Piston skirt, piston rod and trunk piston are three important parts of the piston arrangement in marine diesel engines. In this article we will learn about each one of them in brief. Piston Skirts
Piston skirt is fitted in both two stroke and four stroke engines. It has different function for different engines. In large cross head two stroke engines with uniflow scavenging these skirts are short in length and are fitted to act as a guide and to stabilize the position of the piston inside the liner. It is generally made of cast iron. The diameter of the skirt is usually kept slightly larger than that of the piston. This is done to prevent damage to the liner surface due to the piston movement.
Soft bronze rings are also fitted in the piston skirts. These bronze rings help during the running-in of the engine, when the engine is new, and can be replaced if necessary.
In two stroke engines having loop or cross scavenging arrangements the skirts are slightly larger as these helps in blanking off the scavenge and the exhaust ports in the liner.
In four stroke or trunk piston engines the skirt has arrangement for gudgeon pin, which transmits power from the piston to the gudgeon pin or top end bearing. As there are no cross head guides in four stroke engines, these skirts help in transferring the side thrust produced from the connecting rod to the liner walls.
Piston rods are generally found in large two stroke engines. Piston rods help in transmitting the power produced in the combustion space to the cross head and the running gear of the engine.
The lengths of these rods depend on the length of the engine stroke and the manufacturers design. The top end of the rod is flanged or attached to the underside of the piston and the bottom end is connected to the cross head.
The piston rod passes through the piston gland or stuffing box so the rod should have smooth running surface and low coefficient of friction. For cooling of the piston the rods consists of two through and through concentric holes. These holes are for supply and return line of cooling oil.
Trunk piston is a term usually given to the pistons in four stroke medium speed engines. These pistons have composite design which comprises of thin sectioned alloy steel piston crown along with aluminum alloy skirt. These pistons are light, strong and rigid in construction and are capable of resisting high temperatures and corrosion.
The piston is forged and the space inside is provided for arrangement of cooling spaces, which is done by cooling oil. The skirt consists of space for gudgeon pin which transmits power to the connecting rod. The skirt also helps in transferring the side thrust produced by the connecting rod.
The piston consists of rings grooves for fitting piston rings. The landing of piston rings is hardened and plated with chrome to reduce wear. The top surface of the crown may be recessed to provide clearance for inlet and exhaust valves. Compression rings are fitted in the crown and are generally plasma coated whereas other rings are chrome plated. The oil control ring is fitted in the top of the piston skirt.