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.

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