11.2.
Functions of the Lubricating System
The engine lubricating system supplies oil to all moving parts. A typical engine lubricating system is shown in Fig. 11.1. The oil pump picks up oil from the oil pan and sends it up through the oil passages to the main bearings of the crankshaft. Oil from the main bearings moves through oil holes drilled in the crankshaft to the rod bearings. Oil then passes through an oil line to the cylinder head where it flows through an oil gallery to camshaft bearings and valve train parts. The pistons, piston rings, and piston pins receive oil thrown off the connecting-rod bearings. This oil falls on the cylinder walls thereby reaches the pistons, rings and pins. Then the oil drops back down in to the oil pan.
Lubrication of bearings is necessary to achieve maximum service life of the engine. However, their lubrication is quite simple and is easily met through properly designed bearings and by using oil with the correct viscosity. The highest unit pressures and the most difficult lubrication actually occur between the cam lobes and valve lifters. Present day motor oil formulation is based on the oil’s ability to minimise lifter scuffing and wear. Cam lobes are not lubricated with positive pressure but depend on oil thrown from the connecting rods and on oil, which drains back from the rocker and lifter chambers. Valve assemblies, pistons, piston pins, oil pump-distributor drives and cam drives require only a surface film of oil. Since the loads are relatively light the oil received from splash is usually adequate. Oil under slight pressure is usually sent to the rocker arms. It is not the amount of pressure, but the definite delivery of oil to the moving surface needing lubrication is important. Some engines provide an oil flow to the cam drive. This oil helps to cushion the drive and reduce noise. Automobile engines also utilize engine oil
to operate hydraulic valve lifters, which requires the engine oil to have a different kind of characteristic. Hydraulic lifters are manufactured with extremely close tolerances to minimise leakage. The engine oil keeps the lifter clean, minimizing deposit formation to avoid lifter sticking.
Fig. 11.1. Engine lubrication system.
One of the functions of the engine lubrication system is to maintain a positive and continuous oil supply to the bearings. Engine oil pressure is high enough to get the oil to the bearing with sufficient force to produce adequate oil flow for proper cooling. Normal engine oil pressure range is from 207 to 414 kPa while the hydrodynamic film pressures developed in the high-pressure areas of the engine bearing may be over 6895 kPa.
