LO 8-9 Explain the purpose, function, and construction of lubrication circuits for the crankshaft

1. Crankshaft Lubrication

Crankshaft Lubrication

To lubricate the main and rod bearings, pressurized engine oil needs to be directed in sufficient quantities to lubricate, cool, and clean the crankshaft journals and bearings. Engine oil is delivered to the oil galleries in the block and then distributed to the main bearings using holes present in the upper main bearing shells Figure 62. A cross-drilled hole in the crank’s main bearing journal uniformly distributes oil around the crankshaft main journal through a groove in the bearing shell. The crankshaft also has passages drilled from the main bearing cross-drilling to the rod journal, connecting each main bearing to an adjacent rod bearing. The exception to this is the rear main, which in some cases has a slightly smaller oil hole into the block for lubrication of only one main journal. This relieves lubrication oil pressure exerted against the rear main seal, which minimizes potential leaks and oil weepage from the rear main seal.

FIGURE 08-62: A. Drilled passageways though the crankshaft distribute pressurized oil to the rod bearings and piston pins. B. Passageways drilled between the oil rail and the main bearing supply oil to the crankshaft.

FIGURE 08-63: The crankshaft accounts for close to 25% of all engine friction. The piston and rings, valve train, and accessory drive system running injectors and the oil and water pumps account for the bulk of the remaining friction.

The latest engines use very narrow oil clearances compared to traditional oil clearances. Narrow, smaller oil clearances are necessary to enable the use of energy-saving, lower-viscosity oils. Lower-viscosity oils reduce fuel consumption because they flow with less resistance and do not require as much power for the oil pump to pressurize as much oil volume when operating clearances are smaller. More significantly, lower-viscosity oil, in thinner operating clearances, does not resist crankshaft rotation from shafts shearing thick films of oil. Given that the crankshaft and its bearings contribute to around 25% of total engine friction, fuel consumption is reduced by as much as 2% Figure 63.

When bearings are serviced or when a crankshaft is installed in an engine, it is important to clean out the oil passages by running a narrow wire brush through the cross-drilling. If the crankshaft is on a bench, compressed air can also be used to blow out any residue of sludge or metal filings left over from machining processes. Any metal particles or sludge left in the cross-drillings after service can not only quickly destroy a bearing or starve the journal of oil but also can result in catastrophic engine failure.