10.2 Main Engine Structural Components & Moving Parts

10.2 Main Engine Structural Components & Moving Parts

A large two-stroke marine propulsion diesel engine is a monumental structure weighing upwards of 1,000 to 2,000 tonnes and standing several stories tall. To withstand combustion pressures exceeding $150\text{ bar}$ and dynamic mechanical forces generated by reciprocating components, the engine is constructed from heavy-duty cast and fabricated steel components. For DG Shipping GP Rating trainees, learning the static structural framework, cylinder components, running gear, and thrust bearings is essential for safe watchkeeping, maintenance operations, and oral board examinations.

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1. Static Structural Framework

The stationary framework of a marine engine must maintain rigid running alignment between the cylinder liners, crosshead guides, and crankshaft under all operating conditions, including ship hogging, sagging, and wave-induced hull flexing.

+-------------------------------------------------------------------------+
|                    MAIN ENGINE STRUCTURAL FRAMEWORK                     |
+-------------------------------------------------------------------------+
                                     
         [ CYLINDER COVER / HEAD ]  <--- Forged steel, houses exhaust valve
       ==============================    and fuel injectors
       |                            |
       |    CYLINDER BLOCK / JACKET |  <--- Cast iron; houses cylinder liners
       |                            |       and scavenge air receiver
       ==============================
       |    \                  /    |
       |     \    A-FRAMES    /     |  <--- Fabricated steel columns;
       |      \  (ENTABLATURE/      |       carry crosshead guide bars
       |       \  COLUMNS)  /       |
       ==============================
       |   [MAIN BEARING SADDLE]    |  <--- Welded steel box girders;
       |          BEDPLATE          |       houses lower main bearing shells
       ==============================
       |   [SUMP TANK / TANK TOP]   |  <--- Chocked onto inner bottom hull

       * HIGH-TENSILE TIE RODS run continuously from bedplate transverse
         girders to the top of the cylinder block, clamping the structure
         in permanent compressive prestress.

The Bedplate

The bedplate forms the rigid foundation of the entire engine, bolted onto engine seating chocks resting directly over the ship's double bottom tank top:

• Construction: Fabricated from welded mild steel longitudinal box girders and heavy cast or forged steel transverse saddles (girders).
• Main Bearing Saddles: The transverse girders house the lower main bearing shells, which support the journals of the massive crankshaft. Modern engines utilize thin-walled, tri-metal or white-metal (babbitt) lined steel bearing shells.
• Oil Sump Pan: The bottom of the bedplate forms an inclined oil trough or collects lube oil in a drain pan, allowing oil to drain freely by gravity into the engine room double bottom drain tank (dry sump system).

A-Frames (Entablature / Columns)

Mounted directly on top of the bedplate transverse girders are the A-frames (or columnar entablature):

• Construction: Triangular fabricated steel structures bolted to the bedplate.
• Crosshead Guide Bars: The internal vertical surfaces of the A-frames carry precision-machined vertical guide bars (guide plates). The crosshead slippers (guide shoes) reciprocate up and down against these lubricated guide bars, absorbing $100\%$ of the transverse horizontal side thrust exerted by the oscillating connecting rod.

Cylinder Block (Cylinder Jacket)

The cylinder block (or jacket) is bolted on top of the A-frames:

• Construction: Cast iron block formed in individual units or multi-cylinder monoblocks bolted together with fitted bolts.
• Functions: It provides bore recesses to support and house the cylinder liners, forms the enclosed cooling water jackets around the upper part of each liner, and houses the scavenge air manifold receiver at its base.

Through-Bolts (Tie Rods)

Combustion loads tend to separate the cylinder cover from the bedplate. High-tensile tie rods carry these loads while their initial tension holds the engine framework in compression:

• Arrangement: Long, high-tensile alloy steel bolts (tie rods) extend from the underside of the bedplate transverse saddles, passing through dedicated vertical holes in the A-frames and cylinder block, terminating at the top deck of the block.
• Pre-Tensioning: Tie rods are stretched and tightened under immense tension using hydraulic tensioning jacks during assembly. This pre-stresses the entire engine framework (bedplate, A-frames, and cylinder block) into continuous mechanical compression.
• Operational Safety: Firing loads act against the initial compression. Correct tie-rod tension helps keep the structural joints compressed, reducing cyclic tensile stress and the risk of fatigue cracking. Preload and joint condition still require inspection.

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2. Cylinder Liners & Cylinder Lubrication

The cylinder liner forms the cylindrical inner wall of the combustion chamber in which the piston reciprocates.

+-------------------------------------------------------------------------+
|                   BORE-COOLED CYLINDER LINER STRUCTURE                  |
+-------------------------------------------------------------------------+
          +-------------------------------------------+
          |  BORE-COOLED UPPER COLLAR                 |
          |  - Angled drilled cooling passages (FW)   |  <--- Max Pressure & Temp
          |  - Resists combustion thermal stress      |
          +-------------------------------------------+
          |                                           |
          |  CYLINDER LUBRICATION QUILLS              |
          |  - Timed injection of high-BN oil         |  <--- Neutralizes H2SO4
          |  - Non-return valves prevent gas blowback |
          +-------------------------------------------+
          |                                           |
          |  RUNNING SURFACE (MID-STROKE)             |
          |  - Pearlitic grey cast iron               |  <--- Graphite flakes
          |  - Vanadium / titanium alloyed            |       retain oil film
          +-------------------------------------------+
          |                                           |
          |  SCAVENGE AIR INLET PORTS                 |
          |  - Angled machined ports at bottom        |  <--- Imparts uniflow
          |  - Uncovered by descending piston crown   |       rotational swirl
          +-------------------------------------------+

Metallurgy and Bore-Cooling

• Material: Liners are cast from high-grade pearlitic grey cast iron alloyed with vanadium, titanium, and molybdenum. The microstructure contains hard vanadium carbides to resist abrasive wear and graphite flakes that act as natural micro-reservoirs to retain lubricating oil films.
• Bore-Cooling: The upper collar of the liner is exposed to extreme temperatures ($>1000^\circ\text{C}$ gas temperature) and maximum firing pressure ($P_{\max} > 140\text{ bar}$). Making the liner wall thick enough to resist mechanical pressure would cause excessive thermal gradients and severe thermal stress cracking. Modern liners feature bore cooling: a ring of closely spaced, angled holes drilled into the thick upper collar through which treated jacket cooling water circulates close to the combustion face, removing heat close to the combustion surface while keeping liner temperatures within the engine maker's limits.
• Scavenge Ports: Machined into the lower portion of two-stroke liners, angled tangentially to produce a swirling air vortex during scavenging.

Cylinder Lubrication and Base Number (BN)

Because the crosshead engine's diaphragm plate isolates the cylinder from the crankcase, the cylinder liner cannot be lubricated by splash from the sump:

• Lubricator Quills: Multiple lubricator quills fitted with spring-loaded non-return valves are spaced radially around the liner circumference. Mechanical or electronically controlled lubricators inject precise, timed pulses of cylinder oil directly onto the piston ring pack as it ascends and descends.
• Base Number (BN) and Acid Neutralization: Merchant vessels frequently burn Heavy Fuel Oil (HFO) or Very Low Sulfur Fuel Oil (VLSFO) containing sulfur ($S$). Combustion produces sulfur dioxide ($SO_2$) and sulfur trioxide ($SO_3$), which combines with moisture to form sulfuric acid ($H_2SO_4$). This acid condenses on cool liner walls, causing devastating corrosive wear. Cylinder oil is formulated with high alkalinity, measured as Base Number (BN) (ranging from BN 40 up to BN 100). The calcium carbonate additives in the alkaline oil chemically neutralize the sulfuric acid, protecting the metal.

Liner Wear Limits and Ovality Gauging

During engine overhauls, liners are calibrated using an inside micrometer mounted on an extension rod:

• Measurements are taken at several vertical positions: Top Dead Center (piston top ring reversal point—where wear is always greatest due to peak gas pressure and thinnest oil film), mid-stroke, and bottom.
• At each vertical height, two orthogonal readings are recorded: Fore-and-Aft (F-A) and Port-and-Starboard (P-S). The mathematical difference between F-A and P-S is the ovality.
• Wear Rate and Renewal Limit: A rough large-engine liner wear-rate example is $0.05\text{--}0.1\text{ mm per }1,000\text{ running hours}$. Compare measured wear, ovality and surface condition with the engine maker's renewal criteria. For example, if the specified diametral wear limit is $0.8\%$ for an $800\text{ mm}$ liner, the permitted increase is $800\times0.008=6.4\text{ mm}$. The percentage must come from the applicable engine instructions.

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3. Cylinder Head (Cylinder Cover) & Mounted Fittings

The cylinder head (or cover) is a massive forged or cast alloy steel block bolted to the top of the cylinder block using hydraulically tensioned studs, enclosing the top of the combustion chamber.

+-------------------------------------------------------------------------+
|               CYLINDER COVER FITTINGS & VALVES (TOP VIEW)               |
+-------------------------------------------------------------------------+

                        [ INDICATOR COCK ]
                                |
             +------------------+------------------+
             |                                     |
     [ FUEL INJECTOR 1 ]                   [ FUEL INJECTOR 2 ]
             |       +---------------------+       |
             +-----> |                     | <-----+ 
                     |    EXHAUST VALVE    |         
                     |   (Nimonic Spindle, |         
                     |    Hydraulic Act.,  |         
                     |    Air Spring)      |         
             +-----> |                     | <-----+ 
             |       +---------------------+       |
     [ FUEL INJECTOR 3 ]                   [ STARTING AIR VALVE ]
             |                                     |
             +------------------+------------------+
                                |
                  [ CYLINDER SAFETY RELIEF VALVE ]

Key Mounted Fittings and Valves

1. Exhaust Valve Assembly:
   • Spindle Material: Manufactured from heat-resistant Nimonic alloy (nickel-chromium-cobalt superalloy) or Stellite hard-faced alloy to resist high-temperature exhaust gas erosion and hot corrosion.
   • Hydraulic Actuation: Opened hydraulically by high-pressure oil delivered from a camshaft pump or electro-hydraulic rail.
   • Air Spring: Modern exhaust valves eliminate mechanical steel coil springs (which suffer from fatigue and resonance surge). Instead, they utilize a pneumatic air spring (compressed air at $7\text{ bar}$) to return and seat the valve firmly.
   • Valve Rotator (Rotocap / Vane Wing): Fitted with aerodynamic vane wings on the stem or a mechanical rotator that rotates the valve spindle slightly upon every opening. Rotation evens out temperature distribution around the valve face, prevents localized hot spots, and wipes carbon particles off the seat.
2. Fuel Injectors (Fuel Valves):
   • Two or three fuel injectors are arranged symmetrically around the central exhaust valve to ensure uniform fuel distribution throughout the large combustion chamber bore.
3. Cylinder Starting Air Valve:
   • A pneumatically operated poppet valve that admits high-pressure starting air ($30\text{ bar}$) directly into the cylinder to crank the engine during starting.
4. Cylinder Safety Relief Valve:
   • A spring-loaded relief valve set to lift at $10\%\text{--}15\%$ above the normal maximum peak firing pressure ($P_{\max}$), typically around $150\text{--}160\text{ bar}$.
   • Protects the cylinder against dangerous overpressure caused by hydraulic lock (water or fuel accumulation in the cylinder) or fuel pump rack malfunction.
5. Indicator Cock:
   • A small manually operated needle valve connecting the combustion space to an external connection thread for attaching mechanical indicator instruments or electronic pressure transducers.
   • Open indicator cocks for turning-gear checks with starting air isolated. Blow through on air only after the gear is disengaged and personnel are clear, following the engineer's procedure.

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4. Reciprocating and Rotating Running Gear

The moving components convert the chemical energy of burning fuel into linear kinetic energy and subsequently into rotating shaft torque.

+-------------------------------------------------------------------------+
|               RUNNING GEAR OF A TWO-STROKE CROSSHEAD ENGINE             |
+-------------------------------------------------------------------------+

            [ PISTON CROWN ]  <--- Forged steel, internal oil cooling
            [ PISTON SKIRT ]  <--- Cast iron with bronze wear bands
                   |
              [PISTON ROD]    <--- High-tensile forged steel
                   |
             [STUFFING BOX]   <--- Diaphragm plate boundary
                   |
        =====[CROSSHEAD PIN]=====  <--- Top-end bearing
        |   [ GUIDE SHOES ]     |  <--- Reciprocates on vertical guide bars;
        |     (SLIPPERS)        |       absorbs ALL side thrust
        =========================
                   |
            [CONNECTING ROD]  <--- Transmits thrust; converts linear
                   |               to rotary motion
             [CRANKPIN BRG]   <--- Bottom-end bearing
                   |
             [ CRANKSHAFT ]   <--- Transmits torque; main bearings in bedplate
                   |
            [ THRUST BLOCK ]  <--- Michell tilting pads; absorbs propeller thrust
                   |
           [ PROPELLER SHAFT ]

Piston Assembly

• Piston Crown: Forged chrome-molybdenum alloy steel with a dished combustion profile. The crown is exposed to combustion gas temperatures exceeding $1500^\circ\text{C}$. It is internally cooled via the "cocktail-shaker" method, where lubricating oil (supplied up through the hollow piston rod) is tossed against the hot under-crown by piston reciprocating inertia, removing heat before draining.
• Piston Skirt: Cast iron skirt guiding the piston inside the liner. In crosshead engines, the skirt is short and fitted with bronze wear bands (rubbing rings) inserted into circumferential grooves to prevent steel-on-iron scoring during running-in.
• Piston Rings: 4 to 5 compression rings fitted into machined ring grooves in the crown. Rings are manufactured from nodular cast iron coated with chromium or cermet (ceramic-metal) on the running face to resist abrasive wear and gas blow-by.

Piston Rod, Crosshead & Guide Shoes

• Piston Rod: A ground forged-steel shaft with internal cooling-oil passages, rigidly bolted to the piston crown, passing through the stuffing box and terminating at the crosshead pin.
• Crosshead Pin: A large hardened forged steel cylindrical pin that forms the oscillating pivot between the piston rod and connecting rod.
• Guide Shoes (Slippers): White-metal lined sliding blocks mounted on both ends of the crosshead pin. They slide vertically between the lubricated guide bars on the engine A-frame, absorbing the entire horizontal side thrust produced by the angularity of the connecting rod.

Connecting Rod & Crankshaft

• Connecting Rod: High-tensile forged steel rod with a top-end bearing (crosshead bearing) and a split bottom-end bearing (crankpin bearing) lined with replaceable thin-wall bearing shells.
• Crankshaft: Huge forged steel shaft converting reciprocating motion into rotational torque. In large slow-speed engines, crankshafts are semi-built: forged crankpins and webs are heated to expand and shrunk onto forged main bearing journals with an interference fit. Heavy counterweights (balance weights) are bolted to crank webs to balance reciprocating and rotating masses.

Thrust Block (Michell Tilting-Pad Bearing)

The propeller produces axial thrust along the shafting. The thrust bearing transfers this force from a rotating thrust collar through its pads and support into the hull while controlling axial movement. In an integrated arrangement, the collar forms part of the engine crankshaft:

• Location: Positioned at the aft end of the engine bedplate or directly aft of the flywheel in the shaft tunnel.
• Principle of Operation (Michell Bearing): An integral forged collar on the thrust shaft rotates against a stationary ring of kidney-shaped tilting pads (Michell pads). Each pad is supported on a pivot off-center. As the thrust collar rotates, lubricating oil is dragged into the narrowing wedge between the collar and the tilted pad faces, forming a high-pressure hydrodynamic oil wedge capable of supporting hundreds of tonnes of axial load with zero metal-to-metal contact.
• Hull Transmission: The thrust block housing is solidly bolted to heavy longitudinal hull girders, transferring the propeller thrust directly into the ship's keel structure.