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How Internal Forces in Marine Engines Affect Their Operation?

10 Jun 2014
Understanding the operations of intricately designed marine diesel engines involves knowledge of stresses, transmission and reversal in each of such engine type.

The connecting rod of the engine swings about the crosshead pin in 2 stroke engines and about the gudgeon pin in 4 stroke engines. The swinging movement is restricted by the crankpin bearing.

The mass of the connecting rod sets up an inertial load in the transverse direction, which demands the connecting rod to be light in medium and high speed diesel engines, mainly 4 stroke diesel engines. Thus, the connecting rods are manufactured in I or H sections to reduce the weight in spite of higher manufacturing costs.

Different types of internal forces acting inside the 4 stroke marine engine are:

4 Stroke marine diesel engines experience compressive and tensile forces alternatively.
Immediately after combustion, the gases expand and force the piston downwards, thrusting the gases compressively until the inlet valve opens.
The piston and the connecting rod now experience a tensile or centrifugal force which is in direct proportion to the mass of components and the square of velocity.
The inlet valve and the exhaust valve both stay open and overlap each other for a small period, thereby relieving the components of compressive forces.
Further, the exhaust valve closes during the induction stroke and the inlet valve closes on the compression stroke. The piston and the connecting rod are once again in compression, though the magnitude is minimal.
Contrarily in 2 Stroke engines, the piston and connecting rod are always in compression i.e. during the expansion stroke and the compression stroke.
The magnitude of compressive force is greater in expansion stroke as compared to the compression stroke as the gases expand after combustion and force the piston downwards.
It could thus be inferred that the components of a 4 stroke diesel engines (mainly piston and connection rod) are in compression and tension, alternatively for each revolution of the crankshaft. Due to the large centrifugal force experienced by the piston and the connecting rod, the bottom end bearing bolts that are always in compression, try to shear off as the piston and connecting rod have a tendency to fly off.

The bottom end bearing bolts thus have a restricted life and needs to be replaced depending on the manufacturer’s recommendations. There could be unprecedented, disastrous consequences, if the bottom end bearing bolts fail in their operation. Special care should be exercised when they’re removed from the engine during major overhauls. They should be inspected for cracks on the surface as well.

Furthermore, the piston in 4 stroke engines is made of heat resistant Aluminium alloy, to keep the mass as low as practically possible to minimize the effect of high tensile centrifugal force and reduce the whip loading. These considerations are not significant in 2 stroke engines as they’re always in compression.

Moreover, in 2 stroke engine, the connecting rod is in line to the piston rod and hence there’s an angularity when the crank moves from the top and bottom dead centre positions. The transverse thrust set up is transmitted by the guide slippers on to the engine or cylinder guides. This transverse thrust is termed as guide load, which comprises of the resultant of the piston rod and the connecting rod loads caused by the cylinder pressures (static load) and the dynamic loads caused by the inertia of moving parts. The alignment of the guides is very important as is the clearance between the guides and slippers. If the clearances become excessive then exorbitant wear will occur between the piston rod and the stuffing box and the piston and the cylinder liner.

The transverse thrust or the guide load is calculated by the triangle law of forces, drawn up from the force acting downwards on the piston rod along the line of the piston stroke and the reaction from the upper part of the connecting rod. When the line of the connecting rod is identical to the line of piston rod, then the magnitude of the guide load is zero i.e. when the piston is at its bottom dead centre or top dead centre. The magnitude of the guide load varies when the piston moves during its expansion stroke and acts in one particular direction. Similarly, when the piston is moving upwards during the compression stroke the guide load varies but acts in opposite direction.

Contrarily, in 4 stroke engines the side thrust from the piston pin or the gudgeon pin is transferred to the side of the piston skirt or trunk. The thrust from the side of the piston skirt is then transmitted to the liner and is balanced by the equal reaction from cylinder liner. The piston skirt should be carefully and effectively designed so that the area coming in contact with the cylinder liner is adequate for the loads it has to meet. The aim of designing should be such that when the piston comes up to its working temperature, then there should at least be a 90 degrees circumferential arc of contact on each side of the piston skirt and this circumferential arc should extend over the length of the piston skirt.

This is a general overview of the practical considerations of the stress reversal in 4 stroke and 2 stroke engines. The transmission of thrust requires the designers to work out precisions to minimize the losses and reduce the weight to power ratio, while maintaining the strength and sturdiness of the engine.

Info gathered from Marine Insight

Image credits: directindustry
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