Cylinder liners should be gauged at regular intervals during overhaul (normally at 16000 hrs) to accurately measure the increase in bore. Continuous record of gauging should be maintained on board for each cylinder.
The liner must be cleaned and inspected. A careful examination must be made to ascertain if lubrication is adequate and for any surface cracks. Liner is gauged with a inside micrometer and extension bar. Gauging are taken at number of vertical positions (normally 8 to 10) over the area swept by piston rings.
Readings are taken in fore and aft and in athwart ships directions. To ensure readings are taken at same corresponding points, a template is used, which consist of a flat bar with holes drilled through were measurements are to be taken.
Gauging figures are noted in a tabular form as shown in the figure. Total wear from original and mean rate of wear since the last recordings can be calculated.
Types of Liner wear:
Abrasive wear – It occurs when abrasive particles enter the combustion space with scavenge air or a result of poor quality or contaminated fuel. High abrasive wears are noticed if fuel oil is contaminated by catalytic fines.
Corrosive wear : It occurs when burning heavy fuel which contains significant amount of sulfur. As the fuel burns the sulfur combines with oxygen to produce oxides of sulfur which form sulfuric acid on contact with water. To minimize the formation of acids it is important that the cylinder liner temperatures are maintained above dew point. (It is the temperature at which the air can no longer hold all its water vapor, and some of its vapor condenses into liquid).
Adhesive Wear : (Also refered as “Scuffing”)Occurs when the rough surface on Liner and Piston ring have metal to metal contact, resulting in local welding and metal deformation. Rougher the surface greater the likelihood of adhesive wear. It can occur if the lubricating film between ring and liner is removed due to excessive temperature, insufficient supply of oil, blow past, etc.
Reasons for abrasive wear are:
1. presence of abrasive particles.
2. high liner surface temperature.
3. failure of lubrication
Frictional Wear
Whenever two surfaces glide/moves over each other, Rubbing occurs, causing the frictional wear of the two surfaces. Coating the wear surfaces, the piston rings slide on the cylinder bush. The friction wear depends on several factors, such as the speed of movement between the surfaces, the material, temperature, engine load, pressure, maintenance, lubrication, and combustion efficiency.
Microseizure :This occurs due to breakdown of lubrication film which may be due to insufficient quantity of cylinder oil or excessive load. Normally occurs during running-in period.
Cloverleafing :It is the term used to describe high corrosive wear which occurs in the liner between the oil injection points.
AS the cylinder oil enters the cylinder it will start neutralizing the acids, becoming less alkaline. If the TBN of the oil is too low then its alkalinity may be depleted before it has completely covered the liner. Further contact with acids may lead to oil itself becoming acidic. This will lead to cloverleafing.
Possible errors and causes
The pattern of wear over the length of the liner tends to be greatest at the top of the stroke adjacent to combustion space, where pressure and temperature are greatest. This reduces towards the lower end of the stroke, but will increase at the exhaust and scavenge ports due to boundary lubrication, reduction in surface area and oil being blown out into scavenge space.
 |
| Record of cylinder Liner wear |
The pattern of wear over the length of the liner tends to be greatest at the top of the stroke adjacent to combustion space, where pressure and temperature are greatest. This reduces towards the lower end of the stroke, but will increase at the exhaust and scavenge ports due to boundary lubrication, reduction in surface area and oil being blown out into scavenge space.
Liner Wear Rate :
The rate of wear varies over the life of liner. It is high during the initial running-in period after which it should reduce to an almost constant rate for the most useful life of the liner. Finally the rate will progressively increase as wear becomes excessive.
Max permissible wear of the liner is about 0.6 – 0.8% of the bore. A wear rate of 0.05 – 0.07 mm/1000 hours is generally accepted.
Author Amit
Posts
Post a Comment