Alpha Lubricating System

Cylinder lubricating oil feed rate

• Cylinder oil feed rates should be set with a reasonable margin of safety on the recommendations of the engine builder/designer. A high oil feed rate, far greater than 150 per cent nominal, is seldom required other than for a short period during running-in or when special circumstances justify it. 
• Over-lubrication can in fact be harmful in certain adverse conditions. Unless there are abnormal circumstances, it is pointless to pursue prolonged operation with an excess of 2.72 g/kWh, especially in conjunction with low Sulphur fuel, 70 BN cylinder oil and an engine series well designed against corrosive wear and with no need for excessive BN. Even for a highly detergent oil, over-lubrication can lead to excessive calcium carbonate deposits in the remote top edge of the hot piston crown land. The BN of all commercial cylinder oils today is derived largely from calcium carbonate. In adverse conditions it is sometimes not the oil detergency that cannot cope but rather there is an enormous generation of calcium carbonate deposit. 
• An engine well designed against corrosive wear can become sensitive to excessive feed rate on 70 BN oil when burning fuel with a normal sulphur content. This is because if the BN is not used, the unused BN can result in hard calcium carbonate deposits (used BN results in a softer calcium sulphate). 
• A high cylinder oil feed rate, can lead to a disturbed pressure drop across the ring pack and negatively affect piston running. 
• Excessive cylinder oil also tends to increase the formation of deposits on the top land of a piston. A solid crust is formed, which is softer or harder depending on the additives used in the oil, the type of fuel and its sulphur content, as well as its combustion characteristics. 
• An excessive feed rate can, in fact, also lead to the destruction of a scuffing liner within hours. Another reason for caution over excessive feed rates is that modern engines are significantly more powerful, burn more fuel and use more cylinder oil per cylinder than before. The cylinder oil ash level could be more than five times higher than in engines of the 1970s and 1980s.

‘Intelligent’ cylinder lubrication 

• Demands placed on cylinder oil in two-stroke engines vary widely and depend on the operational conditions. The cylinder of a large bore engine is typically fed with just one gram of lubricant during each stroke, and this small amount must be spread correctly to fulfil all requirements. 
• Studies of the relationship between wear and lube oil dosage reveal that interaction between operational factors such as load variations, fuel and lube oil qualities, and atmospheric humidity exert a strong influence on the wear rates of piston rings, ring grooves and cylinder liners. It has also become clear that over lubrication has a considerable negative effect on the tribological conditions of liners and rings. 
• Traditional cylinder lubrication systems, whereby the lube oil is injected at a fixed rate proportional to the engine rev/min or mean effective pressure and perhaps adjusted occasionally based on a scavenge port inspection, may yield a reasonable average condition. Such a condition, however, will result from long periods of excessive lubrication and shorter periods of oil starvation because the sum of influencing factors will change on a daily or sometimes even on an hourly basis. Furthermore, there is the risk of ruining the cylinder condition if unfavorable operational factors are not detected and properly counteracted by the engine operators. 
• Operational conditions beyond the norm are unavoidable, and many shipowners simply over-lubricate their engines under the false assumption that they will thus always be ‘on the safe side’. But overlubricating is not only expensive; it may even be counterproductive in promoting scuffing through excessive carbon deposits and/or ‘bore polished’ running surfaces. 
• An optimal basic setting of cylinder lubricators should be proportional to the engine load and the fuel oil sulphur content. Feed rate control proportional to the load is one of two standard options of Alpha Lubricator system; the other is to control lubrication in proportion to the mean effective pressure. 
• Lubricator control in relation to the fuel oil sulphur content may either be carried out automatically based on a feed-forward signal from the fuel inlet line or manually, based on the sulphur content from the bunker receipt or fuel oil analysis data. 

Alpha Lubricating System

• The electronically-controlled Alpha Lubricator was developed to inject the oil into the cylinder directly on the piston ring pack at the exact time that the effect is optimal. 
• The system features a number of injectors that inject a specific amount of lubricant into the cylinder every four (five, six etc.) revolutions of the engine. 
• The lubricator has a small piston for each quill in the cylinder liner, with power for injecting the oil provided from system pressure generated by a pump station.

 

• The properties of cylinder oil scraped from the liner wall reflect the chemical environment in the cylinders as well as the physical condition of rings and liner; and there is a direct relationship between some of the key parameters in the scrape-down oil and the actual cylinder condition. 
• A lubrication algorithm based on scrape-down analysis data, cylinder oil dosage, engine load and cylinder wear rate can thus be created. 
• Automatic optimization of lube oil dosage and cylinder lubrication efficiency is facilitated by on-line monitoring of the scrape-down oil composition from each cylinder, feeding the results into a computer (along with the above algorithm), and sending signals to each Alpha Lubricator. 
• Corrosive wear control can be based on either feed rate control or control of the cylinder oil base number; the latter approach calls for two or more lube oil tanks or blending facilities onboard. 
• Alpha Lubricators allowing significantly reduced feed rates over mechanical lubricators. Large bore engines are equipped with two such lubricators for each cylinder, while smaller bore engines have one unit. 

• Further reductions in cylinder oil consumption are promised from a so-called ‘sulphur  handle’ which, in conjunction with the Alpha Lubricator system, feeds the lubricant in proportion to the amount of sulphur entering the cylinder (Alpha Adaptive Cylinder Oil Control). The load dependent rate is applied rather than the rev/min or mean effective pressure dependent rate of other lubricator systems. Two basic requirements have to be addressed: 
• the cylinder oil dosage must not be lower than the minimum needed for lubrication; 
• and the additive amount supplied through the BN (the alkalinity throughput) must be only sufficient for neutralization and for keeping the piston ring pack clean. 

• As the second criterion usually overrides the first, the following determine the control: 
• the cylinder oil dosage shall be proportional to the engine load (that is, the amount of fuel entering the cylinders), and 
• proportional to the sulphur percentage in the fuel. 

• A minimum cylinder oil dosage is set to address the other duties of the lubricant, such as securing sufficient oil film detergency.
  

Question & Answers