Piston Materials and Types

     

          

Materials

Piston crowns attain a running temperature of about 450^oC and in this zone there is a need for high strength and minimum distortion in order to maintain resistance to gas loads and maintain the attitude to the rings in relation to the liner. The heat flow path from the crown must be uniform otherwise thermal distortion will cause a non-circular piston resulting in reduced running clearance or even possible contact with the liner wall.

In addition to this thermal stress they are also subject to compressive stress from combustion and compression loads, as well as inertial loads.

Materials such as pearlitic, flake and spheroidal cast iron, alloy cast irons containing Nickel and chromium, and aluminium alloys may be used.

The determining factor is the design criteria for the engine.

For a modern slow speed engine steel forging or castings of nickel-chrome steel or molybdenum steel are common. The weight of the material is not normally a governing factor in this type of engine although resistance to thermal stress and distortion is. Efficient cooling is a required to ensure the piston retains sufficient strength to prevent distortion.

For medium and high speed engines the weight of the material becomes important to reduce the stresses on the rotating parts. The high thermal conductivity of aluminium alloys allied to its low weight makes this an ideal material. To keep thermal stresses to a reasonable level cooling pipes may be cast into the crown, although this may be omitted on smaller engines.Where cooling is omitted, the crown is made thicker both for strength and to aid in the heat removal from the outer surface.

Hard landings are inserted into the ring groves to keep wear rated down.Composite pistons may be used consisting of an cast alloy steel crown with an aluminium-alloy or cast iron body.

Annealing

After casting or forging the component is formed of different material thicknesses. The thinner parts will cool more quickly thereby setting up internal stresses. Annealing removes or reduces these stresse as well as refining the grain structure.

Cooling

Oil Cooled	Water cooled
High specific heat capacity therefore removes more heat per unit volume	Low specific heat capacity
Requires chemical conditioning treatment to prevent scaling	Does not require chemical treatment but requires increased separate and purification plant
Larger capacity cooling water pump or separate piston cooling pump and coolers although less so than with oil	Larger capacity Lube oil pump, sump quantity and coolers
Special piping required to get coolant to and from piston without leak	No special means required and leakage not a problem with less risk of hammering and bubble impingement.
Coolant drains tank required to collect water if engine has to be drained.	Increased capacity sump tank required
Pistons often of more complicated design	Thermal stresses in piston generally less in oil cooled pistons
Cooling pumps may be stopped more quickly after engine stopped	Large volumes of oil required to keep oxidation down and extended cooling period required after engine stopped to prevent coking of oil

Wear rings

Wear rings are found on some slow speed engines employing loop or cross flow scavenging although they may be found in most designs. They are made of a low coefficient of friction material and serves two main purposes. To provide a rubbing surface and to prevent contact between the hot upper surfaces of the piston and the liner wall.In trunk piston engines wear rings to negate the distortion effect caused by the interference fit of the gudgeon pin .

The ring may be inserted in two pieces into the groove then lightly caulked in with good clearance between the ends.