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The Well Travelled Engine
Thoughtful design practices can be found in nearly every part of a modern ships engine room and no more so than in the engines themselves. These have to turn, pump, cool, inject, breath, exhaust and power in a very reliable manner. During this endeavour they are corroded, eroded, worn, burnt but otherwise generally treated tenderly by the ships staff.
Take for example the piston rings on two typical engines:
The first is a generator engine, 200mm bore, 220mm stroke 1000 rpm. Now this engine runs at constant speed, 80% MCR for (say) 300 days per year. How far do the piston rings travel in that time ?
Distance per: Stroke = 0.22 x 2 = 0.44m
Minute = 0.44 * 1000 = 440 m
Day = 440 * 60 * 24 = 633600 m
Year = 633600 * 300 = 190,000 km
That is in 1 year each piston ring will travel 190,000 kilometres or about 5 times round the world (probably about as far as the ship itself)
Now let us consider the main engine, 850m bore, 2500mm stroke 100 RPM, at sea for 300 days per year.
Distance per:
Stroke = 2.4 * 2 = 5m
Year = 216, 000 km
Or in 1 year the rings travel about the same distance as those for the generator. This is not a coincidence as the maximum piston speed of most engines is selected for durability reasons to around 10 m/s irrespective of engine size.
Now following on from this, ponder upon the following : The oil that lubricates the rings and liners is scraped into a very thin film. At mid stroke this film is at its thickest, being about 10 micron or 10 * 10-6 meters. That is less than the thickness of a human hair. At the extremities of the stroke this thickness reduces to nearly zero. So in the engine you have a large number of piston rings travelling constantly for very great distances. They are separated from imminent destruction by an oil film thinner than a human hair. They perform this magical feat constantly and reliably, often without even a thought from the engineers walking past. The only reason this can happen is because of the oil film itself.
It pays to look after your oil!
Take for example the piston rings on two typical engines:
The first is a generator engine, 200mm bore, 220mm stroke 1000 rpm. Now this engine runs at constant speed, 80% MCR for (say) 300 days per year. How far do the piston rings travel in that time ?
Distance per: Stroke = 0.22 x 2 = 0.44m
Minute = 0.44 * 1000 = 440 m
Day = 440 * 60 * 24 = 633600 m
Year = 633600 * 300 = 190,000 km
That is in 1 year each piston ring will travel 190,000 kilometres or about 5 times round the world (probably about as far as the ship itself)
Now let us consider the main engine, 850m bore, 2500mm stroke 100 RPM, at sea for 300 days per year.
Distance per:
Stroke = 2.4 * 2 = 5m
Year = 216, 000 km
Or in 1 year the rings travel about the same distance as those for the generator. This is not a coincidence as the maximum piston speed of most engines is selected for durability reasons to around 10 m/s irrespective of engine size.
Now following on from this, ponder upon the following : The oil that lubricates the rings and liners is scraped into a very thin film. At mid stroke this film is at its thickest, being about 10 micron or 10 * 10-6 meters. That is less than the thickness of a human hair. At the extremities of the stroke this thickness reduces to nearly zero. So in the engine you have a large number of piston rings travelling constantly for very great distances. They are separated from imminent destruction by an oil film thinner than a human hair. They perform this magical feat constantly and reliably, often without even a thought from the engineers walking past. The only reason this can happen is because of the oil film itself.
It pays to look after your oil!
