(a) Forces due to combustion load

• Combustion forces exert alternating tension and compression loads on the engine structure.
• These forces act on the piston, crankshaft, and bedplate.
• The gas load is transmitted from the cylinder head through tie bolts to the bedplate.
• The bedplate then transfers the load to the ship's hull via holding-down bolts and resin chocks.

Structural Components to Withstand Forces:

Bedplate: Made of mild steel (MS) plates and steel castings, it is assembled and welded to ensure high longitudinal and transverse strength. It also resists twisting forces.

Cross Girders: Cast steel cross girders house the main bearings and provide additional transverse strength and resistance to twisting.

Chocks: Installed between the bedplate and the ship's double-bottom tank top, these absorb shocks and cyclic stresses, ensuring smooth load distribution.

The angular motion of the connecting rod (con-rod) during the engine cycle creates guide forces. At the top and bottom of the stroke, the con-rod is aligned with the crankshaft, but at other positions, it is inclined, generating horizontal forces. These horizontal forces are absorbed by the guides in two-stroke engines, creating a guide force moment.

• In two-stroke engines, the horizontal forces are absorbed by the guide shoes and transmitted through the engine structure.
• In four-stroke engines, the thrust on the gudgeon pin is absorbed by the piston skirt and transmitted through the cylinder liner to the engine body.

In order to counteract the possible impact from guide force moments, it is recommended to install a set of TOP BRACES between the upper gallery of the engine and hull structure. These braces increase the natural frequency of the vibration system to such an extent that resonance occurs above the running range of engine speed, and guide force moment seems harmless.

(c) Inertia Forces

The inertia forces are categorised into those acting on rotating masses and reciprocating masses:

1. Inertia Forces on Rotating Masses:
   • These forces have a constant magnitude when the engine speed is steady, but their direction changes with rotation.
2. Inertia Forces on Reciprocating Masses:
   • These forces depend on the actual position of the piston, even if the engine speed remains constant.

Unbalanced inertia forces, originating from the rotating and reciprocating masses of the engine, create external moments that are unbalanced. This requires effective countermeasures to mitigate their impact on the hull and engine operation.

Resonance can occur when these external moments coincide with the natural frequency of the system within the engine's operating speed range.

• First-Order Moment: One cycle per revolution.
• Second-Order Moment: Two cycles per revolution.

These forces are managed through flywheel design to smooth out rotational speed fluctuations and the addition of counterweights to balance the drive chain, reducing vibrations and ensuring stable operation.