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Piston for Marine Diesel Engine

Piston forming the lower part of the combustion chamber: The diesel engine is a type of internal combustion engine which ignites the fuel by injecting it into hot, high-pressure air in a combustion chamber. In common with all internal combustion engines the diesel engine operates with a fixed sequence of events, which may be achieved either in four strokes or two, a stroke being the travel of the piston between its extreme points. Each stroke is accomplished in half a revolution of the crankshaft.

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Piston forms the lower part of the combustion chamber. It seals the cylinder and transmits the gas pressure to the connecting rod. The piston absorbs heat of combustion and this heat must be conducted away if the metal temperature is to kept in safe limits. The Piston comprises of two pieces; the crown and the skirt.

The crown is subject to the high temperatures in the combustion space and the surface is liable to be eroded/burnt away. For this reason the material from which the crown is made must be able to maintain its strength and resist corrosion at high temperatures.
piston
Fig: Piston


Handling piston
Fig:Handling piston

Cylinder liner
Fig: Cylinder liner

Steel, alloyed with chromium and molybdenum is used, and some pistons have a special alloy welded onto the hottest part of the crown to try and reduce the erosion caused by the burning fuel. The crown also carries the 4 or 5 piston ring grooves which may be chrome plated.

The cast iron skirt acts as a guide within the cylinder liner. It is only a short skirt on engines with an exhaust valve (known as uniflow scavenged engines), as unlike a trunk piston engine, no side thrust is transmitted to the liner (that's the job of the crosshead guides).

The stresses to which a piston is subjected to are as follows:

(mechanical & thermal stresses)
Compressive and tensile stress caused by bending action due to gas pressure
Inertia effect - movement up and downwards

Thermal stresses - rapid temperature change

The crown of a piston is subjected to a very high gas pressure which will subject the top surface of the crown to compressive loading and the lower surface of crown will be under tensile loading. The piston crown will be like a uniformly loaded beam

As the piston moving upward, towards the end of its stroke its velocity will be reducing.The inertia effect will tend to cause the piston to bow upwards, so that the surface of the crown along with sides will be under tensile loading and lower surface of the crown will be under compressive loading When the piston is retarded on its approach downwards to BDC, the inertia effect will be reversed

The thermal stresses set up in a piston are caused by the different temperature across a section.The free expansion of the hot side is restricted by the cooler surface of the piston. Maximum safe temperature at the three most critical zones for alluminium-alloy piston are Crown 370 degree to 400 degree C. Top ring groove and gudgeon pin bosses 200 degree to 220 degree C.

Mechanical and thermal stresses should be considered to gather as they tend to be complimentary to each other. Top and sometimes 2nd ring grooves is tapered up to 2 times normal axial clearance thus:

i) Clearance allows for carbon deposit

Clearance avoids piston crown edge touching liner due to thermal stress

If the crown temperature exceeds 400 degreeC, failure will probably occur from cracking. If the top ring groove temperature exceeds 220 degreeC, for any length of time, trouble may be expected from:
i)Stuck piston rings
ii) Formation of carbon at the bottom of the ring groove, causing the ring to be packed out

Cracking of crown – due to thermal and mechanical stresses. Cracking through piston wall especially in way of top ring groove – due to fluctuating gas load, excessive thermal stresses. Crack starts from inside wall. Cracking may take place due to the following reasons apart from the reasons mention above:

i) Unsuitable material for the rating of the engine or inadequate machining
ii) Excessive scaling on the cooling side, cavitaion erosion
iii) High coolant temperature
iv) Local impingement
v) Poor atomization, high penetration of fuel
vi) High water content in fuel


Summarized below more guidance for marine diesel engine piston handling safety procedure:
  1. Construction of 4 stroke piston

    2. 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 prevent distortion.
    More.....

  2. Piston oil cooling method - basic principle

    3. 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 ( specific heat of water approximately 4 and lubricating oil 2 and temperature difference 14 deg C for water and 10 deg C for lube oil) .

    More .....

  3. Function of piston ring in a marine diesel engine

    4. The efficiency of the engine depends upon the effective sealing between the piston and liners. Leakage will reduced compression pressure and power will lost. Piston rings seal the gas space by expanding outwards due to the gas pressure acting behind them. They also spread the lubricating oil up and down the cylinder liner and transfer heat to the liner walls Three to six power or compression rings are fitted to the piston, the number depending on weather the engine operates on the 2-stroke or 4-stroke cycle.

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  4. Requirement of piston ring

    5. Piston forms the lower part of the combustion chamber in a marine diesel engine. It seals the cylinder and transmits the gas pressure to the connecting rod.

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  5. Piston designing considerations

    6. Piston forms the lower part of the combustion chamber in a marine diesel engine. It seals the cylinder and transmits the gas pressure to the connecting rod.

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    Troubles of piston to be avoided
    i) Piston seizure; all too often followed by an explosion of the lubricating oil – air mixture in the crankcase. ii) Cracking of piston crown and side walls, due to cyclic variation of pressure stress superimposed on stress due to temperature gradient.

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  6. Detection of piston overheating

    7. most common reasons of Piston running hot : i) Inadequate circulation of cooling media and or supply not sufficient. ii) Excessive deposit in cooling space (scale or carbon). iii) Lubrication not sufficient.

    More .....



Marine diesel engine related other useful articles:
  1. Four stroke cycle diesel engines operational guideline

    2. The four-stroke cycle is completed in four strokes of the piston, or two revolutions of the crankshaft. In order to operate this cycle the engine requires a mechanism to open and close the inlet and exhaust valves
    More .....

  2. Two stroke cycle diesel engines operational guideline

    3. The two-stroke cycle is completed in two strokes of the piston or one revolution of the crankshaft. In order to operate this cycle where each event is accomplished in a very short time, the engine requires a number of special arrangements.
    More .....

  3. Power measurement for marine diesel engine - The engine indicator

    4. There are two possible measurements of engine power: the indicated power and the shaft power. The indicated power is the power developed within the engine cylinder and can be measured by an engine indicator. The shaft power is the power available at the output shaft of the engine and can be measured using a torsionmeter or with a brake.
    More .....

  4. Supply of fresh air and removal of exhaust gases by a gas exchanger

    5. A basic part of the cycle of an internal combustion engine is the supply of fresh air and removal of exhaust gases. This is the gas exchange process. Scavenging is the removal of exhaust gases by blowing in fresh air.
    More .....

  5. The fuel oil system for a diesel engine

    6. The fuel oil system for a diesel engine can be considered in two parts—the fuel supply and the fuel injection systems. Fuel supply deals with the provision of fuel oil suitable for use by the injection system.
    More .....

  6. Lubricating oil system for a marine diesel engine - how it works

    7. The lubrication system of an engine provides a supply of lubricating oil to the various moving parts in the engine. Its main function is to enable the formation of a film of oil between the moving parts, which reduces friction and wear. The lubricating oil is also used as a cleaner and in some engines as a coolant.
    More .....

  7. Cooling of ships engine - how it works , requirement of fresh water & sea water cooling system

    8. Cooling of engines is achieved by circulating a cooling liquid around internal passages within the engine. The cooling liquid is thus heated up and is in turn cooled by a sea water circulated cooler. Without adequate cooling certain parts of the engine which are exposed to very high temperatures, as a result of burning fuel, would soon fail.
    More .....

  8. Starting air system for diesel engine - how it works

    9. Diesel engines are started by supplying compressed air into the cylinders in the appropriate sequence for the required direction. A supply of compressed air is stored in air reservoirs or 'bottles' ready for immediate use. Up to 12 starts are possible with the stored quantity of compressed air.
    More .....

  9. Governor-Function of governors controlling speed of marine diesel engine

    10. The principal control device on any engine is the governor. It governs or controls the engine speed at some fixed value while power output changes to meet demand. This is achieved by the governor automatically adjusting the engine fuel pump settings to meet the desired load at the set speed.
    More .....

  10. Cylinder relief valve of a marine diesel engine - operational guideline

    11. The cylinder relief valve is designed to relieve pressures in excess of 10% to 20% above normal. The operation of this device indicates a fault in the engine which should be discovered and corrected.
    More .....

  11. Explosion relief valve of a marine diesel engine

    12. As a practical safeguard against explosions which occur in a crankcase, explosion relief valves or doors are fitted. These valves serve to relieve excessive crankcase pressures and stop flames being emitted from the crankcase. They must also be self closing to stop the return of atmospheric air to the crankcase.
    More .....

  12. Turning gear operational guideline
    The turning gear or turning engine is a reversible electric motor which drives a worm gear which can be connected with the toothed flywheel to turn a large diesel. A slow-speed drive is thus provided to enable positioning of the engine parts for overhaul purposes.
    More .....

  13. Couplings, clutches and gearboxes of a marine diesel engine

    14. The principal control device on any engine is the governor. It governs or controls the engine speed at some fixed value while power output changes to meet demand. This is achieved by the governor automatically adjusting the engine fuel pump settings to meet the desired load at the set speed.
    More .....

  14. MAN B&W diesel engine - Basic principles and operational guideline

    15. It is one of the MC series introduced in 1982, and has a longer stroke and increased maximum pressure when compared with the earlier L-GF and L-GB designs.
    More .....

  15. Crankcase oil mist detector of a marine diesel engine

    16. It is one of the MC series introduced in 1982, and has a longer stroke and increased maximum pressure when compared with the earlier L-GF and L-GB designs.
    More .....

  16. Various Heat exchanger for running machinery on board cargo ships

    17. Shell and tube heat exchangers for engine cooling water and lubricating oil cooling have traditionally been circulated with sea water. The sea water is in contact with the inside of the tubes, tube plates and water boxes.
    More .....

  17. Guideline for Turbochargers safety and operational requirement

    18. Shell and tube heat exchangers for engine cooling water and lubricating oil cooling have traditionally been circulated with sea water. The sea water is in contact with the inside of the tubes, tube plates and water boxes.
    More .....

  18. Function of Piston & piston rings

    19. Piston forms the lower part of the combustion chamber. It seals the cylinder and transmits the gas pressure to the connecting rod. The Piston comprises of two pieces; the crown and the skirt.The crown of a piston is subject to mechanical and thermal stresses.
    More .....







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