Piston In Marine Diesel Engines:Types, Material, Diagram,Crosshead and Trunk piston engines

                    Pistons are key components of marine engines. It work by transferring the force output of expanding gas in the cylinder to a crankshaft. Pistons are directly exposed to heat and gas load, so the material of the piston is not only thick enough to bear the mechanical load but also thin enough to minimize the thermal stress.
            Piston is the combination of two pieces; crown and skirt. Crown is subjected to high temperature in the combustion space. That is why  the material of the crown is of high strength and resistant to corrosion at high temperature.
 There are two types of pistons used in marine engines Crosshead piston and Trunk piston

Crosshead Piston (Used in 2 Stroke Engines)
     In crosshead type, piston is not directly coupled to the crank shaft ie, piston is connected to the crosshead bearing by piston road and through connecting road its connected to crankshaft. Through the crosshead guides, transverse stress is  transmitted, there by reducing the side trusts on piston
     Based on cooling medium crosshead pistons are divided into water cooled pistons and oil cooled piston. Water cooled pistons are old design. Below shown is a schematic diagram of oil cooled crosshead piston.
Commonly used materials for Crosshead Piston
> Crown: Cast  chrome nickel molybdenum alloy steel
> Skirt: Cast Iron
> Pisto Rod: Forged steel

Trunk Piston (Used in 4 Stroke Engines)

      Here piston is directly connected to the connecting road through gudgeon pin. So, the elongated skirt (trunk) take the side trust and transmit to cylinder liner

Commonly used materials for Trunk Piston
> Crown- Cast Iron
> Skirt -    Ai-Si Alloy or nodular CI
> Gadgeon pin- Carburised steel
> Connecting Road- Forged steel

Difference between Trunk piston and Crosshead piston (Difference between 2 stroke and 4 stroke pistons)

Crosshead piston
> Used in two stroke engines
> Its having connecting rod and piston rod
> Piston is rigidly connected to piston rod
>  Side thrust is transmitted  through crosshead guides to the engine structure.
> Cost and space required is high
> High torque in lower speed
Easier lubrication
> Reduced liner wear

Trunk Piston
Used in four stroke engines
> Only Connecting rod, which connects piston and bearing
> Height and cost of manufacturing is low
> Skirt receives the side trust from crank and connecting rod
You can answer following Questions
> What is the material of piston
> Difference between trunk piston and cross head piston in marine engines
> Different between trunk piston engine and cross head piston engine in marine engines

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