A Multi-Variable Study of Diesel Geartrain NVH Performance, 03-R9861

Principal Investigators
George E. Bailey
Douglas R. Fussner

Inclusive Dates:  10/01/08 – 04/01/10

Background - Geartrain noise can be a significant contribution to the overall sound level of diesel engines. Some engine manufacturers employ isolation solutions such as sound deadening covers and foam panels to combat the problem, but these add cost. Little has been published on geartrain noise reduction, and public standards for diesel geartrain design and development are not available. This effort is addressing these problems by developing a mathematical model for designing diesel engine geartrains with improved noise, vibration and harshness (NVH) characteristics.

Approach - A lumped-parameter vibration model was developed using an open-source scientific software package. The model was verified through experiments performed on a four-cylinder diesel engine with a geartrain consisting of six gears. Eleven degrees of freedom were modeled including the rotations of the six gears, the radial and axial displacement of the crankshaft, and three axial displacements of the main idler gear. Recently published gear tooth mesh stiffness and damping models were used as a starting point and modified as necessary to correlate with test results. Two additional sets of gears were designed with different tooth geometries to provide different input geometries to the model and different rattle behavior in the engine. These gear sets were prototyped and tested in the engine along with the engine's original gear set. Bearing clearance and backlash, both well-known to influence gear NVH, were varied and tested.

Accomplishments - Verification of the lumped parameter vibration model required inputs that included the motion of the crankshaft. The rotation of the crankshaft was measured with an optical encoder, and the translational motions were measured with lasers, as shown in Figure 1. The two new gear sets were designed. A transient engine dynamometer profile was developed to provide a repeatable test to load the geartrain at three different levels throughout the operating speed range of the engine. The experimentation quantified the influence of an anti-backlash idler gear in reducing gear rattle noise and revealed that a key path for gear rattle noise transmission is through an idler gear journal bearing shaft. A backlash investigation showed that increasing backlash increases rattle, with a greater penalty at lower speed. While definite conclusions could not be made, there was evidence that changing to a different idler gear material with increased internal damping properties had an effect in reducing gear rattle noise. Increasing the contact ratio in a helical gear set and increasing the idler gear journal bearing clearance allowed the anti-backlash gear to be eliminated while maintaining the same overall noise level. The gear rattle sound became audible, however. The analytical model takes the gear tooth design parameters into account and was able to predict the observed differences in dynamic behavior between the various gear designs and parameter variations. This analytical model is now available to aid in the design of quiet diesel engine geartrains.

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