DieselShip

Piston

Manufacturing and materials

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

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.

B&W LMC oil cooled piston

The piston has a concave top. This is near self supporting and reduces the need for internal ribbing. It prevents the cyclic distortion of the top when under firing load. This distortion can lead to fatigue and cracking

Pistons may be cooled by oil or water. Oil has the advantage that it may be supplied simply from the lubrication system up the piston rod. Its disadvantage are that maximum temperatures is relatively low in order to avoid oxidised deposits which build up on the surfaces. In addition the heat capacity of oil is much lower than that of water thus a greater flow is required and so pumps and pipework must be larger. Also, if the bearing supply oil is used as is mainly the case a greater capacity sump is required with more oil in use.

Water does not have these problems, but leakage into the crankcase can cause problems with the oil (such as Micro Biol-Degradation). The concave or dished piston profile is used for most pistons because it is stronger than the flat top for the same section thickness

Sulzer watercooled piston (rnd)

Increasing section thickness would result in higher thermal stress.

Sulzer piston require a flat top because of the scavengeing and exhaust flow arrangement (loop scavengeing of RND etc). in order to avoid thicker sections internal support ribs are used. However these ribs cause problems in that coolant flow is restricted. The flow of water with an RD piston is directed to and from the piston by telescopic pipes. The outlet is positioned higher than the inlet within the cooling cavity and on the opposite side of the support rib in order to ensure positive circulation.

With highly rated engines overheating occurred in stagnant flow areas between the ribs and so a different form of cooling was required. The cocktail shaker effect has air as well as water in the cooling cavity as the piston reciprocates water washes over the entire inner surface of the piston just as in a cocktail shaker. Unfortunately air bubbles become trapped in the water and flow to outlet reducing the air content and removing the cocktail shaker effect. To avoid this problem air must be supplied to the piston some engine builders use air pumps feeding air to the inlet flow. The sulzer engine allows air to be drawn into the flow at a specially designed telescopic transfer system.

The telescopic arrangement is designed to prevent leakage and allows air to be drawn into the coolant flow to maintain the cocktail shaker effect. Consider the inlet telescopic, a double nozzle unit is fitted to the top of the standpipe. Small holes allow connection from the main seal to the space between the nozzles. Water flowing through the lower nozzle is subject to pressure reduction and a velocity increase. The space between the nozzles is therefore at a lower pressure than other parts of the system. Any water which leaks past the main seal is drawn through the radial holes into the low pressure region and hence back into the coolant flow.

The pumping action of the telescopic draws air past the lower seal and this is also drawn through the radial holes into the coolant flow. This maintains the air quantity in the piston and so maintains the cocktail shaker effect.

The sulzer water cooled piston differs from that of the Oil cooled variety by the method it uses for distributing the cooling medium. In tis case the piston is not continually flooded but instead contains a level governed by the outlet weir. Cooling of the crown occurs during change of direction at the top of the stroke by so called 'Cocktail shaker' action.

Composite pistons

With medium speed and higher speed engines considerable inertia forces are placed on the conn rod and bearings as the piston changes direction at the ends of the stroke. The amount of force is a factor of the speed and rotating mass. To reduce this force whilst maintaining the same engine speed it is necessary to reduce this rotating mass.

Aluminium, with its lower density than steel is used when alloyed with silicon for extra strength. Even alloyed the aluminium has less mechanical strength than the steel, therefore damage is possible due to gas pressure acting on crown and piston rings. The piston could deform sufficiently to prevent proper operation of the rings in their grooves. Some engine manufacturers fit cast iron inserts into the grooves but more generally the piston is made in two parts with a cast steel crown containing two grooves.

Aluminium has a better coefficient of heat transfer than steel thus overheating is not a problem. Its lower coefficient of friction avoids the problems of fitting bushes for the gudgeon pin, thus a floating gudgeon pin may be used. The higher coefficient of expansion could lead to the need for greater piston/liner clearance. However, as the main body is not subject to the high temperatures of combustion this expansion is not a problem.

Sulzer rotating piston

This piston rotates as it reciprocates. The rotation being brought about by the swing of the con rod. This causes two spring loaded palls located in the spherical top end to oscillate. These palls engage with a toothed rim which is connected to the piston by means of a compensating spring. As the conrod swings the palls act on the toothed rim causing it, and hence the piston, to rotate. The amount of rotation is limited to one tooth pitch every engine rev and the action is similar to that of a ratchet mechanism. The advantage of this is that local overheating of the piston or the liner due to blow past is prevented. Running in characteristics are improved and liner wear are improved. There is a better spread of oil brought about by the piston rotation. A spherical top end is required but this provides better support for the piston which does not distort as much as one fitted with a gudgeon pin. Piston to liner clearance may therefore be reduced.

Transfer of gas loads from crown to piston rod

Is usually transmitted from the reinforced crown to the piston rod by internal mechanism avoiding possible distortion of the ring belt.

The tops of pistons are made dome shaped or have strong internal ribbing.

Thermal distortion of Piston

Anti-Polishing rings

High topland ( the 1st piston ring is positioned will below the upper surface of the piston) with asociated reduced ring heat load has given better ring pack performance by improving working conditions for the cylinder lube oil. The disadvantage of this system is that a coke build up can occur aboth the piston which leads to 'bore polishing'. This polishing reduces the ability of the cylinder lube oil to 'key' into the liner therefore increased cylinder lube oil consumption/increased liner wear can result. To combat this piston cleaning rings are incorporated into the liner. These slightly reduce the bore removing the depoisits.

Modern Design

The top piston ring is moved further down the piston. This allows the crown to enter deeper into the crown reducing temperature and pressure on the liner. The top piston ring is a 'Controlled Pressure relief' (CPR) ring. This design has several oblique shallow grooves in the piston ring face allowing some gas presure to pass through to the 2nd ring thereby reducing load on the top ring. To reduce blowpast an 'S' type joint is formed n the ring ends