PISTON COOLANT AND PROPERTIES

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Piston Coolant:

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. This can be seen from the fact that specific heat of water is approximately 4 whilst the specific heat of lubricating oil is about 2 (Both in S I Units).

Further the temperature range (t₂ – t₁) of cooling water passing through a piston may be of the order of 14*C while for cooling oil it will be 10*C for similarly rated engine.

Let Q = Quantity of heat removed in any given time.

Q = Weight of coolant used in time T x (t₂-t₁) x Specific heat.

If weight of water used is unity.

Q_W (Heat removed by water) = 1 x 14 x 4 = 56.

If weight of oil used in the same time is W_O

Q_O (Heat removed by oil) = W_O x 10 x 2.

If same amount of heat removed:

Q_O = Q_W.             W_O20 = 56.

                             W_O = 56/20 = 2^.8.

So it can be seen for same cooling effect amount of oil circulated is about 3 times the water. In actual designing practice there are many other factors to be taken into account.

Fresh and Distilled Water Piston Cooling system:

Advantage:

1. The main advantage of cooling pistons by water is the ability of water to absorb large amounts of heat.
2. Relatively easy to obtain.

Disadvantage:

1. The piston cooling water conveyance pipes and attendant gear must be kept out of the crankcase as far as possible, due to the danger of contamination of the crankcase lubricating oil by water leakage. Because of possible contamination of Jacket cooling water with oil, the jacket cooling water system must be made separate from the piston cooling system. This necessitates duplication of cooling water pumps, piping, motors, starters, coolers and control equipment.
2. When an engine has water-cooled pistons, the piston cooling space should be drained of water after the engine is shut down for an extended period. A drain tank is necessary for the same purpose Cascade type filter is often incorporated for separation of oil and water.
3. There is risk of scaling and corrosion if water is not properly treated and maintained.

Lubricating Oil Piston Cooling System:

Advantage:

1. The piston cooling oil pump is combined with the lubricating oil pump and piston cooling oil cooler is combined with the lubricating oil cooler. This makes overall simplicity in the system.
2. Internal stress within the material of the Piston is generally less in oil-cooled piston than in water-cooled piston. Good design in water-cooled piston can improve its condition of working.
3. No risk of crankcase-system oil contamination, even when piston cooling oil conveyance piping is fitted inside the crankcase.
4. Simpler arrangements for cooling-oil conveyance piping with less risk of ‘hammering’ in piping and bubble impingement attack.

Disadvantage:

1. Larger power requirements for pumping cooling oil.
2. Larger amount of lubricating oil required giving some cooling effect.

Flow Pattern of the Coolant:

The flow is such that piston cooling oil or water enters at the lowest part of the cooling space and leaves from the uppermost part. It should move in such a manner that upward movement of coolant is uniform on opposite sides of the piston to give even cooling without causing distortion due to unequal expansion. The flow direction is arranged in this manner so that the piston is always full of coolant and the underside of piston crown is always in contact with it. This is particularly important in slow speed propulsion engines, as when the piston is running at dead slow speed the coolant in piston is not ‘shaken up’ the way it is done when the engine is running at full speed.

If the coolant flow took place in the opposite direction, it would be possible at very slow speed for the coolant to drain from the piston and lose contact with the crown. The piston could become overheated. Some water-cooled pistons have the outlet for the water at approximately half the cooling space height. When running slow, the piston is half full of water and piston movement agitates this water in the piston and the water gets splashed on the underside of crown and piston wall.

When the engine is stopped a jet action from the piston cooling pipe nozzle directs cooling water onto the piston crown, thus removing residual heat and catering for an emergency stop at full speed. The splash method of cooling is called "cocktail shaker cooling"

Quality Requirements for cooling water:

Engine cooling water is a consumable store, which should be carefully selected, treated and continually watched. If this is neglected, corrosion, erosion and cavitation may occur on the watersides of the cooling system and deposits may be formed. These deposits impair heat transfer and may cause thermal overloading of the engine parts, which have to be cooled. Therefore, the water should be treated before the engine is put into operation. During operation care should be taken that the specified concentration is always used.

Corrosion and cavitation on the thrust side of the cylinder liners may occur in all water-cooled combustion engines. This is caused by the concerted action of corrosion and cavitation. The cylinder liner is set into vibrating motion with varying amplitudes and accelerations by the piston during working stroke resulting in negative and excess pressures at the interface of liner wall and cooling water. When the liquid is reaching vapour pressure, it forms vapour bubbles, which as they collapse at the subsequent pressure rise in the course of positive stroke of the vibratory cycle of the liner wall, produce high local pressure and temperature peaks. The impact intensity, which acts the liner into vibrating motion, depends on engine revolutions. This explains why cavitation is less frequent occurrence in medium and low speed engines, than in high-speed engines (revolutions 700 r.p.m).

Vibration Fissure* Corrosion is a damaging mechanism which is caused by dynamic and corrosive load simultaneously. This can be the cause for the formation of cracks and rapid progress of cracks in water- cooled mechanical loaded engine parts, due to a faulty water treatment. Corrosion attack is avoided, when a cohesive protective coating or surface film is formed on the metal cooling surfaces. This protective coating can be obtained by adding corrosion inhibiting oil or a chemical corrosion inhibitor to the cooling water. (*Fissure - narrow opening or crack of some length and depth.)

Laboratory tests and practical experiences confirm, that certain emulsifiable corrosion inhibiting oils are better in reducing successfully vibration fissure corrosion and cavitation than chemical inhibitors. Corrosion preventive oil forms an oil-in-water emulsion, and the emulsifier in the oil provides for a protective layer on the metal cooling surfaces, which prevents corrosive damage.

Characteristic of water should be Within the Following Limits:

Type of water: Fresh water, free of impurities.

Total hardness: Maximum 100 German Hardness*

PH-valve at 20*C: 8.

Chlorine ion content: Maximum 50 mg/L

(* - 1* German Hardness = 10 mg CaO in 1 L water).

Total hardness of water combines temporary and permanent hardness. The calcium and magnesium salts mainly define it. The hydrogen- carbonate part of the calcium and magnesium salts determines the temporary hardness and the remaining calcium and magnesium salts (sulphates) determine the permanent hardness. The temporary (carbonate) hardness is determining for the formation of calcium deposits in the cooling system. Water with a total German hardness of more than 10*should be diluted with distilled water or rainwater or can be softened by chemicals. If the water has a hardness which is lower than specified by the manufacturer of the adding inhibitors, the water should be hardened by mixing with hard water or by adding certain chemicals.

When distillate (i.e. from a fresh water generator) or non-saline water is available, this should be used as engine cooling water. However a slight hardening will then be necessary, depending upon the additive used. This water is free from calcium, and mineral salts so that there will be no formation of deposits reducing-heat transfer and impairing the cooling effect. On the other hand it will be more corrosive than normal hard water, because it will not develop a thin scale, which provides for a temporary protection against corrosion. Consequently water distillates should be treated with special care and concentration constantly watched.

Cooling water Additives:

Only these additives to be used which give adequate protection of the engine against corrosion and cavitation, both in service and during standstill, and which do not attack the materials and seals of the cooling system.

The conditions for the effective application of corrosion inhibitors are:

a)      A clean cooling system.

b)      Suitable water.

c)      Properly prepared cooling water for initial fill.

d)     Continual supervision of the concentration.

e)      The condition of the cooling system.

If it is additives prime task to prevent cavitations, an emulsifiable corrosion inhibiting oil should be selected. As deposits have an adverse effect on the activity of the additive and i.e. the stability of emulsion - it is essential that all surfaces in contact with the cooling water are free from rust and other contaminants before the cooling system put in service. If deposits are found to be present, the entire system should be flushed or cleaned with solvent. This is done most effectively by special firm, or supervised by an expert from the supplier of the solvents. The cleaning agent should not attack the material or seals in cooling system. When cooling water additives are used, the manufacturer's instructions concerning the water quality to be used, additions, concentration and storage should be carefully followed. For low speed engines lower concentrations are usually allowed than for high-speed engines. When draining the treated cooling waters observe environmental protection regulations.

Anti-Corrosion Oil:

This inhibitor is an emulsifiable mineral oil containing additional agents. A thin protective oil film, which does not affect heat transfer and prevents deposits, is formed on the metal surfaces of cooling system. Frothing can occur with oil emulsions, but this may be corrected by maintaining the water pH value of the solution between 8 and 9. Adding hardening powder, such as calcium sulphate and 10% magnesium sulphate does this.

Note: Anti-corrosion oils are not recommended and not suitable when there is a possibility of cooling water temperature dropping below 0*C or rising above 95*C.

Chemical Inhibitors:

Additions of sodium-nitride and sodium-nitride-borate basis have shown to be satisfactory. The new regulation for waste-water disposal and the possibility of cooling water (Fresh water) leakage into the sea water side prohibits the use of chromate in cooling water system. Nitride and nitrate are not suitable for galvanised pipes or in a cooling water system where cooling side is protected by Zn-anodes.

Note: Corrosion inhibiting oils mixed with chemical additions may cause deposits in the cooling system and reduce the heat transfer. In case the cooling water treatment is changed from oil to a chemical inhibitor or the other way round, the entire system should be carefully cleaned first.

Anti-Freeze Agents:

When the engine is operating at temperatures below the freezing point of water, an anti-freeze agent should be added. Suitable anti-freeze agents (Glysantin 3059) protect against corrosion and are also effective in protecting against cavitation. The additional water treatment is then not necessary. An adequate corrosion protection is obtained when the concentration is adjusted for a low-temperature protection level. Any type of anti-freeze medium agents in use, causing corrosion in the cooling system it should be used only during one winter. Anti-freeze agents must not be mixed with each other.

Note: When cooling water contains a corrosion inhibiting oil, no anti-freezing agent should be added, otherwise the emulsion will break and decompose at once. Chemical additives are normally compatible with anti-freeze agents, but with the later added, a different concentration of chemical additive may be required.

Damage on piston crown:

A diesel engine piston may be damaged by:

i)                    Direct oxidation at high temperature at the skin owing to flame impingement,

ii)                  Catalytic oxidation promoted by fuel ash in a corrosive environment,

iii)                Wet corrosion by sulphuric and sulphurous acids during low temperature operation or during stand-by periods.

The proper selection of material and its treatment is one measure of prevention against such deteriorative damage. Liners are made of alloyed cast iron, but could he chrome plated. Pistons are steel forging or castings containing small additions of chromium and molybdenum. Ring grooves may be plain quench-hardened; chromium clad or fitted with hardened steel or cast iron inserts.

Direct Oxidation:

Present day diesel engines burn a low grade of fuel oil containing sulphur. During the combustion, the crown of piston comes in contact with products of combustion and air containing oxygen, steam, carbon dioxide, sulphur dioxide, sulphur trioxide etc. Metals exposed to such condition will be coated with an oxide film. High rate of cooling or a protective coating by chrome will prevent the peeling of the layer. If the layer is allowed to thicken it will splinter under the effect of flame impingement. A new surface will be exposed to damaging action and thus the wastage will penetrate.

Catalytic oxidation:

Residue oils contain vanadates and compounds of sodium and sulphur. The vanadates are oxidised forming vanadium pentoxidc. Free sulphur or sulphur compounds are oxidised forming sulphuric and sulphurous acids. The attack of molten oil ash containing vanadates and sulphate could be severe. The severity of the damage is associated with overheating.

Wet corrosion:

The presence of sulphur is responsible for this type of damage; it is the dew paint of sulphur trioxide with steam that matters as regards this type of corrosive damage.

Overheated Piston:

Knocking at both ends of the piston travel associated with drop in engine revolutions, rise in cylinder and piston cooling temperatures, rise in exhaust temperature, smoke in exhaust, will indicate a hot piston working with high friction against the liner surface.

A piston can he overheated owing to the following:

1. Failure of coolant circulation.
2. High friction on liner caused by rings seized in the groove, insufficient ring clearance, long skirt touching the liner body.
3. Failure of cylinder lubrication.
4. Improper combustion caused by sticky, leaky or broken rings loss of compression, worn liner, worn injector holes, incorrect fuel timings, unsuitable fuel, insufficient air,
5. Unbalanced cylinder load.
6. Continued overload operation.

Whenever a hot piston is detected:

a)      The engine should be slowed down without stopping.

b)      This measure will immediately reduce heat generation both frictional as well as from combustion of fuel.

c)      Identify the affected cylinder by observation of temperatures, noise, etc. The fuel supply is terminated by lifting the pump plunger.

d)     The unit is cooled down by maintaining circulation of coolant in piston and liner.

e)      Increase lubrication in affected cylinder.