Piston The piston comprises of two pieces, the crown and skirt. The crown is subjected to high temperatures in the combustion space and is liable to be eroded/burnt away. For this reason, the material from which is the crown is made, must be able to maintain its strength and resist corrosion at high temperatures. Steel alloyed with chromium and molybdenum is used.

Some pistons have a special alloyed steel welded on to the hottest part of the crown to try and reduce the erosion caused by the burning fuel (The MAN B&W piston has a 8mm thick heat resisting nickel chrome alloy called Inconel welded to the hottest part of the crown). The crown also carries four or five piston ring grooves. These ring grooves may be chrome plated. The cast iron skirt acts as a guide within a liner. It is a short skirt on engines having exhaust valve. No side thrust is transmitted to the liner (that is the job of the crosshead guides).

A forged piston rod is bolted to the underside of the piston A forged piston rod is bolted to the underside of the piston. The other side of the piston rod is attached to the crosshead pin. Pistons are cooled either using water or the crankcase lube oil. Water has a better cooling effect; there is a risk of leakage of water into the crankcase. Modern diesel engines have lube cooled pistons. The piston rods are utilized to carry the lube oil to and from the piston. The rod is hollow and it has a tube running up at its centre. This gives an annular space, which with the central bore, allows supply and return.

An alternative method of cooling uses nozzle plate and nozzles An alternative method of cooling uses nozzle plate and nozzles. Note that the oil goes up the annular space, formed between the oil tube and the bore in the piston rod, and returns down the centre. The oil is sprayed up matching bores 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 effect caused by the reciprocating motion, gives efficient cooling.

When overhauling the piston, it is important to check the thinning of piston crown due to burning/erosion/corrosion. The piston should be dismantled to check the cooling space. If there is a carbon buildup(in an oil cooled piston) or scale (in case of water cooled piston), then this may lead to thermal stressing of the piston resulting finally in cracking of the piston crown. If the cooling oil is allowed to leak into the combustion space, it will have disastrous effect.

Piston rings Piston rings are made from cast iron alloyed with chromium, molybdenum, vanadium, titanium, nickel and copper. They are harder than cylinder liner in which they run to give them a maximum life before they are renewed. Piston rings have the following jobs to perform seal the gas space by expanding outwards due to gas pressure acting from behind. spread the lube oil up and down the liner transfer heat from the piston to the cylinder liner (from there heat will be carried to the jacket cooling water) When overhauling the piston, it is important to check the piston rings grooves for wear and piston ring condition. The piston ring butt clearance and axial clearance between ring and groove is measured and recorded.