Lub

K. V Karanth, M.I.T Manipal.

ENGINE LUBRICATION Purpose of lubrication 1. To reduce friction between the moving parts. 2. To reduce wear of the moving parts by way of lubricating the surfaces and keeping them apart. 3. Remove heat from engine parts by acting as a cooling agent. To keep down the temperature of the moving parts and thus prevent scuffing and seizure. 4. To act as a seal and prevent leakage between the parts such as pistons, piston rings and cylinders. 5. Absorb shocks between bearings and other engine parts, thus reducing engine noise and extending engine life. 6. Acts as a cleaning agent. To wash away acidic accumulation and abrasive metal worn from the friction surfaces.

Crankcase ventilation In the case of four stroke engines, air must circulate through the crankcase, when the engine is running. This removes the hot lubricating oil vapors and blow by from the crankcase. For entry of air and exit of the gases, the crankcase is provided with a breather tube. The air entry into the crankcase also cools the oil to some extent. However, discharging the crankcase vapors into the atmosphere causes air pollution. To prevent atmosphere pollution, modern engines have a positive crankcase ventilation (PCV) system. In this case, the crankcase is connected by a pipe to the inlet manifold. This enables the oil vapors to be sucked into the inlet manifold during engine suction and burned.

Methods of lubrication: The different parts of an engine are lubricated by anyone combinations of the following methods. 1. Mechanical system – Splash lubrication 2. Pressure lubrication system. a. Wet sump lubrication system b. Dry sump lubrication system. 3. Semi pressure lubrication system.

Splash lubrication: In the splash lubrication system, the oil retained in the oil pan is churned and splashed up by the internal parts of the engine (connecting rod big end and crankshaft) into a combination of liquid and mist. This oil mist is sprayed over the interior of the engine i.e. on the cylinder walls and on the underside of the piston crown. Some of the oil splashed gets collected in pockets over the main bearings. The collected oils then flow through the bearings by means of oil hole and grooves in the bearing surfaces.

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K. V Karanth, M.I.T Manipal.

In one design, the connecting rod big end caps have scoops. These scoops pick up oil from the oil pan, during the lower ends of the connecting rod travel. The connecting rod caps have drilled holes. Through these holes, part of the oil picked up by the scoop reaches the end bearings. The remaining oil lubricates the rest of the engine parts by splash i.e. the throwing of oil by the connecting rods, crankshaft and other oiled moving parts. In another design, the connecting rod big end cap has thin projections. These projections improve oil splashing.

Pressure lubrication system: In the pressure lubrication system, the lubricating oil is pumped under pressure to the various engine bearings. The oil is delivered by an oil pump under a pressure of 1.30 to 1.40 kscm into an oil gallery or distributor duct. This gallery distributes oil to the various engine parts and bearings. The oil pump is driven by the engine crankshaft or camshaft. This system can be seen in figure. The crankshaft main bearing (each one) is pressure fed from the main oil gallery. Internal drilled holes (or ducts) in the webs of the crankshaft conduct oil from the main bearings to the connecting rod big end bearings. The oil flows from the big end bearing through the long hole drilled in the shank of the connecting rod to the wrist. pin.

Semi pressure lubrication system The semipressure lubrication system is a combination of splash and pressure lubrication system. Most automotive engines use this system. This is more simple and less costly than the complete pressure lubrication system. This system also enables bearing loads and engine speeds than for the splash system.

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K. V Karanth, M.I.T Manipal.

1. Wet sump lubrication In the wet sump lubrication system, the main oil supply is kept in the sump which is below the engine cylinder in the crankcase. The oil pump draws oil from the crankcase and forces it through the lubricating oil filters to the various parts of the engine. This system is widely used in cars and trucks. In some engines, the oil pump is submerged in the sump oil. Some designs use tiling intake pipe and strainer unit. Where there is no oil hole in the connecting rod shank, the cylinder walls and piston pins are lubricated by the spray of oil that is forced out of the connecting rod bearings and thrown by the revolving cranks. This spray also lubricates many exposed internal parts. A separate line supplies lubricant to the accessories and their drive shafts, valves and rocker arms. Used oil drains back into the crankcase by gravity for recirculation.

2. Dry sump lubrication: The dry sump lubrication is used in more expensive cars. In the dry sump pressure lubrication system, there is no oil sump in the crankshaft chamber. In this system, the oil is kept either in a separate tank or reservoir, provided with cooling fins. Two pumps are used in this system. One pump sucks the oil from the reservoir and forces it under pressure to the various bearings of the engine, as in the wet sump system. The other pump (also called scavenger oil pump) is of large capacity. This pump sucks oil which drains down to the bottom of the crankshaft chamber, and returns it to the oil reservoir. The main advantage of the dry sump lubrication system is that the oil is cooled during its circulation. As such, the oil has better lubricating value. Dry sump system is suitable for engines fitted to vehicles which may have to work in inclined positions.

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