Marine Engineering Study Materials

The coolant used for removing and conveying the heat from a piston may be either fresh water, distilled water or lubricating oil. Water has the ability to remove more heat than lubricating oil. his can be seen from the fact that the specific heat of water (in SI unit) is approximately 4 while specific heat of lube oil is about 2. Further, the temperature range ( t₂-t₁ ) for cooling water passing through a piston may be of the order of 14 degree Celsius while for cooling oil i will be 10 degree Celsius for a similarly rated engine.

Let,

Q = quantity heat removed in any given time

Then

Q = weight of coolant used in time T x ( t₂-t₁) x specific heat

Assume the weight of water passing through the piston in time T is equal to unity. Then

Q₀ = 1.0 x 14 x 4

Assume the weight of oil passing the piston in time T is W₀. Then

Q₀ = W₀ x 10 x 2

If the same same amount of heat is removed from the oil cooled piston as from the water cooled piston in the equivalent time T then,

Q₀ = Q_w

and W₀ x 10 x 2 = 1.0 x 14 x 4

So,

W0 = (1.0 x 14 x 4) / (10 x 2)

= 56 / 20

= 2.8

From this it can be seen that for the same cooling effect, the amount of cooling oil circulated will be nearly three times the amount of water. In actual design practice there are many other factors to be taken into account when designing and comparing the relative merits of water or oil for piston cooling systems.

Piston Cooling Media

Fresh Water

Advantages

• Relatively easy to obtain, and does not require special reserve storage facility.

Disadvantages

• Contain Hardness salts which could form scales on internal surface of the piston.

Distilled Water

Advantages

• Absence of scale forming matters.

Disadvantages

• Must be produced by evaporation of fresh or salt water and condensation of vapour produced.
• Strict supervision of evaporation control necessary to keep the contaminants carried over to a low order – in parts per million (ppm)

Fresh and Distilled Water Piston Cooling Systems

Advantages

• Water has the ability to absorb large amounts of heat

 Disadvantages

• The piston cooling water conveyance pipes and attendant gear must be kept out of the crankcase as far as possible, because of the danger of contamination of the crankcase lubricating oil by water leakage. Because of possible contamination of he jacket cooling water with oil, the jacket cooling water system must be made separate from the piston cooling system. The necessities of duplication of cooling water pumps, pipings, motors, wiring, starters, coolers and control equipment.
• When an engine has water cooled pistons the piston cooling system should be drained off water after the engine is shut down for an extended period. A drain tank is necessary to hold the piston cooling water. This is often incorporated with a cascade type filter for separation of oil and scum from the piston cooling water.
• Generally, water cooling of piston makes for added complication, and a higher risk of contamination of the crankcase lubricating oil system.

 Lubricating oil Piston Cooling Systems

Advantages

• The piston cooling oil pump is combined with the lubricating oil pump and the piston oil cooler is combined with the lubricating oil cooler. This makes for overall simplicity in ancillary pumping and piping system, and in the control equipments associated with these systems.
• Internal stress within the material of the piston is generally less in oil cooled pistons than in water cooled pistons, but good design in a water cooled piston can negate this advantage to some degree.
• No risk of crankcase system oil contamination, even if piston cooling oil telescopic pipes are fitted in crankcase.
• Simpler arrangement such as telescopic pipes for cooling oil conveyance piping with less risk of hammering in pipings and a bubble impingement attack.

Disadvantages

• Large power requirements for pumping cooling oil.
• Larger amounts of lubricating oil required in lubricating oil system, if oxidation is to be kept down.
• Increased period of time to cool down after stopping main engine, if coking in piston is to be avoided.

 

References

“Lamb’s Questions and Answers on the Marine Diesel Engine” by Stanley G Christensen