piston used in marine diesel engines

piston converts the force produced by the combustion gases to mechanical power through its reciprocating motion.

The Piston is usually of composite construction consisting of Piston crown and skirt.

Crown : The crown must withstand high gas load and transmit the force to the piston rod. It should be able to withstand the fluctuating mechanical and thermal stresses. Material of crown must be able to maintain its strength, resist corrosion at high temperature and readily conduct heat for cooling. The shape of the crown is generally concave to give an efficient combustion space.

Piston crown are cast in chromium - molybdenum alloy steel. Chromium, molybdenum and nickel are added for strength, heat resistance and higher thermal conductivity. In some MAN B&W Engines, Piston has 8 mm thick heat resisting layer of a hard nickel chrome alloy called Inconel, welded to the hottest part of the crown to resist the “burning” of crown. The crown has 4 or 5 piston ring grooves which are chromium plated to resist the wear. The layer thickness is increased suitably to give higher corrosion resistance.

Skirt : The skirt acts as guide within the cylinder liner. It is usually made of cast iron as the mechanical and thermal load acting on the skirt is much less as compared to crown. The skirt is much smaller in modern engines, as the side thrust due to the connecting rod angularity is taken up by the crosshead guide and guide shoes. Brass rubbing bands may be fitted on skirts to assist running in.

A forged steel piston rod is bolted to the underside of the piston. The other end of the piston rod is attached to the crosshead.

Piston Cooling : Cooling of the piston is necessary to remove excess heat from combustion and to limit thermal stress. Modern engines have oil cooled pistons. The piston rod is utilized to carry the oil to and from the piston. The rod is hollow and has tube running up its centre. Oil is forced through telescopic pipe via crosshead and piston rod to an annular space around the groove. It returns via the crosshead to a collector where its flow and temperature are monitored.

An alternative method of cooling uses a nozzle plate and nozzles. The oil goes up the annular space formed between oil tube and bore in the piston rod, and returns down the centre. The oil is sprayed up onto the underside of the crown. This allows the crown to be made as thin as possible, to allow for maximum heat transfer while maintaining strength, and combined with cocktail shaker caused by the reciprocating motion, giving efficient cooling. 

PISTON RINGS

Piston rings : The function of a piston ring is to

• Provide a seal to combustion chamber, by expanding outwards due to gas pressure acting behind them. They prevent the gases and combustion products passing the piston.
• Conduct heat away from piston to liner.
• Control the lubricating oil.

Piston rings must have

• good strength,
• elasticity
• resistance to wear and corrosion and
• readily transfer heat.

Piston rings are made of cast iron alloy with chromium, molybdenum, vanadium, titanium, nickel, copper. Top ring which is subjected to greatest load and temperature, may be chrome or plasma plated. Ring clearances are necessary to allow movement and thermal expansion.

Reasons for failure of Piston rings :

1. Insufficient piston ring groove clearances causing ring to jam in the groove.
2. Insufficient lubrication causing abrasive wear.
3. Excessive wear / ovality of liner
4. Ring gap inadequate leading to breakage.