylinder starting-air connection. In a non-reversing arrangement, this protection is at the manifold supply inlet.
• Isolation Non-Return Valve: Fitted at the engine starting-air supply connection, it limits reverse flow from the engine toward the receivers. A leaking cylinder starting valve can still admit hot gas into its branch pipe.
• Air Receiver Drain Valves: Compressed air naturally condenses atmospheric moisture and carries traces of compressor lube oil into receivers. Watchkeepers must manually drain water and oil from receiver bottom drain valves every watch to eliminate the fuel source required for explosions.