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Modern marine diesel engines have large pistons being made up of the crown and the skirt; the grooves for the piston rings being machined into the crown.
The skirt is bolted to the crown the studs also passing through the piston rod end, the other end of the piston rod being connected to the crosshead bearing.
The following sections examine the pistons used on large diesel engines, the first section giving the functions of the piston.
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Function of the Piston in Two and Four Stroke Engines.
- The piston converts the force produced by the combustion gases to mechanical power through its reciprocating motion.
- Piston crown forms part of combustion chamber.
- The shape of piston crown is governed by shape of the combustion chamber in the cylinder head.
- In a trunk type engine the piston skirt is long; taking up the side thrust due to connecting rod angularity, as well as acting as guide for the piston in the liner.
- In a crosshead type engine although the skirt is much shorter, it still acts as a guide. However in this case the side thrust due to the connecting rod angularity is taken up by the crosshead guide and guide shoes.
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Design Features of the Large Piston
As we have seen, the composite piston is composed of two parts; being used in slow and medium speed two and four - stroke engines. Its design features are discussed below,
- Crowns can be of a concave or convex contour.
- Modern crowns have an inconel alloy layer welded to the top of the crown.
- Convex crown is suitable for the high mechanical stress due to thickness of the material.
- A thin crown can be used when there is a low thermal stress present.
- All crowns are subjected to high mechanical and thermal stresses.
- The crown carries a set of compression and oil rings in machined grooves, the grooves are normally chrome-plated.
- Heat resistance material is used such as; Cr-Mo / alloy steel is used in manufacture of the crowns.
- Extensive cooling is essential for the piston crown.
- In modern engines, the thicker crowns have lube oil cooling supplied and discharged up through the centre of a hollow piston rod.
- In trunk engines it takes up the side thrust due to connecting rod angularity.
- Guides piston in the liner and prevents rocking.
- In a four-stroke engine the skirt accommodates an oil scraper ring.
- Can be provided with wear ring in some design.
- Made of wear resistance and low co-efficient cast iron.
A sketch of a piston at TDC in the cylinder liner is shown below;
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Design and Mechanical Properties of a Piston Crown
- Crown Thickness Requirement
In modern high output marine engines the piston crown is required to withstand high temperatures and pressures from the increased fuel injected. This is achieved by increasing the thickness of the crown, and leads to;
- Higher strength and heat resistance properties of material.
- Rigidity to withstand thermal and mechanical loading without distortion.
- Good heat dissipating qualities.
However, the component’s mass must be controlled to limit inertia forces.
2. Mechanical Properties of Piston Crown
As we have seen; the crown is cast from steel alloyed with molybdenum and chrome; this promotes the following mechanical properties.
- Higher mechanical and fatigue strength.
- Higher thermal conductivity and creep resistance.
- High hardness and anti corrosive
- Low co-efficient of expansion.
3. Piston Crown Cooling Design
- The piston crown is exposed to the high temperatures of combustion, and therefore requires to be cooled either by oil or water.
- The modern method is cooling by oil from the main engine lube oil sump. The oil is circulated around the crown by passages being supplied and extracted through piping in the hollow piston rod.
- Because of the added thickness of the crown, the oil passageways must be as close as possible to the outside of the crown without compromising the strength of the component.
- Other cooling designs use a nozzle and nozzle plate to circulate the cooling oil; this promotes splashing due to piston reciprocating motion aiding circulation; being termed cock tail shaker design.
A sketch of one method of piston oil cooling follows;
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