piston

The Piston comprises of two pieces; the crown and the skirt. The crown is subject to the high temperatures in the combustion space and the surface is liable to be eroded/burnt away. For this reason the material from which 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, and some pistons have a special alloy welded onto the hottest part of the crown to try and reduce the erosion caused by the burning fuel. The crown also carries the 4 or 5 piston ring grooves which may be chrome plated. The cast iron skirt acts as a guide within the cylinder liner. It is only a short skirt on engines with an exhaust valve (known as uniflow scavenged engines), as unlike a trunk piston engine, no side thrust is transmitted to the liner (that's the job of the crosshead guides). 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 pin. Pistons are cooled either using water or the crankcase oil. Water has a better cooling effect than oil, but there is a risk of leakage of water into the crankcase.

Modern engines have oil cooled pistons. The piston rod is utilised to carry the oil to and from the piston. The rod is hollow, and has a tube running up its centre. This gives an annular space which, with the central bore, allows a supply and return.

An alternative method of cooling uses a 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 the "cocktail" shaker effect caused by the reciprocating motion, gives efficient cooling.

When overhauling the piston it is important to check the thinning of the piston crown due to burning/erosion/corrosion. The piston should be dismantled to check the cooling space. If this is subject to a build up of carbon (in an oil cooled piston) or scale (in the case of a water cooled piston) then this may have led to thermal stressing of the piston, which in its turn can lead to cracking of the piston crown. If the cooling oil is allowed to leak into the combustion space then the consequences could be disastrous.

PISTON RINGS

The Piston Rings are made of alloying cast iron with chromium, molybdenum, vanadium, titanium, nickel and copper. They are harder than the cylinder liner in which they run to give them a maximum life.

Piston rings seal the gas space by expanding outwards due to the gas pressure acting behind them. They also spread the lubricating oil up and down the cylinder liner and transfer heat to the liner walls.

When overhauling the piston it is important to check the ring grooves for wear and the piston ring condition. The axial and butt clearances should be measured and recorded.