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Crankshaft Peak Firing Pressure Bearing Capability Enhancement
Abstract:
To uprate a 6-Cylinder In-line engine from 123 kW to 165 kW in power and upgrade the emission from Euro-2 to Euro-3 it was required to go for higher peak-firing pressures (PFP). The capability of Engine's Crankshaft to withstand the PFP was increased from 125 bar to 150 bar, maintaining the same cylinder centre distance. A crank-train model was used to achieve the required crankshaft strength for infinite fatigue life. The three aspects of crankshaft design, namely, crank strength, bearing selection, journal-pin lubrication and torsional vibration were considered during the design stage.
The strength to withstand 150 bar PFP was achieved by increasing the crank web-thickness. To maintain the same cylinder centre distance, crankpin and main-journal lengths were reduced. Increased throw stiffness was achieved by increasing the crankpin diameter to improve crankshaft torsional behaviour. Fatigue factor of safety for the webs were calculated as per the FVV (Forschungsvereinigung Verbrennungsmotoren) method. Bearings were modeled as hydrodynamic bearings and the calculations were carried out with Sommerfeld number as per Butenschoen procedure. Torsional vibration calculations were carried out as per the procedure in BICERA (British Internal Combustion Engine Research Association) handbook and the maximum shaft twist was controlled using an appropriate damper.
The new crankshaft passed the fatigue test in a test-rig for 150 bar PFP. Crankshaft lubrication and bearing life were validated by an engine endurance cycle to accelerate the failure. Torsional vibration measurement was carried out and the maximum free-end torsional deflection was within 0.2 deg. The results from crank-train simulation agreed well with the experimental results. The new crankshaft design is now in series production and has been proven in field for over a million km.