PISTON

MARINESHELF publishes articles contributed by seafarers and other marine related sites solely for the benefit of seafarers .All copyright materials are owned by its respective authors or publishers.

PISTON.

Piston Construction:  

In the majority of highly rated 2 stroke engines, the piston is either of two or three part construction Cast steel crown and cast iron skirt are combined in the piston shown below to obtain the strength and heat resistance of steel in the upper section where these properties are important and the good wearing properties of cast iron in the lower section, where piston bears against the cylinder. The piston ring grooves in the piston crown are chromium-plated. Old engines used to have pistons of cast iron, then with the switch from air blast fuel injection to direct airless injection around 1930, firing pressure rose from some 40 kg/cm² to 50 kg/cm² and on towards 60 kg/cm². This higher loading led to the use of forged steel piston crowns.

Piston Materials:

Cast Iron:

There was a time this was the most common material for piston. Cracking of cast iron piston was much reduced by the use of iron castings of pearlitic structure, with fewer tendencies to growth; as well as oil cooling of the larger sizes of piston. The increasing mean pressures, piston speeds of industrial turbo-charged Diesel engines are making it increasingly difficult to obtain a satisfactory safety factor for single piece pistons. High tensile spheroidal graphite cast iron is also not very suitable because its creep properties are not as good as Flake graphite cast iron. Flake graphite present in the structure gives good wearing properties it does not stand very high thermal stress. It is now mainly used for small low rated engines. Commonly used for skirt of composite pistons.

Forged Steel:

Chrome molybdenum steel has become quite common these days for piston crown. It is heat resisting, strong and ductile. It has Poor wearing properties.

Aluminum Alloy:

Mostly used for smaller pistons of medium and high-speed engines. The piston saves in weight i.e. much lighter and gives better heat flow. The drawback to aluminum-alloy pistons is high rate of expansion of the material. This means that an appreciable clearance has to be allowed when the piston is cold to ensure a safe running clearance when the piston reaches maximum temperature.

Piston Cooling:

Let us trace the development of the mode of cooling used by one large marine engine manufacture.

1. At first the pistons were cooled with seawater, which was led in and out through telescopic ‘trombone’ pipes whose glands never sealed perfectly. Consequently seawater used to get into the crankcase, causing corrosion on parts of the running gear. Corrosion on the crank and cross head pins gave rise to bearing troubles. Moreover corrosive attack inside the piston cooling space could never be ruled out entirely despite protective coatings.
2. Improvement came with the adoption of close fresh water cooling circuit, enabling heat extracted from the engine heat to be rejected to the seawater via the exchanger. Though this improved matters, corrosion could still not be completely prevented on pistons and bearings.
3. With the adoption of oil as standard piston coolant in the early 1940’s, this problem was circumvented. But with the advent of supercharging in the mid 1950’s and the attendant increased thermal loads, the piston cooling oil showed a growing tendency to carbonise at the hot spots in the piston cooling space. This obstructed the heat transfer leading to the overheating of the piston surface facing the combustion chamber and eventually to burning away of piston material. In view of the further increase in mean pressure and still greater cylinder boxes anticipated, it was concluded that oil cooling had reached its limits.
4. Studies and measurements on a test cylinder with 76 cm bore confirmed that the return to water- cooling for the pistons of supercharged engines would be beneficial with regard to piston temperature.



Modified type to allow cooling water to reach the crown lower part to remove the heat of the crown material and keep it with in the designed thermal stressescaption

5. However the advantages of water-cooling for the pistons could be exploited only after ways and means were found to prevent water leaking into the crankcase. As before the water is led into and out of the piston through telescopic pipes but this are now placed inside a watertight enclosure. In this way any leakage from the glands is collected and led off and there is no possibility of water getting into the crankcase and causing corrosion there.   

   

   Early Piston of a large slow speed diesel engine (Cocktail shaker Type