BMW engines have a so-called circulating lubrication system. The oil pump takes oil through the oil intake from the oil sump and supplies it to the main oil filter. There is a pressure reducing valve at the outlet of the oil pump. The valve opens when the pressure is too high and part of the oil flows back into the oil sump. Oil enters the main channel along the central axis of the oil filter cartridge. When the oil filter is clogged, the bypass valve opens and supplies unfiltered oil to the main channel.
A bypass valve is installed behind the oil filter (oil pressure control valve), which keeps the oil pressure at approximately 4 bar. When the pressure rises, the valve opens and part of the oil flows back into the oil sump.
Lubrication channels for the crankshaft bearings branch off from the main channel. Through inclined holes in the crankshaft, oil enters the connecting rod bearings and is sprayed from there onto the piston pins and cylinders.
At the same time, through inclined channels, engine oil is supplied to the cylinder head to the camshaft bearings, rocker shafts and rocker arm supports. Check valves ensure that sufficient oil is always supplied to lubricate these parts in the cylinder head.
In the presence of a chain drive, oil is also supplied to lubricate the timing chain, as well as the oil pump drive chain.