to Engine bearings
Dr. Dmitri Kopeliovich
Engine parameters determine the loads applied to the bearings. The load is the resultant force created from the combustion pressure and inertia forces of the moving engine parts.
The load applied to the bearing at any angle of the crankshaft may be calculated and so too can the hydrodynamic bearing parameters (min. oil film thickness, energy loss, oil temperature rise, oil flow, etc.).
The engine factors determining the load applied to the bearing are as follows:
Compression ratio. The higher the C.R. the higher the pressure of the gas in the TDC (top dead center) and the higher the bearing load.
Displacement per one cylinder. Larger displacement means larger amount of fuel-air mixture is intaken into the cylinder in each engine cycle and larger energy is discharged in each cycle.
Fuel type. Gasoline fuel with higher octane number allows working with higher compression ratio without the risk of detonation. Therefore higher octane fuel produces higher load. Methanol fuel may be used with even higher compression ratio (e.g. 16:1).
Engine type. Diesel engines especially diesel engines with direct injection produce highest pressure in the combustion cylinder (up to 220 bar) and highest load applied to the bearings.
Aspiration type. Turbo- and supercharged engines use compressed air in the air-fuel mixture. Compressed air allows to intake larger air amount and larger fuel amount in each engine cycle since air-to-fuel ratio is approximately constant. So the charge of energy is higher than in normally aspirated engines and the load is also higher.
Bore diameter. The same cylinder pressure produces higher absolute load in the cylinder with higher bore diameter.
Piston and conrod weights and rotation speed. Inertia forces developed by the moving piston and conrod decrease the resultant force acting on the upper conrod bearing. At the same time they increase the force acting on the lower part. The inertia forces are proportional to the weights of the moving parts and to the squared rotation speed therefore the effect of inertia forces is particularly high at high RPM (6000 – 10000).