Force Feed Systems Except for petrol two-stroke, and small four-stroke auxiliary engines, most marine engines use a ‘force feed’ lubrication system (Figure 23). Typical components of a force feed system are: oil supply tank or sump oil pump oil filter oil galleries feeding critical engine components oil cooler pressure and temperature gauges, if required. In the force feed lubrication system, oil is pumped from a sump or tank, filtered, andsupplied under pressure through oil galleries to critical components (bearings, etc.)of the engine. Less critical parts of the engine (gears, chains, etc.) are lubricated byoil splashed from bearings and other oil fed parts. The used oil drains into the sump of the engine, where it is returned to the oil pump or supply tank, to be used over and over again. Most force feed systems have the oil reserve stored in the sump of the engine (like car engines). These are called ‘wet-sump’ lubrication systems. Others have the oil reserve stored in an external supply tank. These are called ‘dry-sump’ systems The system in Figure 26 uses a ‘wet sump’. The oil supply is stored in the sump below the engine. A pump in the sump circulates the oil to the engine. After lubricating the engine, used oil drains back to the sump to be constantly re-circulated through the engine. Figure 27 shows a ‘dry sump’ system— oil is returned to the external oil tank by a scavenge pump. The scavenge pump is larger than the delivery pump to stop oil ‘pooling’ in the sump. Dry sump systems have larger oil tank capacity, and are more reliable in heavy seas. The oil also remains cooler, and is less likely to leak past engine seals.
Figure 26: ‘Wet-Sump’ Force Feed Lubrication System
Figure 27: ‘Dry-Sump’ Force Feed Lubrication System
Notes The oil delivery pump can develop extremely high pressures. A relief valve on the oil pump will open to limit the oil pressure, to prevent damage to the engine.
If the oil filter blocked up, no oil would get to the engine. The bypass valve will let unfiltered oil past a blocked filter to supply the engine. (Slightly dirty oil will do less damage than no oil at all.) Acids and contaminants build up in the oil. Always replace the oil and filters when indicated by the service schedule, or earlier if they need it. Use the correct type and grade of lubricant, and never mix them. Oils and greases with different chemical bases may interact and cause expensive damage. Lubrication of Two-Stroke Engines Diesel two-stroke engines normally have a conventional ‘wet-sump’ force feed system as just described. Most two-stroke petrol engines are lubricated by ‘petrol’ or ‘outboard’ mix as it passes through the crankcase during induction. To ensure proper lubrication, and to prevent starting and running problems, you must make sure that: only the correct type of two-stroke oil is used the correct quantity of oil is added to the petrol the petrol and oil are thoroughly mixed together before being used A few high performance two-strokes, force feed a small amount of undiluted oil directly into the bearings and critical components from an oil supply tank. This is called ‘oil injection’. The used lubricating oil is eventually burned with the fuel, and the oil tank must be occasionally topped up to replace the lost oil. Straight petrol is used with oil-injected two-strokes.
Sistem Pengisian Udara Kecuali untuk dua-stroke, dan kecil empat-stroke bantu mesin bensin, mesin-mesin kelautan yang kebanyakan menggunakan u2018force % yang memberi makan sistem pelumasan % u2019 (gambar 23). Komponen yang khas dari kekuatan yang memberi makan sistem adalah:
